JP2003152381A - Radio absorptive material and method of manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a radio absorptive material which is formed by bonding, without the generation of gaps at boundary surface, a radio absorptive layer using foaming particles of volcanic products as a binder and a radio reflective layer as a metal plate or metal foil via a bonding layer which is extremely thinner than a conventional bonding layer and also provide a method of manufacturing the same material. SOLUTION: In the radio absorptive material and the method of manufacturing the same, the radio absorptive material is formed by bonding, via a bonding layer, the radio absorptive layer which is formed by placing carbonic acid gas in contact with a radio absorptive mixture mixing the foaming particles of volcanic products, a radio losing material, and an alkali aqueous solution of silicic acid and by solidifying the mixture and the radio reflective layer as the metal plate or metal foil. Moreover, the bonding layer is the mixture of the foaming particles of volcanic products and an alkali aqueous solution of silicic acid which is formed by placing the carbonic acid gas in contact with the bonding mixture not including the radio losing material and then solidifying the bonding mixture.

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 having a radio wave absorption layer using foamed particles of volcanic ejecta and a method for manufacturing the same.

[0002]

2. Description of the Related Art In recent years, the band of radio waves used has been expanding, and it is expected that the use of radio waves with extremely short wavelengths such as millimeter waves, which were conventionally used only for special purposes, will increase. . In particular, it is considered that this tendency becomes remarkable with the development of ITS (Intelligent Transport System), which is currently being promoted. As a radio wave absorber having excellent absorption performance for radio waves having such a short wavelength, a radio wave absorber including a radio wave absorption layer using foamed particles of volcanic ejecta is known.

The foamed particles of volcanic ejecta have a lower effective dielectric constant than organic polymers such as rubber and plastic which are often used as binders in conventional radio wave absorbers. In the layer, unlike the case where the organic polymer is used as a binder, it is not necessary to strictly control the thickness of the radio wave absorption layer according to the wavelength even when applied to radio waves having a short wavelength such as millimeter waves. Further, there is an advantage that molding and solidification can be easily performed. Further, since it is a non-combustible material, it has an advantage that it does not emit smoke unlike organic polymers.

[0004]

The electromagnetic wave absorber has a conductor such as a metal plate or a metal foil on the back side of the electromagnetic wave absorbing layer for absorbing electromagnetic waves (the side opposite to the side where the electromagnetic waves are intended to be incident). It must be provided as a radio wave reflection layer. This radio wave reflection layer plays a role of canceling the incoming radio waves by reflecting the radio waves that have reached the back side while being incident on the radio wave absorption layer and being attenuated. The presence of voids or foreign matter at the interface with and becomes a factor that significantly reduces the electromagnetic wave absorption characteristics.

However, when a radio wave reflection layer such as a metal plate or a metal foil is provided on the radio wave absorption layer using the foamed particles of the volcanic ejecta, the radio wave absorption layer has a weak adhesive force to be directly adhered to the radio wave reflection layer. , Need to be glued through. The surface properties of the radio wave absorption layer are similar to those of concrete, and the radio wave reflection layer is often provided by adhering the surface with an epoxy adhesive which is generally used for adhering concrete.

However, when the epoxy adhesive is used, there is a problem that the radio wave absorbing layer and the radio wave reflecting layer cannot be bonded with sufficient adhesive force unless the adhesive layer has an average thickness of more than 2 mm. . That is, when the average thickness of the adhesive layer is 2 mm or less, sufficient adhesive force cannot be obtained, and a gap is apt to be formed at the interface between the radio wave absorbing layer and the radio wave reflecting layer, which results in remarkable radio wave absorbing characteristics. There is a problem that the reduction easily occurs. However, if the adhesive layer has an average thickness exceeding 2 mm, the distance between the radio wave absorbing layer and the radio wave reflecting layer may exceed 2 mm, which significantly affects the radio wave absorbing characteristics. . Further, when the above epoxy adhesive is used, it is difficult to control the thickness of the adhesive layer, and it is difficult to design a (two-layer type) radio wave absorber having two radio wave absorption layers.

The present invention has been made in order to solve the above problems, and includes a radio wave absorption layer using foamed particles of volcanic ejecta and a radio wave reflection layer which is a metal plate or a metal foil.
It is an object of the present invention to provide a radio wave absorber in which an adhesive layer having a thickness remarkably smaller than that of a conventional one is adhered without forming a gap at an interface, and a manufacturing method thereof.

[0008]

