Radio wave absorber and method for manufacturing the same

Abstract

Provided is a radio wave absorber having excellent radio wave absorption characteristics in a wide band up to 40 GHz. 【Solution means】A radio wave absorber obtained by firing a molded body containing ferrite, petalite, feldspar, and perlite, and optionally further containing dolomite and glaze, wherein the molded body contains, by mass%, 70.0% or more and 85.0% or less of ferrite, 1.5% or more and 12.0% or less of petalite, 1.5% or more and 12.0% or less of feldspar, 1.5% or more and 12.0% or less of perlite, 0.0% or more and 8.0% or less of dolomite, and 0.0% or more and 9.0% or less of glaze.

Description

Technical Field

[0001] The present invention relates to a radio wave absorber and a method for manufacturing the same.

Background Art

[0002] Electromagnetic compatibility (EMC) is required for electronic devices so that electromagnetic waves generated from the electronic devices do not cause malfunction in other devices, or conversely, the electronic devices do not malfunction due to external electromagnetic waves. To evaluate EMC, a measurement room called an anechoic chamber is required. The outer wall of the anechoic chamber is covered with a metal plate to prevent external electromagnetic waves from entering the chamber and electromagnetic waves generated from the measuring devices inside the chamber from radiating outside. In addition, radio wave absorbers are attached inside the chamber to prevent reflection of unnecessary electromagnetic waves.

[0003] Conventionally, the frequency range measured in an anechoic chamber was from 30 MHz to 1 GHz. However, with the diversification of communication devices such as mobile phones and RF tags, the upper limit of the measurement frequency has also been expanded. For this reason, there is a demand for a radio wave absorber having excellent radio wave absorption characteristics in a wide frequency band exceeding 30 MHz to 1 GHz. Along with such a trend of higher frequencies, the allowable value setting frequency described in the standard has also been revised to 18 GHz or less. Furthermore, with the change to the era of large-capacity high-speed communication and the market introduction of 5G, the frequency band used in electronic devices has changed to 28 GHz in the millimeter wave range. Along with this, excellent radio wave absorption characteristics in a frequency band from 18 GHz or more to at least 40 GHz have been demanded for radio wave absorbers.

[0004] Patent Document 1 describes "a radio wave absorber characterized by firing a molded body containing ferrite powder, silicon carbide powder, perlite powder, and water glass."

Prior Art Documents

Patent Documents

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-18854 [Summary of the Invention] [Problems to be Solved by the Invention]

[0006] In Patent Document 1, the radio wave absorption characteristics in the frequency band of 10 to 18 GHz are improved. However, there is room for improvement in the radio wave absorption characteristics in the wide band up to 40 GHz exceeding 18 GHz.

[0007] Therefore, in view of the above problems, an object of the present invention is to provide a radio wave absorber having excellent radio wave absorption characteristics in a wide band up to 40 GHz and a method for manufacturing the same. [Means for Solving the Problems]

[0008] As a result of intensive studies to solve the above problems, the present inventors have obtained the following findings. By firing a molded body containing a predetermined amount of ferrite, petalite, feldspar, and perlite, and optionally further containing a predetermined amount of dolomite and glaze, a radio wave absorber having excellent radio wave absorption characteristics in a wide band up to 40 GHz can be obtained.

[0009] That is, the gist configuration of the present invention is as follows.

[0010] [1] A radio wave absorber obtained by firing a molded body containing ferrite, petalite, feldspar, and perlite, and optionally further containing dolomite and glaze, wherein the molded body contains, by mass%, 70.0% or more and 85.0% or less of ferrite, 1.5% or more and 12.0% or less of petalite, 1.5% or more and 12.0% or less of feldspar, 1.5% or more and 12.0% or less of perlite, 0.0% or more and 8.0% or less of dolomite, and 0.0% or more and 9.0% or less of glaze characterized in that it contains.

[0011] [2] The shaped body is further the radio wave absorber according to [1] above, containing 3.00% or less of hemp fiber by mass%.

