JP2000228599A - Electromagnetic wave absorber using matching thickness and method of manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a sheet-shaped electromagnetic wave absorber in which a soft magnetic metal powder is dispersed in a rubber or synthetic resin matrix, and to use a matching thickness in which divergence of electromagnetic waves is observed. And an electromagnetic wave absorber having a high-performance electromagnetic wave absorption performance with a return loss of 35 dB or more.
Another object of the present invention is to provide a design method for realizing a high-performance electromagnetic wave absorber using a matching thickness at an arbitrary frequency. SOLUTION: In a high frequency region of 1 GHz or more, a matching thickness at which radio wave absorption reaches a peak at a specific frequency (divergence frequency) determined by an average particle diameter of a soft magnetic metal powder and a volume filling rate of a matrix thereof. Is the thickness.
The method of manufacturing this is based on given conditions of the divergence frequency and the range of thickness required for the electromagnetic wave absorber, and based on them, the average particle size of the soft magnetic metal powder to be used and its matrix. And the matching thickness made possible under the selected conditions is adopted as the thickness.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electromagnetic wave absorber utilizing a matching thickness and a method for manufacturing the same.

[0002]

2. Description of the Related Art Electromagnetic shielding using an electromagnetic wave absorber is performed for the purpose of preventing electromagnetic waves from being emitted from various electronic devices to the outside and preventing interference from external electromagnetic waves. Many of the structures of the electromagnetic wave absorber are obtained by dispersing and fixing soft magnetic metal powder in a rubber or synthetic resin matrix, and are usually used after being processed into a sheet form.

In general, such an electromagnetic wave absorber is designed so that attenuation of an electromagnetic wave passing therethrough is maximized in a specific frequency region, depending on electronic devices to be shielded. To discuss the degree of attenuation, electromagnetic waves incident on the surface of the sheet with a certain intensity, passed through the sheet, reflected on the back surface and returned, were measured, and the ratio between the two was called "return loss." Expressed in dB. The frequency characteristic of the sheet-like electromagnetic wave absorber is
It is determined by the average particle size and volume filling rate of the soft magnetic metal powder present in the sheet, and the thickness of the sheet. Under such circumstances, for example, if the sheet thickness is varied while keeping the average particle size and volume filling rate of the soft magnetic metal powder constant, the frequency at which the return loss peaks is
The thicker the sheet, the lower the frequency shifts.

In the process in which the peak of the return loss shifts as the thickness of the sheet is changed, the value of the return loss is specifically increased, and it can be seen that there is a thickness at which the divergence of electromagnetic waves can be observed. Was issued. This is called "matching thickness", and at such a sheet thickness,
It is considered that the electromagnetic waves traveling in the sheet almost completely cancel each other in the round trip. One example is shown in the graph of FIG. 1, the Fe-Cr-Al alloy is employed as the soft magnetic metal, the average diameter D ₅₀ of 18μm powder, the kneaded material at a ratio of volume filling rate of 33% in a matrix of chlorinated polyethylene rubber, Represents the return loss of a 3.5mm, 4.0mm or 4.5mm thick sheet. Here, the sheet having a thickness of 4.0 mm exhibits a strong attenuation exceeding 45 dB at a frequency slightly higher than 4 GHz, which is much higher than the attenuation of 30 dB or less exhibited by sheets of other thicknesses. Is a singular point. For this frequency, a thickness of 4.0 mm is the matching thickness.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to provide an electromagnetic wave absorber having a very high electromagnetic wave absorbing performance utilizing the above-mentioned matching thickness in which the electromagnetic wave diverging phenomenon is recognized in the electromagnetic wave absorbing sheet. Is to do. It is also an object of the present invention to provide a design method for realizing a high-performance electromagnetic wave absorber using a matching thickness at an arbitrary frequency.

[0006]

The electromagnetic wave absorber utilizing the matching thickness of the present invention is an electromagnetic wave absorber obtained by dispersing a soft magnetic metal powder in a rubber or synthetic resin matrix, and has a frequency of 1 GHz or more. In the high frequency region, the matching thickness at which the radio wave absorption reaches a peak at a specific frequency (divergence frequency) determined by the average particle size of the soft magnetic metal powder and the volume filling rate of the matrix, And a return loss at a diverging frequency of 35 dB or more.

The method of manufacturing an electromagnetic wave absorber using a matching thickness according to the present invention is based on a condition that a divergence frequency intended to be realized in a high frequency region of 1 GHz or more and a range of a thickness required for the electromagnetic wave absorber are given. Based on them, the average particle size of the powder of the soft magnetic metal to be used and the volume filling rate of the matrix thereof are selected, and the matching thickness that can be obtained under the selected conditions is designed as the thickness. Become.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, specific examples will be described.
An embodiment of the present invention will be described. The following four types of soft magnetic metal powders were prepared: Name Composition Particle size (μm) FeCrAl coarse powder Cr 7%, Al 9%, Fe residue 42 FeCrAl fine powder Same as above 18 SUS410L powder Cr 12%, Fe residue 10 Carbonyl iron Powder Pure iron 3.4 These powders are filled into a chlorinated polyethylene matrix in various amounts within a range of 23 to 45% by volume, and the kneaded material is formed into a sheet having a thickness of 1.0 to 7.3 mm. did. The frequency characteristics of these prototype sheets as an electromagnetic wave absorber were measured, and the divergence frequency was determined. The graph of FIG. 2 was obtained by plotting the relationship between the volume filling factor and the divergence frequency. The data in FIG. 2 shows that at the same volume filling ratio, the smaller the particle size of the soft magnetic powder, the higher the divergence frequency, and the higher the volume filling ratio, the closer the divergence frequency approaches a constant value. .