The present inventors have completed the present invention as a result of intensive research to solve the above problems. That is, the present invention is as follows. (1) Radio wave absorption layer formed by contacting carbon dioxide to a radio wave absorption mixture obtained by mixing foamed particles of volcanic ejecta, radio wave loss material, and alkali silicate aqueous solution, and a radio wave reflection layer that is a metal plate or metal foil Is a radio wave absorber formed by adhering via an adhesive layer, wherein the adhesive layer is a mixture of foamed particles of volcanic ejecta and an alkali silicate aqueous solution, and does not contain a radio wave loss material. An electromagnetic wave absorber made by contacting and solidifying carbon dioxide gas. (2) The electromagnetic wave absorber according to (1) above, wherein the adhesive layer has an average thickness of 0.3 mm to 2 mm. (3) The radio wave absorber according to (1) or (2) above, wherein at least one of the radio wave absorption mixture and the adhesion mixture further contains alumina cement. (4) 1 part by weight to 700 parts by weight of the radio wave loss material, 50 parts by weight to 200 parts by weight of an aqueous solution of alkali silicate, and 10 parts by weight of the mixture for absorbing the radio waves to 100 parts by weight of the foamed particles of the volcanic ejecta. Parts to 50 parts by weight of alumina cement, and the mixture for bonding is 50 parts by weight to 2 parts by weight with respect to 100 parts by weight of expanded particles of volcanic ejecta.
00 parts by weight aqueous solution of alkali silicate and 10 parts by weight to 5 parts by weight
The radio wave absorber according to (3) above, which is obtained by mixing 0 part by weight of alumina cement. (5) The radio wave absorber according to any one of (1) to (4) above, wherein the radio wave loss material is conductive carbon. (6) Conductive carbon is 100 foam particles of volcanic ejecta
The radio wave absorber according to the above (5), characterized in that 1 part by weight to 10 parts by weight is blended with respect to parts by weight. (7) A step of press-molding a bonding mixture, which is a mixture of foamed particles of volcanic ejecta and an aqueous solution of alkali silicate and does not contain a radio wave loss material, on a radio wave reflection layer which is a metal plate or a metal foil, After press-molding the adhesive mixture, press-molding a radio wave absorbing mixture of foamed particles of volcanic ejecta, radio wave loss material, and alkaline silicate aqueous solution, and adhering mixture and radio wave on the radio wave reflection layer. A step of bringing carbon dioxide into contact with the absorbing mixture to solidify them, and a method for producing a radio wave absorber.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below.
FIG. 1 is a sectional view showing a simplified radio wave absorber 1 of a preferred example of the present invention. The radio wave absorber 1 of the present invention basically includes a radio wave absorption layer 2, a radio wave reflection layer 3, and an adhesive layer 4 interposed therebetween. The radio wave absorbing layer 2 used in the present invention is formed by contacting and solidifying carbon dioxide gas to a radio wave absorbing mixture obtained by mixing foamed particles of a volcanic ejecta, a radio wave loss material, and an alkali silicate aqueous solution. A metal plate or metal foil is used as the radio wave reflection layer 3 in the present invention. In the radio wave absorber 1 of the present invention, such a radio wave absorption layer 2 and a radio wave reflection layer 3 are a mixture of foamed particles of a volcanic ejecta and an alkali silicate aqueous solution, and do not contain a radio wave loss material. The mixture has a structure in which carbon dioxide gas is brought into contact with the mixture to be solidified through an adhesive layer 4 which is solidified.

The adhesive layer 4 in the present invention is the radio wave absorbing layer 2.
Similar to the formation of the above, the carbon dioxide gas is formed by contacting the bonding mixture containing the foamed particles of the volcanic ejecta and the alkali silicate aqueous solution. That is, dehydration condensation of silanol bonds (formation of siloxane bonds) occurs due to contact between the adhesive mixture and carbon dioxide gas, and silica gel and alkali carbonate are produced and solidified to form an adhesive layer. According to the present invention, a radio wave absorbing layer having a surface texture similar to concrete is adhered to a radio wave reflecting layer which is a metal plate or a metal foil, and is used for binding foam particles of volcanic ejecta in the radio wave absorbing layer. Since the adhesiveness of the aqueous solution of alkali silicate to the metal is utilized, the radio wave absorbing layer and the radio wave reflecting layer can be formed without forming an adhesive layer between the radio wave absorbing layer and the radio wave reflecting layer using another adhesive such as an epoxy adhesive. It becomes possible to bond with.

Thus, in the present invention, unlike the prior art, it is possible to realize the radio wave absorber 1 which can be bonded with a sufficient adhesive force even if the average thickness D1 is 2 mm or less. It should be noted that "sufficient adhesive force" means that the adhesive force is greater than the bending stress of the radio wave absorption layer, in other words, after the radio wave reflection layer and the radio wave absorption layer are bonded via the adhesive layer, When the layer is peeled off, it means that the wave reflection layer has a large adhesive force such that the wave absorption layer is broken (cracked) before being peeled off from the wave absorption layer. As described above, in the present invention, it is possible to bond the radio wave absorbing layer and the radio wave reflecting layer with a sufficient adhesive force without making the adhesive layer have an average thickness of more than 2 mm. It is possible to realize a radio wave absorber having stable radio wave absorption characteristics without forming a gap at the interface between the layers and preventing an excessive gap between the radio wave absorption layer and the radio wave reflection layer.

Further, in the radio wave absorber of the present invention, the thickness of the adhesive layer can be easily controlled, so that only the original advantage of using the foamed particles of the volcanic ejecta as the binder for the radio wave absorber is utilized. It is possible to obtain an excellent electromagnetic wave absorber. In other words, unlike conventional wave absorbers that use organic polymers such as rubber and plastics as binders, they have a low effective dielectric constant and even when applied to radio waves with short wavelengths such as millimeter waves It is not necessary to strictly control the thickness of the electromagnetic wave in accordance with the wavelength, and it is possible to preferably realize a radio wave absorber having an advantage that molding and solidification can be easily performed. With the radio wave absorber of the present invention, in particular, even at a frequency of 5.8 GHz, a radio wave absorber having a good radio wave absorption amount of about 20 dB to 40 dB (measured by the arch method as described later) is easily obtained. It is possible to use such a wave absorber,
Non-stop automatic toll collection system (ETC (Electronic Toll Col
It is extremely useful as a radio wave absorber for a (light) system).

The radio wave absorber 1 of the present invention is not particularly limited in its manufacturing method, and may be obtained by any manufacturing method as long as it has the above-mentioned constitution. Then, a suitable manufacturing method of the radio wave absorber 1 is proposed.

FIG. 2 is a simplified sectional view for explaining the manufacturing method of the present invention. The manufacturing method of the present invention basically includes the following steps. In the first step, the bonding mixture 11 is press-molded on the radio wave reflection layer 3 which is a metal plate or a metal foil (FIG. 2A). The press-molding is performed, for example, by applying the adhesive mixture 11 on the radio wave reflection layer 3, placing it in a mold, and press-molding. In FIG. 2A, a press plate 15 is provided in a mold 14 for press molding of an appropriate size, which is, for example, a substantially rectangular parallelepiped box-shaped body that opens in only one direction.
An example in which the above-mentioned press molding is performed is shown below. The press molding is performed under normal pressing conditions in the art, that is, a temperature condition of 0 ° C. to 50 ° C. and a pressure of 0.1 MPa.
It may be performed under a pressure condition of 0.5 MPa. The adhesive mixture after press molding may be appropriately dried before the following steps. The drying may be carried out to such an extent that the adhesive mixture 11 after press molding is made to be a dry material which is not compatible even if a liquid material having the same composition is applied thereon. The drying is performed, for example, in a constant temperature drying furnace at 60 ° C to 80 ° C for 4 hours.
Do about 6 hours. In addition, after applying the adhesive mixture 11, natural drying may be performed for about 8 hours to 10 hours.

