JP2016046483A - Conductive slurry for electromagnetic wave absorber and electromagnetic wave absorber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a conductive slurry for an electromagnetic wave absorber capable of manufacturing a compact electromagnetic wave absorber excellent in electromagnetic wave absorption performance over a broad band of frequencies of 1GHz to 110GHz.SOLUTION: The conductive slurry, used for manufacturing an electromagnetic wave absorber, includes a carbon material, a non-conductive fiber, and a solvent.SELECTED DRAWING: None

Description

  The present invention relates to a conductive slurry used for manufacturing a radio wave absorber for an anechoic chamber and a radio wave absorber manufactured using the conductive slurry.

  In recent years, many electromagnetic sources such as electronic devices, communication devices, and information systems coexist and radiate various electromagnetic waves. Electromagnetic waves generated from these devices may affect peripheral devices, and the devices themselves are also affected by the peripheral devices. In order to allow a wide variety of devices to coexist in such an environment, EMC (Electro-Magnetic Compatibility) measures that do not affect the peripheral devices and are not affected by the peripheral devices themselves are required. Yes.

  With the changing times, electronic devices and communication devices have shifted from low frequency bands to products that use high frequency bands. In recent years, products using microwaves (radio waves having a frequency of 1 GHz to 30 GHz) and millimeter waves (radio waves having a frequency of 30 GHz to 300 GHz) are increasing. For example, a 4th generation mobile phone (4 GHz), an ultrahigh-speed wireless LAN (60 GHz), an in-vehicle millimeter wave radar (77 GHz), and the like can be given. In addition, microwaves and millimeter waves are also being used in high power irradiation radars used in the aerospace business and military related fields, and microwave and millimeter wave radio waves have begun to be used in a wide range of fields. Yes.

  As the number of products used increases, standards for electromagnetic interference have been developed. Until now, most of the allowable value setting frequencies shown in the standard are 1 GHz or less, and EMC measurement has been mainly 1 GHz or less. However, in recent years, as described above, the frequency of electronic devices has been increased, and the allowable value setting frequency described in the standard has also been revised to 18 GHz or less. In the future, it will be easy to imagine that measurement in a higher frequency band will be required before entering a full-fledged ubiquitous society.

  As described above, in recent years, various products having different operating frequencies have been developed, and accordingly, the measurable frequency required in an anechoic chamber has been widened. In particular, a radio wave absorber used in a microwave / millimeter wave band anechoic chamber is required to have a radio wave absorber excellent in radio wave absorption performance (radio wave absorption amount of 25 dB or more) over a wide band (1 GHz to 110 GHz).

  Conventionally, there exist radio wave absorbers that absorb microwaves and millimeter waves. This is a radio wave absorber obtained by molding a material made of foamed polyurethane, foamed polypropylene or foamed polyethylene into a pyramid shape by kneading or impregnating a carbon-based conductive material such as carbon black or graphite. .

  In recent years, a flame retardant type electromagnetic wave absorber made of an inorganic material and a conductive material such as carbon black or graphite has been developed. In Patent Document 1, a base material formed by forming a hydrous inorganic compound such as sepiolite, aluminum hydroxide, and calcium hydroxide into a pyramid shape is prepared, and the base material is immersed in a carbon black paint. And a radio wave absorber imparted with electrical conductivity.

JP 2003-115633 A

  However, these radio wave absorbers are only the type of radio wave absorbers that have been molded to a certain height with a certain amount of carbon-based conductive material added. It has become. The commercialized microwave and millimeter wave band absorbers are only of this type.

  Even in the radio wave absorber of Patent Document 1, the radio wave absorption amount at 1 GHz is certainly 26 dB, and excellent radio wave absorption performance can be obtained. However, since the height of the radio wave absorber is 300 mm, the radio wave absorption performance is not sufficient when it is molded in a smaller size.

