JP2008016783A - Radio wave absorbing material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide radio wave absorbing material having practical radio waves absorbing performance and capable of preventing generation of a harmful gas even in a case of fire. <P>SOLUTION: The radio wave absorbing material contains 20-30 wt.% carbon powder having an average grain size 1-100 μm and volatile component of not more than 0.5 wt.%, 15-40 wt.% a hydraulic cement, and the residual portion of a silica material, and has radio wave absorbing performance of not less than 20 dB in frequency bands of S band and C band. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a radio wave absorber that absorbs S-band and C-band radio waves used in ITS (Intelligent Transport System), wireless LAN (Wireless Local Area Network), and the like.

  Recently, for example, with the introduction of ITS, introduction of services using wireless communication such as DSRC (Dedicated Short Lange Communication) has been promoted. In offices and homes, the construction of a network of a plurality of terminals using a wireless LAN is becoming mainstream.

  In order to operate such DSRC or wireless LAN without any trouble, it is necessary to suppress the reflection of the radio waves used on the road surface, the vehicle roof, the wall surface of the building, and the like. Therefore, as a countermeasure against radio wave interference, a method of filling a road surface, a wall surface of a building, a partition or the like with a radio wave absorber is used, and various radio wave absorbers have been proposed (see, for example, Patent Documents 1 to 3). .

  For example, radio wave absorbers used indoors in buildings have high radio wave absorption characteristics, as well as characteristics that do not generate harmful gases when a fire occurs, assuming a fire. Therefore, it is necessary to be composed of only an inorganic material that does not generate toxic gas by thermal decomposition.

On the other hand, the radio wave absorbers disclosed in Patent Documents 1 and 3 have not obtained practical radio wave absorption performance of 20 dB or more, and have not obtained sufficient radio wave absorption performance. In addition, the radio wave absorber disclosed in Patent Document 2 uses a chlorinated organic substance in the binder substantially, and there is a possibility that harmful gas is generated in a fire.
Republished patent WO2003 / 064780 (Table 1 etc.) JP 2002-353014 A (paragraph [0018] etc.) JP 2003-218579 A (paragraph [0007], Table 1 etc.)

  An object of the present invention is to provide a radio wave absorber that has practical radio wave absorption performance and does not generate harmful gas even in the event of a fire.

  The radio wave absorber of the present invention has an average particle size of 1 to 100 μm and a volatile content of 0.5% by weight or less of carbon powder of 20 to 30% by weight, and hydraulic cement of 15 to 40% by weight. The remainder is made of a siliceous raw material, and the radio wave absorption performance in the frequency band of S band and C band is 20 dB or more.

  The radio wave absorber is preferably a molded body having a thickness of 2 mm to 5 mm, a specific gravity of 1 to 2, and a bending strength of 5 MPa or more. Thereby, workability can be improved.

  According to the radio wave absorber of the present invention, radio wave absorption performance of 20 dB or more is obtained, and no harmful gas is generated even in the event of a fire. Moreover, the high intensity | strength molded object which has the outstanding workability and durability can be manufactured.

  Hereinafter, embodiments of the present invention will be described in detail.

  In the radio wave absorber of the present invention, the carbon powder is 20% by weight to 30% by weight, the hydraulic cement is 15% by weight to 40% by weight, and the balance is made of a siliceous raw material. The radio wave absorber is made of an inorganic material in this way, and does not contain elements such as chlorine that may generate harmful gases even if it is exposed to a heating environment, so it is nonflammable and excellent in safety. .

  This radio wave absorber comprises (a) adding water to a raw material powder (carbon powder, hydraulic cement powder, siliceous raw material powder) having a predetermined composition and mixing it with a mixer or the like to form a slurry. (B) Pour into a mold, solidify by the action of a hydraulic binder (hydraulic cement) contained in the raw material, cure until sufficient strength, (c) demold and take out the cured product and dry, It can be manufactured by the process.

  The radio wave absorber manufactured in this way exhibits radio wave absorption performance of 20 dB or more in the frequency band of S band (2 GHz to 4 GHz) and C band (4 GHz to 8 GHz).

  As the carbon powder, one having an average particle diameter of 1 μm to 100 μm and a volatile content of 0.5% by weight or less is used. When the average particle size is less than 1 μm, the amount of water required for producing the slurry in the production process of the radio wave absorber described later is remarkably increased, and as a result, a good molded article cannot be obtained. On the other hand, when the average particle size exceeds 100 μm, the radio wave absorption performance is lowered, and the radio wave absorption performance of 20 dB cannot be obtained.

  The reason why carbon powder with a volatile content of 0.5% by weight or less is used is that if the volatile content is large, the hygroscopicity (water absorption) becomes high, the shape maintaining characteristics of the molded product deteriorate, and it is exposed to rainwater. This is because it cannot be used in a difficult environment. In other words, since the volatile content is extremely small, the water absorption is extremely small, and the durability in an environment where it is exposed to rainwater can be improved, and the radio wave absorption performance can be maintained for a long time.

  If the content of the carbon powder is 20% by weight to 30% by weight, if less than 20% by weight, radio waves in the S-band and C-band frequency bands cannot be absorbed by 20 dB or more, exceeding 30% by weight. Then, the amount of water required when producing the slurry is remarkably increased, and as a result, a good molded article cannot be obtained.

