JP2010232247A - Electromagnetic wave control material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electromagnetic wave control material with which high electromagnetic wave control effects can be obtained. <P>SOLUTION: In the electromagnetic wave control material 1, formed of a composite material where carbon powder 3, is distributed in a matrix 2 formed of resin, the carbon powder 3 is covered with a covering material 4 of electrical insulation properties. When the carbon powder 3 is covered with the covering material 4, carbon powders 3 are prevented from being brought into direct contact with each other. Thus, conductivity does not show itself in the composite material constituting the electromagnetic wave control material 1, even if the content of carbon powder 3 is made high. Consequently, a dielectric constant can be made high in the electromagnetic wave control material 1, in a state where non-conductivity is maintained, namely, in a state with an electric field formed. Thus, high electromagnetic wave control effects can be obtained. It is desirable that the covering material 4 have a comparatively high thermal conductivity of 20 W&times;m<SP>-1</SP>&times;K<SP>-1</SP>or more, for example. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention relates to an electromagnetic wave suppression material, and particularly to an electromagnetic wave suppression material made of a composite material in which carbon powder is dispersed in a matrix made of resin.

  For suppressing electromagnetic waves, for example, ferrite ceramics are used. However, in the case of ferrite ceramics, the magnetic permeability decreases as the frequency range increases, so that the electromagnetic wave suppression effect in the high frequency range is small. This is explained by the existence of Snoek's critical frequency.

  Therefore, in order to effectively suppress electromagnetic waves in a high frequency range, composite materials composed of a magnetic material and a resin are disclosed in, for example, Japanese Patent Application Laid-Open No. 2002-324863 (Patent Document 1) and Japanese Patent Application Laid-Open No. 2007-258432 (Patent Document 2). In addition, a composite material composed of a dielectric material and a resin is disclosed in, for example, Japanese Patent Application Laid-Open No. 2004-253640 (Patent Document 3), and a composite material composed of a conductive material (carbon) and a resin is disclosed. For example, it is disclosed in Japanese Patent Laid-Open No. 2003-124011 (Patent Document 4).

  However, the techniques described in Patent Documents 1 to 4 have problems to be solved.

  First, in the composite material composed of the magnetic material and the resin described in Patent Document 2, when spherical powder is used as the magnetic material, the magnetic permeability decreases due to the influence of diamagnetism generated at the grain boundary due to eddy current. There's a problem. On the other hand, Patent Document 1 discloses a technique for orienting a flat magnetic material powder in the surface direction of a sheet in order to reduce the influence of eddy currents. However, in the case of the composite material described in Patent Document 1, it is necessary to take measures for the orientation as described above, and the method for forming the composite material becomes complicated. There is also a problem that directivity is generated in the effect of suppressing electromagnetic waves depending on the orientation direction of the flat powder. Further, when using a magnetic material like the composite materials described in Patent Documents 1 and 2, the specific gravity of the composite material is about 4 because the specific gravity of the magnetic material is large (about 5 for ferrite and about 7 for metal powder). The problem of increasing the weight and hindering the weight reduction of equipment using this composite material as an electromagnetic wave suppression material is also encountered.

Next, Patent Document 3 describes, more specifically, ACu ₃ B ₄ O ₁₂ (A: alkaline earth metal or a mixture of alkaline earth metals. B: Ti, Zr, Hf, Fe, or a mixture thereof. The composite material which consists of dielectric material powder which has the composition represented by this, and resin is disclosed. In such a composite material made of dielectric material powder and resin, in order to effectively suppress electromagnetic waves, it is necessary to increase the dielectric constant of the composite material. For this purpose, it is necessary to increase the PVC (Pigment Volume Concentration) in the composite material, but as the PVC increases, the flexibility or flexibility of the composite material increases. And the specific gravity of the composite material increases.

  Next, as described above, Patent Document 4 discloses a composite material composed of a conductive material (carbon) and a resin. However, even in such a composite material composed of a conductive material and a resin, a composite material is disclosed. In order to increase the dielectric constant, it is necessary to increase the PVC in the composite material. However, as the PVC is increased, the carbons as the conductive material are more likely to come into contact with each other. Therefore, an electric field is not formed and the problem that the electromagnetic wave suppression effect falls is encountered. Patent Document 4 discloses the use of carbon nanotubes as a technique that can increase the dielectric constant even when a small amount of carbon is added. However, carbon nanotubes have the disadvantage that they are more expensive than ordinary carbon powder. In addition, when carbon nanotubes are used, there is a problem that directivity occurs in the effect of suppressing electromagnetic waves depending on the orientation direction.

