JP7705266B2 - Radio wave absorber - Google Patents

Description

The present invention relates to a radio wave absorber.

Traditionally, various types of radio wave absorbers have been developed, and pyramidal radio wave absorbers, in particular those made by impregnating a base material such as foamed polyurethane with conductive paint and then drying and hardening, have a long history and are widely used in large quantities. Although foamed polyurethane has continuous air bubbles, the size of the bubbles is small, so there is almost no air circulation, and the thermal conductivity is also low. As a result, when a radio wave absorber is irradiated with strong radio waves and the power density increases, the temperature inside the radio wave absorber becomes high, and since most foamed polyurethane radio wave absorbers are organic, a large amount of decomposition gas, including flammable gas, is generated.

Another radio wave absorber has been developed, in which a flat base material made mainly of glass fiber is impregnated with conductive paint containing carbon powder, which is then dried and hardened. As this is mainly made of inorganic matter, it generates a relatively small amount of decomposition gas, including combustible gas. However, due to its flat shape, it has narrowband characteristics in which its absorption performance is only demonstrated in a specific frequency band, making it unsuitable for use in radio wave anechoic chambers.

In addition, some of the currently popular radio wave absorbers are non-flammable, high-power radio wave absorbers that combine inorganic materials and carbon fiber. However, while they are completely non-flammable and have excellent power resistance, they have the problem of a high bulk density. Furthermore, radio wave absorbers in which a pyramidal structure with a honeycomb structure made of organic materials as the skeleton is impregnated with conductive paint containing carbon powder are also used for high-power applications, but there have been reports of fires (explosions) caused by pyrolysis gases from the organic materials. In this situation, for example, Patent Document 1 discloses an invention for a "radio wave absorber" in which carbon is attached to ceramic fibers formed into a pyramidal shape to improve fire resistance.

Japanese Patent Application Publication No. 9-307268

However, conventional radio wave absorbers, including the invention disclosed in Patent Document 1, are still susceptible to the risk of fire (explosion) accidents due to the combustion of conductive paint when irradiated with high-power radio waves, or the generation of pyrolysis gas containing a large amount of combustible gas when exposed to high temperatures. In addition, although stable radio wave absorption performance is demonstrated over a wide band when four mats made of isosceles triangular glass fiber material with a specified thickness are assembled into a pyramidal shape, the temperature rise is still drastic when irradiated with high-power radio waves, and it is not possible to improve the power resistance.

In view of the above-mentioned problems, the present invention aims to provide a radio wave absorber with high heat resistance and power resistance.

(1) A radio wave absorber having a board in which a conductive paint is impregnated and dried on a glass wool base material, the conductive paint contains carbon powder with silicone resin as a binder, and the board is molded into a hollow pyramidal or wedge shape.

(2) The board is an electromagnetic wave absorber as described in (1) above, in which a V-groove is formed on the inside of the corner of the pyramidal or wedge shape.

(3) The board is a radio wave absorber as described in (1) or (2) above, in which ventilation holes are formed through the board on the surface of the pyramidal or wedge shape.

(4) The radio wave absorber according to (3) above, in which the ventilation holes are formed at and/or near the bottom of the pyramidal or wedge-shaped surface.

(5) The radio wave absorber according to any one of (1) to (4) above, wherein the content of silicone resin in the conductive paint is 2% by weight or less.

According to the invention related to (1) above, the board is formed by impregnating and drying a conductive paint containing carbon powder with a non-flammable glass wool base material using silicone resin as a binder, which makes it possible to reduce the amount of organic matter contained, and to obtain a radio wave absorber with extremely high heat resistance and power resistance performance. Furthermore, by forming the board into a hollow pyramidal or wedge shape, it is possible to obtain a lightweight, wideband characteristic, and it can be suitably used in radio wave anechoic chambers, etc.

In addition, according to the invention related to (2) above, the boards are integrally formed, and a V-groove is provided inside the corner of the pyramidal or wedge shape, so that the boards can be folded at the corners and the cross sections of the boards are joined at the V-groove, preventing the end faces of the boards from being exposed on the surface of the radio wave absorber. This makes it possible to effectively suppress delamination, which is particularly likely to occur in boards made of glass wool, and to obtain a durable radio wave absorber.

In addition, according to the invention related to (3) above, ventilation holes penetrating the board are provided on the surface of the pyramidal or wedge shape, which promotes natural cooling, and reduces the temperature rise, further improving the power resistance of the radio wave absorber.