In the subsequent step, the radio wave absorbing mixture 12 is further press-molded on the adhesive mixture 11 after the press molding (FIG. 2 (b)). Radio wave absorption mixture 1
In the press molding of No. 2, as in the case of the press molding of the admixture 11 for bonding, for example, the mixture 12 for electromagnetic wave absorption is applied onto the admixture 11 after press molding, and the mixture is put in a mold and pressed. This is done by molding. The press molding in the process of step (1) is also a press condition usually used in the art, that is, a temperature condition of 0 ° C. to 50 ° C. and 0.1
It may be performed under a pressure condition of MPa to 0.5 MPa. When the mixture 12 for absorbing radio waves is applied, it is particularly preferable to apply it while appropriately stirring so that the radio wave loss material can be dispersed substantially uniformly in the mixture 12. The radio wave absorbing mixture 12 after press molding may also be appropriately dried after being applied, as in the case of the admixture 11 described above.

In the next step, carbon dioxide is brought into contact with the mixture 11 for adhesion and the mixture 12 for absorption of radio waves on the radio wave reflection layer 3 to solidify them. Adhesive mixture 1
1 and the mixture 12 for electromagnetic wave absorption and carbon dioxide gas,
It may be carried out so that each of these mixtures is sufficiently solidified, but it is preferably carried out so that all of the respective mixtures 11, 12 are uniformly solidified. In a preferred embodiment, the outer diameter is 1 mm
A rod of about 3 mm is pierced so as to reach the center of the inside of the mixture 12 for electromagnetic wave absorption, a thin tube (carbon dioxide gas injection nozzle) 13 is inserted into the hole formed by this, and carbon dioxide gas is introduced into the mixture from the thin tube. A mode of supplying is mentioned (FIG. 2 (c)). If a large number of holes are provided in the tube wall of the thin tube 13, the carbon dioxide gas will contact the mixture 11 and 12 more uniformly, which is a more preferable mode. Further, instead of the rod material, a thin tube may be directly inserted into the mixture to supply carbon dioxide gas. Carbon dioxide supply pressure is 2 kg / cm ² ~
It is preferably about 5 kg / cm ² . If the surface of the mixture comes into contact with air before it comes into contact with carbon dioxide, the surface may be solidified by carbon dioxide in the air, making it impossible to pierce it with a rod or a thin tube. Therefore, it is preferable that a resin or metal film is attached to the surface of the radio wave absorbing mixture, or a coating film is provided on the surface, and a rod or a thin tube is pierced on the surface. The holes created by the sticking of rods and capillaries can be used as they are, but after solidification, a mixture of expanded particles of volcanic ejecta and a binder (organic binder such as CMC or inorganic binder such as water glass). It is preferable to fill and fill in to maintain the strength.

In the manufacturing method of the present invention, the radio wave absorber 1 of the present invention as shown in FIG. 1 can be preferably manufactured through the steps 1 to 3 described above. It should be noted that the radio wave absorbing mixture 12 solidified in the above step is the radio wave absorbing layer 2
Thus, the adhesive mixture 11 solidified in the above step becomes the adhesive layer 4.

According to the manufacturing method of the present invention, the adhesive mixture 1 in the state of being laminated on the radio wave reflection layer 3 in the above steps.
It is preferable to further press-mold the 1 and the radio wave absorbing mixture 12. FIG. 2C shows a case where press molding is performed by the press plate 15 at the same time when the carbon dioxide gas is contacted in the step of. By performing the press molding, the thicknesses of the radio wave absorbing layer, the adhesive layer, and the radio wave reflecting layer can be formed more uniformly, and the radio wave absorber 1 having particularly stable radio wave absorbing characteristics can be manufactured. . Also in the press molding performed at the same time as the contact with the carbon dioxide gas, the molding conditions usually performed in the art, that is, the temperature condition of 0 ° C. to 50 ° C. and 0.1 MPa to
It is preferable to carry out under a pressure condition of 0.5 MPa.

According to such a manufacturing method of the present invention, it becomes possible to efficiently form the electromagnetic wave absorbing layer and to bond the electromagnetic wave absorbing layer and the electromagnetic wave reflecting layer with good workability, and to bond the electromagnetic wave absorbing layer to the electromagnetic wave absorbing layer. There is the advantage that a stronger adhesion between the layers can be obtained.

The radio wave absorber of the present invention may be manufactured by a method including the following steps (1) to (3). FIG. 3 is a simplified cross-sectional view for explaining another manufacturing method of the present invention. First, in the first step, carbon dioxide is brought into contact with the radio wave absorbing mixture 16 and at the same time, press molding is performed by the press plate 20 in the mold frame 19 to previously form the radio wave absorbing layer 2 (FIG.
(A)). The conditions of contact with carbon dioxide gas and the conditions of press molding when forming the radio wave absorption layer 2 may be the same as those in the above-described manufacturing method.

