JP2002151881A - Radio wave absorbing body - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a radio wave absorbing body having excellent absorbing characteristics over wide range of frequency bands. SOLUTION: The body is constructed by a composite, in which thermoplastic resin of 3-15 wt.%, a ferrite powder of 65-95 wt.% and carbon fiber of 2-20 wt.% are made to contain there in, and the average aspect ratio of the carbon fiber is set to be within the range of 2-25.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a countermeasure against unnecessary radiation of an antenna, a countermeasure against a TV ghost, a countermeasure against a radar ghost,
Anechoic chamber, anechoic box, medical equipment, digital information equipment,
Regarding radio wave absorbers used for building wall materials and tiles,
In particular, in recent years, digital information devices such as E,
The present invention relates to a radio wave absorber used as an MI countermeasure component.

[0002]

2. Description of the Related Art Conventionally, buildings, facilities,
Also, inside and around portable information terminals such as mobile phones, digital cameras, and notebook computers, radio wave absorbers are installed at locations where unnecessary reflection, scattering, and interference of radio waves occur, and the energy of Various troubles caused by unnecessary radio waves are suppressed by converting radio waves into thermal energy to absorb radio waves.

In general, there is known a radio wave absorbing material in which a magnetic powder or a soft magnetic metal powder is dispersed and contained in an insulator, and an unnecessary radio wave is removed by attenuating the radio wave by utilizing the magnetic loss.

[0004] The material constituting the radio wave absorber is as follows.
A composite material in which a magnetic powder or a soft magnetic metal powder is blended in a predetermined ratio in a rubber, a thermoplastic resin, or a thermosetting resin is used (see JP-A-4-80274).
It is manufactured by an injection molding method, a rolling method, a hot press molding method, or the like.

Generally, the frequency at which a radio wave absorber easily absorbs radio waves is determined by the material of the radio wave absorber, and the material of the radio wave absorber is a material that efficiently absorbs radio waves in the frequency band used. In recent years, with the demand for smaller and lighter devices represented by digital information devices and the higher speed and higher functionality of various devices, a radio wave absorber that can efficiently absorb unnecessary radio waves over a wide frequency band is strongly desired. ing. When the frequency band of unnecessary radio waves to be absorbed is wide, it is difficult for a single material to efficiently absorb radio waves over a wide frequency range. To obtain a composite material, or to form them separately and then to laminate them.

For example, Japanese Patent Application Laid-Open No. 58-188192 discloses that a radio wave absorber is obtained by mixing ferrite and steel fiber with a phenol resin.
Also, JP-A-4-26195, JP-A-5-206
No. 677, as shown in FIG. 4, radio wave absorbing layers 8 and 9 having different absorption characteristics are laminated with an adhesive, and a radio wave reflecting layer 10 made of a metal plate, a metal foil, a conductive fiber or a conductive resin is used. A backed radio wave absorber 7 is disclosed.

[0007]

However, the radio wave absorber disclosed in Japanese Patent Application Laid-Open No. 58-188192 has a high frequency band capable of efficiently absorbing radio waves of 9 to 12 GHz, which is necessary for EMI measures of digital information equipment. 100M
In the frequency band of Hz to 1000 MHz, unnecessary radio waves cannot be efficiently absorbed.

Further, the radio wave absorber 7 of FIG. 4 disclosed in JP-A-4-26195 and JP-A-5-206677 increases the cost by adding a bonding or thermocompression step.
There is a problem such as peeling due to the difference in the coefficient of thermal expansion, and in the field of digital information equipment, which has been miniaturized in recent years due to the increase in thickness due to lamination, it has been difficult to install it inside the equipment.

[0009]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned drawbacks, and has a thermosetting resin of 3 to 15% by weight.
A radio wave absorber made of a composite material containing 70 to 95% by weight of ferrite powder and 2 to 20% by weight of carbon fibers, wherein the carbon fibers have an average aspect ratio of 2 to 25.

