JP2008258253A - Material excellent in electromagnetic wave shielding property and heat dissipation and molded good - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a functional resin material having heat dissipation and electromagnetic wave shielding property, in particular, a material that can shield electromagnetic wave in a wide band. <P>SOLUTION: The functional material having superior heat dissipation and electromagnetic wave shielding property can be made by uniformly dispersing a resin ceramics powder that is selected from a group consisting of alumina powder, aluminum nitride powder, silicon carbide powder, silicon nitride powder, and boron nitride powder and a carbon fiber that is selected from a group consisting of pitch-based carbon fiber, pitch-based ultra-high elasticity carbon fiber and carbon nanotube. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a material excellent in electromagnetic wave shielding and heat dissipation. In particular, the present invention relates to a material capable of shielding electromagnetic waves in a wide band exceeding 1 MHz to 1 GHz.

  In the field of electronic equipment, heat dissipation of the heating element is an important issue. Moreover, since it causes malfunctioning, it is strongly required to protect devices and elements from electromagnetic waves.

For heat dissipation of the heat generating element, for example, a heat sink made of copper or aluminum having excellent thermal conductivity is used. Further, as the actual temperature of the heat generating element rises, the use of heat pipes loaded with a cooling liquid is increasing in addition to the heat sink. However, in the above-described heat sink made of metal such as copper or aluminum, it is necessary to make the fin of the heat sink thinner and taller in order to improve heat dissipation. At this time, an eddy current is likely to be generated, and it is necessary to separately shield an electromagnetic wave, which is a factor that hinders the cost increase and thinning of the electronic device.
In addition, since the metal heat dissipation material has a large specific gravity, it is a factor that hinders weight reduction. In addition, the heat sink made of metal has high thermal conductivity and is easy to move the heat of the heating element. On the other hand, the heat sink is cooled because it is not self-dissipating and it is difficult to dissipate heat even if it absorbs heat. In some cases, it is always necessary to cool the surface of these metal heat sinks using a cooling fan.

As electromagnetic wave shielding bodies for shielding electromagnetic waves, metal plates, foils, meshes, conductive films, composite materials mixed with conductive fillers, conductive surface treated products such as plating, vapor deposition, and painting are usually used. ing.
Conventionally, separate members have been used for the heat radiation member and the electromagnetic wave shielding member, which has increased the cost and hindered the thickness reduction of the product.

  In order to solve the above-described problems, there are some techniques that shield electromagnetic waves and provide materials having excellent thermal conductivity. For example, Patent Document 1 and Patent Document 2 propose a sheet having a multilayer structure including a conductive layer, an insulating layer, and an electromagnetic wave shielding layer. Further, Patent Document 3 proposes a heat conductive sheet in which a small amount of ferrite powder and carbon nanotubes are added to a resin and excellent in heat conductivity and static electricity removing effect.

However, in the sheets of Patent Documents 1 and 2, in order to form a multilayer sheet, the adhesion of each layer becomes a problem. Moreover, since it is necessary to make each layer separately, there exists a problem that it is difficult to make thickness thin and it is difficult to produce the product of complicated shape. Moreover, since it is necessary to make each layer combining, it becomes a factor of a cost increase. In addition, since metal is used in these sheets, self-heat dissipation is poor, and combined use with a heat sink is common. In addition, although it has the property of shielding electromagnetic waves, it does not have the property of absorbing electromagnetic waves. Therefore, it is necessary to perform a conductive surface treatment with a metal material on the entire inner surface of the housing, which increases costs.
On the other hand, although the sheet of Patent Document 3 does not have a multilayer structure, the thermal conductivity is 5 W / m · K or less, which does not indicate a value that can be used as a heat sink, and the heat dissipation effect is not sufficient.
JP 2001-168573 A JP 10-313191 A JP 2004-47965 A

  In addition, for shielding electromagnetic waves, there is a demand for shielding characteristics in a wide band. However, although there are many materials that exhibit shielding properties at specific frequencies, a material that exhibits shielding characteristics in a wide band that exceeds 1 MHz to 1 GHz is found. This is not easy, and there has been a demand for a material that is excellent in the effect of lowering heat and exhibits electromagnetic wave shielding properties in a wide band.

