JPH0834354B2 - Radio wave absorber containing ferrite - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a radio wave absorber, more specifically, a two-layer type including a matrix layer containing at least ferrite powder and a matrix layer containing at least carbon fibers and / or carbon beads. Of the electromagnetic wave absorber, especially the electromagnetic wave absorber made of concrete.

[Prior Art] In recent years, TV ghost interference caused by reflection of TV radio waves from a building or the like has become a major social problem. In particular, since concrete, which is an excellent structural material for structures, has extremely poor electromagnetic wave absorption characteristics, a structure having a concrete wall surface often causes electromagnetic interference.

As one of the countermeasures against this radio wave interference, there is a method of providing a radio wave absorbing plate that absorbs incoming radio waves and hardly generates reflected radio waves on the building wall surface. Conventionally, foamed polystyrene containing carbon, rubber containing carbon, rubber containing ferrite powder, ferrite tile and the like have been used as the radio wave absorber material.

As a result of research on a building material that does not cause such radio interference, the present inventor has found that by incorporating carbon fiber into concrete, effective radio wave absorption characteristics can be imparted to concrete, and The body has already been developed.
58-108603). Moreover, especially for reinforced concrete,
In light of the fact that the concrete layer on the outside of the reinforcing bar is preferably thin, it is necessary to study concrete with sufficient electromagnetic wave absorption characteristics despite its thin thickness.
We have developed a concrete electromagnetic wave absorber that exhibits good absorption characteristics in the range of 130 MHz to 170 MHz in the VHF television frequency band (Japanese Patent Laid-Open No. 58-108602).

[Problems to be solved by the invention] Commercial VHF with a thickness of about 200 mm or less as a building material for electromagnetic wave absorption
Although it is considered that those exhibiting excellent absorption characteristics over a range of television frequencies from 100 MHz to 200 MHz are suitable for practical use, when conventional absorbers are to have good characteristics over the above frequency range, There was no choice but to make the wall thickness thicker than 200 mm, which was a little inconvenient to use as an actual building material.

The present inventor has studied the radio wave absorbing concrete panel, a composite building material panel consisting of a layer not containing these and a layer filled with carbon fibers and carbon beads, the reflection coefficient in the above frequency band is 0.6 or less, particularly 100
We devised an absorber panel with bimodal absorption characteristics in which the reflection coefficient is 0.1 or less around MHz and 200 MHz (Journal of the Television Society, Vol.40, No.8, 780-790, 1986). However, the thickness of this panel was close to 24 cm in total of the two layers, and did not reach the target of 20 cm or less.

The radio wave absorption panel previously developed by the present inventor aimed at the absorption effect due to dielectric loss. The present inventor further tried to add a magnetic loss effect, and by incorporating a ferrite powder into at least one of the two layers, a composite (2) having an excellent absorption effect in a wide frequency range of 100 to 200 MHz was obtained. The inventors have found that a (layer type) panel can be obtained and arrived at the present invention.

[Means for Solving the Problems] The two-layer type electromagnetic wave absorber provided by the present invention has a matrix made of concrete containing at least carbon fibers and / or carbon beads and a matrix made of concrete. And a layer containing ferrite powder.

 Hereinafter, the present invention will be described in detail with reference to the drawings.

FIG. 1 schematically shows the two-layer type electromagnetic wave absorber of the present invention.
In the figure, the layer in contact with the metal plate is referred to as the first layer, and the layer on the side on which the outside radio wave is incident is referred to as the second layer.

Concrete, which is an excellent building material, is used as the matrix. That is, the present invention relates to a concrete two-layer electromagnetic wave absorber.

The concrete used as a matrix in the radio wave absorber of the present invention may be a commonly used one. Cement, water, fine aggregate and, if necessary, coarse aggregate are mixed in an appropriate ratio, kneaded well, and further cured. Is obtained. Concrete here means mortar in addition to normal concrete.
It also includes lightweight concrete.

The ferrite powder used in the present invention may be a powder which is usually commercially available.

The carbon fiber used in the present invention may be an ordinary carbon fiber, for example, a fiber diameter of 5 to 30 μ, a fiber length of 0.3 to 1
5 mm can be used.

The carbon beads used in the present invention are generally spherical carbon particles having a particle size of 50 to 1500 μm, but in the present invention, powdery carbon having a particle size much smaller than these carbon particles, and powdered carbon. It includes the aggregated carbon which is aggregated by adding a binder to the aggregated carbon to a particle size of 50 to 1500 μm.

The content of each component in the electromagnetic wave absorption of the present invention can be varied within a wide range depending on various factors such as the thickness of the product, desired electromagnetic wave absorption characteristics, intended use of the product, and installation location.

The radio wave absorber of the present invention can be manufactured for each layer in the same manner as, for example, ordinary concrete manufacturing. For example, one layer may be formed (cured) and then the other layer may be formed thereon, or the two layers may be separately formed and then laminated.
At that time, the carbon fibers, the ferrite powder, and the carbon beads are uniformly mixed with the matrix raw material at any stage before curing.

For example, the ferrite powder, carbon fibers and carbon beads may be first mixed with cement, or may be mixed with cement, water and fine aggregate or simultaneously with cement, water, fine aggregate and coarse aggregate. Further, the ferrite powder, the carbon fibers and the carbon beads may be mixed separately at different stages of kneading. The mixing ratio of cement, water and fine aggregate can be changed in the same manner as in ordinary concrete. Further, it is also possible to mix steel fibers or the like which are usually used as a reinforcing material.