In the next step, the radio wave absorbing layer 2 obtained in the above step is laminated on the radio wave reflecting layer 3 which is a metal plate or a metal foil with the adhesive mixture 17 interposed. The method of stacking is not particularly limited, and the radio wave reflection layer 3, the adhesive mixture 17, and the radio wave absorption layer 2 may be stacked in this order from one side (for example, the lower side) to the other side (for example, the upper side) that remains. (Example of FIG. 3B), or conversely, the radio wave absorption layer 2, the adhesive mixture 17, and the radio wave reflection layer 3 may be laminated in this order from one side to the other side. In any of the stacking methods, the adhesive mixture 17 is applied onto the radio wave absorbing layer 2 or the radio wave reflecting layer 3 and press molding is performed. The devices and conditions used for these operations are as described above. The manufacturing method may be the same as that described above.

In the subsequent step, carbon dioxide is brought into contact with the adhesive mixture 17 in the laminate formed in the step to solidify it. The adhesion mixture 17 and the carbon dioxide gas may be contacted so that the mixture 17 is sufficiently solidified, and it is preferable that the whole mixture 17 is uniformly solidified. The contact between the bonding mixture and carbon dioxide gas is performed by inserting the thin tube (carbon dioxide gas injection nozzle) 18 so as to reach the center of the inside of the bonding mixture 17, as in the step of the above-described manufacturing method. Carbon dioxide may be supplied to the inside of the mixture (FIG. 3 (c)).

In the radio wave absorber manufactured by such a method, the radio wave absorbing mixture 16 previously solidified in the above step becomes the radio wave absorbing layer 2, and the adhesive mixture 17 solidified in the above step is the adhesive layer 4 Becomes

In this manufacturing method as well, from the same viewpoint as in the manufacturing method described above, it is preferable to further perform press molding during the step. The press molding conditions may be the same as those described above. FIG. 3C shows a case where press molding is performed by the press plate 20 at the same time when the carbon dioxide gas is contacted in the step of.

In the radio wave absorber 1 of the present invention, the foamed particles of the volcanic ejecta in the radio wave absorbing mixture for forming the radio wave absorbing layer 2 are fine particles of volcanic debris such as shirasu, pumice, mullet, and perlite. , 800 ℃ ~ 1000 ℃
It refers to the one obtained by heating and foaming. The expanded particles thus obtained are composed of SiO ₂ , A
It is a composite compound made of 1 ₂ O ₃ , CaO, MgO, etc. and having a large number of pores inside.

The foamed particles of the volcanic ejecta in the radio wave absorbing mixture preferably have a bulk specific gravity of 1.0 or less from the viewpoint that they can be easily molded and solidified by contact with carbon dioxide gas. It is more preferably about 0.5.
When the bulk density of the expanded particles exceeds 1.0, the weight of the obtained radio wave absorption layer 2 increases, and the lightness, which is an advantage of using expanded particles of volcanic ejecta, tends to be impaired. Therefore, it is not preferable. The foamed particles of volcanic ejecta in the radio wave absorbing mixture preferably have a particle size of 30 μm to 500 μm, and more preferably 35 μm to 180 μm, from the viewpoint of strength and weight (specific gravity) of the molded body.
When the particle size of the expanded particles is less than 30 μm, a dense and strong molded body can be obtained from the obtained radio wave absorption layer 2, but the bulk density of the expanded particles increases and the molded body weight increases. It is not preferable because it tends to occur. The particle size is 5
When the thickness exceeds 00 μm, the obtained radio wave absorption layer 2 tends to easily develop cracks and tends to be brittle and easily broken, which is not preferable.

As mentioned above, the foamed particles of the volcanic ejecta are Shirasu foamed particles because the solidification of the radio wave absorbing mixture (formation of the radio wave absorbing layer 2) can be easily performed by contact with carbon dioxide gas. Preferably. "Shirasu"
Is a general term for unfused parts of pumice flow deposits, white sandy deposits such as pumice fall (white, coarse volcanic ejecta) and secondary deposits derived from them. As for the particle size composition of shirasu, as a general tendency, the sand particles (50 μm to 2.0 mm) belong to the largest proportion of 70% to 80%, and the silt components (3.9 μm to 62.5 μm) and the viscosity components are Few. As for the mineral composition, 70% to 80% is glassy, and the rest is crystalline such as feldspar, pyroxene, quartz and magnetite (Kenichiro Nomoto: “Kyushu Mining Society Journal” 35, p85-99 (196).
7)). The chemical composition is silicon dioxide (SiO ₂ ) 65%-
73%, aluminum oxide (Al ₂ O ₃ ) 12% to 18
%, Iron oxide (Fe ₂ O ₃ , FeO) 1% to 3%, calcium oxide (CaO) 2% to 4%, sodium oxide (Na ₂
O) is stable in the range of 3% to 4% and potassium oxide (K ₂ O) is 2% to 3% (Tadaharu Kuroiwa: “Journal of the Japan Mining Association” 9
1, p625-634 (1975)).

The radio wave loss material contained in the radio wave absorbing mixture is particularly granular as long as it has a function of giving a loss and attenuation to the radio wave to be absorbed, such as dielectric loss, conduction loss, magnetic loss. Is not limited. Examples of the radio wave loss material include various conventionally known materials such as conductive carbon, magnetic powder (for example, ferrite), and metal powder (for example, iron powder). As the radio wave loss material, the above materials may be used alone or in appropriate combination. Above all, moderate conductivity (resistivity)
From the viewpoint of providing a conductive loss that has a wide selection of conductivity, Ketjen black, acetylene black, furnace black, graphite,
It is preferable to use conductive carbon such as expanded graphite.