  Therefore, an object of the present invention is to provide a material excellent in heat dissipation and electromagnetic wave shielding properties, particularly a material exhibiting excellent shielding characteristics in a wide band exceeding 1 MHz to 1 GHz.

  As a result of various studies to solve the above-mentioned problems, the inventors of the present invention have excellent heat dissipation and shield broadband electromagnetic waves by uniformly mixing a specific ceramic powder and carbon fiber with a resin. We succeeded in manufacturing the material and solved the above problems.

  That is, the present invention is a material having heat dissipation and electromagnetic wave shielding properties, wherein ceramic powder and carbon fiber are uniformly dispersed in a resin, and the ceramic powder is made of alumina powder, aluminum nitride powder, carbonized carbon. It is selected from the group consisting of silicon powder, silicon nitride powder and boron nitride powder, and the carbon fiber is selected from the group consisting of pitch-based carbon fiber, pitch-based ultrahigh modulus carbon fiber and carbon nanotube. .

  Ceramics, unlike metals, have a feature of self-heat dissipation and a low coefficient of thermal expansion. Carbon has thermal conductivity and electromagnetic wave absorption. Alumina, aluminum nitride, silicon carbide, silicon nitride, and boron nitride powder are used as ceramics, and pitch-based carbon fibers, pitch-based ultrahigh modulus carbon fibers, and carbon nanotubes are used as carbon fibers, and both are uniformly dispersed in the resin. By doing so, a material having both good heat dissipation and electromagnetic wave absorption can be obtained. Further, the carbon fibers are uniformly dispersed in the gaps between the ceramic powders, part of the carbon fibers are in contact with the surface of the ceramic powders, and the carbon fibers are entangled with each other, thereby further improving the heat dissipation.

  The material according to the present invention is excellent in both heat dissipation and electromagnetic wave shielding properties, and can exhibit good electromagnetic wave shielding characteristics particularly in a wide band.

The resin used in the present invention may be either a thermoplastic resin or a thermosetting resin. In the thermoplastic resin, a polyolefin resin, a polyamide resin, an elastomer (styrene, olefin, PVC, urethane, ester) , Amide-based) resin, acrylic resin, engineering plastic, etc. are used. In particular, polyethylene, polypropylene, nylon resin, ABS resin, acrylic resin, ethylene acrylate resin, ethylene vinyl acetate resin, polystyrene resin, polyphenylene sulfide resin, polycarbonate resin, polyester elastomer resin, polyamide elastomer resin, and liquid crystal polymer are selected. Among these, nylon resin, polyester elastomer resin, polyamide elastomer resin, ABS resin, polypropylene resin, polyphenylene sulfide resin, and liquid crystal polymer are preferable because of heat resistance and flexibility.
Moreover, an epoxy resin, a melamine resin, a phenol resin, a silicone resin, a urethane resin, etc. are used for a thermosetting resin. Especially, an epoxy resin, a silicone resin, and a urethane resin are suitable from heat resistance and a softness | flexibility.
Dispersants, lubricants, and plasticizers may be added to these resins. In particular, by using fatty acid esters as the dispersant, the filling rate of ceramics and carbon fiber materials can be increased and the characteristics can be improved. it can.

