JPH0613780A - Radio wave absorbent - Google Patents

Abstract

PURPOSE:To obtain a radio wave absorbent capable of absorbing electromagnetic energy by a method wherein the absorbent is formed through such a manner that metal powder is dispersed into a ferrite sintered body. CONSTITUTION:A radio wave absorbent 1 is a composite body composed of Ni-Zn ferrite sintered body 4 as parent material and TiC powder 5 of conductive ceramics dispersed in the sintered body 4, the conductive ceramic powder has an electric resistivity rho represented by a formula, rho>=100muOMEGA-cm, powder is 10 to 100mum in maximum axis, 1 to 30mum in minimum axis, over 50 in aspect ratio, and needle-like or fiber-like in shape. If the volume ratio of metal powder 5 to ferrite sintered body 4 is less than 5%, the absorbent 1 can not be enhanced in dielectric constant, and if the ratio exceeds 30%, metal powder is lessened in contact resistance, so that the absorbent can not kept high in characteristics. Therefore, an absorbent of this design can be set not only in complex permeability as required but also in complex dielectric constant for a certain frequency band, and consequently it is able to absorb electromagnetic energy damping both the magnetic field component and electrical field component of electromagnetic waves.

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 for absorbing radio waves in a high frequency range.

[0002]

2. Description of the Related Art A radio wave (electromagnetic wave) is a wave having an electric field component E and a magnetic field component H as shown in FIG. 5, and the ratio of the electric field component E and the magnetic field component H is the spatial impedance Zo.
And becomes 377Ω in the far field. The magnetic loss of a substance contributes to the absorption of a magnetic field component, and the dielectric loss contributes to the absorption of an electric field component.

Generally, the magnetic loss μ ^* and the dielectric loss ε ^* of a substance in a high frequency range represent the basic characteristics of this substance. The former is complex permeability μ ^* = μ′−jμ ″
(Or loss angle tan δ = μ ″ / μ ′), the latter is complex permittivity ε ^* = ε′−jε ″ (or loss angle tan δ = ε ″ /
It is known to be represented by ε '). If the complex permeability and complex permittivity are known, the reflectance of this substance (the ratio of the incident electromagnetic wave reflected on the substance surface when it is vertically incident on the substance)
Or the skin depth (thickness of the absorption layer until the incident electromagnetic wave is attenuated to 1 / e in the substance) is obtained, and the absorption characteristic of the substance can be known as the characteristic of the radio wave absorber 1 shown in FIG. it can.

Conventionally, as a radio wave absorber, a ferrite utilizing magnetic loss, a carbon impregnated with urethane having a dielectric loss, and the like have been commercialized. However, these are characterized by absorbing electromagnetic wave energy by causing loss of only one of the two components of the electromagnetic field, the magnetic field component and the electric field component (ferrite has only the magnetic field component, and carbon has only the electric field component). As shown in FIG. 4, it is necessary to mount the reflector 3 on the back side of the absorber 1 (the side opposite to the incident side of the radio wave 2). In addition, the matching thickness (thickness that can be used as an absorber) is complex permeability μ ^* and complex permittivity ε ^*.
There was a problem that it was inevitably decided by.

[0005]

SUMMARY OF THE INVENTION In view of the above problems of the conventional electromagnetic wave absorber, the present invention eliminates two components of an electromagnetic wave, that is, two components, a magnetic field component and an electric field component, so that electromagnetic wave energy is lost. It is an object of the present invention to provide a radio wave absorber that can absorb the electromagnetic wave and does not need to consider the multiple reflection by the reflector and the matching thickness.

[0006]

A radio wave absorber of the present invention for solving the former problem is characterized in that conductive ceramics are dispersed in a ferrite sintered body.

A radio wave absorber of the present invention for solving the latter problem is characterized in that, in the radio wave absorber having the above-mentioned structure, the magnetic loss and the dielectric loss are made equal to each other.

[0008]

[Function] By forming a radio wave absorber by dispersing conductive ceramics in a ferrite sintered body, a desired complex magnetic permeability (magnetic loss) μ ^* can be obtained in a target frequency range, and a complex dielectric constant ( (Dielectric loss) ε ^* can also be generated, and as a result, both the magnetic field component and the electric field component of the electromagnetic wave can be lost to absorb the electromagnetic wave energy, and the absorption characteristics are improved.

The impedance Z when an electromagnetic wave is vertically incident on a substance is given by the following equation. Z = √μ ^* / ε ^{* In} the case of vacuum μ ^* = μ ₀ = 4π × 10 ⁻⁷ ε ^* = ε ₀ = 8.85 × 10 ⁻¹² Therefore Z = 377Ω Impedance of a material is Permeability μ ₀
And the relative permittivity (ratio with the permittivity ε _{0 of} vacuum) from the above equation. And magnetic loss μ ^* and dielectric loss ε
^{If *} is equal, it matches the spatial impedance and there is no reflection. Therefore, it is not necessary to consider multiple reflections by the reflector and matching thickness, and the design of the electromagnetic wave absorber is simplified.