Further, according to the present invention, the average particle size of the ferrite powder is 1 to 300 μm, the maximum particle size is 500 μm or less,
The carbon fiber has an average diameter of 3 to 50 μm.

Further, the present invention is characterized in that the average length of the carbon fiber crystal in the c-axis direction is 1 nm or more.

[0012] The present invention also provides a composite material having a frequency of 10
The attenuation constant at 0 MHz is 1 dB / cm or more, and the attenuation constant at 1000 MHz is 7 dB / cm or more.

Further, the present invention is characterized in that the carbon fibers are substantially non-oriented.

[0014]

Embodiments of the present invention will be described below in detail.

As shown in FIG. 1, a radio wave absorber 1 of the present invention is obtained by mixing a thermosetting resin 2 such as a phenol resin or an epoxy resin, a ferrite powder 3 such as a Ni—Zn ferrite, and a carbon fiber 4. After adding and mixing an organic solvent or the like according to the above, the mixture is molded into a predetermined shape by an injection molding method, a rolling method, a hot press molding method, an extrusion molding method, a casting method, etc., and at a predetermined temperature for a predetermined time. It is obtained by heating and curing.

The thermosetting resin 2 in the cured product
Is 3 to 15% by weight, preferably 5 to 10% by weight, and ferrite powder 3 is 70 to 95% by weight, preferably 77 to 92% by weight.
2 to 20% by weight, preferably 5 to 20% by weight, and the number of the aspect ratio L / D, which is the ratio of the length L to the diameter D of the carbon fiber 4 shown in FIG. The average value is 2 to 25, preferably 5 to 25.

Here, the ratio of the thermosetting resin 2 is 3% by weight.
If the amount is less, the strength of the molded body is remarkably reduced, and it is difficult to use the molded article as the radio wave absorber 1. If the amount exceeds 15% by weight, the amount of the thermosetting resin 2 becomes excessive, and It is not practical because it causes voids and swelling.
In particular, the thermosetting resin 2 preferably has a ratio of 5 to 10% by weight from the viewpoint of the strength of the molded body and the prevention of deformation, voids and swelling during curing.

As the thermosetting resin 2 constituting such a radio wave absorber 1, a phenol resin or an epoxy resin is suitable, but any thermosetting resin can be used without being limited to the above. .

When the ratio of the ferrite powder 3 is 70% by weight or less, the radio wave absorption characteristics are deteriorated mainly in a high frequency band of 100 MHz to 1000 MHz. Or the amount of the carbon fiber 5 decreases, and the radio wave absorption characteristics deteriorate mainly in a low frequency band of 100 MHz or less. In particular, in consideration of the radio wave absorption characteristics and the strength of the molded body, the ferrite powder 3 preferably has a ratio of 77 to 92% by weight.

The ferrite powder 3 includes Ni—Zn ferrite, Ni—Zn—Cu ferrite, Mn—Z
There are n-type ferrite, M-type hexagonal ferrite, and the like. In the present invention, one or more of these ferrites can be used in combination. Particularly, Ni-Zn-based ferrite, Ni-Zn-Cu-based Ferrite has good absorption characteristics in the required frequency band, and good results can be obtained.

When the proportion of the carbon fiber 4 is less than 2% by weight, the radio wave absorption characteristics are deteriorated mainly in a low frequency band of 100 MHz or less, and when it exceeds 20% by weight, it is difficult to uniformly disperse the carbon fiber 4. In this case, the carbon fibers 4 are likely to be shed in the molded product, which is not practical. In particular, it is preferable that the carbon fiber 4 has a ratio of 5 to 20% by weight from the viewpoint of improving the radio wave absorption characteristics and preventing the occurrence of particle shedding.