The content ratio of the radio wave loss material in the radio wave absorption mixture may be appropriately set according to the wavelength of the radio wave to be absorbed by the obtained radio wave absorption layer 2 and the required radio wave absorption characteristics. 1 for 100 parts by weight of foamed particles
It is preferable that the composition is blended in an amount of from 700 to 700 parts by weight. When the content of the radio wave loss material in the radio wave absorption mixture is less than 1 part by weight based on 100 parts by weight of the foamed particles of the volcanic ejecta,
As a radio wave absorber, sufficient dielectric loss cannot be obtained and the radio wave absorption characteristics tend to deteriorate, which is not preferable. Further, when the content of the radio wave loss material in the radio wave absorption mixture exceeds 700 parts by weight with respect to 100 parts by weight of the foamed particles of the volcanic ejecta, the strength decreases, and as a result, the radio wave absorption layer tends to be easily broken. Therefore, it is not preferable. The preferable blending amount of the radio wave loss material varies depending on the type within the above range. For example, when the radio wave loss material is conductive carbon, 1 part by weight to 10 parts by weight is added to 100 parts by weight of the foamed particles of the volcanic ejecta. Is preferred, and more preferably 1 to 5 parts by weight are blended. Also, for example, when the radio wave loss material is ferrite, it is preferable to mix 20 to 700 parts by weight with respect to 100 parts by weight of the foamed particles of the volcanic ejecta,
It is more preferable to add 50 to 200 parts by weight.

The aqueous solution of alkali silicate contained in the radio wave absorbing mixture of the present invention functions as a binder for bonding the foamed particles. Examples of the alkali silicate preferably used in the present invention include potassium silicate, sodium silicate, and lithium silicate. The aqueous alkali silicate solution used in the radio wave absorbing mixture in the present invention may contain two or more kinds of potassium silicate, sodium silicate and lithium silicate.

The aqueous solution of alkali silicate in the radio wave absorbing mixture has an alkali silicate concentration (if two or more alkali silicates are contained, their total concentration) is 30.
It is preferably from 50% by weight to 35% by weight.
It is more preferably 45% by weight. If the concentration of the above-mentioned alkali silicate is less than 30% by weight, the radio wave absorption layer tends to become brittle, which is not preferable. On the other hand, if the concentration of the alkali silicate exceeds 50% by weight, the radio wave absorbing layer becomes heavy and the installation tends to be difficult, which is not preferable.

When the alkali silicate is potassium silicate, the aqueous alkali silicate solution has, for example, a SiO ₂ concentration of 2
5.5 wt% to 27.5 wt%, K ₂ O concentration of 12.5
% To 14.5% by weight aqueous solution and SiO ₂ concentration of 27
Examples include an aqueous solution having a weight% to 29 weight% and a K ₂ O concentration of 21 weight% to 23 weight%.

When the alkali silicate is sodium silicate, the alkali silicate aqueous solution has a concentration of 28% by weight.
˜48 wt%, especially 36 wt% to 38 wt% concentrated aqueous solution of alkali silicate is exemplified, among which commercially available NO. No. 1 to NO. No. 4 water glass is preferably used. The main component of water glass is the general formula Na ₂ O · nS
It is represented by iO ₂ (n = 2 to 4). Incidentally, in general, NO.
No. 1 water glass has a specific gravity of 38 Baume to 59 Baume, a composition of 21% by weight to 38% by weight of SiO _{2 and a} content of 10% to 18% by weight of Na ₂ O. NO. No. 2 water glass has a specific gravity of 42-49 Baume and a composition of SiO _{2 of} 26.5.
% By weight to 32% by weight, Na ₂ O content 10.5% by weight to 1
It is 2.7% by weight. NO. No. 3 water glass has a specific gravity of 4
The composition is 0 baume or more, the composition is SiO ₂ content of 28% by weight to 30% by weight, and the Na ₂ O content is 9% by weight to 10% by weight. NO. Four
No. water glass has a specific gravity of 30 baume or more and a composition of SiO ₂
The content is 23% to 25% by weight, and the Na ₂ O content is 6% to 7% by weight.

When the alkali silicate is lithium silicate, the alkali silicate aqueous solution has, for example, a SiO ₂ concentration of 15 wt% to 25 wt% and a Li ₂ O concentration of 1 wt% to.
1.5 wt% aqueous solution and SiO ₂ concentration of 15 wt% to 2
For example, an aqueous solution having 5 wt% and a Li ₂ O concentration of 2 wt% to 3.5 wt% is exemplified.

The content of the aqueous alkali silicate solution in the radio wave absorbing mixture may be appropriately set in accordance with the wavelength of the radio wave to be absorbed by the obtained radio wave absorbing layer 2 and the required radio wave absorbing characteristics, and is not particularly limited. Is preferably mixed in an amount of 50 parts by weight to 200 parts by weight, and more preferably 100 parts by weight to 150 parts by weight, based on 100 parts by weight of the foamed particles of the volcanic ejecta. The content of the aqueous solution of alkali silicate in the mixture for absorbing radio waves is 100 foam particles of volcanic ejecta.
If it is less than 50 parts by weight with respect to parts by weight, the strength of the obtained electromagnetic wave absorption layer 2 is not sufficiently imparted and it tends to become brittle, which is not preferable. Further, when the content of the alkali silicate aqueous solution in the radio wave absorbing mixture exceeds 200 parts by weight with respect to 100 parts by weight of the foamed particles of the volcanic ejecta, the viscosity of the radio wave absorbing mixture becomes too large and the moldability at room temperature becomes poor. It is not preferable because it tends to become impossible to give sufficient strength to the radio wave absorption layer after the injection of carbon dioxide gas.

In the radio wave absorbing layer 2 of the present invention, carbon dioxide is brought into contact with a radio wave absorbing mixture obtained by mixing the foamed particles of the volcanic ejecta, the radio wave loss material and the aqueous solution of alkali silicate, and the mixture is mixed. It is formed by solidifying. When carbon dioxide comes into contact, silicate gel and alkali carbonate are produced (for example, when the alkali silicate aqueous solution is a sodium silicate aqueous solution, silicate gel and sodium carbonate are produced).
And the mixture is solidified. As described above, the radio wave absorbing layer 2 of the present invention has a configuration in which the radio wave loss material is substantially uniformly dispersed in the binder using the solidified material containing the foamed particles of the volcanic ejecta, the silica gel and the alkali carbonate as the binder. It is realized in layers.