FIG. 3 shows a plot of the relationship between the matching thickness and the filling factor of each prototype sheet. The data in FIG. 3 shows that the matching thickness becomes smaller as the soft magnetic metal particle size becomes smaller, the width of the matching thickness is smaller when the average grain size is smaller, and when the average grain size is not so large, the filling rate is smaller. Is about 30% by volume or more, which shows that a certain matching thickness, which depends on the particle size and is independent of the filling factor, can be recognized.

[0010] In designing the electromagnetic wave absorber sheet of the present invention, first of all, regarding the soft magnetic metal powder to be used, the change of the divergence frequency accompanying the change of the volume filling rate is known in advance. According to the experience of the inventors, the action of the soft magnetic metal to be filled is almost determined by the average particle size, and does not depend on the kind of the metal or the alloy composition. Therefore, when using a soft magnetic metal powder whose powder has an average particle diameter almost identical to the data shown in FIG. 2, the data in FIG. 2 can be used immediately. When a powder having an average particle diameter different from the example of FIG. 2 is used, a curve corresponding to the intended average particle diameter is drawn by interpolation based on the data of FIG. By making up, data can be obtained on the relationship between the divergence frequency and the volume filling factor.

On the other hand, the thickness of the electromagnetic wave absorber sheet usually has an allowable or desirable range according to its use. Therefore, the divergence frequency that the electromagnetic wave absorber sheet to be manufactured should have, taking into account the desired thickness,
The average particle size of the soft magnetic metal powder to be used and the volume filling rate of the powder in the matrix are selected from FIG. 2 or similar data. Next, the matching thickness derived according to them will be confirmed by referring to FIG.

[0012]

EXAMPLES An electromagnetic wave absorber sheet having divergent frequencies of 5 GHz, 10 GHz and 15 GHz, respectively, was manufactured. FIG.
According to the data of 5
SUS410L powder (average particle size of 10 μm) was selected for GHz and 10 GHz, and carbonyl iron powder (average particle size of 3.4 μm) was selected for 15 GHz. Similarly, the volume filling ratio corresponding to each divergence frequency was obtained from the data of FIG. 2, and the matching thickness recognized at the volume filling ratio was selected from FIG. 3 to determine the following sheet manufacturing conditions.

No. Divergence frequency Soft magnetic metal powder Average particle size Volume filling rate Sheet thickness 15 GHz SUS410L powder 10 μm 35% 2.4 mm 2 10 GHz SUS410L powder 10 μm 27% 1.8 mm 315 GHz carbonyl iron powder 3.4 μm 26% 1 .5mm using chlorinated polyethylene rubber as matrix material,
An electromagnetic wave absorber sheet was manufactured. Each return loss was measured to obtain the results shown in FIG. The maximum value of the return loss was 35 dB or more in each of Nos. 1 to 3, and reached 50 dB in the case of a high value (No. 3), and it was confirmed that a high degree of electromagnetic wave absorption was realized.

[0014]

The electromagnetic wave absorber of the present invention utilizes the divergence of electromagnetic waves at a matching thickness in a sheet-like absorber, so that an attenuation of 35 dB or more as a return loss value is ensured and excellent. In this case, it reaches 50 dB, and almost ideal electromagnetic wave absorption is realized. According to the present invention, according to the present invention, the production of the electromagnetic wave absorber sheet first selects a soft magnetic metal powder having an appropriate average particle size and a volume filling rate thereof according to a desired divergence frequency, and a sheet having a matching thickness based on them. Can be easily implemented by following the procedure of designing

[Brief description of the drawings]

FIG. 1 is a graph for explaining a divergence phenomenon in which a return loss value is significantly increased in a sheet-like electromagnetic wave absorber and the existence of a “matching thickness” which is a unique sheet thickness realized by the phenomenon.

FIG. 2 is data for showing an embodiment of the present invention with a specific example, and shows a relationship between a volume filling rate of a soft magnetic metal powder dispersed in an electromagnetic wave absorber and a divergence frequency;
3 is a graph showing soft magnetic metal powders having various compositions and average particle diameters.

FIG. 3 is a graph similar to FIG. 2 and showing the relationship between the volume filling ratio of the soft magnetic metal powder and the matching thickness for soft magnetic metal powders having various compositions and average particle diameters.

FIG. 4 is a graph showing return loss frequency characteristics of three types of electromagnetic wave absorber sheets, which are data of an example of the present invention.

Claims (2)

[Claims] 1. An electromagnetic wave absorber comprising a soft magnetic metal powder dispersed in a rubber or synthetic resin matrix,
In the high-frequency region of 1 GHz or more, the matching thickness at which radio wave absorption reaches a peak at a specific frequency (divergence frequency) determined by the average particle size of the soft magnetic metal powder and the volume filling rate of the matrix is defined as the thickness. And a return loss at a diverging frequency of 35 dB or more. 2. A soft magnetic metal powder to be used based on given conditions of a divergence frequency intended to be realized in a high frequency region of 1 GHz or more and a range of thickness required for an electromagnetic wave absorber. A method for producing an electromagnetic wave absorber using a matching thickness, comprising selecting an average particle size and a volume filling rate of the matrix into a matrix, and designing a matching thickness enabled under the selected conditions as a thickness.