When the carbon fiber 4 having an average aspect ratio of less than 2 is used, sufficient radio wave absorption characteristics cannot be obtained particularly in a low frequency band of 100 MHz or less. Is difficult, and the carbon fibers 4 are likely to be shed in the molded product, which is not practical. In particular, it is preferable to use the carbon fiber 4 having an average aspect ratio of 5 to 25 from the viewpoints of improving the radio wave absorption characteristics, improving the dispersibility, and preventing the falling of particles. Here, the average aspect ratio of the carbon fiber 4 is determined by observing the fracture surface of the radio wave absorber 1 of the present invention with a scanning electron microscope and measuring the length L and the diameter D of the carbon fiber 4 in an arbitrary area. Their ratio L
/ D can be obtained by numerically averaging.

As the carbon fiber 4, PAN-based carbon fiber and pitch-based carbon fiber can be used, but it is preferable to use PAN-based carbon fiber having a small variation in diameter and aspect ratio.

The average particle size of the ferrite powder 3 is 1 to
It is preferable to use one having a size of 300 μm, preferably 10 to 100 μm. If the average particle diameter of the ferrite powder 3 is 1 μm or less, a pulverization process for a long time is required, and the cost of the raw material is increased.
If the average particle diameter of the particles exceeds 300 μm, the particles are likely to fall during handling of the molded body.

The maximum particle size of the ferrite powder 3 is 50
It is preferable to use a material having a size of 0 μm or less, preferably 200 μm or less. This is because if the maximum particle size of the ferrite powder 3 is larger than 500 μm, degranulation tends to occur during handling of the molded body.

The carbon fiber 4 preferably has a number average diameter D of 3 to 50 μm, and more preferably 5 to 20 μm. This is carbon fiber 4
When the average diameter D is in the range of 3 to 50 μm, the raw material cost is low due to easy production, and the dispersibility is good, so that the obtained molded body is hardly degranulated during handling, and the yield in the production process is high. Is improved. Here, the average diameter of the carbon fiber is determined by observing the fracture surface of the radio wave absorber 1 of the present invention with a scanning electron microscope, measuring the diameter D of the carbon fiber 4 in an arbitrary area, and averaging the numbers. I can do it.

The carbon fiber 4 preferably has an average crystal c-axis length of 1 nm or more, preferably 10 nm or more. This is the average crystal c
When the axial length is 1 nm or more, 100 MHz to 1000
This is because the electromagnetic wave absorption characteristics in the frequency band of MHz become particularly good. Here, average carbon fiber c
The axial length can be measured by an X-ray diffraction method from an arbitrary broken surface of the carbon fiber of the radio wave absorber 1 of the present invention.

In the radio wave absorber 1 of the present invention, the attenuation constant at a frequency of 100 MHz is 1 dB / cm or more, and the attenuation constant at a frequency of 1000 MHz is 7 dB / cm or more. Unnecessary radio waves of 1000 MHz can be efficiently absorbed, and can be more suitably used as EMI countermeasure parts. The damping constants referred to here are the real part μ _r ′, ε _r ′ and the imaginary part μ _r ″, ε _r ′ of the complex relative permeability and the complex relative permittivity measured by a high-frequency current-voltage method using a network analyzer. Using the value of ', it can be calculated by Equation 1.

[0029]

(Equation 1)

Further, in the radio wave absorber 1 of the present invention, it is preferable that the carbon fibers 4 are substantially non-oriented. Therefore, there is no anisotropy in the radio wave absorption characteristics, and it is possible to efficiently absorb unnecessary radio waves incident from any direction. Therefore, in addition to unnecessary self-radiated radio waves from various places on the circuit board, it is necessary to efficiently absorb various unnecessary radiated radio waves oscillated from other electronic devices in the vicinity.
It can be suitably used as an EMI countermeasure component for digital information equipment and the like.