In the present invention, in the radio wave absorbing layer 2, from the viewpoint of radio wave absorbing characteristics, the radio wave loss material is conductive carbon and has a particle size of 15 nm to 45 nm, especially 2
It is preferable that the conductive carbon of 5 nm to 35 nm is dispersed substantially uniformly. The radio wave absorption layer 2
If the particle diameter of the conductive carbon is less than 15 nm, it tends to be difficult to disperse the carbon in the electromagnetic wave absorption layer 2 uniformly, which is not preferable. If the particle size of the conductive carbon in the radio wave absorption layer 2 exceeds 45 nm, it tends to be difficult to obtain good radio wave absorption characteristics, which is not preferable. The particle size of the conductive carbon in the radio wave absorption layer 2 is, for example, a scanning electron microscope (SE
Take a photo of the powder itself at high magnification using M),
It can be measured by a microscopic method in which a circle equivalent to the area of the shape of the powder photographed is assumed and the diameter of the circle is used as the particle size. As a scanning electron microscope, specifically, JS
M-5610LV (manufactured by JEOL Ltd.) can be preferably used, and the magnification is 100,000 times, for example. The conductive carbon used in the present invention has an average particle size of 3 when the number of particles measured is 100 (n = 100).
It is preferably around 0 nm.

It should be noted that the radio wave loss material needs to be dispersed in the binder as evenly as possible in order to achieve absorption performance of radio waves of extremely short wavelength such as millimeter waves, but in the present invention, the radio wave loss material is an aqueous solution of alkali silicate. Since it is mixed with the expanded particles in a mixed state, it can be dispersed in the binder extremely uniformly.

From the viewpoint of the strength of the radio wave absorption layer 2, the radio wave absorption layer 2 has a substantially uniform thickness and is 5 m.
It is preferably realized with an average thickness of m to 30 mm, in particular 10 mm to 20 mm. The radio wave absorption layer 2
If the average thickness is less than 5 mm, sufficient strength of the radio wave absorption layer 2 cannot be obtained, and the electric wave absorption layer 2 tends to crack easily, which is not preferable. The average thickness of the radio wave absorption layer 2 is 30 m.
When it exceeds m, the weight per unit area increases, and the handleability tends to deteriorate, which is not preferable. The above-mentioned "generally uniform thickness" refers to a uniform thickness including a manufacturing error. The above-mentioned manufacturing error is, specifically, the difference between the maximum thickness and the minimum thickness of the radio wave absorbing layer 2 is preferably 1000 μm or less, more preferably 500 μm or less.

The radio wave reflection layer 3 in the present invention is realized by a metal plate or a metal foil. In the present specification, a metal plate refers to a flat material formed of a metal material and having a thickness of 100 μm or more, and a metal foil refers to the flat material having a thickness of less than 100 μm. I shall. The metal material forming the radio wave reflection layer is not particularly limited as long as it is a material that is a solid phase at room temperature and has a conductivity that can be generally called a conductor, and aluminum, iron,
Copper, silver, gold and the like, and alloys thereof are mentioned, but among them, the aluminum plate or the aluminum foil is particularly preferable from the viewpoint of relatively low specific gravity and resistivity and low cost. The thickness of the metal foil is
From the viewpoint of high-frequency propagation characteristics, it is preferably about 15 μm or more. From the viewpoint of handleability, the metal plate preferably has a thickness of 1 mm or less.

As the foamed particles of the volcanic ejecta and the alkali silicate aqueous solution used in the above-mentioned admixture, the same foamed particles and the alkali silicate aqueous solution of the volcanic ejecta used in the above radio wave absorption mixture are used. do it.

The adhesive layer 4 in the present invention is formed by mixing foamed particles of volcanic ejecta and an aqueous solution of alkali silicate, and the carbon dioxide is brought into contact with the adhesive mixture containing no radio wave loss material to form a silica gel and an alkali carbonate. Is generated, the mixture is solidified and formed. That is, the adhesive layer 4 of the present invention is realized by a layer composed of expanded particles of volcanic ejecta, a silicate gel, and a solidified product containing alkali carbonate.

The adhesive layer 4 in the electromagnetic wave absorber 1 of the present invention has a substantially uniform thickness, and the average thickness D1 is preferably 0.3 mm to 2 mm, and 0.3 mm to
It is particularly preferred that it is realized to be 0.5 mm.
When the average thickness D1 of the adhesive layer 4 is less than 0.3 mm,
It is not preferable because a large amount of voids are formed between the radio wave absorption layer 2 and the adhesive layer 4, and it tends to be difficult to obtain sufficient adhesive force. If the average thickness D1 of the adhesive layer 4 exceeds 2 mm, the radio wave absorption performance tends to deteriorate, which is not preferable. Note that the above-mentioned “generally uniform thickness” means a uniform thickness including a manufacturing error, as in the case of the radio wave absorption layer 2 described above. Specifically, the above-mentioned manufacturing error is such that the difference between the maximum thickness and the minimum thickness of the adhesive layer 4 is preferably 100 μm or less, more preferably 50 μm.
m or less.

In the radio wave absorber 1 of the present invention, carbon dioxide gas is brought into contact with the adhesive mixture containing the foamed particles of the volcanic ejecta and the aqueous solution of alkali silicate to solidify the adhesive layer 4, and A radio wave absorbing layer 2 formed by bringing carbon dioxide into contact with a radio wave absorbing mixture containing foamed particles of a volcanic ejecta, a radio wave loss material and an aqueous solution of an alkali silicate, and a radio wave reflecting layer 3 which is a metal plate or a metal foil. However, it is possible to realize the bonding with a sufficient bonding force within the above-mentioned range of the average thickness.

