Method for manufacturing radio wave absorber and radio wave absorber

Abstract

To solve a problem on an electronic device manufacturing line in which a conventional broadband flat radio wave absorber is affected by reflection from a coating resin in the millimeter wave band, and external vibration or contact can cause carbon or ferrite powder dust to separate and fall from the radio wave absorber.SOLUTION: One side of a cover layer made of an open-cell foamed resin is impregnated with an adhesive, one side of an absorbing layer made of closed-cell or open-cell foamed resin with carbon powder or ferrite powder dispersed therein is coated with the adhesive, and the adhesive-impregnated side of the cover layer and the adhesive-coated side of the absorbing layer are joined together. The foamed resin of the cover layer and the absorbing layer is polystyrene foam, polyurethane foam, or polyethylene foam, and the adhesive is an acrylic adhesive, silicone adhesive, urethane adhesive, or rubber adhesive.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for manufacturing a broadband, flat radio wave absorber for the millimeter wave band using a foamed resin, and to a radio wave absorber manufactured by the manufacturing method. [Background technology]

[0002] In the manufacturing process of wireless devices, it is necessary to inspect the radiated radio wave strength and radiation directivity, etc., and for high-priced, low-volume industrial wireless devices, a large anechoic chamber installed in a wide space is often used. In contrast, low-priced, high-volume consumer wireless devices often require a small anechoic box that can be inserted into multiple manufacturing lines arranged in a complex manner in a limited space. Meanwhile, the radio wave absorbers affixed to the inner surface of the anechoic box come in pyramid, wedge and flat types, but for anechoic boxes inserted into the manufacturing lines of consumer wireless devices, flat radio wave absorbers are often used, since this allows a large amount of effective space to be secured inside the small anechoic box, and there is less risk of the radio wave absorber being damaged by contact with tools etc. when constructing and adjusting the production line. For the reasons mentioned above, there is a demand for a broadband, planar radio wave absorber that can be used in the millimeter wave band, even in consumer wireless devices that use the broad millimeter wave band, which has become increasingly widespread in recent years. Here, an effective method for realizing a planar radio wave absorber over a wide band is to uniformly increase the relative dielectric constant from space to the inside of the radio wave absorber, and to use a material in the radio wave absorbing region in which lossy material such as carbon powder or ferrite powder is dispersed at low density in foamed resin. Patent Documents 1 and 2 show examples of increasing the dielectric constant uniformly from space to the inside of the radio wave absorber. In Patent Document 1, multiple dielectric plates with different dielectric constants are stacked so that the dielectric constant gradually increases from space to the depth of the radio wave absorber, and the stacked multiple dielectric plates are integrated by coating them with a thin film of polycarbonate. In Patent Document 2, multiple mixtures with different dielectric constants are prepared from a binder, carbon black and / or graphite, and pre-foamed beads, and these mixtures are stacked so that the dielectric constant changes continuously, and are integrated by hot air drying. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Publication No. 60-048294 [Patent Document 2] Japanese Patent Application Publication No. 04-018798 Summary of the Invention [Problem to be solved by the invention]

[0004] In the above-mentioned Patent Document 1, the entire radio wave absorber consisting of multiple dielectric plates is surrounded by a thin film of polycarbonate or the like in a non-foamed state, so that reflection by the thin film of polycarbonate or the like cannot be ignored, particularly for millimeter waves with short wavelengths, and therefore use in the millimeter wave band is difficult. Also, in the above-mentioned Patent Document 2, when vibration is applied to the radio wave absorber from the outside or when something touches the surface of the radio wave absorber, powder dust of carbon black or graphite separates and falls from the radio wave absorber, which is a problem especially in the manufacturing line of electronic devices in the millimeter wave band. Specifically, in electronic devices in the millimeter wave band, even if small powder dust such as carbon adheres to the waveguide opening or transmission line, the transmission characteristics change and the product becomes defective during inspection on the manufacturing line, and there is also a problem that the powder dust is mixed in the product and shipped, and moves to and adheres to the waveguide opening or transmission line during use, causing a sudden deterioration in performance or malfunction. [Means for solving the problem]