As the ceramic powder, alumina, aluminum nitride, silicon carbide, silicon nitride, or boron nitride powder is used. The particle size is preferably 0.1 μm or more and 100 μm or less. When the particle size is smaller than 0.1 μm, the specific surface area increases, so the amount that can be added to the resin is reduced. When the particle size is larger than 100 μm, the gap between the powders is increased. Thus, the self-heat dissipation performance is reduced. In particular, from the viewpoint of characteristics, a preferable particle diameter is 0.3 μm or more and 50 μm or less, more preferably 0.5 μm or more and 40 μm or less, and further preferably 1 μm or more and 20 μm or less.
The particle shape is preferably spherical. As the ceramic material, alumina ceramic is particularly preferable from the viewpoint of cost, and aluminum nitride is preferable from the viewpoint of self-heat dissipation and thermal conductivity.
The ceramic powder contained in the material of the present invention may be of one kind or plural kinds, and the ratio of the ceramic powder in the material is preferably 5% by volume or more and 60% by volume or less, and preferably 10% by volume. % To 50% by volume is more preferable, and 20% to 45% by volume is particularly preferable.

Pitch-based carbon fibers, pitch-based ultrahigh modulus carbon fibers, and carbon nanotubes are used as the carbon fibers. In particular, in order to improve thermal conductivity, the pitch-based ultrahigh modulus carbon fiber is preferable because it has a thermal conductivity of 500 W / m · K or more of copper and aluminum. Carbon nanotubes have the property of absorbing electromagnetic waves in a small amount. However, since carbon nanotubes are very expensive, it is desirable to add pitch-based ultrahigh elastic modulus carbon fibers and carbon nanotubes, mainly pitch-based carbon fibers, in view of cost.
Moreover, it is preferable to add carbon fiber so that it may become 1-60 volume% of the total amount of material by the total amount. When the amount of carbon fiber added is less than 1% by volume, sufficient thermal conductivity and electromagnetic wave absorbing ability cannot be obtained, and when it is more than 60% by volume, sufficient fluidity during molding cannot be obtained. In particular, 5 volume% or more and 40 volume% or less are preferable, and 15 volume% or more and 35 volume% or less are preferable.
In the present invention, the pitch-based ultrahigh modulus carbon fiber refers to one having a tensile modulus of 500 GPa or more.

Moreover, the preferable average length of the pitch-based carbon fiber and the pitch-based ultrahigh modulus carbon fiber is 1 μm or more and 500 μm or less, more preferably 5 μm or more and 500 μm or less, and particularly preferably 10 μm or more and 100 μm or less. The average diameter is preferably 1 μm or more and 50 μm or less, more preferably 3 μm or more and 20 μm or less.
The preferred average length of the carbon nanotubes is 1 μm or more and 50 μm or less, and the preferable average diameter is 5 nm or more and 100 nm or less.

In the material of the present invention, the preferred volume ratio of the ceramic powder and the carbon fiber is 95: 5 to 20:80, the more preferred ratio is 80:20 to 25:75, and the particularly preferred ratio is 70:30 to 30:70. .
The total proportion of carbon fiber and ceramic powder in the material is preferably 40% by volume to 80% by volume, more preferably 50% by volume to 70% by volume, and particularly preferably 60% by volume to 70% by volume. It is.

Mixing and dispersing of the thermoplastic resin, ceramics, and carbon fiber is performed using a heating kneader, a multi-screw extruder, a heating roll, or the like. Further, when a thermosetting resin is used as a base material, it is performed using a mixer, a vacuum mixer, a multi-screw extruder or the like.
The obtained material can produce a molded article having a desired shape by injection molding, sheet molding, extrusion molding or press molding. The resulting molded product contains carbon fiber, so it has high strength and contains a lot of ceramic powder, which prevents the carbon fiber from being oriented in one direction during molding, improving the strength of the material evenly and ensuring uniform heat conduction. And absorption of electromagnetic waves can be realized.
In the molding method, in particular, by using an injection molding method, it is possible to mold a heat sink having a complicated three-dimensional shape at a low temperature with high dimensional accuracy as compared to a heat sink made of copper or aluminum. In addition, a three-dimensional heat sink having a wall thickness of 1 mm or less can be easily molded as compared with a case where a copper or aluminum heat sink is molded by a die casting method.