In the present invention, the above-mentioned radio wave absorbing mixture
And / or the adhesive mixture
It is preferable that it further contains Lumina cement.
Good Alumina cement is fused cement, ban soil sem
Calcium aluminate, the main mineral
And refers to cement. Aluminase used in the present invention
As a ment₂O₃And CaO as the main components
SiO₂, Fe₂O ₃, TiO₂, MgO, etc.
There are some. Containing Alumina Cement
The strength of the electromagnetic wave absorption layer 2 that is more improved and less likely to break.
The adhesive layer 4 can be realized.

When the mixture for absorbing radio waves according to the present invention contains alumina cement, its content is preferably 10 parts by weight to 50 parts by weight with respect to 100 parts by weight of expanded particles of volcanic ejecta. More preferably, it is from 30 parts by weight to 30 parts by weight. When the admixture for bonding in the present invention contains alumina cement, its content is 10 to 5 parts by weight with respect to 100 parts by weight of expanded particles of volcanic ejecta.
It is preferably 0 part by weight. When the content of the alumina cement in the mixture for absorbing radio waves and the mixture for bonding is less than the lower limit described above, it is difficult to obtain sufficient strength of the molded body, and a reaction with moisture in the air causes a phenomenon of flaming. This is not preferable because it tends to occur.

In the radio wave absorber 1 of the present invention, it is preferable that the radio wave absorbing layer 2 and the adhesive layer 4 are realized by using a radio wave absorbing mixture and an adhesive mixture which both contain alumina cement. That is, the radio wave absorber 1 of the present invention
Means that the radio wave absorption layer 2 is 1 to 700 parts by weight of the radio wave loss material with respect to 100 parts by weight of the foamed particles of the volcanic ejecta.
An adhesive layer 4 which is formed by bringing carbon dioxide into contact with a radio wave absorbing mixture obtained by mixing 100 parts by weight to 150 parts by weight of an alkali silicate aqueous solution and 10 parts by weight to 50 parts by weight of alumina cement and solidifying the mixture. However, with respect to 100 parts by weight of the foamed particles of the volcanic ejecta, 100 parts by weight to 150 parts by weight of an aqueous solution of alkali silicate and 10 parts by weight to 50 parts by weight of alumina cement are mixed to make a bonding mixture contact with carbon dioxide gas. It is preferably realized by a solidified material.

The radio wave absorber of the present invention may have only a structure in which the radio wave absorption layer 2 and the radio wave reflection layer 3 are bonded to each other via the adhesive layer 4 as shown in FIG. However, like the radio wave absorber 21 of another preferable example shown in FIG. 4, the radio wave absorption layer 2 is further adhered to the cosmetic surface sheet material 22 via the second adhesive layer 23 (that is, radio wave reflection). Layer 3, adhesive layer 4, radio wave absorbing layer 2, second adhesive layer 2
3, a laminated body of the cosmetic surface sheet material 22).

The surface sheet material 22 for makeup is not particularly limited as long as it has radio wave permeability (radio wave loss, that is, radio wave transmission attenuation is small), and is made of resin (vinyl chloride, acrylic, etc.). Examples of the sheet material include a film provided with an adhesive layer. In addition, the cosmetic surface sheet material is
The thickness is preferably 0.05 mm or less.

Further, in the radio wave absorber 21 shown in FIG. 4, the second adhesive layer for adhering the radio wave absorbing layer 2 and the cosmetic surface sheet material is the same as the adhesive layer 4 with the foamed particles of the volcanic ejecta and the alkali silicate aqueous solution. It is sufficient if it is formed by contacting and solidifying carbon dioxide gas to an adhesive mixture containing no radio wave loss material, which has the same composition as the adhesive layer 4 (mixing ratio of each component, etc.). It is particularly preferable that

The radio wave absorber 21 further including the above-mentioned cosmetic surface sheet material 22 further has effects such as improvement in appearance and prevention of falling of radio wave absorption layer fragments.

The radio wave absorbing layer in the radio wave absorber of the present invention may be solidified and then baked.
Baking after solidification is preferable because the strength of the obtained radio wave absorption layer is improved. The firing of the radio wave absorbing layer may be performed after the step of the first manufacturing method, between the step of the second manufacturing method, and the step of the second manufacturing method. The firing temperature is preferably 500 ° C to 900 ° C, more preferably 600 ° C to 800 ° C.

[0053]

The present invention will be described in more detail with reference to the following examples, but these are merely examples and do not limit the scope of the present invention. Example 1 Expanded particles of Shirasu (Shirasu balloon (SKB-600
0), manufactured by Towa Naoto Co., Ltd., bulk density: 0.18 to 0.2,
(Particle size: 120 μm to 180 μm) 5 parts by weight of 100 parts by weight of conductive carbon powder, furnace black having an average particle size of 30 nm (measured by a microscope), an aqueous solution of alkali silicate (water glass of No. 1). 100 parts by weight and 30 parts by weight of alumina cement were mixed and stirred in a propeller-type stirrer to prepare a mixture for absorbing radio waves. In addition, foamed particles of volcanic ejecta (Shirasu Balloon (SKB-600
0), manufactured by Towa Naoto Co., Ltd., bulk density: 0.18 to 0.2,
Particle size: 120 μm to 180 μm) With respect to 100 parts by weight,
100 parts by weight of 40% by weight alkali silicate aqueous solution and 20 parts by weight of alumina cement were mixed and stirred in a propeller-type stirrer different from the above to prepare a bonding mixture. As the radio wave reflection layer 3, an aluminum plate having a thickness of 0.5 mm was used. The adhesive layer 4 is press-molded on the radio wave reflection layer 3 by the manufacturing method as shown in FIG.
C., 1.4 MPa), and further, the radio wave absorption layer 2 was press-molded (30.degree. C., 0.2 MPa), a resin film was attached to the surface of the radio wave absorption mixture, and a bar (diameter:
3 mm) was inserted and perforated, and a thin tube was inserted into each hole.
The thin tube used was one having a plurality of holes in the tube wall so that carbon dioxide gas was also diffused from the periphery of the thin tube. Next, carbon dioxide gas was applied to the thin tube for 60 seconds (gas supply pressure: 2 kg / cm ^{2 to} 5 kg / cm ² ) to solidify the adhesive mixture and the radio wave absorbing mixture, and the resin film was peeled off. Thus, the radio wave absorber 1 having the structure shown in FIG. 1 was produced. In the obtained electromagnetic wave absorber 1, the adhesive layer 4 had an average thickness D1 of 1.0 mm, and the electromagnetic wave absorber 2 had an average thickness of 6.8 mm.