[0005] In order to solve the problems associated with the conventional wave absorbers, the method for producing a wave absorber of the present invention involves permeating one surface of a cover layer made of resin foamed into open cells with an adhesive so that the concentration decreases from the surface toward the inside, applying the adhesive to one surface of an absorbing layer made of resin foamed into closed or open cells in which carbon powder or ferrite powder has been dispersed, bringing the surface of the cover layer permeated with the adhesive into contact with the surface of the absorbing layer coated with the adhesive, and bonding the cover layer and the absorbing layer together with the adhesive on their respective surfaces. Furthermore, in the method for producing a radio wave absorber of the present invention, the foamed resin of the cover layer and the absorbing layer is foamed polystyrene, foamed polyurethane, or foamed polyethylene, and the adhesive is an acrylic adhesive, a silicone adhesive, a urethane adhesive, or a rubber adhesive. Furthermore, in the method for producing a radio wave absorber of the present invention, the adhesive is mixed with a volatile organic solvent and sprayed in the form of a mist using liquefied gas or compressed gas to permeate one surface of the cover layer. Effect of the Invention

[0006] According to the method for manufacturing a radio wave absorber of the present invention, the following multiple effects can be simultaneously achieved. The first effect is to solve the problem of the reflection of the non-foamed resin thin film in the above-mentioned Patent Document 1, which makes it difficult to use the radio wave absorber in the millimeter wave band. That is, according to the method for manufacturing a radio wave absorber of the present invention, the dielectric constant of the cover layer can be made to approach the dielectric constant of the space by reducing the foaming density of the resin, and the adhesive that has permeated the cover layer is distributed so that the concentration increases in the direction from the space toward the inside of the radio wave absorber, so that the dielectric constant of the radio wave absorber also increases uniformly and continuously according to the change in the concentration of the adhesive. Furthermore, by adjusting the foaming density of the absorbing layer and the dispersion density of the carbon powder or ferrite powder, the dielectric constant of the absorbing layer can be made to approach the dielectric constant of the joint surface between the cover layer and the absorbing layer, so that the dielectric constant from the surface of the cover layer as viewed from the space to the absorbing layer can be increased uniformly and continuously. As a result, a planar radio wave absorber in a wide band can be realized. The second effect is to solve the problem caused by powder dust such as carbon, which is found in the above Patent Document 2. That is, according to the manufacturing method of the radio wave absorber of the present invention, since the cover layer of the radio wave absorber does not contain carbon powder or ferrite powder, the powder dust of carbon or ferrite does not separate and fall from the radio wave absorber even if something touches the surface of the radio wave absorber. Also, even if vibration is applied from the outside to the absorber layer containing carbon powder or ferrite powder and the powder dust of carbon or ferrite separates from the absorber layer, the powder dust is not captured by the adhesive that has permeated the cover layer and passes through to the surface of the cover layer and does not fall. Therefore, it is possible to prevent problems such as defects occurring on the manufacturing line of millimeter wave electronic devices, and powder dust moving and adhering to the waveguide opening or transmission line inside the device during use after being shipped while still mixed in the product, causing sudden performance deterioration or malfunction. The third effect is that a broadband, flat-type radio wave absorber can be manufactured with fewer components. In other words, as described above, in order to make a radio wave absorber broadband, it is necessary to uniformly increase the dielectric constant from the space toward the inside of the radio wave absorber, but in the above-mentioned Patent Document 1, at least three or more dielectric plates are required to say that the dielectric constant of a plurality of dielectric plates with different dielectric constants gradually increases from the space toward the depth of the radio wave absorber. Also, in the above-mentioned Patent Document 2, at least three or more mixtures are required to say that a plurality of mixtures with different dielectric constants are laminated so that the dielectric constant changes continuously. In contrast, according to the manufacturing method of the radio wave absorber of the present invention, only two layers, a cover layer and an absorbing layer, are prepared, and further, the dielectric constant can be uniformly increased by utilizing the fact that the penetration concentration of the adhesive for integrating the cover layer and the absorbing layer becomes continuous. The fourth effect is that it can be manufactured by a simple method. That is, in the above-mentioned Patent Document 1, since a thin film of polycarbonate or the like is coated so as to surround a plurality of dielectric sheets, it is necessary to apply the thin film from all directions. Also, in the above-mentioned Patent Document 2, it is necessary to stack a plurality of mixtures in order in a mold, introduce hot air, and then remove them from the mold. In contrast, according to the manufacturing method of the radio wave absorber of the present invention, the adhesive is permeated or applied into the cover layer and the absorbing layer in only one direction, and the cover layer and the absorbing layer can be integrated by simply bringing the adhesive-covered surfaces of the cover layer and the absorbing layer into contact with each other. As described above, the present invention provides an excellent method for manufacturing a radio wave absorber that not only has the effect of solving the problems of the conventional techniques disclosed in Patent Documents 1 and 2, but also has the effect of enabling the manufacture of the radio wave absorber with fewer components and a simple process. [Brief description of the drawings]