Here, the fact that the carbon fiber 4 is substantially non-oriented means that there is no orientation at all, or that the degree of orientation is 20 degrees.
% Or less. The degree of orientation here refers to an arbitrary cross-section, a cross-section orthogonal to the cross-section, and a cross-section orthogonal to the two cross-sections. In each section, an arbitrary direction is defined as 0 degree, and the direction of the long axis when the direction of the long axis of the carbon fiber 4 in the section is defined as being in a range of 0 to 180 degrees is 0 to 45 degrees. 4
When counting the numbers in four ranges of 5 to 90 degrees, 90 to 135 degrees, and 135 to 180 degrees, when the number of counts in the angle range of the minimum frequency is subtracted from the number of counts in the angle range of the maximum frequency It means that the difference is equal to or less than 20% with respect to the sum of the count numbers in each angle range.

As a mode of use of the radio wave absorber 1 according to the present invention, as shown in FIG. 3A, for example, the radio wave absorber 1 is attached to the top or inside of an IC package 5 mounted on a circuit board 6 in a plate shape, It can be pasted on a line cable, made into a casing that covers a circuit board, or pasted on a casing. As shown in FIG. 3 (b), a cap shape covers the entire IC package 5, a tube shape passes a high-frequency line cable therein, and a case shape covers a circuit or an element of a digital information device or the like. Can be implemented as follows.

Further, wall materials for an anechoic chamber, office wall materials,
Use for building materials, outer wall materials such as tiles, tableware, accessories, medical equipment, gaskets, isolators, attenuators, terminators, circulators, high-frequency magnetic shields around optical elements, absorption of unnecessary electromagnetic waves such as photoelectric transmission modules, etc. Can be.

[0034]

EXAMPLE 1 First, the respective ratios of the thermosetting resin 2, the ferrite powder 3, and the carbon fiber 4 and the carbon fiber 4
In order to examine the appropriate range of the average aspect ratio of the radio wave absorbers 1 having different ratios of the respective components and the average aspect ratio of the carbon fiber 4 to be used, 100 MHz
The radio wave absorption characteristics at ~ 1000 MHz were examined.

Specifically, a resol type phenol resin is used as the thermoplastic resin 2 and Ni-Z is used as the ferrite powder 3.
n-type ferrite powder, carbon fiber 4
AN-based carbon fiber was used. These are charged into a high-speed mixer at a predetermined ratio to perform granulation, and the obtained granules are subjected to powder pressure molding at room temperature under a pressure of 300 MPa, mold release, heat curing at a high temperature, and testing. I got a piece. next,
The three-point bending strength of the obtained test piece was measured according to JIS-K6911.
Was measured by Next, the complex relative permeability and the complex relative permittivity of the obtained test piece were measured by a high-frequency current-voltage method using a network analyzer, and the values were substituted into Equation 1 for attenuation at 100 MHz to 1000 MHz. Constants were calculated.

First, an experiment was conducted in which the ratio of the ferrite powder 3 and the carbon fiber 4 was within the range of the present invention, and the ratio of the thermosetting resin 2 was changed in the range of 2 to 17% by weight. The average aspect ratio of the carbon fiber 4 was unified to 5, and the average length of the crystal of the carbon fiber 4 in the c-axis direction was 1 nm. Table 1 shows the experimental results.
In addition, when the molded body after curing was swollen, it was indicated by x, and when it did not, it was indicated by o.

According to Table 1, the proportion of the thermoplastic resin 2 in the range of 3 to 15% by weight (N
o. 2-No. In 5), the strength of the molded body was 10 MPa or more, and the molded body had sufficient strength as a radio wave absorber, and the molded body did not swell. However, no. 5
Is No. 2-No. In comparison with No. 4, there were more voids on the surface of the molded body. On the other hand, a product (No. 1) in which the ratio of the thermoplastic resin 2 is less than 3% by weight, which is out of the range of the present invention, has a molded body strength as low as 10 MPa or less and is not practical as a radio wave absorber. The ratio of the thermoplastic resin 2, which is also outside the scope of the present invention, exceeds 15% by weight (No.
In the case of 6), the molded product after curing swelled and was not practical as a radio wave absorber.