  In view of the above problems, the present invention provides a conductive slurry for a radio wave absorber capable of producing a radio wave absorber excellent in radio wave absorption performance over a wide frequency band of 1 GHz to 110 GHz, and a radio wave produced using the slurry. It aims at providing an absorber.

  In order to achieve the above object, the present inventors have focused on adding non-conductive fibers to a conductive slurry containing a conductive carbon material. Carbon material has been widely used as a conductive material for electromagnetic wave absorbers, but there is a limit to the amount of addition by itself, and the height of the electromagnetic wave absorber is increased to ensure electromagnetic wave absorption performance. There was a need. On the other hand, a non-conductive fiber alone does not exhibit radio wave absorption performance at all. Nevertheless, the inventor has surprisingly found that the radio wave absorber manufactured using the composite slurry using both the carbon material and the non-conductive fiber significantly improves the radio wave absorption performance, and the present invention has been completed. It came to do.

  That is, the conductive slurry used for producing the radio wave absorber of the present invention is characterized by containing a carbon material, non-conductive fibers, and a solvent. In the present invention, the surface of the non-conductive fiber is coated with the carbon material.

  In the present invention, the carbon material is preferably carbon black or carbon nanotube. The non-conductive fiber is at least one inorganic fiber selected from glass fiber, alumina fiber and silica fiber, or at least one organic fiber selected from polyester fiber, aramid fiber, polyethylene fiber and acrylic fiber. Preferably there is.

  In this invention, it is preferable that the content rate of the said carbon material is 3.0-6.0 mass%, and the content rate of the said nonelectroconductive fiber is 0.1-1.0 mass%.

  In the present invention, the nonconductive fibers preferably have an average fiber length of 1 mm to 10 mm and an average fiber diameter of 10 μm to 200 μm.

  The radio wave absorber of the present invention is produced using the above electroconductive slurry for radio wave absorber.

  According to the conductive slurry for a radio wave absorber of the present invention, it is possible to produce a radio wave absorber that is excellent in radio wave absorption performance over a wide frequency band of 1 GHz to 110 GHz even if it is small. In addition, the radio wave absorber of the present invention is excellent in radio wave absorption performance over a wide frequency band of 1 GHz to 110 GHz even if it is small.

1 is a perspective view of a radio wave absorber 10 according to an embodiment of the present invention. 2 is an SEM image of the radio wave absorber manufactured in Example 1. FIG.

  Hereinafter, embodiments of the conductive slurry for a radio wave absorber and the radio wave absorber of the present invention will be described.

  A conductive slurry for a radio wave absorber according to an embodiment of the present invention (hereinafter simply referred to as “conductive slurry”) is used, for example, to produce a radio wave absorber used in an anechoic chamber. It contains conductive fibers and a solvent. The electroconductive slurry is kneaded with an organic binder such as a resin or an inorganic binder such as cement, and the kneaded product is molded to produce a radio wave absorber having a predetermined shape. For example, it can be a pyramid shape like the radio wave absorber 10 shown in FIG.

  As the carbon material, carbon having high conductivity, fine particles, and a large surface area is preferable. Examples of the fibrous carbon material include carbon fibers and carbon nanotubes. In particular, since carbon nanotubes have a very low bulk density compared to carbon fibers, high conductivity can be obtained in a smaller amount than carbon fibers. Examples of the particulate carbon material (carbon particles) include carbon black and graphite. In particular, since carbon black has a very large specific surface area, high conductivity can be obtained with a smaller amount than graphite. These carbon materials are materials that exhibit high conductivity by being uniformly dispersed.

  The role of the conductive carbon material in the present embodiment is to form a connection between carbons within the radio wave absorber and to create a path for the radio wave. When the radio wave absorber 10 is irradiated with radio waves, a conductive loss and a dielectric loss occur due to the combination of the conductive portion and the dielectric portion (non-conductive material or void), and the radio wave is converted into heat. The radio wave absorber 10 of the present embodiment can absorb microwave and millimeter wave radio waves based on such a principle.