JP 2002-324863 A JP 2007-258432 A Japanese Patent Laid-Open No. 2004-253640 Japanese Patent Laid-Open No. 2003-12401

  Accordingly, an object of the present invention is to provide an electromagnetic wave suppressing material that can solve the above-described problems.

  The present invention is directed to an electromagnetic wave suppression material made of a composite material in which carbon powder is dispersed in a matrix made of resin. In order to solve the above technical problem, the carbon powder is an electrically insulating coating material. It is characterized by being covered with.

The coating material preferably has a relatively high thermal conductivity, such as a thermal conductivity of 20 W · m ⁻¹ · K ⁻¹ or higher.

  The carbon powder is preferably a spherical powder.

  In the electromagnetic wave suppression material according to the present invention, it is preferable that the resin occupies 30 to 50% by volume, and the carbon powder coated with the coating material occupies 50 to 70% by volume.

  According to the electromagnetic wave suppressing material according to the present invention, since the carbon powder is coated with the electrically insulating coating material, the carbon powder is not directly in contact with each other. Therefore, even if the carbon powder content is increased, the composite material constituting the electromagnetic wave suppression material can be prevented from exhibiting electrical conductivity. Therefore, according to the electromagnetic wave suppression material according to the present invention, it is possible to increase the dielectric constant while maintaining non-conductivity, that is, in a state where an electric field is formed, and as a result, a high electromagnetic wave suppression effect can be obtained. it can.

  In addition, according to the electromagnetic wave suppressing material according to the present invention, the specific gravity of the carbon powder can be reduced because the specific gravity of the carbon powder is about 2. Therefore, weight reduction of the apparatus using this electromagnetic wave suppression material can be achieved.

  In the present invention, when a coating material having a relatively high thermal conductivity is used, the heat dissipation of the electromagnetic wave suppressing material can be improved. Therefore, the heat generated by the electromagnetic wave suppressing effect can be released efficiently.

  In the present invention, when a spherical powder is used as the carbon powder, the electromagnetic wave suppressing material can be prevented from having directivity with respect to the electromagnetic wave suppressing effect. Therefore, when the electromagnetic wave suppressing material is formed into a plate shape, for example, the degree of freedom with respect to the forming method is high. In addition, the electromagnetic wave suppressing material can be used without any problem like a paint at the stage before the resin is cured.

It is a figure which expands and shows the electromagnetic wave suppression material which concerns on this invention enlarged.

  The principle of electromagnetic wave absorption is that most of incident electromagnetic wave energy is converted into thermal energy inside an electromagnetic wave suppressor (sometimes referred to as “electromagnetic wave absorber”). For this reason, in an electromagnetic wave suppression body, both the energy reflected forward and the energy permeate | transmitted back can be made small. The mechanism of conversion into thermal energy is mainly classified into three types of “conduction loss”, “dielectric loss”, and “magnetic loss”, and the electromagnetic wave absorption energy P per unit volume at this time is expressed by the following equation: It is represented by (1).

P = 1/2 · fμ ″ | H | ² + 1/2 · fε ″ | E | ² + 1/2 · σ | E | ² (1)
Here, P: electromagnetic wave absorption energy, E: electric field, H: magnetic field, f: frequency, and σ: conductivity.

  If the electric field changes at a certain rate, the dielectric constant is not a constant, but is shown as a dielectric function that is a function of the frequency of the electric field. Since loss due to electrical conduction or interband transition occurs in the dielectric function, it is generally a complex function as shown in the following equation (2).

ε * = ε ′ + iε ″ (2)
Here, ε * is called a complex dielectric constant, ε ′ is a real part of the complex dielectric constant, and ε ″ is an imaginary part thereof.

  Similarly, in the magnetic material, when the magnetic field changes at a speed of a certain level or more, the magnetic permeability becomes a complex function and is expressed by the following equation (3).

μ * = μ ′ + iμ ″ (3)
Here, μ * is called a complex permeability, μ ′ is a real part of the complex permeability, and μ ″ is an imaginary part thereof.

