JPH04275403A - Complex sintered material for electromagnetic wave absorption - Google Patents

Abstract

PURPOSE:To obtain a ferrite complex sintering material which has electromagnetic wave absorbing properies excellent in a wide frequency band, especially, in the TV frequency band. CONSTITUTION:The complex sintering material is constituted as complex ceramics where conductive particles having the shape that the electric resistivity is 1X10-2-10OMEGA.cm and that the anti-electric-field coefficient becomes 0.2 or below are mixed 5-30% by volume ratio in the oxide magnetic substances of ferrite and are sintered.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a composite sintered material used as an electromagnetic wave absorbing material in a high frequency band.

0002

2. Description of the Related Art Ferrite-based magnetic materials, such as Ni--Zn ferrite and Mn--Zn ferrite magnetic materials, have been put into practice or proposed as electromagnetic wave absorbing materials in high frequency bands. All of these ferrite-based magnetic materials are ferrimagnetic non-metallic magnetic materials, and are used as ceramic sintered bodies, often in the form of tiles.

The electromagnetic wave absorption effect of these sintered bodies utilizes the magnetic loss of ferrite, but it cannot be said that they have sufficient absorption characteristics. Therefore, in order to improve the absorption characteristics, attempts have been made to use a combination of measures such as lining the tiles with a metal reflective layer and utilizing interference between incident waves and reflected waves. However, since dielectric loss is not utilized for absorption characteristics, the effective frequency band is limited, and the absorption characteristics cannot be said to be sufficient.

[0004] Also, as electromagnetic wave absorbing materials, composite materials made by mixing rubber with ferrite powder or ferrite and carbon powder have been put into practical use or have been proposed.
The absorption characteristics in the band (approximately 100 MHz/1,000 MHz band) are completely inadequate, and it has only been put to practical use in bridges and the like as part of radar countermeasures, and is not compatible with a wide band of frequencies.

[0005]

Problems to be Solved by the Invention In view of the above-mentioned circumstances of the ferrite-based electromagnetic wave absorbing sintered bodies that have been put into practical use or have been proposed, the present invention has been made to provide excellent performance in a wide frequency band, particularly in the TV frequency band. An object of the present invention is to provide a ferrite-based composite sintered material that exhibits electromagnetic wave absorption characteristics.

[0006]

[Means for solving the problems] The composite sintered material of the present invention is
In order to solve the above problems, we developed a ferrite oxide magnetic material with an electrical resistivity of 1 x 10-2 Ω・cm to 10 Ω・c.
The present invention is characterized in that it is made into a composite ceramic in which 5% to 30% by volume of conductive particles having a shape with a diameter of m and an anti-electric field coefficient of 0.02 or less are mixed and sintered.

[0007] As the above-mentioned conductor particles, SiC or S
iC whiskers are available.

[0008]

[Operation] Generally, the electromagnetic wave absorption characteristics of a material in the high frequency range are determined by the composite magnetic permeability μ* = μ, which is the basic property of the material.
'-jμ'', loss angle tanδ=μ''/μ', complex permittivity ε* =ε'-jε'', and its loss angle tanδ
It is well known that the complex magnetic permeability of the former is directly related to the magnetic loss, and the complex dielectric constant of the latter is directly related to the dielectric loss, and determines the absorption characteristics.

[0009] Conventional sintered materials for absorbers using ferrite are materials that have a large magnetic loss in the target TV frequency band, and in particular, materials that have a large value of μ'' (or tan δ), which indicates magnetic loss. However, in the composite sintered body of the present invention, in addition to the excellent magnetic loss possessed by ferrite, dielectric loss caused by the presence of a nonmetallic conductor in the insulator is added, and the absorption properties are improved. In addition, compared to composite materials made by hardening ferrite powder, carbon powder, or short fibers with rubber, etc., the composite sintered material of the present invention has a larger magnetic loss because the ferrite is sintered to form a ceramic. , the absorption characteristics are improved from this point as well.