In addition, according to the invention related to (4) above, ventilation holes are provided at the bottom and/or near the bottom of the pyramidal or wedge-shaped surface, so that it is possible to reduce the temperature near the bottom of the radio wave absorber, which is particularly prone to temperature rise, and the power resistance of the radio wave absorber can be further improved.

In addition, according to the invention related to (5) above, the content of the organic silicone resin in the conductive paint is 2% by weight or less, which significantly reduces the organic content compared to conventional paints, and suppresses the generation of decomposition gases, including combustible gases, making it possible to significantly reduce the risk of fire or explosion.

1 is a cross-sectional view of a radio wave absorber according to a first embodiment of the present invention. The component ratios of conductive paints are as follows: (a) is the component ratio of the conductive paint in the first embodiment of the present invention; (b) is the component ratio of the conductive paint using a chloroprene binder for glass wool; and (c) is the component ratio of the conductive paint using a chloroprene binder for urethane foam. 1 is a graph comparing temperature changes over time when the conductive paint in the first embodiment of the present invention and a conductive paint using a chloroprene binder are irradiated with radio waves. In the first embodiment of the present invention, (a) is a graph showing temperature change over time when irradiated with radio waves of 500 mW/ ^cm2 , (b) is a graph showing temperature change over time when irradiated with radio waves of 600 ^mW /cm2, and (c) is a graph showing temperature change over time when irradiated with radio waves of 460 mW/ ^cm2 using a conductive paint with a chloroprene binder. FIG. 1A is a development view of a board which is a constituent member of a radio wave absorber according to a first embodiment of the present invention, and FIG. 1B is an enlarged cross-sectional view showing a cross section of a V-groove at a corner. 6 is a cross-sectional view showing the cross section BB in FIG. 5. 1 is a cross-sectional view of a radio wave absorber according to a first embodiment of the present invention. 13A and 13B are a side view and a cross-sectional view of a radio wave absorber having four ventilation holes per surface in a second embodiment of the present invention. 13A and 13B are a side view and a cross-sectional view of a radio wave absorber having three ventilation holes on each surface in a second embodiment of the present invention. In an embodiment of the present invention, (a) is a graph showing a temperature change over time when the radio wave absorber shown in FIG. 1 is irradiated with radio waves, (b) is a graph showing a temperature change over time when the radio wave absorber shown in FIG. 8 is irradiated with radio waves, and (c) is a graph showing a temperature change over time when the radio wave absorber shown in FIG. 9 is irradiated with radio waves. 11 is a table showing the temperatures of TC4, TC6 and TC7 35 minutes after irradiation with radio waves, among the data shown in FIG. 10 . FIG. 11A is a schematic cross-sectional view of an example in which ventilation holes are provided on the mounting surface of a radio wave absorber according to another embodiment of the present invention, and FIG. 11B is a rear view of the mounting surface of the radio wave absorber. 13 is a schematic cross-sectional view of another embodiment of the present invention in which a ventilation hole is provided in the vicinity of the mounting surface of the radio wave absorber.

The following describes an embodiment of the radio wave absorber of the present invention with reference to the drawings.

First Embodiment
As shown in FIG. 1, the wave absorber 1 of this embodiment is formed by molding a board having a base material of glass wool made of glass fiber into a pyramidal or wedge shape, and the glass wool board is impregnated with conductive paint using silicone resin as a binder and dried, so that the carbon powder contained in the conductive paint is fixed.

Figure 2 (a) shows the component ratio of the conductive paint using the silicone binder used in this embodiment. The silicone binder is a silicone resin whose main skeleton is inorganic and whose heat resistance temperature is 180°C (equivalent to heat resistance class 180 (H) of JIS C4003:2010), and is not only more heat resistant than other general resins used as binders, but also has a low organic content and is difficult to burn even by itself. And by combining this silicone resin as a binder and impregnating it into a glass mat, which is a non-flammable material, the flame retardant that is usually essential for conductive paints is unnecessary. Therefore, the binder resin as a paste for the flame retardant is also unnecessary, and these combined effects can significantly reduce the coating amount of the conductive paint, that is, the organic content of the entire coating. Therefore, the generation of decomposition gas including flammable gas can be significantly suppressed. As a result, it is possible to obtain a radio wave absorber 1 that is extremely difficult to burn.

Figure 2(b) shows the component ratios of a conventional conductive paint using chloroprene that is impregnated into a glass wool substrate for comparison, and Figure 2(c) shows the component ratios of a conventional conductive paint using chloroprene that is impregnated into a urethane foam substrate.