[0007] [Figure 1] FIG. 1 is a diagram showing a configuration of a radio wave absorber according to the present invention; [Diagram 2] FIG. 1 is a diagram showing an example of a method for permeating an adhesive into the cover layer of the radio wave absorber of the present invention. [Diagram 3] FIG. 1 is a diagram showing an example of a method for applying an adhesive to the absorbing layer of the radio wave absorber of the present invention. [Figure 4] FIG. 1 is a diagram showing an example of the distribution of the relative dielectric constant of a radio wave absorber according to the present invention. [Diagram 5] FIG. 1 is a diagram showing an example of the reflection characteristics when a metal plate is placed on the back surface of each layer and the radio wave absorber of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] Hereinafter, an embodiment of the present invention will be described. EXAMPLES

[0009] (Embodiment 1) An embodiment of the present invention will be described. Fig. 1 shows the configuration of the radio wave absorber of the present invention. In Fig. 1, 101 denotes all the components of the radio wave absorber, 102 denotes a cover layer made of open-cell foamed resin, and 103 denotes an absorbing layer in which carbon powder or ferrite powder is dispersed in open-cell or closed-cell foamed resin. Also, 104 denotes a surface of the cover layer 102 into which the adhesive is permeated, and 105 denotes a surface of the absorbing layer onto which the adhesive is applied. Also, 106 denotes an arrow indicating the direction from which the radio wave to be absorbed arrives, and the radio wave absorber 101 is structured to absorb radio waves arriving from the space opposite to the surface 104 of the cover layer 102 into which the adhesive is permeated. Here, an example of a method for permeating the adhesive into the cover layer is shown in FIG. 2. In FIG. 2, 201 is a spray device that sprays the adhesive onto the surface 104 of the cover layer, and the adhesive is mixed with a solvent inside the spray device 201 and sprayed together with liquefied gas or compressed gas. Although FIG. 2 shows an example in which only one can-shaped spray device is used, it is also possible to spray the adhesive efficiently and uniformly by using a special device that simultaneously sprays the adhesive over a wide range from multiple nozzles. In FIG. 2, the adhesive 202 sprayed from the spray device 201 is a low-viscosity liquid in which the adhesive is dissolved in the solvent until the solvent evaporates, and therefore permeates from the surface 104 of the cover layer into the inside of the cover layer due to osmotic pressure. Since the low-viscosity liquid adhesive is captured little by little by the resin of the cover layer and the osmotic pressure increases as the amount of adhesive decreases, the concentration of the adhesive is highest on the surface 104 of the cover layer and permeates so that the concentration decreases uniformly toward the inside of the cover layer. In other words, when looking at the inside of the cover layer from the air side, which is the direction of arrival of the radio waves 106 in Fig. 1, the adhesive penetrates so that the concentration is uniformly high. Note that if the adhesive is sprayed while being sucked from the air side using a suction device 203 as shown in Fig. 2, it is possible to penetrate deeper. Next, an example of a method for applying adhesive to the absorbent layer is shown in Figure 3. In Figure 3, 301 is an applicator that applies the adhesive to surface 105 of the absorbent layer, and 302 is the adhesive applied to the applicator. Note that Figure 3 shows an example in which a roll brush is used as the applicator, but it is also possible to apply the adhesive efficiently and uniformly by using a special device that squeezes out small amounts of adhesive from multiple nozzles installed on a flat surface and presses them down. Up to this point, the concentration of the adhesive in the cover layer 102 in Fig. 1 decreases from the surface 104 toward the inside, and the adhesive is also applied to the surface of the surface 104, and the adhesive is also applied to the surface 105 of the absorbent layer 103 in Fig. 1. Therefore, by pressing the surface 104 of the cover layer 102 and the surface 105 of the absorbent layer against each other, the adhesive layers are held in contact with each other and are joined together. FIG. 4 shows an example of the distribution of the dielectric constant inside the cover layer 102 and the absorbing layer 103, and 401 is the distribution line of the dielectric constant. As shown in FIG. 4, when the inside of the cover layer 102 is viewed from the air side, which is the direction 106 from which the radio waves arrive, the adhesive penetrates so that the concentration is uniformly high, and the dielectric constant also increases uniformly in response to the concentration of the adhesive. By adjusting the dielectric constant of the absorbing layer 103 so that it coincides with the dielectric constant of the boundary between the cover layer 102 and the absorbing layer 103, which are held in contact with each other by the adhesive, it is possible to realize the distribution of the dielectric constant as shown in FIG. 4. In other words, the dielectric constant changes uniformly and continuously from the surface to the inside of the radio wave absorber, and the internal absorbing layer can absorb radio waves, thereby realizing a flat foam-type radio wave absorber having good absorption characteristics in a wide band. FIG. 5 shows an example of the reflection characteristic of a radio wave absorber using a 5 mm thick sheet of open-cell foamed polyethylene as the cover layer, a 14 mm thick sheet of open-cell foamed polyurethane with carbon powder dispersed therein as the absorbing layer, and an adhesive made of styrene butadiene rubber as the main component, methylpentane and isobutane as solvents, and sprayed with liquefied propane gas. Note that FIG. 5 shows the reflection characteristic when a metal plate is placed on the back of the measurement object, and the measurement was performed using the free space method. Also, in FIG. 5, 501 shows the result of measuring only the cover layer without permeating the adhesive, 502 shows the result of measuring only the absorbing layer without applying the adhesive, and 503 shows the result of measuring the radio wave absorber of this embodiment. As shown in FIG. 5, the cover layer that does not contain carbon powder or ferrite powder has a relative dielectric constant close to that of air and is close to lossless, so the reflection characteristic is about 0 dB, but the absorption layer that contains carbon powder has a reflection characteristic of -21 dB to -25 dB. In contrast, in the radio wave absorber of this embodiment having an adhesive with a uniform concentration gradient inside the cover layer, the gradient of the dielectric constant of the adhesive has the effect of matching the impedance of the cover layer and the absorbing layer, and it is found that the reflection characteristics are improved by at least 3 dB and at most 15 dB compared to the reflection characteristics of only the absorbing layer. Note that, in the embodiment in which the reflection characteristics of the radio wave absorber of this embodiment have the best reflection characteristics at 72.5 GHz, the reason for the occurrence of ripples is that the adjustment to the dielectric constants at the boundary between the cover layer and the absorbing layer was not made to match in the example in which the actual measurement values ​​of the reflection characteristics are shown in Fig. 5, and therefore the loss increased due to multiple reflections between the boundary between the cover layer and the absorbing layer and the metal plate on the back side. Although the embodiment of the method for manufacturing a planar radio wave absorber having a wide band in the millimeter wave band according to the present invention has been described above, it goes without saying that the present invention can also be applied to radio wave absorbers in other frequency bands. [Industrial Applicability]