According to research including many experiments conducted by the present inventor, the above μ' and μ'' can be determined by determining the frequency of electromagnetic waves, the volume ratio V of the conductor particles, the electrical resistivity R, and the conductor particles. It depends on the anti-electric field coefficient N, which is determined by the major axis/minor axis ratio k of .

According to a large number of experimental results, when the electrical resistivity of conductive particles is 10-2 Ω·cm or less, the desired μ″ and t
anδ cannot be obtained, desired magnetic loss cannot be obtained, and
If it exceeds 10 Ω·cm, it is not preferable because it loses its characteristics as a conductive material. Therefore, the desirable range of electrical resistivity R is 1×10-2 Ω・cm to 10 Ω・c
It is m. Further, the appropriate mixing ratio of the conductor particles is 5 to 30% by volume. Further, it is desirable that the anti-electric field coefficient N is 0.2 or less. Note that Table 1 shows the size ratio of typical particle shapes and the values of anti-electric field coefficients.

0012

[Table 1]

According to Table 1, K such that the value of N is 0.2 or less is determined as long as the ratio l1 /l2 of the major axis l1 to the minor axis l2 is approximately 5 or more.

From the above, the most suitable conductor material is SiC short fibers or whiskers having an electrical resistivity R of 2×10 −1 Ω·cm. Although bulk particles of SiC satisfy the electrical resistivity, they are unsuitable in terms of the N value. Also, carbon powder and short fibers have an electrical resistivity of approximately 10-
3Ω·cm, and no favorable results were obtained.

[0015]

[Example] Below, a composite sintered material for absorbing electromagnetic waves as an example of the present invention will be compared with a currently used electromagnetic wave absorbing material, and its properties will be shown in Tables 2 and 4.

Table 2 shows the electromagnetic wave absorption characteristics of a composite sintered material obtained by mixing Ni--Zn ferrite with various conductive particles at a volume ratio of 15% at a frequency of 200 MHz.

[0017]

[Table 2]

The dimensions, shapes, N values, and electrical resistivities of the various conductive particles in Table 2 are as shown in Table 3.

[0019]

[Table 3]

Table 4 also shows the electromagnetic wave absorption characteristics at a frequency of 200 MHz of composite sintered materials prepared by mixing Ni--Zn ferrite of various compositions with SiC whiskers as conductive particles at a volume ratio of 10%. In addition, Si to be mixed
The dimensions, shape, N value, and electrical resistivity of the C whiskers are the same as in Table 3. The electromagnetic wave absorbing materials in columns 2, 3 and 4 of Table 2 and columns 5 and 6 of Table 4 are comparative materials. Further, the thickness of the electromagnetic wave absorption layer (skin depth S) is expressed as the thickness of the absorption layer for attenuating the electromagnetic wave intensity to 1/e.

[0021]

[Table 4]

As seen from Tables 2 and 4, the material of the present invention has a lower electromagnetic wave reflectance and a smaller thickness of the absorption layer than the comparative material.

[0023]

[Effects] As described above, the present invention has excellent absorption characteristics in a wide frequency band, especially in the TV frequency band,
A ferritic composite sintered material that can reduce the thickness of the absorption layer can be obtained.

Claims (3)

[Claims] Claim 1: In a composite sintered material used as an electromagnetic wave absorbing material in a high frequency band, the ferrite oxide magnetic material has an electrical resistivity of 1 x 10-2 Ω·cm to 10 Ω·c.
1. A composite sintered material, characterized in that it is made of a composite ceramic obtained by mixing 5% to 30% by volume of conductive particles having a shape of m and an anti-electric field coefficient of 0.02 or less and sintering the mixture. 2. The composite sintered material according to claim 1, wherein the conductor particles are SiC. 3. The composite sintered material according to claim 1, wherein the conductor particles are SiC whiskers.