  In the case where the conductive material is a fibrous carbon material, the aspect ratio is preferably 100 or more, and in the case of carbon particles, the particle diameter is preferably 0.2 μm or less. When the same mass is added, it is easier to obtain a connection between fibers when the aspect ratio is larger. In addition, since the number is smaller when the particle size is smaller, it is easier to form a conductive network. In this specification, “aspect ratio” means a value obtained by averaging the length / width ratio values of each fibrous carbon with respect to 10 particles in the field of view when observed with an SEM. “Particle size” means a particle size at an integrated value of 50% (50% cumulative particle size: D50) in a particle size distribution obtained by a laser diffraction / scattering method.

  The nonconductive fiber is not particularly limited as long as it is a fiber made of a nonconductive material. Examples of the inorganic fiber include glass fiber, alumina fiber, and silica fiber. Examples of the organic fiber include polyester fiber, aramid fiber, polyethylene fiber, and acrylic fiber.

  Examples of the solvent include water, ethanol, IPA and the like.

  The present embodiment is characterized in that the conductive slurry contains non-conductive fibers in addition to the carbon material. The role of the non-conductive fiber in the present embodiment is not only to improve the strength of the radio wave absorber but also to improve the radio wave absorption performance of the radio wave absorber. Non-conductive fibers basically do not have the property of absorbing radio waves, but when added to a slurry in which carbon is dispersed, the non-conductive fibers change to a material having a large radio wave absorption performance. The effect is a combination of a plurality of actions and exerts a tremendous effect on radio wave absorption. Therefore, even if the radio wave absorber produced using the conductive slurry of this embodiment is small, it exhibits excellent radio wave absorption performance over a wide frequency band of 1 GHz to 110 GHz.

  The first radio wave absorption effect is a radio wave absorption effect due to the material itself. When non-conductive fibers are added to the carbon slurry under predetermined conditions, the surface of the non-conductive fibers is thinly coated with the carbon material. Thereby, the center part which is a nonelectroconductive fiber becomes a dielectric, and the surface part which consists of carbon becomes a dielectric loss body, and it can be in a state like what is called a hollow carbon fiber. In this way, when radio waves reach the surface of a non-conductive fiber coated with a carbon material, the radio waves are transmitted along the surface, and the radio waves are converted into heat by the electrical loss of the conductive carbon material. Encourage absorption.

  The second radio wave absorption effect is a radio wave absorption effect due to the fiber shape. The higher the dielectric constant, the shorter the wavelength of the radio wave that propagates through the material. Basically, since the radio wave absorber has a high dielectric constant, the wavelength of the radio wave propagating therein becomes very short. In particular, the wavelength of radio waves in the microwave / millimeter wave band is in units of several millimeters. When the wavelength and the length of the nonconductive fiber coincide with each other, or become a half wavelength or a quarter wavelength, an electric wave propagates on the nonconductive fiber, and an effect of promoting radio wave absorption is obtained. From such a viewpoint, in the present embodiment, the non-conductive fiber preferably has an average fiber length of 1 mm to 10 mm.

  The third radio wave absorption effect is a radio wave absorption effect due to radio wave scattering in the nonconductive fiber. As described above, in the conductive slurry of the present embodiment, the central portion which is a non-conductive fiber is a dielectric, and the surface portion made of carbon is a dielectric loss body, which is in a state like a so-called hollow carbon fiber. Therefore, when a radio wave enters the non-conductive fiber, the radio wave is reflected many times in the non-conductive fiber, thereby canceling the radio wave and absorbing the radio wave. For this purpose, the average fiber diameter of the nonconductive fibers is important, and it is preferably 10 μm to 200 μm.

  In this specification, the “average fiber length” of non-conductive fibers refers to the length of an arbitrary 100 fibers included in the field of view when the non-conductive fibers are observed with a scanning electron microscope (SEM) or a microscope. Average. In addition, the “average fiber diameter” of the non-conductive fibers is the average of the diameters of any 30 fibers included in the field of view by burying the non-conductive fibers with a resin, laying out the surface, and observing with an SEM or the like. .