It is known that the permittivity of a conductor is infinite at high frequencies. However, in that case, the magnetic permeability becomes 0, and it cannot be used as a loss material. Therefore, it is necessary to make the permittivity a finite value by insulating the conductor or in a state close to it.

The electrical resistivity of the conductive ceramics used in the present invention is preferably ρ ≧ 100 μΩ-cm. If it is less than this, the contact resistance between the dispersed ceramics decreases, and a desired dielectric loss cannot be obtained.

The longest axis of the conductive ceramics has a shape of 1
Desirably, the shape is needle-like or fiber-like with 0 to 100 μm, the shortest axis of 1 to 30 μm, and an aspect ratio of 50 or more. If the particles have a small aspect ratio, the dielectric constant does not increase. The length of the longest axis is 100μ
If it is more than m, there is a problem that the dispersion in the ferrite sintered body having a thickness of several mm does not work properly, and the dimensions are set as described above.

If the volume ratio of the conductive ceramics is 5% or less, the dielectric constant cannot be increased, and if it exceeds 30%,
The contact resistance between ceramics decreases, and the characteristics cannot be maintained.

When conductive ceramics such as TiC (sintering temperature of about 3140 ° C.) is used, the melting point is Ni--
Since the temperature is higher than the sintering temperature of Zn ferrite (around 1300 ° C.), a stable sintered body can be produced without deforming the shape during sintering or reacting with the base material.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Specific embodiments of the present invention will be described in detail below with reference to the drawings. The radio wave absorber 1 has a Ni-Zn ferrite sintered body 4 as a base material, as conceptually shown in FIG.
It was configured as a composite body in which TiC powder 5 which is a conductive ceramic was dispersed.

As the TiC powder, two kinds of fiber powder (A) and granular powder (B) are used, and the volume ratio is 5% to 40%.
By making various changes within the range up to the above, radio wave absorbers having a thickness of 10 mm were created, and the absorption rate was measured for radio waves with a frequency of 200 MHz.

The measurement results are shown in FIG. In the figure, the mark indicates the electromagnetic wave absorber using the fiber powder (A), and the mark x indicates the electromagnetic wave absorber using the granular powder (B). From this figure, it can be seen that the one using the granular powder does not have a high absorptivity, and that the one using the fiber powder is 5
It can be seen that the characteristics cannot be maintained at less than 30% and at 30% or more.

FIG. 3 shows the absorptance measured by changing the frequency of the radio wave for the radio wave absorber having the TiC fiber powder dispersed in the ferrite sintered body and having a powder volume ratio of 15%. It is the electromagnetic wave absorption characteristic curve created. From this figure, the frequency of the target radio wave to be absorbed is 300
In the vicinity of MHz, the volume ratio of TiC fiber powder is 15
It can be seen that the one with% dispersion is optimal.

[0019]

As described above, according to the present invention, it is possible to absorb electromagnetic wave energy by making radio waves in a target frequency range lose two components, a magnetic field component and an electric field component, and improve absorption characteristics. be able to.

[Brief description of drawings]

FIG. 1 is a conceptual diagram showing a configuration of a radio wave absorber of the present invention.

FIG. 2 is a curve diagram showing the radio wave absorption characteristics relating to the volume ratio of the conductive ceramics of the radio wave absorber of the present invention for two types of powder shapes.

FIG. 3 is a curve diagram showing an example of the radio wave absorption characteristics of the radio wave absorber according to the present invention with respect to the radio frequency.

FIG. 4 is an explanatory diagram illustrating the arrangement of a radio wave absorber and a reflection plate for radio waves.

FIG. 5 is an explanatory diagram illustrating an electric field component and a magnetic field component of a radio wave.

[Explanation of symbols]

 1 Radio wave absorber 2 Radio wave 4 Ferrite 5 Conductive ceramics

Claims (5)

[Claims] 1. A radio wave absorber formed by dispersing conductive ceramics in a ferrite sintered body. 2. The conductive ceramics according to claim 1, which has an electric resistance of 100 μΩ-cm or more, a longest axis of 10 to 100 μm, a shortest axis of 1 to 30 μm, and an aspect ratio of 50 or more. The electromagnetic wave absorber according to 1. 3. The volume ratio of the conductive ceramics is 5
The radio wave absorber according to claim 1, wherein the radio wave absorber is -30%. 4. The radio wave absorber according to claim 1, wherein the conductive ceramic is TiC fiber. 5. The radio wave absorber according to claim 1, wherein the magnetic loss and the dielectric loss are equal to each other.