[0012] [3] A step of mixing and shaping a raw material containing ferrite, petalite, feldspar, and perlite, and optionally further containing dolomite and glaze to obtain a shaped body; A step of firing the shaped body to obtain a radio wave absorber; having The shaped body contains, by mass%, 70.0% or more and 85.0% or less of ferrite, 1.5% or more and 12.0% or less of petalite, 1.5% or more and 12.0% or less of feldspar, 1.5% or more and 12.0% or less of perlite, 0.0% or more and 8.0% or less of dolomite, and 0.0% or more and 9.0% or less of glaze A method for manufacturing a radio wave absorber, characterized by containing the above.

[0013] [4] The shaped body is further the method for manufacturing a radio wave absorber according to [3] above, containing 3.00% or less of hemp fiber by mass%.

Advantages of the Invention

[0014] According to the present invention, it is possible to provide a radio wave absorber excellent in radio wave absorption characteristics in a wide band up to 40 GHz and a method for manufacturing the same.

Brief Description of the Drawings

[0015]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Mode for Carrying Out the Invention

[0016] Hereinafter, embodiments of the radio wave absorber according to the present invention will be described. Note that the embodiments described below are examples of embodying the present invention, and do not limit the configuration of the present invention with their specific examples.

[0017] The shape of the radio wave absorber according to an embodiment of the present invention is not particularly limited, but it is preferably a shape in which the ratio of the volume of the radio wave absorber per unit volume increases from the end on the electromagnetic wave incident side toward the other end. For example, a wedge shape, a polygonal pyramid shape, and a conical shape can be mentioned, and a wedge shape or a pyramid shape (square pyramid) is preferably used. When the radio wave absorber has a pyramid shape, the base can be about 10 to 20 cm and the height can be about 5 to 10 cm. Further, the radio wave absorber may have a flat base portion, and may have a shape in which the above-described pyramid shape is provided on the base portion.

[0018] The radio wave absorber according to an embodiment of the present invention is obtained by firing a molded body containing ferrite, petalite, feldspar, and perlite, and optionally further containing dolomite and glaze. The molded body preferably further contains hemp fiber. In the following description, the % of the content is mass %.

[0019] The formed body shall contain ferrite. In order for a radio wave absorber formed by firing the formed body to obtain desired radio wave absorption characteristics, it is a prerequisite that the formed body contains ferrite. The material of the ferrite is not particularly limited, and one or more selected from the group consisting of NiO / ZnO-based, LiO / ZnO-based, NiO / ZnO / CuO-based, MnO / ZnO-based, etc. can be used. From the viewpoint of obtaining good radio wave absorption characteristics in a wide band, it is preferable to use NiO / ZnO-based or LiO / ZnO-based ferrite. Also, the average particle size of the ferrite powder at the raw material stage can be 35 to 45 μm, and the particle size distribution can be about 10 to 120 μm. Note that the average particle size and particle size distribution of each raw material are values measured by the laser diffraction method.

[0020] If the ferrite content of the formed body is less than 70.0%, the desired radio wave absorption characteristics cannot be obtained. Therefore, the ferrite content shall be 70.0% or more. On the other hand, if the ferrite content of the formed body exceeds 85.0%, the balance with the content of other raw materials is disrupted, and there is a possibility that the desired radio wave absorption characteristics cannot be obtained. Therefore, the ferrite content shall be 85.0% or less.

[0021] The formed body shall contain petalite (lepidolite). Petalite is a kind of silicate mineral, and its chemical formula is LiAl(Si4O 10) It is represented by. Note that spodumene (hedenbergite) or lithium aluminosilicate may be used as a substitute for petalite. By the molded body containing petalite, in the radio wave absorber, effects such as improvement of radio wave absorption characteristics in the GHz band, ensuring conductivity, and maintaining the shape of the sintered body can be obtained. If the content of petalite in the molded body is less than 1.5%, the desired radio wave absorption characteristics in the GHz band cannot be obtained. Therefore, the content of petalite is 1.5% or more, and preferably 2.0% or more. On the other hand, if the content of petalite in the molded body exceeds 12.0%, the balance with the content of other raw materials is disrupted, and there is a possibility that the desired radio wave absorption characteristics in the GHz band cannot be obtained. Therefore, the content of petalite is 12.0% or less, and preferably 10.0% or less. Petalite at the raw material stage is preferably in powder form, and the average particle size of the petalite powder can be 20 to 30 μm, and the particle size distribution can be about 5 to 70 μm.