  Graphite powder is particularly preferably used as the carbon powder, and its radio wave absorption performance is greater than when other carbon powders are used. The cause of this is not clear, but the graphite powder has a scale-like shape, and the radio wave absorption performance can be improved by orienting the powder so that the main surfaces of the powder are parallel to each other in the molded body. It is presumed that

  The hydraulic cement functions as a binder when the raw material powder is molded in the manufacturing process of the radio wave absorber, and bears the strength of the radio wave absorber (molded body) to be manufactured. The content of the hydraulic cement is 15% to 40% by weight. If the content is less than 15% by weight, the strength of the radio wave absorber (molded product) decreases, and if it exceeds 40% by weight, the strength of the radio wave absorber increases. However, since the content of the siliceous raw material and the carbon powder is reduced, the radio wave absorption performance and the water absorption prevention performance are remarkably lowered, and the amount of the powder is increased, so that cracking is likely to occur and the workability is lowered. It is.

  The hydraulic cement is a cement that can form a hardened body by reaction with water. Examples thereof include Portland cement, blast furnace cement, fly ash cement, alumina cement, and silica cement.

  The siliceous raw material is used as an admixture. Although the siliceous raw material itself is not hydraulic, pozzolans that form an insoluble compound by gradually combining at room temperature with calcium hydroxide (derived from hydraulic cement) dissolved in the water when the slurry is produced. Activity can be expected. Silica fume is preferably used as the siliceous raw material, but fly ash, quartz sand, quartzite powder, blast furnace slag, diatomaceous earth, and the like can also be used.

  The thickness of the radio wave absorber is preferably 2 mm to 5 mm. If the thickness is less than 2 mm, the workability is lowered. Conversely, if the thickness exceeds 5 mm, the construction space is widened, and the weight of the radio wave absorber itself is increased. Is unfavorable because it increases.

  The specific gravity of the radio wave absorber is preferably 1 to 2, and the bending strength is preferably 5 MPa or more. In order to manufacture the radio wave absorber having a specific gravity of less than 1, a special manufacturing method such as foaming is required, and the bending strength is extremely small, which makes the handling inconvenient. On the other hand, when the specific gravity is more than 2, there is a problem that the weight of the radio wave absorber itself increases and the workability is lowered. In such a wave absorber having a thickness of 2 mm to 5 mm and a specific gravity of 1 to 2, the type and blending ratio of the raw material powder are adjusted so that the bending strength is 5 MPa or more from the viewpoint of obtaining good workability. It is preferable.

  Graphite powder (manufactured by Nippon Graphite Industry Co., Ltd.) as carbon powder, hydraulic cement (alumina cement, manufactured by Electrochemical Industry Co., Ltd.), and silica fume (manufactured by Elchem Japan Co., Ltd.) as siliceous raw material Were weighed so as to have the composition shown in Table 1 and charged into a mixer, and a predetermined amount of water was added and mixed to prepare a slurry.

  The slurry was poured into a mold and allowed to solidify in a sealed state for 24 hours. The solidified product was demolded and dried at room temperature for 12 hours and at 110 ° C. for 12 hours. Thus, the radio wave absorber samples of Examples 1 to 7 and Comparative Examples 1 to 6 shown in Table 1 were produced.

  The three-point bending strength of these samples was measured according to JIS R1601. The specific gravity of these samples was determined by measuring the shape and determining the volume, and by measuring the weight. The radio wave absorption characteristics were evaluated by the absorption rate when a back surface (one main surface) of a 3 mm thick radio wave absorber sample was lined with a metal aluminum plate and linearly polarized light was vertically incident at a frequency of 5.8 GHz. .

The results are also shown in Table 1. As shown in Table 1, the carbon powder is 20 wt% to 30 wt%, the hydraulic cement is 15 wt% to 40 wt%, the balance is made of a siliceous raw material, and the average particle size of the carbon powder is 1 μm to 100 μm In the sample of the example satisfying the condition, a high radio wave absorption performance of 20 dB or more was obtained. Further, the strength also shows a high value of 6 MPa or more. Since the average particle diameter of the carbon powder in Comparative Example 1 is small, the carbon powder content in Comparative Example 4 is large, so both require a large amount of water for slurry preparation, and therefore a good molded body is obtained. I could not. In Comparative Example 5, since the amount of cement was small, sufficient strength could not be obtained, and a good molded product could not be obtained. In Comparative Example 6, since the amount of powder was large, cracks occurred and a good molded product could not be obtained. Therefore, the characteristics of the samples of Comparative Example 1, Comparative Example 4, Comparative Example 5, and Comparative Example 6 are not evaluated. Since Comparative Example 2 had a large average particle size of carbon powder, and Comparative Example 3 had a low carbon powder content, both could not obtain a radio wave absorption performance of 20 dB.

Claims (3)

Carbon powder having an average particle size of 1 μm to 100 μm and a volatile content of 0.5 wt% or less is 20 wt% to 30 wt%, hydraulic cement is 15 wt% to 40 wt%, and the balance is made of siliceous raw material. ,
1. A radio wave absorber characterized by having a radio wave absorption performance in a frequency band of S band and C band of 20 dB or more.   2. The radio wave absorber according to claim 1, wherein the thickness is 2 mm to 5 mm, the specific gravity is 1 to 2, and the bending strength is 5 MPa or more.   3. The radio wave absorber according to claim 1, wherein the carbon powder is graphite powder.