[0038]

[Table 1]

Example 2 Next, the ratio of the thermosetting resin 2 was set within the range of the present invention, and the ratio of the ferrite powder 3 and the carbon fiber 4 was changed in the range of 65 to 97% by weight and 0 to 20% by weight, respectively. I let it. In addition, the average aspect ratio of the carbon fiber 4 was unified to 5. Table 2 shows the results.

According to Table 2, the ferrite powder 3 having a ratio of 70 to 95% by weight and the carbon fiber 4 having a ratio of 2 to 20% by weight (Nos. 8 to N) are within the scope of the present invention.
o. In 11), there is no shedding during the handling of the cured molded article, and the attenuation constant at 100 MHz is 1 dB /
cm, the attenuation constant at 1000 MHz is 7 dB / cm
That was all. On the other hand, when the ratio of the ferrite powder 3 exceeds 95%, the ratio of the thermoplastic resin 2 or the ratio of the carbon fibers 4 is out of the range of the present invention, but the ratio of the thermoplastic resin 2 is less than 3% by weight. And No. 1 in Table 1.
As shown in FIG. 1, the strength of the molded body was 10 MPa or less, and was not practical as a radio wave absorber. When the ratio of the carbon fiber 4 is less than 2% (No. 7),
The attenuation constant at 100 MHz was less than 1 dB / cm. When the ratio of the ferrite powder 3 was less than 70% (No. 13), the attenuation constant at 1000 MHz was 7 dB.
/ Cm. On the other hand, when the ratio of the carbon fiber 4 exceeded 20% by weight (No. 12), the carbon fiber 4 was shed during the handling of the molded product, and was not practical as a radio wave absorber.

[0041]

[Table 2]

Example 3 Next, the average aspect ratio of the carbon fiber 4 was set to 1 to 3.
Changed to zero. The proportions of the thermosetting resin 2, the ferrite powder 3, and the carbon fiber 4 were unified to 5% by weight, 85% by weight, and 10% by weight, respectively. Table 3 shows the results.

According to Table 3, the carbon fiber 4 having an average aspect ratio in the range of 2 to 25 (No. 15 to No. 18), which is within the scope of the present invention, is not treated during the handling of the cured molded product. No shattering, 100M
The attenuation constant at 1 Hz is 1 dB / cm or more, 1000 MHz
In which the average aspect ratio is in the range of 5 to 25 (No. 16 to No. 1).
In 8), the attenuation constant at 100 MHz and 1000 MHz was large. On the other hand, the carbon fiber 4 having an average aspect ratio of less than 2 (No. 14), which is out of the range of the present invention, has an attenuation constant of 1 d at 100 MHz.
B / cm, the attenuation constant at 1000 MHz is 7 dB /
cm. In the case where the average aspect ratio of the carbon fiber 5 out of the range of the present invention is more than 25 (No. 19), the powder is not practical as a radio wave absorber because the particles are shed during the handling of the cured molded article. Was.

[0044]

[Table 3]

From the above experimental results, the ratio of the thermoplastic resin 2 is 3 to 15% by weight, more preferably 5 to 10% by weight.
%, The ratio of the ferrite powder 3 is preferably 70 to 95% by weight, more preferably 77 to 92% by weight, and the ratio of the carbon fiber 4 is preferably 2 to 20% by weight, more preferably 5 to 15% by weight. The average aspect ratio of the carbon fiber 4 used is 2 to 2
It can be said that it is preferably in the range of 5, more preferably 5 to 15. Example 4 Next, in order to verify the appropriate ranges of the average particle diameter of the ferrite powder 3 and the average diameter D of the carbon fibers 4, an experiment was performed by changing the average particle diameter of the ferrite powder 3 and the average diameter D of the carbon fibers 4. went.