[0022] The molded body shall contain feldspar. Feldspar is a kind of mineral mainly composed of aluminosilicate. Aluminosilicate is a silicate having a structure in which silicon and oxygen are connected in a three-dimensional network-like manner, and in which Si 4+ is replaced by Al 3+ and the positive charge lost by the replacement is compensated in the form of alkali metal ions (M +) It contains cations such as, and its chemical formula is represented by xM2O·yAl2O3·zSiO2·nH2O (x, y, z, n are arbitrary integers). Examples of the alkali metal M include Na, Ca, K, etc. Examples of feldspar include Kishu feldspar, Tsushima feldspar, and Heizu feldspar. When the molded body contains feldspar, in the radio wave absorber, effects such as improvement of radio wave absorption characteristics in the GHz band, ensuring conductivity, and maintaining the shape of the sintered body can be obtained. If the content of feldspar in the molded body is less than 1.5%, the desired radio wave absorption characteristics in the GHz band cannot be obtained. Therefore, the content of feldspar should be 1.5% or more, and preferably 1.8% or more. On the other hand, if the content of feldspar in the molded body exceeds 12.0%, the balance with the content of other raw materials is disrupted, and there is a possibility that the desired radio wave absorption characteristics in the GHz band cannot be obtained. Therefore, the content of feldspar should be 12.0% or less, and preferably 10.0% or less. Feldspar at the raw material stage is preferably in powder form, and the average particle size of the feldspar powder can be 10 - 20μm, and the particle size distribution can be about 5 - 40μm.

[0023] The molded body shall contain perlite. Perlite is a siliceous volcanic rock, mainly composed of amorphous silicon dioxide (SiO2) and a small amount of water. As perlite, it is preferable to use obsidian-based perlite obtained by heat-treating obsidian, which is a glassy igneous rock, at a high temperature. When the molded body contains perlite, in the radio wave absorber, effects such as improvement of radio wave absorption characteristics in the GHz band, ensuring conductivity, and maintaining the shape of the sintered body can be obtained. If the content of perlite in the molded body is less than 1.5%, the desired radio wave absorption characteristics in the GHz band cannot be obtained. Therefore, the content of perlite should be 1.5% or more, preferably 2.0% or more, and more preferably 2.8% or more. On the other hand, if the content of perlite in the molded body exceeds 12.0%, the balance with the content of other raw materials is disrupted, and there is a possibility that the desired radio wave absorption characteristics in the GHz band cannot be obtained. Therefore, the content of perlite should be 12.0% or less, preferably 10.0% or less, and more preferably 8.0% or less. Perlite at the raw material stage is preferably in powder form, and the average particle size of the perlite powder can be 145 - 155μm, and the particle size distribution can be about 20 - 460μm.

[0024] Furthermore, the molded body preferably contains dolomite. Dolomite mainly consists of a double salt of calcium carbonate and magnesium carbonate, and its chemical formula is represented by CaMg(CO3)2. Since dolomite is an optional component in the present invention, the lower limit of the dolomite content in the molded body is not particularly limited, and the dolomite content may be 0.0%. However, when the dolomite content in the molded body is 1.5% or more, in the radio wave absorber, effects such as improvement of radio wave absorption characteristics in the GHz band, ensuring of sinterability, and suppression of vitrification of the surface of the radio wave absorber can be preferably obtained. Therefore, the dolomite content is preferably 1.5% or more, more preferably 1.6% or more. On the other hand, when the dolomite content in the molded body exceeds 8.0%, the desired radio wave absorption characteristics in the GHz band cannot be obtained. Therefore, when the molded body contains dolomite, the dolomite content is 8.0% or less, preferably 6.0% or less. Dolomite at the raw material stage is preferably in powder form, and the average particle size of the dolomite powder can be 25 to 35 μm, and the particle size distribution can be about 5 to 100 μm.