  In the conductive slurry of this embodiment, the content of the carbon material is preferably 3.0 to 6.0% by mass, and more preferably 4.0 to 4.5% by mass. By setting it as 3.0 mass% or more, a carbon material can be disperse | distributed uniformly and a favorable conductive path can be formed, and sufficient electroconductivity can be obtained. If it is 6.0 mass% or less, the high oil absorption of carbon does not make it difficult to form a slurry.

  In the conductive slurry of this embodiment, the content of nonconductive fibers is preferably 0.1 to 1.0% by mass, and more preferably 0.3 to 0.8% by mass. If it is 0.1 mass% or more, the effect which improves electromagnetic wave absorption performance can fully be acquired. When it exceeds 1.0 mass%, the addition amount below that and the radio wave absorption performance do not change.

  In the conductive slurry, a dispersant for dispersing the carbon material and water glass for adjusting the viscosity of the conductive slurry can be added. The content of the dispersant is preferably 0.1 to 4.0% by mass. If it is 0.1% by mass or more, the carbon can be sufficiently dispersed, and if it is 4.0% by mass or less, the volume ratio of the dispersing agent becomes too large and adversely affects strength and radio wave absorption performance. There is nothing. Moreover, water glass plays the role which makes the viscosity of electroconductive slurry small and disperse | distributes nonelectroconductive fiber uniformly. Since the viscosity is small, there is also an effect that the non-conductive fibers are not easily broken during kneading. In this respect, the content of water glass is preferably 0.1 to 1.0% by mass.

  The remainder other than these materials in the conductive slurry becomes a solvent.

  The conductive slurry of this embodiment can be suitably produced by the following method. First, a carbon material dispersant and water glass are dissolved in a solvent, and the carbon material is added thereto. This mixed liquid is kneaded with a rotary ball mill or the like to produce a carbon slurry. The rotational speed of the ball mill can be 40 to 200 rpm, and the kneading time can be about 10 to 50 hours.

  Next, non-conductive fibers and water glass are added to the produced carbon slurry and sufficiently kneaded with an ordinary mixer such as a hand mixer to obtain the conductive slurry of this embodiment. For example, kneading is carried out for 300 seconds using a kneading deaerator (Mazerustar KK-2000, Kurashiki Boseki Co., Ltd.) with a rotation speed ratio of 6: 9. Thus, after the carbon is sufficiently uniformly dispersed, the non-conductive fibers are added to the carbon slurry, and after the addition, the surface of the non-conductive fibers is made by kneading under the predetermined conditions. Can be coated with carbon material.

  Here, a method for manufacturing a radio wave absorber using the conductive slurry of this embodiment will be described. There is a method in which the conductive slurry of this embodiment is kneaded with an organic binder such as a resin or an inorganic binder such as cement, and the kneaded product is molded into a predetermined shape. The kneading may be performed with a normal mixer such as a hand mixer, and then molded in a vacuum defoamer.

  In addition, when adding a nonelectroconductive fiber and water glass to a carbon slurry, you may add together organic binders, such as resin, or inorganic binders, such as cement. In that case, what is necessary is just to knead | mix with the kneading deaerator which has high dispersion | distribution capability by revolution and rotation, and shape | mold after that. In order to coat the surface of the nonconductive fiber with the carbon material, the kneading time is preferably 100 to 1000 seconds. The rotational speed ratio between revolution and rotation is preferably 2: 1 to 1: 2.

  Dispersant (Sannopco Roma D) and water glass were dissolved in water, and carbon black (Lion Corporation, Ketjen Black) was added thereto. This mixture was kneaded for 24 hours with a rotary ball mill (rotation speed: 80 rpm) to obtain a carbon slurry. Next, the non-conductive fibers shown in Table 1 and water glass are added to the produced carbon slurry, and the rotational speed ratio between revolution and rotation is determined using a kneading deaerator (Mazerustar KK-2000, Kurashiki Boseki Co., Ltd.). At 6: 9, the mixture was kneaded for 300 seconds to obtain a conductive slurry. Table 1 shows the content of each component of the conductive slurry. In addition, ECS06-670 manufactured by Central Glass Fiber Co., Ltd. was used as the glass fiber in Table 1. In addition, Twaron made by Teijin Limited was used as the aramid fiber in Table 1.