As a result, when the ferrite powder 3 has an average particle diameter of 1 to 300 μm, a granulated body can be prepared relatively easily by the method of Example 1, and even when the molded body after curing is handled. No shedding was observed. On the other hand, with the ferrite powder 3 having an average particle size of less than 1 μm, it was difficult to produce a granulated body, and the yield was poor. Further, the ferrite powder 3 having an average particle size of less than 1 μm has a disadvantage that the price is high because the pulverization time during production is long. Similarly,
In ferrite powder 3 having an average particle diameter of more than 300 μm, ferrite powder 3 having a maximum particle diameter of more than 500 μm, carbon fiber 4 having an average diameter D of 3 μm or less, or carbon fiber 4 having an average diameter D of more than 50 μm, It is difficult to uniformly disperse the granules, and the yield of granules and threshing during handling of cured molded products,
In each item of the surface roughness of the molded body, it was confirmed that the characteristics were slightly inferior.

Therefore, the average particle size of the ferrite powder 3 is 1 to 300 μm, more preferably 10 to 200 μm.
m, maximum particle size 500 μm or less, more preferably 300 μm
m or less, and the average diameter D of the carbon fiber 4 is 3
It has been confirmed that the thickness is preferably 50 μm to 50 μm, more preferably 5 μm to 25 μm.

Next, in order to verify the appropriate range of the average c-axis length of the crystal of the carbon fiber 4, the average c-axis length of the crystal of the carbon fiber 4 is set to 0.5 to 50 nm.
Was changed within the range. Others were the same as in Example 1.

The ratio of the thermosetting resin 2, the ferrite powder 3, and the carbon fiber 4 was 5% by weight, respectively.
85% by weight and 10% by weight were used, and the carbon fiber 4 having an average aspect ratio of 5 was used. Table 4 shows the results
Shown in

According to Table 4, the carbon fiber 4 crystals having an average c-axis length of 1 nm or more (No. 21 to No. 21)
No. 23), the attenuation constant at 100 MHz and 1
The attenuation constant at 000 MHz is improved, and is particularly remarkable in the case of 10 nm or more (Nos. 22 to 23). Therefore, it was confirmed that it is preferable to use a carbon fiber 4 having an average length in the c-axis direction of 1 nm or more, more preferably 10 nm or more.

[0051]

[Table 4]

Example 5 Next, in order to confirm the degree of orientation of the carbon fiber 4 and the anisotropy of the radio wave absorption characteristics, test pieces were prepared by various molding methods. As a molding method, four types of powder pressing method, coater method, injection molding method, and rolling method were performed. Regarding the powder pressing method, a test piece was prepared in the same manner as in Experiment 1. In the coater method and the injection molding method, a predetermined amount of the thermoplastic resin 2, ferrite powder 3, and carbon fiber 4 are mixed, and toluene is added to form a slurry.
After molding with a coater or an injection molding machine, the toluene content was dried and then heated and cured at 250 ° C. to obtain a test piece. Regarding the rolling method, the mixture was supplied between rolls of a rolling molding machine to form a sheet, and was heated and cured at 250 ° C. to obtain a test piece.

The ratio of the thermosetting resin 2, the ferrite powder 3, and the carbon fiber 4 was 5% by weight, respectively.
The weight ratio was set to 85% by weight and 10% by weight, and the average aspect ratio of the carbon fiber 4 was 5 and the average length of the crystal in the c-axis direction was 1 nm.

The degree of orientation was determined in each of three cross sections of an arbitrary cross section of the obtained test piece, a cross section orthogonal to the cross section, and a cross section orthogonal to the two cross sections. Observing an arbitrary range, defining an arbitrary direction in each section as 0 degree, and defining a direction of the major axis of the carbon fiber 4 in the section in a range of 0 degree to 180 degrees, Direction 0-45 degrees, 45
The number is divided into four ranges of ~ 90 degrees, 90-135 degrees, 135-180 degrees, and the difference is obtained by subtracting the count number of the minimum frequency range from the count value of the maximum frequency range. This is the maximum value of the value calculated from the ratio of the count number of each angle range to the total value, and was obtained by observing each cross section with a scanning electron microscope. Table 5 shows the results.