In order to obtain a material having heat dissipation comparable to aluminum, the amount of resin is preferably 60% by volume or less of the entire material. A preferable amount of the resin is 20 to 60% by volume of the total amount of the material, more preferably 25 to 50% by volume, and particularly preferably 30 to 40% by volume. If the particle size of the ceramic powder is too small, it is necessary to increase the amount of resin added, and therefore the particle size of the ceramic powder is preferably 0.1 to 100 μm.
In particular, a spherical alumina powder having a particle diameter of 1 μm to 40 μm is calculated and blended so as to be finely packed, and by adding carbon fiber to this, it becomes possible to reduce the amount of resin to 40% by volume or less, A molding material comparable to a metallic heat dissipation material can be obtained. In addition, the presence of carbon fibers randomly in the finely packed ceramic powder can reduce the orientation of the carbon fibers generated by sheet molding, injection molding, and extrusion molding, and heat dissipation generated in the heat sink using carbon fibers. The direction dependency of the effect can be reduced, and a uniform electromagnetic wave absorption effect can be obtained.

  Hereinafter, based on an Example, the material of this invention is demonstrated in detail.

Examination of raw materials, etc. Various prototypes were manufactured and properties were evaluated for the purpose of obtaining materials with excellent heat dissipation and electromagnetic wave shielding properties.

Comparative product 1 (sintered product)
A heat-dissipating alumina ceramic or aluminum nitride ceramic powder is mixed with an organic binder together with an electromagnetic wave-absorbing carbon material to produce a molding material. Using this molding material, a heat sink-shaped molded body is formed by injection molding. Obtained, degreased and sintered to produce a heat sink made of a sintered body.
However, since the sintered density was not improved and the pores existed in the sintered body at about 10%, the expected heat dissipation effect could not be obtained. It was confirmed that this was because there was no wettability between the ceramics and the carbon material and the carbon material prevented the ceramics from sintering during sintering, and the subsequent experiments were abandoned.

Comparative product 2 (sintered product)
A heat-dissipating alumina ceramic or aluminum nitride ceramic powder is mixed with an organic binder together with an electromagnetic shielding copper powder to produce a molding material. Using this molding material, a heat sink-shaped molded body is formed by injection molding. Obtained, degreased and sintered to produce a heat sink made of a sintered body.
However, since the sintered density was not improved during sintering and the pores existed in the sintered body at about 10%, the expected heat dissipation effect could not be obtained. This is because there is no wettability between ceramics and copper powder, and because the melting point of copper is 500 ° C lower than the sintering temperature of ceramics, the copper melts during sintering and prevents ceramics from sintering. The subsequent experiment was abandoned.

Comparative product 3 (resin material)
The resin was kneaded with ceramic powder with high heat dissipation and ferrite powder with excellent electromagnetic shielding properties to form a molding material, which was used for sheet molding and injection molding.
When the heat dissipation was measured, the heat conduction of the ferrite powder was low, so heat was accumulated inside, and a heat dissipation comparable to a metal heat dissipation material could not be obtained.

Comparative product 4 (resin material)
An attempt was made to produce a material in which ceramic powder with high heat dissipation and copper powder with electromagnetic shielding properties were mixed with resin. In order to increase thermal conductivity and electromagnetic wave shielding, heat kneading was performed using copper powder of 20 μm or less, but the copper powder rapidly oxidized during kneading, so that the expected electromagnetic wave shielding characteristics and heat dissipation can be obtained. There wasn't.

Comparative product 5 (resin material)
An attempt was made to obtain a material with stable properties and excellent thermal conductivity by kneading with a resin using ceramic powder and a carbon material.
When graphite powder is used as the carbon material, the particle size is very small and the surface area is large. Therefore, it is difficult to produce a molding material with a resin addition amount of 40% by volume or less. The characteristics could not be obtained.