  In the above formulas (2) and (3), the ratio of the real part to the imaginary part, that is, tan δ = ε ′ / ε ″ (tan δ = μ ′ / μ ″) is called dielectric loss (magnetic loss).

  The electromagnetic wave suppression material according to the present invention intends to suppress an electromagnetic wave by changing the electromagnetic wave into thermal energy based on the above-described dielectric loss. More specifically, an unnecessary electromagnetic wave generated from an electronic device in a 1 to 5 GHz band is suppressed by using dielectric loss, and an adverse effect due to such an electromagnetic wave is to be prevented.

  As shown in FIG. 1, an electromagnetic wave suppressing material 1 according to the present invention is made of a composite material in which carbon powder 3 is dispersed in a matrix 2 made of resin, and the carbon powder 3 is an electrically insulating coating material. 4 is covered.

  As the resin constituting the matrix 2, for example, an epoxy resin, a silicone resin, or the like can be used.

  Moreover, as the carbon powder 3, for example, a spherical graphite powder having an average particle diameter of about 6.0 μm can be advantageously used. Thus, by using the spherical carbon powder 3, directivity is produced in the effect of suppressing electromagnetic waves depending on the orientation direction of the carbon powder that can be encountered when using non-spherical carbon powder such as flakes or carbon nanotubes. Such a problem can be avoided.

As the electrically insulating coating material 4, for example, a material having a volume resistivity of 10 ⁸ Ω · cm or more can be used. The covering material 4 preferably has a relatively high thermal conductivity. Examples of the heat conductive material suitable as the coating material 4 include boron nitride, aluminum hydroxide, aluminum nitride, and alumina. For reference, for example, alumina has a volume resistivity of 10 ¹⁵ Ω · cm and a thermal conductivity of 20 W · m ⁻¹ · K ⁻¹ , and aluminum nitride has a volume resistivity of 10 ¹⁴ Ω · cm and a thermal conductivity. The rate is 90 W · m ⁻¹ · K ⁻¹ , and boron nitride has a volume resistivity of 10 ¹⁶ Ω · cm and a thermal conductivity of 40 W · m ⁻¹ · K ⁻¹ .

  Examples of a method for obtaining a state in which the surface of each particle of the carbon powder 3 is coated with the electrically insulating coating material 4 include a mechanical coating method and a sol-gel reaction coating method. The mechanical coating method is a method in which a material to be coated (here, carbon powder) and a material to be coated (for example, alumina powder) are mixed and given a strong mechanical energy to cause a mechanochemical reaction, This is a surface coating method.

  The content of the resin constituting the matrix 2 in the electromagnetic wave suppression material is preferably 30 to 50% by volume. When the content is less than 30% by volume, molding is difficult, and a flexible or flexible electromagnetic wave suppressing material cannot be obtained. On the other hand, when the content exceeds 50% by volume, the dielectric constant becomes too small, and the electromagnetic wave suppression effect becomes insufficient.

  The content of the carbon powder 3 coated with the coating material 4 in the electromagnetic wave suppression material is preferably 50 to 70% by volume. If the content exceeds 70% by volume, molding is difficult and a flexible or flexible electromagnetic wave suppressing material cannot be obtained. On the other hand, if the content is less than 50% by volume, the dielectric constant becomes too small, and the electromagnetic wave suppression effect becomes insufficient.

  Further, the coating material 4 preferably covers 50% or more of the surface area of the carbon powder 3. This is because if the coverage is less than 50%, the carbon powders are easily brought into contact with each other, and the electromagnetic wave suppressing material 1 may exhibit conductivity.

[Experimental example]
Next, experimental examples carried out to confirm the effects of the present invention will be described.

  10 parts by weight of alumina powder was blended with 100 parts by weight of carbon powder, and the surface of the carbon powder was coated with alumina powder by a mechanical coating method. In the case of the above blending ratio, theoretically, the alumina powder covers 50% of the surface area of the carbon powder. In this experimental example, spherical graphite powder having an average particle diameter of 6.0 μm was used as the carbon powder, and alumina powder having an average particle diameter of 0.2 μm was used as the alumina powder.

  In addition, in order to produce a sample serving as a comparative example, the carbon powder and the alumina powder were also left in a state where they were not blended with each other.

