JP2003060383A - Electromagnetic wave absorbing sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electromagnetic wave absorbing sheet which displays superior absorption characteristics over a wide high frequency band and is easily thinned. SOLUTION: The absorptive sheet 1, 1a comprises a sheet body 2 made of polypropylene (insulation material), and a powder 6, 8 of soft-magnetic metal or alloy of 3 μm or less embedded in the body 2 at a proportion of 25-40 vol.%.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an electromagnetic wave absorption sheet having excellent absorption characteristics over a wide frequency band.

[0002]

2. Description of the Related Art A conventional broadband electromagnetic wave absorber is
A so-called continuous pyramid type with many quadrangular pyramids,
There are ferrite-based absorbers, laminated sheet types, etc., which are used for anechoic chambers and for preventing radio interference. The above ferrite-based absorber has a high magnetic permeability at 500 MHz or less, but has a thickness of about 5 in order to obtain sufficient absorption characteristics.
Since it is mm, it is not suitable for use in a narrow space such as an electronic device. Further, the above-mentioned laminated sheet type is one in which metal powder having an average particle size of several tens of μm is embedded in a rubber sheet or the like and a plurality of such sheets are laminated, and exhibits excellent absorption characteristics in the GHz level band, but has a thickness About 3-5m
Since it is m, the applicable range is also limited.

[0003]

DISCLOSURE OF THE INVENTION The present invention solves the problems in the prior art described above, exhibits excellent absorption characteristics in a wide high frequency band of several hundred MHz to several tens GHz, and is easily thinned. An object is to provide an electromagnetic wave absorbing sheet. In the present specification, the term "electromagnetic wave" includes a case where the electric wave is initially a single wave or a single magnetic wave.

[0004]

In order to solve the above-mentioned problems, the present invention has been made by paying attention to defining the particle size and the amount of metal powder to be embedded in a sheet body made of an insulating material. is there. That is, the electromagnetic wave absorbing sheet (claim 1) of the present invention comprises a sheet body made of an insulating material, and a powder made of a soft magnetic metal or alloy having an average particle size of 3 μm or less, which is embedded in the sheet body in an amount of 25 to 40 vol%. ,including,
It is characterized by

According to this, since the extremely fine powder having the average particle diameter of 3 μm or less is used, the deterioration of the magnetic permeability due to the eddy current generated by the electromagnetic wave to be absorbed can be reduced. As a result, microwave band (several hundred MHz to several tens of MHz)
High magnetic permeability can be maintained in the entire range of (GHz),
Moreover, the dielectric constant can be suppressed low. Therefore, even a thin sheet having a thickness of, for example, about 0.5 to 2 mm can exhibit excellent electromagnetic wave absorption characteristics in the above wide microwave band and can be easily applied to a narrow space such as an electronic device. . If the burying amount of the above powder in the sheet body is less than 25 vol%, the distribution density becomes sparse and the absorption characteristics deteriorate, while the burying amount is 45 vol%.
When it exceeds, the permittivity becomes too high and the broadband characteristics cannot be obtained. In order to prevent these, the burying amount of the powder is within the above range. Further, various rubbers or synthetic resins are applied to the insulating material forming the seat body.

Further, according to the present invention, the electromagnetic wave absorbing sheet, wherein the powder has a flat shape having a major axis and a minor axis, and the major axis is embedded substantially along the plane direction of the sheet body (claim Item 2) is also included. According to this, since the powder to be embedded is flat, the powder is more easily magnetized and the magnetic permeability is higher than that of the spherical powder. As a result, the electromagnetic wave absorption characteristics can be exhibited more stably, particularly in a lower frequency range. The flat powder is formed by stirring a substantially spherical powder together with a large number of steel balls in an attritor, and the ratio of the major axis to the minor axis of the flat powder (aspect ratio) is About 1.1: to about 3
It is a degree. The term “almost along” also includes a range of inclination of ± 20 degrees or less centered on the plane direction of the sheet body (direction orthogonal to the thickness direction).

