JP2002171089A - Electromagnetic wave absorbing sheet having no anisotropy, and its manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electromagnetic wave absorbing sheet having an absorption performance of electromagnetic wave incident to the sheet not different according to a polarization direction, without anisotropy in absorption characteristics in the sheet obtained by molding a powder or soft magnetic metal dispersed into a matrix material of a rubber or a plastic by rolling by a roll, and to produce a method for manufacturing the same. SOLUTION: The method for manufacturing the electromagnetic wave absorbing sheet comprises the steps of blending the powder of the soft magnetic metal with the matrix material of the rubber or the plastic, kneading and roughly rolling the mixture by a mixing roll, and then precisely rolling the material by a calender roll. In this case, the rough rolling by the mixing roll is stopped to a thickness of the sheet not les than 4 mm, and transferred to the precise rolling by the calender roll. The material is cross-rolled in the precise rolling to a desired sheet thickness, and the electromagnetic wave absorbing sheet having low anisotropic of its permittivity is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to relates to an electromagnetic wave absorbing sheet having no anisotropy in absorption characteristics. Specifically, it is a sheet-like electromagnetic wave absorber, and its electromagnetic wave absorbing performance with respect to electromagnetic waves incident on this sheet from various directions at various angles is as follows.
The present invention relates to an electromagnetic wave absorbing sheet that does not differ depending on the polarization direction.

[0002]

2. Description of the Related Art In order to prevent external electromagnetic waves from interfering with electronic equipment and to suppress radiation of noise electromagnetic waves from the equipment itself, a soft magnetic metal, such as permalloy powder, is used in a rubber or plastic matrix material. Are widely used as electromagnetic wave absorbers. Most of them are in the form of a sheet, and forming into a sheet is usually performed by roll rolling.

[0003] The absorption characteristics of this type of electromagnetic wave absorbing sheet, such as the peak frequency at which the absorption is performed most strongly, depend on the particle size distribution, particle shape, and the like of the soft magnetic metal powder to be dispersed.
For example, the ratio to be mixed in the matrix and the thickness of the sheet
Determined by many factors. In order to suppress the emission of noise electromagnetic waves, it is needless to say that the absorption peak should be matched to the main frequency of the frequency at which the electronic device operates and its harmonics. Various techniques for producing an electromagnetic wave absorbing sheet having a peak have been developed.

However, it has been found that the characteristics of the sheet-shaped electromagnetic wave absorber may have anisotropy depending on the direction in which the sheet is cut out. The inventors who have pursued this phenomenon have found that the anisotropy of the electromagnetic wave absorption characteristics is mainly caused by the anisotropy of the dielectric constant. Since the dielectric constant is an important factor influencing the peak frequency of absorption, when a practical sheet is cut out from the same electromagnetic wave absorbing sheet, the peak frequency of absorption differs depending on the direction.

[0005] In the electromagnetic wave absorber sheet produced by rolling, as shown in FIG. 1, the direction of the rolling direction and the perpendicular direction A, when the rolling direction B direction, when measuring the dielectric constant ε having a sheet, A A relatively high value is obtained in the direction and a relatively low value is obtained in the B direction. (Here, the measurement of the dielectric constant ε was performed by a waveguide method using a “network analyzer HP8510C”.) This phenomenon occurs when an electromagnetic wave having a frequency close to the absorption peak enters the electromagnetic wave absorbing sheet. This means that the state of absorption differs depending on the polarization direction, which results in that the intended absorption is not realized. Such anisotropy is of course not preferred.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems associated with an electromagnetic wave absorbing sheet, and to provide an electromagnetic wave absorbing sheet having no anisotropy in its absorption characteristics. However, an object of the present invention is to provide an electromagnetic wave absorbing sheet that does not differ depending on the polarization direction, and a method for manufacturing the same.

[0007]

The electromagnetic wave absorbing sheet of the present invention, which achieves the above object, is obtained by dispersing a soft magnetic metal powder in a rubber or plastic matrix material into a sheet by roll rolling. In the electromagnetic wave absorbing sheet, there is substantially no difference between the measured value of the dielectric constant in the rolling direction and the value measured in the direction perpendicular to the rolling direction. It is a sheet.

According to the method of the present invention for producing an electromagnetic wave absorbing sheet having no anisotropy in absorption characteristics, a soft magnetic metal powder is mixed with a rubber or plastic matrix material, kneaded with a mixing roll and roughly rolled. In the method for producing an electromagnetic wave absorbing sheet comprising fine rolling with a calender roll, rough rolling with a mixing roll is stopped to a thickness not less than 4 mm, and a sheet with a desired thickness is formed by fine rolling with a calender roll. It is characterized by the following.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One of the causes that may cause anisotropy in an electromagnetic wave absorbing sheet is the influence of a roll rolling operation on the sheet. Therefore, the so-called "cross-rolling", that is, an operation of rolling in a certain direction and then rolling in a direction perpendicular thereto is expected to give a sheet with low anisotropy. However, as far as the inventors tried, cross rolling was not very effective as seen in the comparative examples described later.

Looking back at the manufacturing process of the electromagnetic wave absorbing sheet, the kneading of the soft magnetic metal powder and the matrix rubber or plastic is performed by a mixing roll, and the rolling is performed by a rolling roll. Difference between them, roll pair of a mixing roll while the rotation that produces the kneading effect, the roll pair of the rolling rolls is that the kneading effect is not rotating as occurs directly. The inventors who paid attention to this guessed that kneading might cause anisotropy, and tried to discontinue the influence of kneading as soon as possible in the process of sheet formation. Specifically, kneading is stopped at a stage where the thickness of the rough rolled sheet obtained by the mixing roll is large, and the process is shifted to fine rolling by the rolling roll. This attempt was successful, and the anisotropy of the absorption characteristics of the product electromagnetic wave absorbing sheet was significantly reduced.