  Next, an epoxy resin and an amine curing agent are added at a weight ratio shown in Table 1 to each of the carbon powder coated with the alumina powder, the carbon powder alone, and the simple mixture of the alumina powder and the epoxy resin, The mixture was mixed with a planetary mixer to obtain an electromagnetic wave suppressing material paste. In this experimental example, a polyethylene glycol diglycidyl ether type epoxy resin was used as the epoxy resin.

  In Table 1, “PVC” refers to the volume fraction of the filler in the composite material (here, “carbon powder coated with alumina powder”, “carbon powder”, or “carbon powder and alumina powder”). is there. The calculation formula of PVC [%] is as the following formula (4).

PVC = (volume of filler) / (volume of composite material) × 100
= (Filler volume) / {(Filler volume) + (Resin volume)} × 100 (4)
The volume of the filler is calculated by dividing the weight of the filler by the density of the filler. Similarly, the volume of the resin is calculated by dividing the weight of the resin by the density of the resin.

  Next, the electromagnetic wave suppressing material paste obtained by mixing as described above was degassed under reduced pressure, and then sheet-formed by a doctor blade method. Thereafter, heat treatment was performed under conditions of a temperature of 90 ° C. and 1 hour to advance the curing reaction between the epoxy resin and the amine curing agent, and an electromagnetic wave suppression material sheet according to each sample was produced.

  The dielectric constants (ε ′ and ε ″) at 1.1 GHz and 3.8 GHz were evaluated by the S-parameter method for the electromagnetic wave suppression material according to each sample thus obtained. The conductivity was measured by a laser flash method, and the specific gravity was calculated by cutting the electromagnetic wave suppressing material into a predetermined size and measuring the size and weight.

  These results are shown in Table 2. In Table 2, “PVC” shown in Table 1 is shown again.

  In Tables 1 and 2, Samples 7 to 10 are comparative examples outside the scope of the present invention.

  As in Samples 1 to 6 within the scope of the present invention, when carbon powder coated with alumina powder was used, the higher the PVC, the higher the dielectric constant (ε ′ and ε ″).

  On the other hand, when the carbon powder not subjected to the alumina powder coating treatment was used as in Samples 7 to 9, the dielectric constant could not be measured in the high PVC region. Similarly, when the alumina powder and the carbon powder were simply mixed as in Sample 10, the dielectric constant could not be measured. These are considered to be because the carbon powders contacted each other in the composite material to develop conductivity.

  From the above, it can be seen that by using a carbon powder coated with an electrically insulating alumina powder, a high PVC can be achieved without problems, and a high dielectric constant can be advantageously achieved.

  Further, regarding the thermal conductivity, when comparing Samples 1 to 6 within the scope of the present invention with Samples 7 to 9 as comparative examples, the carbon powder coated with alumina powder is not subjected to the coating treatment of alumina powder. Compared with carbon powder, it was found that it contributed to increasing the thermal conductivity of the composite material. This is because alumina has a relatively high thermal conductivity, and it can be considered that the relatively high thermal conductivity of alumina increases the thermal conductivity of the composite material. The effect of improving the thermal conductivity of alumina was also observed in sample 10 in which alumina powder and carbon powder were simply mixed.

  Moreover, since the samples 1-10 produced in this experimental example all use carbon powder having a small specific gravity as the main material of the filler, the specific gravity of the composite material could be 2 or less. For example, in the case of a ceramic used for electromagnetic wave suppression or a composite material of a metal powder and a resin, the specific gravity is approximately 3 or more. It was confirmed that it could be realized.

1 Electromagnetic wave suppression material 2 Matrix 3 Carbon powder 4 Coating material

Claims (5)

  An electromagnetic wave suppressing material comprising a composite material in which carbon powder is dispersed in a resin matrix, wherein the carbon powder is coated with an electrically insulating coating material.   The electromagnetic wave suppression material according to claim 1, wherein the coating material has a relatively high thermal conductivity. The electromagnetic wave suppression material according to claim 2, wherein the coating material has a thermal conductivity of 20 W · m ⁻¹ · K ⁻¹ or more.   The electromagnetic wave suppressing material according to claim 1, wherein the carbon powder is a spherical powder.   The electromagnetic wave suppression material according to claim 1, wherein the resin accounts for 30 to 50% by volume, and the carbon powder coated with the coating material accounts for 50 to 70% by volume.

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