[0025] In addition to the above, the molded body preferably contains a glaze. In the present invention, the glaze is a substance containing various oxides in a predetermined ratio, and preferably has conductivity. The glaze in the present invention can contain 40 to 50% by mass of SiO2, 10 to 20% by mass of Al2O3, 5 to 15% by mass of ZnO, 5 to 15% by mass of CaO, 5 to 15% by mass of Fe2O3, 1 to 10% by mass of Li2O, 1 to 5% by mass of MnO, 1 to 5% by mass of CoO, 1 to 5% by mass of K2O, 1% by mass or less of Na2O, and 1% by mass or less of MgO. Since the glaze is an optional component in the present invention, the lower limit of the content of the glaze in the molded body is not particularly limited, and the content of the glaze may be 0.0%. However, when the molded body contains a predetermined amount of glaze, effects such as improvement of radio wave absorption characteristics in the GHz band, ensuring conductivity, and maintaining the shape of the sintered body can be obtained in the radio wave absorber. Therefore, the content of the glaze is preferably 0.5% or more. However, if the content of the glaze in the molded body exceeds 9.0%, the desired radio wave absorption characteristics in the GHz band cannot be obtained. Therefore, when the molded body contains a glaze, the content of the glaze is 9.0% or less, and preferably 8.0% or less. The glaze at the raw material stage is preferably in powder form, and the average particle size of the glaze powder can be 10 to 20 μm, and the particle size distribution can be about 1 to 40 μm.

[0026] The formed body may further contain hemp fibers. In the present invention, hemp fibers are a type of plant fiber, and commercially available ones can be used. Since hemp fibers are an optional component in the present invention, the lower limit of the content of hemp fibers in the formed body is not particularly limited, and the content of hemp fibers may be 0.00%. However, hemp fibers play a role in maintaining the shape of the formed body before firing. Therefore, the content of hemp fibers in the formed body is preferably 0.10% or more, more preferably 0.20% or more, and even more preferably 0.50% or more. However, if the content of hemp fibers becomes excessive, the filling property of the slurry into the resin mold deteriorates, and the density of the radio wave absorber after firing becomes low, so that suitable radio wave absorption characteristics cannot be obtained. Therefore, the content of hemp fibers is preferably 3.00% or less, and more preferably 1.00% or less. Hemp fibers having a fiber length of 5 to 10 mm and a fiber thickness of about 10 to 50 μm can be used. In addition, as a substitute for hemp fibers, pulp fibers, nylon fibers for concrete additives, etc. can also be used.

[0027] The radio wave absorber obtained by firing the formed body having the above raw materials has excellent radio wave absorption characteristics. Here, excellent radio wave absorption characteristics mean that when the radio wave absorption characteristics are measured, the reflection amount is -20 dB or less in the entire frequency band of 12.4 to 40 GHz.

[0028] The radio wave absorption characteristics can be measured by the following method. A ferrite tile is fixed to the lower surface of the base portion of the radio wave absorber to form a composite radio wave absorber. Using a reflection amount measuring device using a dielectric lens, the radio wave absorption characteristics are measured in the frequency band of 12.4 to 40 GHz. The temperature range during measurement is preferably 20 ± 10 °C, and the humidity range is preferably 30 ± 20%.

[0029] The manufacturing method of the radio wave absorber according to an embodiment of the present invention includes a step of mixing and molding raw materials to obtain a formed body, and a step of firing the formed body to obtain a radio wave absorber. The raw materials include ferrite, petalite, feldspar, and perlite, and optionally further include dolomite and glaze. The raw materials preferably further include hemp fibers. The content of each component in the raw materials is as described above.

[0030] The forming method of the raw materials is not particularly limited, and as an example, it can be formed by the following method. First, weigh a predetermined amount of petalite, feldspar, perlite, dolomite, conductive glaze, and hemp fiber, which are the ceramic substrates, and put them into a container. With respect to the solid content of the ceramic substrate and ferrite, 2% by weight of alginate (5% aqueous solution of Kimica Alginate, manufactured by Kimica Corporation) as a binder, 10% by weight of polyvinyl alcohol (PVA, Celna WF-804, manufactured by Chukyo Yushi Co., Ltd.), 2.8% by weight of stearic acid (Cellosol 920, manufactured by Chukyo Yushi Co., Ltd.), and 1% by weight of polyacrylate (SN Dispersant 5468, manufactured by San Nopco Ltd.) as a dispersant are added. As water, 3.5% by weight of ion-exchanged water is added with respect to the solid content of the ceramic substrate and ferrite, and they are uniformly mixed with a stirrer. Then, ferrite powder is added and stirred and mixed to obtain a slurry. Carboxymethyl cellulose (CMC), water-soluble acrylic resin, acrylic emulsion, or wax emulsion may be used as the binder. An anionic surfactant or polycarboxylate may be used as the dispersant. The obtained slurry is formed into a pyramid shape or a wedge shape by the slip casting method using a mold such as a gypsum board or a non-absorbent resin. A method of pressure molding the mixture of raw materials may also be used.