Comparative Example 1 Using an epoxy adhesive (Hiquick, manufactured by Cemedine Co., Ltd.), the radio wave absorbing layer and the radio wave reflecting layer were adhered,
A radio wave absorber was produced. As the radio wave absorption layer, the same one as the radio wave absorption layer in Example 1 was solidified in advance according to the steps of the second manufacturing method of the present invention. As the radio wave reflection layer, an aluminum plate having a thickness of 0.5 mm was used as in Example 1. The average thickness of the adhesive layer using the epoxy adhesive was 2.2 mm.

(Radio Wave Absorption Characteristics) With respect to the radio wave absorbers obtained in Example 1 and Comparative Example 1, the radio wave absorption amount at a frequency of 5.8 GHz was measured by an arch method (reflected power method) at an incident angle of 10 °. However, in the electromagnetic wave absorber of Example 1, 32
The result was 10 dB for the electromagnetic wave absorber of Comparative Example 1.

[0056]

As is clear from the above description, according to the present invention, a radio wave absorbing layer using foamed particles of volcanic ejecta as a binder and a radio wave reflecting layer which is a metal plate or a metal foil are conventionally used. It is possible to provide a radio wave absorber in which an adhesive layer having a remarkably smaller thickness is adhered to the interface without forming a gap, and a method for manufacturing the same.

[Brief description of drawings]

FIG. 1 is a simplified cross-sectional view showing a radio wave absorber 1 of a preferred example of the present invention.

FIG. 2 is a simplified cross-sectional view for explaining the first manufacturing method of the present invention.

FIG. 3 is a simplified cross-sectional view for explaining the second manufacturing method of the present invention.

FIG. 4 is a schematic cross-sectional view showing a radio wave absorber 21 of another preferred example of the present invention.

[Explanation of symbols]

1,21 Radio wave absorber 2 Radio wave absorption layer 3 Radio wave reflection layer 4 Adhesive layer

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Kazuhiko Kanemitsu             Mitsubishi Electric, 8 Nishino-cho, Higashi-Mukojima, Amagasaki City, Hyogo Prefecture             Line Industry Co., Ltd. (72) Inventor Nozomi Fujita             Mitsubishi Electric, 8 Nishino-cho, Higashi-Mukojima, Amagasaki City, Hyogo Prefecture             Line Industry Co., Ltd. (72) Inventor Toshio Kudo             663 Minoshima, Arita-shi, Wakayama Mitsubishi Electric Works             Minoshima Works Co., Ltd. (72) Inventor Katsunori Hosoya             663 Minoshima, Arita-shi, Wakayama Mitsubishi Electric Works             Minoshima Works Co., Ltd. (72) Inventor Yutaka Uehara             4056-1, Yoshino-cho, Kagoshima City, Kagoshima Prefecture F term (reference) 5E321 BB21 BB25 BB31 BB60 CC16                       GG11

Claims (7)

[Claims] 1. A radio wave absorbing layer formed by contacting and solidifying carbon dioxide gas to a radio wave absorbing mixture obtained by mixing foamed particles of a volcanic ejecta, a radio wave loss material and an alkali silicate aqueous solution, and a radio wave which is a metal plate or a metal foil. A radio wave absorber formed by adhering a reflective layer via an adhesive layer, wherein the adhesive layer is a mixture of foamed particles of volcanic ejecta and an aqueous solution of alkali silicate and does not contain a radio wave loss material. Absorber that is made by contacting the mixture for use with carbon dioxide and solidifying it. 2. The average thickness of the adhesive layer is 0.3 mm to 2 mm.
The radio wave absorber according to claim 1, wherein 3. The radio wave absorber according to claim 1, wherein at least one of the radio wave absorbing mixture and the adhesive mixture further contains alumina cement. 4. The radio wave absorbing mixture comprises 1 part by weight to 700 parts by weight of a radio wave loss material, 50 parts by weight to 200 parts by weight of an aqueous solution of alkali silicate, and 100 parts by weight of expanded particles of volcanic ejecta. 10 parts by weight to 50 parts by weight of alumina cement are mixed, and the above-mentioned bonding mixture is used in an amount of 50 parts by weight to 200 parts by weight of an alkali silicate aqueous solution and 10 parts by weight with respect to 100 parts by weight of expanded particles of volcanic ejecta. The radio wave absorber according to claim 3, which is obtained by mixing 50 parts by weight to 50 parts by weight of alumina cement. 5. The radio wave absorber according to claim 1, wherein the radio wave loss material is conductive carbon. 6. The radio wave absorber according to claim 5, wherein the conductive carbon is mixed in an amount of 1 part by weight to 10 parts by weight with respect to 100 parts by weight of the foamed particles of the volcanic ejecta. 7. A step of press-molding a mixture for admixing foamed particles of volcanic eruption and an aqueous solution of alkali silicate and containing no radio wave loss material on a radio wave reflection layer which is a metal plate or a metal foil. A step of press-molding the adhesive mixture, and further press-molding a radio wave absorbing mixture of foamed particles of volcanic ejecta, radio wave loss material and aqueous solution of alkali silicate, and adhering mixture on the radio wave reflection layer. And a step of bringing carbon dioxide into contact with the mixture for absorbing radio waves to solidify the carbon dioxide gas, and a method for producing the radio wave absorber.