Further, the present invention also includes an electromagnetic wave absorbing sheet (claim 3) in which the thickness of the sheet body is 1 mm or less. According to this, since it is thin and has flexibility, it can be easily arranged in a narrow space in various electronic devices. As a result, required functions can be guaranteed without causing malfunction in the target device, and the devices can be arranged easily and inexpensively. Incidentally, by thinly coating an adhesive layer on either surface of the electromagnetic wave absorption / absorption sheet of the present invention in advance, the sheet is cut into a desired size and easily attached to the surface of the shielded device or its inner surface. It is also possible to paste it.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, preferred embodiments for carrying out the present invention will be described with reference to the drawings. FIG. 1 (A) shows a cross section of an electromagnetic wave absorbing sheet 1 of the present invention. As shown in FIG. 1 (A), the electromagnetic wave absorbing sheet 1 has a sheet body 2 made of a synthetic resin insulating material such as polypropylene, and is embedded in the sheet body 2 at a rate of 25 to 40 vol% substantially uniformly. The powder 6 is made of a soft magnetic metal or alloy having an average particle diameter of 3 μm or less. The sheet body 2 has a thickness t of 1 mm and has a front surface 3 and a back surface 4.

The powder 6 is Fe, which is a soft magnetic metal,
Ni, Co, or a melt of an alloy based on any of these is pulverized by a water atomizing method or a gas atomizing method, and further classified by a sieve having a mesh of 3 μm or less, or mechanically pulverized. It is either finely divided powder or carbonyl iron powder. The electromagnetic wave absorbing sheet 1 is made of synthetic resin such as polypropylene and 2
After kneading 5 to 40 vol% of the powder 6 in a known kneader (not shown), the resulting mixture is passed through a known calender roll (not shown) and rolled to obtain a mixture as shown in FIG. 1 (A). In addition, a thin and flat sheet body 2 having a thickness t of 1 mm and having front and back surfaces 3 and 4 is formed, and a large number of powders 6 are substantially uniformly embedded in the sheet body 2.

According to the electromagnetic wave absorbing sheet 1 as described above, the deterioration of the magnetic permeability of the powder 6 due to the eddy current generated in the powder 6 by the target electromagnetic wave is reduced, so that the high magnetic permeability is maintained in the entire microwave band. In addition, the dielectric constant can be suppressed to be low. Moreover, even a thin sheet body 2 having a thickness t of about 1 mm can exhibit a wide electromagnetic wave absorption characteristic in the microwave band, and thus can be easily applied to a narrow space such as an electronic device.

FIG. 1B shows a cross section of an electromagnetic wave absorbing sheet 1a of the present invention, which is an applied form of the electromagnetic wave absorbing sheet 1.
As shown in FIG. 1 (B), the electromagnetic wave absorbing sheet 1a also has a sheet body 2 made of an insulating material such as polypropylene, and an average particle diameter of 3 μm embedded in the sheet body 2 almost uniformly.
And a flat-shaped powder 8 made of the following soft magnetic metal or alloy. As shown in FIG. 1 (C), the powder 8 is obtained by stirring the above-mentioned atomized powder together with a large number of steel balls in a known attritor (not shown) for several hours to obtain a long axis L and a short axis S. And the ratio
The sheet body 2 is formed into a flat shape with an (aspect ratio) of 1.1: to 3 at a ratio of 25 to 40 vol%.
Buried in.

The flat-shaped powder 8 having the major axis L and the minor axis S is kneaded in a kneader in the same manner as described above, and then rolled by a calender roll to give a thickness t as shown in FIG. 1 (B). A thin and flat sheet body 2 having front and back surfaces 3 and 4 of 1 mm, and a large number of powders 8 in which the major axes L of the sheet body 2 are substantially along the plane direction thereof and which are substantially uniformly embedded. Electromagnetic wave absorption sheet 1 consisting of
a is obtained. According to the electromagnetic wave absorption sheet 1a as described above, the flat powder 8 is easily magnetized and the magnetic permeability is further increased, so that the electromagnetic wave absorption characteristics are further stabilized particularly in a frequency band lower than that of the electromagnetic wave absorption sheet 1. Can be demonstrated.

[0013]

Example: A seat body 2 made of polypropylene (insulating material) having a thickness t of 1 mm, and 40 vo in the seat body 2.
Electromagnetic wave absorption sheet 1 of Example 1 consisting of carbonyl iron powder 6 having an average particle size of 1.6 μm and buried substantially uniformly at 1%
Prepared. Further, an electromagnetic wave absorbing sheet 1 of Example 2 including the same sheet body 2 as described above and the carbonyl iron powder 6 having a mean particle size of 3 μm, which was substantially uniformly embedded in the sheet body 2 at 40 vol%, was prepared. On the other hand, an electromagnetic wave absorbing sheet of Comparative Example 1 was prepared, which was composed of the same sheet body 2 as described above and the carbonyl iron powder 6 having a mean particle size of 9 μm and which was substantially uniformly embedded in the sheet body 2 at 40 vol%.