According to Table 5, the degree of orientation of the carbon fiber 4 in the test piece molded by the powder pressing method or the coater method was 20% or less. The degree of orientation of carbon fiber 4 is 20%
It exceeded the value. In order to confirm the anisotropy of the radio wave absorption characteristics of these test pieces, the absorption characteristics when the direction of the electric field was perpendicular to each of the above three cross sections were measured. As a result, when the degree of orientation was 20% or less, the radio wave absorption characteristics did not show a significant difference in the above three cross sections, and no anisotropy was confirmed. However, when the degree of orientation exceeded 20%,
The radio wave absorption characteristics had a significant difference among the three cross sections described above, and anisotropy was confirmed. From this, it was confirmed that anisotropy did not occur in the radio wave absorption characteristics when the carbon fiber 4 was made substantially non-oriented. The thus-obtained radio wave absorber 1 can efficiently absorb unnecessary radio waves incident from any direction. Therefore, in addition to unnecessary self-radiated radio waves from various places on the circuit board, it is necessary to efficiently absorb various unnecessary radiated radio waves oscillated from other electronic devices in the surroundings. It can be used more preferably.

As a molding method, a powder pressing method or a coater method can be said to be preferable because the carbon fiber 4 can be made substantially non-oriented.

[0057]

[Table 5]

[0058]

According to the present invention, there is provided a composite material containing a thermosetting resin, a ferrite powder, and a carbon fiber in a specific range, wherein the carbon fiber has an average aspect ratio of 2 to 25. By making the body, unnecessary radio waves can be efficiently absorbed.

Further, according to the present invention, the yield in the manufacturing process can be improved, and at the same time, the mechanical strength of the molded body can be improved. Also,
In particular, unnecessary radio waves can be effectively absorbed in a frequency band of 100 MHz to 1000 MHz required for EMI measures of digital information equipment.

Further, according to the present invention, it is possible to provide a radio wave absorber that efficiently absorbs unnecessary radio waves emitted from any direction, and effectively eliminates unnecessary radio waves under various use conditions and mounting methods. Can be absorbed.

[Brief description of the drawings]

FIG. 1 is an enlarged sectional view of a radio wave absorber of the present invention.

FIG. 2 is an enlarged perspective view of a carbon fiber used for a radio wave absorber of the present invention.

FIGS. 3 (a) and 3 (b) are cross-sectional views showing a usage form of the radio wave absorber of the present invention.

FIG. 4 is a cross-sectional view of a conventional radio wave absorber.

[Explanation of symbols]

 1: Radio wave absorber 2: Thermoplastic resin 3: Ferrite powder 4: Carbon fiber 5: IC package 6: Circuit board 7: Radio wave absorber 8: Radio wave absorption layer 1 9: Radio wave absorption layer 2 10: Radio wave reflection layer L: Length D: Diameter

Claims (5)

[Claims] 1. A thermosetting resin of 3 to 15% by weight, a ferrite powder of 65 to 95% by weight, and a carbon fiber of 2 to 95% by weight.
A radio wave absorber comprising a composite material containing 20% by weight, wherein the carbon fiber has an average aspect ratio of 2 to 25. 2. The ferrite powder has an average particle size of 1 to 30.
2. The radio wave absorber according to claim 1, wherein the carbon fiber has 0 μm and a maximum particle size of 500 μm or less, and the carbon fiber has an average diameter of 3 to 50 μm. 3. The radio wave absorber according to claim 1, wherein the carbon fiber has an average crystal length in the c-axis direction of 1 nm or more. 4. An attenuation constant at a frequency of 100 MHz is 1
The attenuation constant at a frequency of 1000 MHz or more and a dB / cm or more of 7 dB / cm or more.
4. The radio wave absorber according to any one of items 1 to 3. 5. The radio wave absorber according to claim 1, wherein said carbon fiber is substantially non-oriented.