When a glass wool substrate of a given size (e.g., thickness 1.0 cm, width 66.6 cm, length 66.6 cm, weight approximately 212.6 g) is impregnated with conductive paint using the silicone binder of this embodiment, approximately 2000 g of conductive paint is impregnated, and approximately 12.4% by weight of the total weight of the radio wave absorber is organic matter made of silicone resin, etc. On the other hand, when a conductive paint using chloroprene binder is impregnated, approximately 2150 g of conductive paint is impregnated, and approximately 32% by weight of the total weight of the radio wave absorber is organic matter made of chloroprene, etc.

Furthermore, when a urethane foam substrate (e.g., 1.0 cm thick, 74.5 cm wide, 15 m long, weighing approximately 2.523 kg) used in a general radio wave absorber is impregnated with a conductive paint using a chloroprene binder, approximately 10.726 kg of conductive paint is impregnated, and approximately 96% by weight of the total weight of the radio wave absorber is organic matter consisting of urethane foam and chloroprene. Therefore, by impregnating a glass wool substrate with a conductive paint using a silicone binder as in this embodiment, it is possible to significantly reduce the amount of organic matter contained in the radio wave absorber 1, and even when irradiated with high-power radio waves and heated to a high temperature, it is possible to obtain a radio wave absorber 1 with high heat resistance and power resistance performance by suppressing the generation of pyrolysis gas.

Figure 3 shows a graph showing the relationship between temperature and time when a thermocouple is attached to a radio wave absorber in which a glass wool substrate is impregnated with a conductive paint using a chloroprene binder and dried, and to a radio wave absorber 1 in which a glass wool substrate is impregnated with a conductive paint using the silicone binder of this embodiment and dried, and radio waves are irradiated. The temperature of a radio wave absorber normally rises smoothly and exponentially with the passage of time during radio wave irradiation, but in the case of the conductive paint using a chloroprene binder, the temperature rises sharply at the boundary of 190°C, deviating from the smooth exponential function.

On the other hand, the radio wave absorber 1 impregnated with the conductive paint using the silicone binder of this embodiment and dried does not exhibit a sudden temperature rise that deviates from a smooth exponential function even when it reaches 250°C, and is therefore found to have high heat resistance. From these test results, the radio wave absorber 1 in which the glass wool substrate is impregnated with the conductive paint using the silicone binder and dried has a heat resistance temperature that is at least 1.3 times (approximately 250°C/190°C) higher than that of the radio wave absorber impregnated with the conductive paint using the chloroprene binder and dried.

Moreover, temperature data of a power resistance test for the radio wave absorber 1 of this embodiment is shown in a graph in Fig. 4. To explain in more detail, as shown in the cross-sectional view of Fig. 1, thermocouples TC1 to TC6 were installed at various positions on the radio wave absorber 1 (TC5 and TC6 were installed in the space of the hollow part), and radio waves with an output of 500 mW/ ^cm2 and radio waves with an output of 600 mW/ ^cm2 were irradiated from a large horn antenna to measure temperature changes over time at the installation positions of the thermocouples TC1 to TC6.

Fig. 4(a) shows the temperature change when irradiated with radio waves having an output of 500 mW/ ^cm2 for 40 minutes, and Fig. 4(b) shows the temperature change when irradiated with radio waves having an output of 600 mW/ ^cm2 for 30 minutes. Also, Fig. 4(c) shows the temperature change when a radio wave absorber using a chloroprene binder that has been impregnated and dried is irradiated with radio waves having an output of 460 mW/ ^cm2 .

From the measurement results, the radio wave absorber 1 of this embodiment, in which a conductive paint using a silicone binder was impregnated into a glass wool substrate and dried, only rose to 204°C even at the bottom (TC4), which has the highest temperature, even when irradiated with radio waves of 500mW/ ^cm2 for 40 minutes, and no phenomenon such as smoke generation was observed. In addition, when irradiated with radio waves of 600mW/ ^cm2 , an odor like burnt rubber was generated when the bottom (TC4) exceeded 220°C, and the temperature rose to 250°C after 30 minutes of irradiation. On the other hand, the radio wave absorber impregnated with a conductive paint using a chloroprene binder and dried showed a result that the temperature of each measurement point exceeded 300°C in only 10 minutes after irradiation with radio waves of 460mW/ ^cm2 . From such results, it is understood that the radio wave absorber 1 of this embodiment has high heat resistance and power resistance performance.

Next, a method for forming the radio wave absorber 1 of this embodiment will be described. The radio wave absorber 1 of this embodiment is formed by impregnating a glass wool board with the conductive paint using the silicone binder described above, drying it, and then forming it into a mold as shown in the development view of FIG. 5(a) into a pyramidal or wedge shape. Therefore, at the stage where a plate-shaped radio wave absorber (board 11) is created by impregnating a glass wool board with the conductive paint and drying it, the attenuation (transmission loss) of radio waves passing through the board 11 can be measured, making it possible to carry out intermediate inspections of electrical performance and to easily readjust if performance is insufficient. This makes it possible to significantly improve the yield of non-defective products after the pyramidal or wedge-shaped radio wave absorber 1 is completed.