Furthermore, the electromagnetic wave absorption of Comparative Example 2 consisting of the same sheet body 2 as described above and the powder 6 of Fe-13 wt% Cr having an average particle size of 9 μm which is almost uniformly embedded in the sheet body 2 at 40 vol%. I prepared a sheet. Examples 1 and 2
The electromagnetic wave absorbing sheet 1 and the electromagnetic wave absorbing sheets of Comparative Examples 1 and 2 are arranged in the vicinity of the center in a free space, and a high frequency of 0 to 20 GHz is perpendicular to the surface 3 of each electromagnetic wave absorbing sheet 1 from a horn antenna (not shown). Electromagnetic waves (plane waves) were individually incident. Then, the return loss of each sheet 1 at each frequency was measured by a network analyzer (not shown). The measurement results are shown in the graph of FIG.

According to the graph of FIG. 2, the electromagnetic wave absorbing sheet 1 of Example 1 has an attenuation peak in the vicinity of about 14 GHz and exhibits a return loss of 10 dB or more in the band of about 8 to 17 GHz. In addition, the electromagnetic wave absorbing sheet 1 of Example 2 has a gradual attenuation peak near about 11 GHz and 10d in the band of about 8 to 14 GHz.
The return loss was B or more. On the other hand, according to the graph of FIG. 2, the electromagnetic wave absorbing sheets of Comparative Examples 1 and 2 are about 10 GH.
Although there was a gentle attenuation peak near z, the return loss was less than 10 dB in all cases.

Further, even with the same carbonyl iron powder 6, the electromagnetic wave absorbing sheet 1 of Examples 1 and 2 having an average particle diameter of 1.6 μm and 3 μm and the electromagnetic wave absorbing sheet of Comparative Example 1 having an average particle diameter of 9 μm are compared. Then, as shown in the graph of FIG. 2, the smaller the average particle size, the greater the attenuation amount and the attenuation in a relatively high frequency region. The larger the average particle size, the smaller the attenuation amount. It is shifting to the extremely low frequency range. That is, the electromagnetic wave absorbing sheet 1 of Examples 1 and 2
The use of the fine carbonyl iron powder 6 having an average particle diameter of 3 μm or less reduces deterioration of magnetic permeability due to eddy currents generated by electromagnetic waves to be absorbed, maintains high magnetic permeability, and keeps the dielectric constant low. Therefore, it was possible to exhibit excellent absorption characteristics in a wide frequency band.
From the above, it is easily understood that the effect of the present invention (Claim 1) is supported.

Next, the same average particle size as in Example 1 was 1.
Three types of flat powders 8 were produced by individually stirring 6 μm carbonyl iron powder 6 together with a large number of steel balls in an attritor (not shown) for 1.5 hours, 3 hours, and 6 hours. The ratio (aspect ratio) of the long axis L and the short axis S in the three types of flat powders 8 is 1.2 for 1.5 hours of stirring, 1.4 for 3 hours of stirring, and 6 hours of stirring for 6 hours. It was 1.8. The above three types of flat powders 8 were kneaded by the kneader and rolled to obtain a thickness t.
Example 3 by individually embedding at 40 vol% in a sheet body 2 made of polypropylene (insulating material) of 1 mm
The electromagnetic wave absorption sheets 1 to 5 were obtained.

The vicinity of the center of the electromagnetic wave absorbing sheet 1 of Examples 3 to 5 is arranged in a free space, and a high frequency of 5 to 20 GHz is perpendicular to the surface 3 of each electromagnetic wave absorbing sheet 1 from a horn antenna similar to the above. Electromagnetic waves (plane waves) were individually incident. Then, the return loss of each sheet 1 at each frequency was measured by a network analyzer (not shown). The measurement results are shown in the graph of FIG. 3 together with the measurement results of Example 1 above. According to the graph of FIG. 3, as compared with Example 1, the electromagnetic wave absorbing sheets 1 of Examples 3 to 5 using the flat powder 8 had a gentle attenuation peak and were shifted toward a relatively low frequency band. . Further, the above tendency became remarkable as the stirring time by the attritor was long and the ratio (aspect) of the long axis L and the short axis S was increased.