[0010] As described above, the manufacturing method for a radio wave absorber of the present invention is capable of manufacturing a planar radio wave absorber that can be used in the millimeter wave band over a wide band, with fewer components and manufacturing procedures, and the radio wave absorber manufactured by the manufacturing method of the present invention is suitable for millimeter wave radars and high-speed wireless devices that use the millimeter wave band, which have become increasingly widespread in recent years. [Explanation of symbols]

[0011] 101 Components of radio wave absorbers 102 Cover Layer 103 Absorption layer 104 Surface of the cover layer to be permeated with adhesive 105 Surface to which adhesive of absorbent layer is applied 106 Direction from which radio waves arrive 201 Spray equipment 202 Sprayed adhesive 203 Suction device 301 Applicator 302 Adhesive 401 Distribution line of relative dielectric constant 501 Reflection characteristics of the cover layer when a metal plate is placed on the back 502 Reflection characteristics of an absorbing layer with a metal plate on the back 503 Reflection Characteristics of an Electromagnetic Wave Absorber with a Metal Plate on the Back

Claims

1. A method for manufacturing a radio wave absorber, comprising: permeating one surface of a cover layer made of a resin foamed into open cells with an adhesive so that the concentration decreases from the surface toward the inside; applying the adhesive to one surface of an absorbing layer made of resin foamed into closed or open cells in which carbon powder or ferrite powder has been dispersed; bringing the surface of the cover layer permeated with the adhesive into contact with the surface of the absorbing layer coated with the adhesive; and bonding the cover layer and the absorbing layer together with the adhesive on their respective surfaces.

2. 2. A method for manufacturing a radio wave absorber according to claim 1, wherein the foamed resin of said cover layer and said absorbing layer is foamed polystyrene, foamed polyurethane or foamed polyethylene.

3. 2. The method for manufacturing a radio wave absorber according to claim 1, wherein the adhesive is an acrylic adhesive, a silicone adhesive, a urethane adhesive, or a rubber adhesive.

4. 2. The method for producing a radio wave absorber according to claim 1, wherein the adhesive is mixed with a volatile organic solvent and sprayed in the form of a mist by liquefied gas or compressed gas to penetrate into one surface of the cover layer.

5. A radio wave absorber manufactured by the method for manufacturing a radio wave absorber according to any one of claims 1 to 4.

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