Figure 5(a) shows an exploded view of the board 11 constituting the radio wave absorber 1 of this embodiment, which is composed of an integrally molded glass wool substrate. In the illustrated example, the radio wave absorber 1 is pyramidal in shape, and at each corner 12, as shown in the A-A cross section of Figure 5(b) and the B-B cross section of Figure 6, a V-groove is provided on the inside of the corner 12 of the radio wave absorber 1 without cutting the board 11. As shown in the figure, this V-groove extends to the top that forms the tip of the radio wave absorber 1, and as shown in the cross section near the top in Figure 6, the V-grooves interfere with each other and the board 11 gradually becomes thinner as it approaches the top.

By forming the board 11 in the above manner, as shown in the cross-sectional view of Figure 7, when the board 11 is folded at the corners 12 to form a three-dimensional shape, the cross sections of the boards can be joined together at the V-groove as shown in the figure. This makes it possible to sharply point the tip of the radio wave absorber 1, and prevents the end face of the board 11 from being exposed on the surface of the radio wave absorber 1, effectively suppressing delamination that is particularly likely to occur with glass wool substrates.

Second Embodiment
In order to further improve the heat resistance and power resistance of the radio wave absorber 1 of the first embodiment described above, the radio wave absorber 1 of this embodiment has a board 11 constituting the radio wave absorber 1 provided with a plurality of ventilation holes 13 penetrating the board 11, as shown in Figs. 8 and 9 .

In this embodiment, as shown in Fig. 8, one ventilation hole 13 with a diameter of 1 cm and three ventilation holes 13 with a diameter of 2 cm are provided from the top side of the surface of the radio wave absorber 1, and these are formed on the four surfaces of the radio wave absorber 1. In the example shown in Fig. 9, one ventilation hole 13 with a diameter of 1 cm and two ventilation holes 13 with a diameter of 2 cm are provided from the top side of the radio wave absorber 1, and these are formed on the four surfaces of the radio wave absorber 1.

Moreover, temperature data of a power resistance test for the radio wave absorber 1 of this embodiment is shown in a graph in Fig. 10. To explain in more detail, as shown in Fig. 8 and Fig. 9, thermocouples TC1 to TC6 were installed at various positions on the radio wave absorber 1 (TC5 and TC6 were installed in the space of the hollow part), and radio waves with an output of 500 mW/ ^cm2 were irradiated from the horn antenna for 35 minutes, and the temperature change over time at each installation position was measured.

Figure 10(a) shows the temperature change of a radio wave absorber without ventilation holes 13 for comparison, Figure 10(b) shows the temperature change of the radio wave absorber shown in Figure 8, and Figure 10(c) shows the temperature change of the radio wave absorber shown in Figure 9. Figure 11 shows the temperature of the base part 14 35 minutes after irradiation with radio waves (see TC4 in Figure 10), the temperature of the hollow part near the bottom of the radio wave absorber (see TC6 in Figure 10), and the ambient temperature equivalent to room temperature (see TC7 in Figure 10).

As shown in the figure, by providing the ventilation holes 13, it is possible to suppress the temperature rise of about 23 to 44°C in the base portion 14, and furthermore, it is possible to suppress the temperature rise of about 76 to 81°C in the hollow portion near the bottom of the radio wave absorber. Therefore, by providing the ventilation holes 13 in the board 11 constituting the radio wave absorber 1, it is possible to promote natural cooling, and it is possible to mitigate the temperature rise and further improve the power resistance performance of the radio wave absorber 1. In particular, it can be said that providing the ventilation holes 13 in the base portion 14 (bottom) of the radio wave absorber 1 and/or the board 11 near the bottom, which are prone to temperature rise, contributes to improving the power resistance performance.

As a further modification, in addition to forming ventilation holes 13 on the four surfaces of the radio wave absorber 1 described above, it is also possible to form a ventilation hole 13 on the bottom surface of the radio wave absorber 1. For example, by providing a ventilation hole 13 in the base portion 14 that leads to the hollow portion of the radio wave absorber 1 and arranging punching metal or the like on the back side to fix the radio wave absorber 1, it is possible to ventilate the hollow portion.