  The produced conductive slurry and cement were kneaded at a mass ratio of 1: 1, and then subjected to a vacuum deaerator. A resin mold capable of forming a pyramid shape having a bottom surface of 100 mm × 100 mm and a height of 120 mm was prepared. The kneaded material is poured into the resin mold, dried at room temperature and normal humidity for 5 days, water in the kneaded material is blown off, and the dried molded body is fired at 1300 ° C. for 5 hours in the atmosphere. In the example, 36 wave absorbers were produced.

  In each of the examples and comparative examples, 36 wave absorbers were installed as 6 × 6 in the manner shown in FIG. 1, and 60 cm × 60 cm specimens were prepared. Using a horn antenna manufactured by Keycom Corporation and a vector network analyzer manufactured by Agilent Technology Co., Ltd., radio wave absorption was measured by the normal incidence method. The measurement results are shown in Table 2.

  As is apparent from Table 2, the amount of radio wave absorption at 1 GHz and 3 GHz is greatly improved in the example compared with the comparative example, and as a result, in the example, good radio wave absorption performance can be obtained over a wide frequency band of 1 GHz to 110 GHz. It was. So far, in order to improve the amount of radio wave absorption at frequencies of 1 GHz and 3 GHz, it has been necessary to raise the height of the radio wave absorber to about 30 cm. However, by using the conductive slurry of the example, the height is 12 cm. Even with the electromagnetic wave absorber, a radio wave absorption amount of 25 dB or more could be realized.

  As a representative example, a scanning electron microscope (SEM) photograph of the radio wave absorber produced in Example 1 is shown in FIG. Specifically, the portion where the nonconductive fibers were exposed in the pores was extracted from the molded radio wave absorber by pulverization, and the surface thereof was observed. Although it was a non-conductive fiber, it could be observed without depositing the conductive film, indicating that the surface of the non-conductive fiber was coated with carbon. In addition, the surface of the non-conductive fiber was analyzed by EDX, and it was confirmed that about 1 μm of particles mainly composed of carbon having a diameter of 100 nm or less were laminated.

  According to the conductive slurry for a radio wave absorber of the present invention, it is possible to produce a radio wave absorber that is excellent in radio wave absorption performance over a wide frequency band of 1 GHz to 110 GHz even if it is small.

  10 Radio wave absorber

Claims (6)

A conductive slurry used for producing a radio wave absorber,
A conductive slurry for a radio wave absorber comprising a carbon material, a nonconductive fiber, and a solvent.   The electroconductive slurry for a radio wave absorber according to claim 1, wherein a surface of the nonconductive fiber is coated with the carbon material. The carbon material is carbon black or carbon nanotube,
The non-conductive fiber is at least one inorganic fiber selected from glass fiber, alumina fiber and silica fiber, or at least one organic fiber selected from polyester fiber, aramid fiber, polyethylene fiber and acrylic fiber. The conductive slurry for a radio wave absorber according to claim 1 or 2.   The content rate of the said carbon material is 3.0-6.0 mass%, The content rate of the said nonelectroconductive fiber is 0.1-1.0 mass%, Either of Claims 1-3. The electroconductive slurry for electromagnetic wave absorbers as described.   The conductive slurry for a radio wave absorber according to any one of claims 1 to 4, wherein the non-conductive fibers have an average fiber length of 1 mm to 10 mm and an average fiber diameter of 10 µm to 200 µm.   The electromagnetic wave absorber produced using the electroconductive slurry for electromagnetic wave absorbers of any one of Claims 1-5.