An attempt was made to obtain a material having stable properties and excellent thermal conductivity by kneading with a resin using the ceramic powder of the present invention and a carbon material.
When carbon fiber was used as the carbon material, it was possible to improve heat dissipation by using pitch-based carbon fiber among PAN-based carbon fiber and pitch-based carbon fiber.
Furthermore, when carbon nanotubes were used, it became clear that the electromagnetic wave shielding properties were excellent even in a small amount of 2 to 5%.
In addition, the use of alumina powder, aluminum nitride powder, silicon carbide powder, silicon nitride powder, and boron nitride powder as the ceramic powder has succeeded in improving heat dissipation.
Moreover, in order to obtain the material excellent in the heat dissipation comparable with aluminum, it turned out that the quantity of resin is 60 volume% or less of the whole material. In particular, by reducing the resin amount to 40% by volume or less, it was possible to obtain a molding material comparable to a metallic heat dissipation material.
For this reason, in order to reduce the distance between the ceramic powders at first, ceramic powders having a particle diameter of less than 0.1 μm were used. However, the smaller the powder particle diameter, the greater the amount of resin added. It has been found suitable to use a powder of 0.1 μm to 100 μm.

Evaluation of Characteristics A sheet made of the material of the present invention and a comparative sheet (including only one of ceramic powder / carbon fiber) were prepared, and the characteristics were examined.
Table 1 shows a blending table of materials used in Examples and Comparative Examples.

Preparation of sheet When using thermoplastic resin, set the resin in advance in a 0.5 L heating kneader to 200 degrees in the case of polypropylene and 250 degrees in the case of nylon resin, mix for 10 minutes and melt sufficiently Then, the ceramic powder and carbon fiber were gradually added and heat-kneaded for 1 hour. The lump taken out was formed into a sheet shape, and then subjected to a pulverizer to obtain a molding material.
Using the obtained molding material, an injection molding machine with a clamping force of 20 tons was used to prepare a molded body of 35 mm × 35 mm × thickness 2 mm for temperature measurement. For measurement of electromagnetic wave absorption, a molded body of 100 mm × 100 mm × thickness 1.5 mm was molded and used.
In the case of a silicone resin which is a thermosetting resin, a two-component addition type liquid silicone rubber was used, and 10 parts by weight of a curing agent was mixed with 100 parts by weight of the main agent. After adding ceramic powder and carbon fiber to this, stirring and mixing were performed for 30 minutes while vacuum defoaming at 25 ° C. using a 1 L vacuum defoaming mixer, and after taking out, 2 mm was added to the polyethylene terephthalate (PET) film. After coating to a thickness of 1.5 mm (for measuring electromagnetic waves) and 1.5 mm (for measuring electromagnetic waves), the film was released from the PET film after 24 hours at 25 ° C. and allowed to stand for 48 hours at 25 ° C., and measurements were taken. The sheet size was as described above.

The alumina powder used in the examples and comparative examples had an average particle size of 10 μm. The pitch-based carbon fibers and pitch-based ultrahigh elasticity carbon fibers used in Examples and Comparative Examples had an average diameter of 10 μm and an average length of 50 μm.
Regarding the measurement of electromagnetic waves, the electromagnetic wave shielding characteristics of 0.1 MHz to 1 GHz were measured using a spectrum analyzer R3132 manufactured by Advantest.
Moreover, about the measurement of the thermal radiation characteristic, it measured by the method shown below. A copper plate having a width of 15 mm, a thickness of 2 mm, and a length of 100 mm was heated to 80 ° C. using a heat source, and after confirming soaking for 30 minutes, a sample (35 mm square, 2 mm thickness) was placed on the copper plate. The temperature after 30 minutes of the copper plate 5 mm behind the sample was measured.

表中に示す電磁波遮蔽特性は透過損失であり、対応する樹脂のみで作製したシートにおける透過量を基準値とし、実施例あるいは比較例のシートにおける透過量の減少値を示す。 The results are shown in Table 1.