Further, according to the measurement results shown in the graph of FIG. 3, since the powder 8 of Example 3 is flat, it is more easily magnetized and has a higher magnetic permeability than the spherical carbonyl iron powder 6 of Example 1. Since the height was higher, the area of the region where the attenuation amount was 10 dB or more increased. However, in Examples 4 and 5 in which the flattened powder 8 was used, the area area of the attenuation amount of 10 dB or more was reduced and the attenuation peak was shifted to the lower frequency range. From such a result, it was found that the flat powder 8 more stably exerts the electromagnetic wave absorption characteristics of the electromagnetic wave absorption sheet 1 by setting the aspect ratio of the long axis L and the short axis S in Example 3 to about the same. And the present invention (claim 2)
The effect of was confirmed.

The present invention is not limited to the embodiments and examples described above. For example, the seat body 2 includes polyethylene, polyamide, acrylic resin,
Synthetic resins such as polystyrene, polyvinyl chloride, ABS resin, phenol resin, unsaturated polyester resin, and polyetherimide, polyamideimide, polysulfone, polyetherketone made of heat resistant polymer may be used. Alternatively, a heat-resistant synthetic rubber such as styrene / butadiene rubber (SBR), isobutylene / isoprene rubber (IIR), or polybutadiene rubber (BR) is also applicable. Further, the soft magnetic metal powders 6 and 8 embedded in the sheet body 2 are not limited to the iron powders and Fe-based alloys, but may be Ni or C.
o, or alloys based on them. Further, at least one of the front surface 3 and the back surface 4 of the sheet body 2 may be covered with an adhesive layer and the surface thereof may be covered with a paper sheet. Thus, by peeling off the paper sheet, it becomes possible to easily attach it to a narrow space such as in an electronic device or the inner wall of the anechoic chamber.

[0021]

According to the electromagnetic wave absorbing sheet of the present invention described above (claim 1), since fine powder having an average particle diameter of 3 μm or less is used, deterioration of magnetic permeability due to eddy current caused by electromagnetic waves to be absorbed. It becomes smaller, high magnetic permeability can be maintained in the entire microwave band, and the dielectric constant can be suppressed low. Therefore, even when a thin sheet having a thickness of, for example, about 1 mm is used, excellent electromagnetic wave absorption characteristics can be exhibited in a wide microwave band, and the sheet can be easily applied to a narrow space such as an electronic device.

Further, according to the electromagnetic wave absorbing sheet of claim 2, since the powder to be embedded is flat, it is more easily magnetized and has a higher magnetic permeability than the spherical shape, and the electromagnetic wave absorbing sheet is absorbed particularly in a lower frequency range. The characteristics can be exhibited more stably. Further, according to the electromagnetic wave absorbing sheet of the third aspect, since it is thin and has flexibility, it can be easily arranged in a narrow space in various electronic devices. Therefore, a desired function can be guaranteed without causing a malfunction in the shield target device, and the shield target device can be arranged easily and inexpensively.

[Brief description of drawings]

1A and 1B are cross-sectional views showing an electromagnetic wave absorbing sheet of the present invention, and FIG. 1C is a schematic view of powder used in the electromagnetic wave absorbing sheet of FIG. 1B.

FIG. 2 is a graph showing return loss characteristics of electromagnetic wave absorbing sheets of Examples and Comparative Examples.

FIG. 3 is a graph showing reflection attenuation characteristics of electromagnetic wave absorbing sheets of different examples.

[Explanation of symbols]

1, 1a ... Electromagnetic wave absorption sheet, 2 ......... Sheet body, 6, 8 ... Powder, L ...
…… Major axis, S ………… Short axis, t…
……… Thickness

Claims (3)

[Claims] 1. An electromagnetic wave comprising: a sheet body made of an insulating material; and a powder made of a soft magnetic metal or alloy having an average particle size of 3 μm or less, which is embedded in the sheet body at 25 to 40 vol%. Absorption sheet. 2. The powder has a flat shape having a major axis and a minor axis, and the major axis is embedded substantially along a plane direction of the sheet body. Electromagnetic wave absorption sheet. 3. The electromagnetic wave absorbing sheet according to claim 1, wherein the sheet body has a thickness of 1 mm or less.