As another modified example, as shown in FIG. 12(a), a ventilation hole 13 may be provided below the top of the radio wave absorber 1, and a mounting surface side ventilation hole that communicates with the hollow part may be provided above the mounting surface of the radio wave absorber 1. That is, the warm air in the hollow part is discharged to the rear side of the mounting surface, which creates an air flow in the hollow part. Then, cool air (outside air) is taken into the hollow part from the ventilation hole 13 near the top. With this configuration, it is possible to effectively suppress the temperature rise of the radio wave absorber 1 and improve the power resistance performance. In addition, as shown in FIG. 12(b), the radio wave absorber 1 is mounted so that the corner part 12 of the radio wave absorber 1 is located upward, and a mounting surface side ventilation hole is provided, so that the warm air in the hollow part can be efficiently collected and discharged. It should be noted that the modified example shown in FIG. 12 has a relatively high temperature reduction effect compared to the modified example shown in FIG. 13 described later, since the warm air discharged from the hollow part does not come into contact with the adjacent radio wave absorber 1.

As another modification, as shown in FIG. 13, a ventilation hole 13 may be provided below near the top of the radio wave absorber 1, and a ventilation hole near the bottom that communicates with the hollow part may be provided above near the bottom of the radio wave absorber 1. That is, the warm air in the hollow part is discharged from the ventilation hole near the bottom, which creates an air flow in the hollow part. Then, cool air (outside air) is taken into the hollow part from the ventilation hole 13 near the top. With this configuration, it is possible to effectively suppress the temperature rise of the radio wave absorber 1 and improve the power resistance performance. In this case, the radio wave absorber 1 is attached so that the corner 12 of the radio wave absorber 1 is located upward, and the above-mentioned ventilation hole near the bottom is provided on the surface of the corner 12 of the radio wave absorber 1 or its vicinity, so that the warm air in the hollow part can be efficiently collected and discharged.

In each of the modified examples shown in Figures 12 and 13 above, by utilizing the so-called "chimney effect," a phenomenon in which warm air rises while cold air is drawn in, it is possible to efficiently generate airflow within the hollow section even with a small opening area, thereby achieving the temperature reduction effect of the radio wave absorber 1. Furthermore, since the above-mentioned temperature reduction effect can be achieved even with a small opening area, there is no need to increase the opening area. Therefore, it is possible to prevent dust and insects from entering the radio wave absorber 1, and it is also possible to effectively prevent a decrease in the strength of the radio wave absorber 1 itself due to the provision of an opening.

The above-mentioned modified examples shown in Figs. 12 and 13 all show the installation of the radio wave absorber 1 on a wall surface, but when the radio wave absorber 1 is installed facing downwards on a ceiling surface, particularly in the modified example shown in Fig. 13, the discharged warm air will drift near the ceiling, which may impair the temperature reduction effect of the group of radio wave absorbers 1. To address this, by applying a breathable material such as punched metal to the mounting surface of the radio wave absorber 1 on the ceiling, it becomes possible to take in cool air (outside air) from the ventilation holes 13 near the top of the radio wave absorber 1 while efficiently discharging the warm air in the hollow space through the punched metal, etc.

In addition, the warm air exhausted to the rear side of the mounting surface of the radio wave absorber 1 as described above can be forcibly exhausted and/or cool air (outside air) can be introduced by adding a separate device, thereby further improving the temperature reduction effect on the radio wave absorber 1.

Although the embodiments of the radio wave absorber of the present invention have been described above based on the drawings and the like, the specific configuration is not limited to these embodiments. The scope of the present invention is indicated by the claims, not by the description of the above embodiments, and further includes all modifications within the meaning and scope of the claims. In addition, the specific materials, dimensions, shapes, etc. described in the above embodiments can be modified within the scope of the invention.

1 Radio wave absorber 11 Board 12 Corner 13 Ventilation hole 14 Base

Claims (5)

The board has a glass mat based on glass wool , which is impregnated with conductive paint and dried,
The conductive paint contains carbon powder with silicone resin as a binder,
The radio wave absorber, wherein the board is formed in a hollow pyramidal or wedge shape. The radio wave absorber according to claim 1 , wherein the board is integrally formed and has a V-shaped groove formed on the inside of a corner of the pyramidal or wedge shape. 3. The radio wave absorber according to claim 1, wherein the board has ventilation holes formed in a surface portion of the pyramidal or wedge shape, the ventilation holes penetrating the board. The radio wave absorber according to claim 3 , wherein the ventilation holes are formed at and/or near the bottom of a surface of the pyramidal or wedge shape. 5. The radio wave absorber according to claim 1, wherein the content of the silicone resin in the conductive paint is 2% by weight or less.

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