The electromagnetic wave shielding characteristic shown in the table is transmission loss, and indicates a reduction value of the transmission amount in the sheet of the example or the comparative example with the transmission amount in the sheet made of only the corresponding resin as a reference value.

  From the measurement results, the sheet made of the material of the present invention is excellent in heat dissipation, with the temperature of the copper plate after 30 minutes being 72 ° C. or less, and the electromagnetic wave shielding property is −10 dB or more in a wide band between 1 MHz and 1 GHz. It was confirmed that excellent electromagnetic shielding properties were exhibited. When the copper plate was measured under the same conditions, the heat release temperature was 71 ° C. and that for aluminum was 73 ° C., indicating that the material according to the present invention has a heat release characteristic (endothermic effect) equal to or higher than that of aluminum and copper. It was. Furthermore, in the electromagnetic wave shielding characteristics, it was confirmed that the material exhibits an excellent value exceeding −30 dB in a low frequency region (100 MHz or less), and is an excellent material exhibiting a stable shielding characteristic up to 1 GHz.

Note that the copper plate emits almost no infrared rays, so when taking data with infrared thermography, the measured temperature is much lower than the actual temperature, but with the material according to the present invention, the measured temperature of the thermography is almost the same as the actual temperature. Therefore, it was confirmed that the thermal emissivity was close to 1. For this reason, it is considered that the heat dissipation characteristics of the material according to the present invention (the effect of reducing the heat of the other party [endothermic effect]) are caused by the high self-heat dissipation property (effect of dissipating its own heat) due to thermal radiation.
Moreover, since the carbon fiber concerning this invention is excellent in an electromagnetic wave absorption characteristic, it is thought that the material concerning this invention is excellent not only in electromagnetic wave shielding but also in electromagnetic wave absorptivity. When an electromagnetic wave is incident on the sample, the relationship of incident amount = reflection amount + absorption amount + transmission amount holds, but high electromagnetic wave shielding means that the transmission amount is small, and that electromagnetic wave absorption is high. It means that the amount of transmission and reflection is small. A material having high electromagnetic wave absorptivity can prevent harmful effects caused by reflection interference.

  Since the material according to the present invention has both excellent heat dissipation and electromagnetic wave shielding properties, it has been possible to integrate two components that have been used in the past, and to reduce the thickness of the product. In addition, since the material according to the present invention has an electromagnetic wave shielding property in a wide band, it is suitable for use only for electromagnetic wave shielding. For example, with the recent development of communication technology, it has been pointed out that electromagnetic waves generated from mobile phones, personal computers, game machines, etc. may adversely affect the human body, but the present invention aims to protect against such electromagnetic waves. It can also be used.

Claims (6)

  A material having electromagnetic shielding properties and heat dissipation properties, wherein ceramic powder and carbon fiber are uniformly dispersed in a resin, and the ceramic powder is alumina powder, aluminum nitride powder, silicon carbide powder, silicon nitride A material selected from the group consisting of powder and boron nitride powder, wherein the carbon fiber is selected from the group consisting of pitch-based carbon fiber, pitch-based ultrahigh modulus carbon fiber and carbon nanotube.   The material according to claim 1, wherein a ratio of the resin in the material is 20 to 60% by volume.   The material according to claim 1 or 2, wherein the ceramic powder has an average particle size of 0.1 µm to 100 µm.   3. The material according to claim 1, wherein an average length of the carbon fibers is 1 μm to 500 μm.   A ratio of the ceramic powder in the material is 5 to 60% by volume, a ratio of the carbon fiber is 1 to 60% by volume, and a total ratio of the carbon fiber and the ceramic powder is 40 to 80% by volume. The material according to any one of claims 1 to 4.   A molded product obtained by injection molding, sheet molding, extrusion molding or press molding using the material according to any one of claims 1 to 5.