The mixing ratio of each component can be changed so as to obtain a desired content in the product. As an example, based on the total 100% by volume of the raw materials of each layer, 0 to 30% by volume of ferrite powder and 0 to 10% by volume of carbon fiber as a matrix raw material, and optionally 0 to 25% by volume of carbon beads. % Can be mixed, but is not limited thereto.

[Operation and Effect of the Invention] The radio wave absorber of the present invention is effective for radio waves in a wide range of wavelengths, but is particularly effective for television waves, that is, VHF and LHF band radio waves.

The electromagnetic wave absorber of Conquenreto of the present invention has a high strength due to its essential characteristics (for example, the compressive strength is about 220 to 3).
Since it is 20 kg / f / cm ² ), it can be used particularly advantageously as a building construction material. For example, it can be used as a panel for a building wall, or can be manufactured and used in the case of a viaduct girder. Further, as will be apparent from the examples described below, the electromagnetic wave absorber of the present invention has sufficient electromagnetic wave absorption characteristics even if the entire thickness is thin, so that it is possible to reduce the weight, and it is particularly advantageously used as a high-rise building panel. it can.

In the composite electromagnetic wave absorber of the present invention, carbon fiber and / or carbon beads are added to one of the layers to reduce the weight while maintaining the strength as a building material, and to reduce the electric loss term of the complex relative permittivity. The electromagnetic absorption performance is improved by making it larger. In addition, by adding ferrite powder to the other layer, the complex relative magnetic permeability is increased to further improve the radio wave absorption performance. The present invention has been completed by finding such a composite effect of a carbon material and ferrite.

Therefore, the present invention generally relates to a method of imparting a radio wave absorption property to a building material by including ferrite powder in the building material matrix.

In designing the electromagnetic wave absorber, it is necessary to know the complex relative permittivity (ε _r ) and the complex relative permittivity (u _r ) of the material.

[Examples] The present invention will be described in more detail with reference to Examples below.

Reference example Cement mortar manufactured by Chichibu Cement Co., sand mixed with ordinary cement, carbon fiber manufactured by Kureha Chemical Industry Co., Ltd. (average fiber diameter 18μ, average fiber length 0.7 mm, abbreviated as C _f ), Kureha Chemical Industry Carbon beads (average particle size)
300μ, abbreviated as C _b ), TDK Corporation's ferrite powder (average particle size 13.4μ, abbreviated as F) were mixed with water in various blends, and the complex relative dielectric constant (ε _r ) of the mortar raw material was obtained. The complex specific magnetic susceptibility (μ _r ) was measured. The results are shown in Table 1.

As is clear from Table 1, the complex relative magnetic susceptibility (μ _r ) generally increases as the mixing ratio of ferrite increases.

Examples and Comparative Examples Based on the results obtained in the reference examples, two-layer concrete panels were manufactured using the materials used in the reference examples, and the frequency characteristics of the reflection coefficient of this absorber were measured. Component mix ratio, material constant and thickness of these concrete panels,
The frequency characteristics are shown in Tables 2, 3 and 2, respectively.

As is clear from the results of Tables 2 and 3 and FIG. 2, in Comparative Example (A) in which the ferrite powder was not contained and only the carbon fiber and the carbon beads were put in the first layer.
In contrast to the bimodal characteristic of having absorption only in the vicinity of 100 MHz and 200 MHz, the embodiment (B) of the present invention in which ferrite powder and carbon fiber are mixed in the first layer and carbon beads are mixed in the second layer is 100 MHz. The absorption characteristics in the frequency band between 20MHz and 200MHz and the total thickness (D) is 19.2cm
And I was able to cut the target of 20 cm. In the example shown by C, carbon fiber and carbon beads are put in the first layer, and ferrite powder is put in the second layer. The thickness is as thin as 15.00 cm, but the frequency characteristic is better than that of B. Rather it was similar to A.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a structure of a two-layer type electromagnetic wave absorber of the present invention, and FIG. 2 is a diagram showing frequency characteristics of two-layer type concrete panels of Examples and Comparative Examples.

Claims (5)

[Claims] 1. A radio wave absorber comprising a two-layer structure comprising a layer of a concrete matrix containing at least carbon fibers and / or carbon beads, and a layer of a concrete matrix containing at least ferrite powder. 2. The electromagnetic wave absorber according to claim 1, wherein the two-layer structure has a thickness of 20 cm or less. 3. When a metal plate is provided on the surface of one of the layers to form a radio wave absorber structure, VHF is formed on the surface of the layer without the metal plate.
The radio wave absorber according to claim 1 or 2, wherein the absolute value of the reflection coefficient in the vicinity of 100 MHz and 200 MHz of the television frequency shows a value of 0.2 or less. 4. A radio wave absorber comprising a two-layer structure comprising a layer of concrete matrix containing at least carbon fibers and / or carbon beads and a layer of concrete matrix containing at least ferrite powder. A method of manufacturing, comprising the steps of (a) mixing a concrete material containing sand and cement with at least ferrite powder to form a layer containing at least ferrite powder in a matrix made of concrete, and (b) sand and Forming a layer in which at least carbon fibers and / or carbon beads are contained in a matrix made of concrete from a concrete material containing cement mixed with at least carbon fibers and / or carbon beads; Manufacturing method characterized by comprising the steps. 5. (a) Casting a mortar in which a ferrite powder and water are mixed into a concrete material containing sand and cement and curing the mortar to form a layer containing the ferrite powder in a matrix made of concrete, and (B) On the above-mentioned layer, a mortar obtained by mixing carbon fiber and / or carbon beads and water into a concrete material containing sand and cement is cast and cured, and at least carbon fiber and / or carbon is added to a matrix made of concrete. The method according to claim 4, comprising the step of forming a layer containing beads.