When the thickness of the transition from kneading / rough rolling using a mixing roll to fine rolling using a rolling roll should be determined, it is surprisingly critical that the final product has a thickness of about 1 mm or more. In the case of, it was clarified that the transition should be made at a position not less than 4 mm.

At the stage of fine rolling by a rolling roll, the above-mentioned cross rolling is useful, and should be performed if an electromagnetic wave absorbing sheet having sufficiently low anisotropy in absorption characteristics is to be obtained.

In practicing the present invention, any technique known in the art can be used. For example, soft magnetic metal powders include Fe-Al-
Powders of Si alloy, Fe-Ni alloy or Fe-Cr alloy can be suitably used. Similarly, as the matrix material, those often used such as chlorinated polyethylene rubber and acrylic rubber are useful.

[0014]

EXAMPLE 67 parts by weight of a powder (average particle diameter: 8 μm) obtained by spraying an Fe-13Cr alloy with water were mixed with 12 parts by weight of chlorinated polyethylene. After kneading with a mixing roll and rough rolling, fine rolling was performed with a rolling roll to obtain an electromagnetic wave absorbing sheet having a thickness of 0.5 mm. At this time, the sheet thickness at which the transition from kneading and rough rolling to fine rolling is
The thickness of the comparative example was 2.5 mm, and that of the example was 4.0 mm. In the fine rolling stage, both were cross-rolled.
From each of the obtained sheets, samples were cut in directions orthogonal to each other as shown in FIG.

The dielectric constant of each electromagnetic wave absorbing sheet is 28-40.
The measurement was performed in the frequency range of MHz, and the result shown in FIG. 2 was obtained in the comparative example, and the result shown in FIG. 4 was obtained in the example. Each absorption characteristic was as shown in FIG. 3 and FIG.

Referring to the graph of FIG. 2 relating to the comparative example, the value of the dielectric constant ε ′ is 6 to 7 in absolute value depending on the direction of the sheet.
There is a big difference. As a result, as shown in FIG. 3, the absorption characteristics of these sheets show that the peak absorption frequency is 37 MHz for one (A direction) and 37 MHz for the other (B direction).
Is different from 39.5 MHz, the return loss RL is also
It stops at 10 dB, but the other reaches -16 dB. On the other hand, in the embodiment, as shown in FIG. 3, the absolute value of the dielectric constant ε ′ is not more than 2 in absolute value, and accordingly, both peak frequencies are 38 MHz as shown in FIG.
And the return loss RL is also 22 to 23 dB.
At a high level.

[0017]

The electromagnetic wave absorbing sheet of the present invention has low anisotropy in the dielectric constant of the sheet, and thus has low anisotropy in the absorption characteristics. Specifically, in the suitably manufactured electromagnetic wave absorbing sheet of the present invention, it is easy to make the difference between the dielectric constants ε equal to or less than 2 in absolute value.
An electromagnetic wave absorbing sheet as designed can be obtained such that the deviation of the peak absorption frequency in the 0 MHz frequency region is within 1 MHz.

[Brief description of the drawings]

FIG. 1 is a plan view showing a relationship between a rolling direction of an electromagnetic wave absorbing sheet and a sample cutting direction.

FIG. 2 is a graph showing data of a comparative example of the present invention and showing anisotropy of dielectric constant of an electromagnetic wave absorbing sheet.

3 is a graph showing data of a comparative example of the present invention, showing a deviation of an absorption characteristic of an electromagnetic wave absorbing sheet whose dielectric constant is shown in FIG. 2;

FIG. 4 is a graph showing the isotropic property of the dielectric constant of the electromagnetic wave absorbing sheet, which is data of the example of the present invention.

5 is a graph showing the coincidence of the absorption characteristics of the electromagnetic wave absorbing sheet showing the dielectric constant in FIG. 4, which is also data of the example of the present invention.

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Claims (5)

[Claims] 1. An electromagnetic wave absorbing sheet obtained by forming a soft magnetic metal powder dispersed in a rubber or plastic matrix material into a sheet by roll rolling, wherein the dielectric constant is measured in a rolling direction. There is virtually no difference between the values measured for the direction perpendicular to it,
Therefore, an electromagnetic wave absorbing sheet having no anisotropy in the absorption characteristics depending on the polarization direction. 2. The soft magnetic metal powder is an Fe—Ni alloy,
The electromagnetic wave absorbing sheet according to claim 1, wherein the sheet is a metal powder selected from an Fe-Al-Si alloy or an Fe-Cr alloy. 3. The electromagnetic wave absorbing sheet according to claim 1, wherein the matrix material is chlorinated polyethylene rubber or acrylic rubber. 4. A method for producing an electromagnetic wave absorber, comprising mixing a soft magnetic metal powder with a rubber or plastic matrix material, kneading and rough rolling with a mixing roll, and then fine rolling with a calender roll. The method for producing an electromagnetic wave absorbing sheet having no anisotropy in absorption characteristics, characterized in that the rough rolling is stopped to a thickness not less than 4 mm and the sheet is rolled to a desired thickness by calendering. . 5. The method according to claim 4, wherein the fine rolling by the calender roll is performed twice or more and the rolling in directions substantially perpendicular to each other is performed.