[0031] By firing the formed body obtained as described above, the radio wave absorber of the present embodiment is obtained. As the firing conditions, the time for raising the temperature to the firing temperature is 9 hours, the firing temperature (the temperature of the atmosphere during firing) is preferably 1000 to 1200 °C, and the firing time (the holding time at the firing temperature) is preferably 2 to 3 hours. Also, the atmosphere can be air firing.

[0032] In addition, conventional methods can be used for the steps and conditions not described in the present invention.

Example

[0033] As raw materials, LiO / ZnO-based ferrite (Li-Zn ferrite, manufactured by Tosyoku Kogyo Co., Ltd.) with an average particle size of 35 to 45 μm and a particle size distribution of 10 to 120 μm, petalite (Petalite #200, manufactured by Maruto Co., Ltd.) with an average particle size of 20 to 30 μm and a particle size distribution of 5 to 70 μm, feldspar (Kamaido feldspar, special grade, manufactured by Kamaido Industrial Cooperative Association) with an average particle size of 10 to 20 μm and a particle size distribution of 5 to 40 μm, perlite (Pacific Perlite No. 5, manufactured by Pacific Material Co., Ltd.) with an average particle size of 145 to 155 μm and a particle size distribution of 25 to 460 μm, dolomite (dolomite powder, manufactured by Tono Agricultural Cooperative Association) with an average particle size of 25 to 35 μm and a particle size distribution of 5 to 100 μm, and a conductive glaze with an average particle size of 10 to 20 μm and a particle size distribution of 1 to 40 μm were prepared. The component composition of the glaze used is shown in Table 2. In addition, as the hemp fiber, hemp fiber for plaster wall material (crack prevention) manufactured by Kinki Wall Material Industry Co., Ltd. was used.

[0034] In each example, the raw materials were mixed at the ratios shown in Table 1, and a binder, a dispersant, and 3.5% by mass of ion-exchanged water were added thereto, and the mixture was mixed with a ball mill to prepare a slurry. This slurry was poured into a non-water-absorbing resin mold and dried at 50°C for 16 hours to remove the dispersion medium in the slurry. As a result, a molded body having a shape in which two pyramids (bottom surface: 47 mm × 47 mm, height: 70 mm) were arranged vertically and horizontally (a total of 4) was obtained on a flat plate (bottom surface: 100 mm × 100 mm, thickness: 15 mm). The molded body removed from the mold was fired under the condition of holding at 1100°C in the air for 2 hours to obtain a radio wave absorber. The obtained molded body was fired under the condition of holding at 1100°C in the air for 2 hours to obtain a radio wave absorber.

[0035]

Table 1

[0036]

Table 2

[0037] For the obtained radio wave absorber, the radio wave absorption characteristics were measured by the method described above. The measurement results are shown in FIGS. 1 to 5. FIG. 1 is a graph showing the radio wave absorption characteristics at frequencies of 12.4 to 40 GHz for Nos. 1 to 4, and visualizes the relationship between the content of the glaze and the radio wave absorption characteristics. FIG. 2 is a graph showing the radio wave absorption characteristics at frequencies of 12.4 to 40 GHz for Nos. 1 and 5 to 7, and visualizes the relationship between the content of petalite and the radio wave absorption characteristics. FIG. 3 is a graph showing the radio wave absorption characteristics at frequencies of 12.4 to 40 GHz for Nos. 1 and 8 to 11, and visualizes the relationship between the content of feldspar and the radio wave absorption characteristics. FIG. 4 is a graph showing the radio wave absorption characteristics at frequencies of 12.4 to 40 GHz for Nos. 1 and 12 to 14, and visualizes the relationship between the content of dolomite and the radio wave absorption characteristics. FIG. 5 is a graph showing the radio wave absorption characteristics at frequencies of 12.4 to 40 GHz for Nos. 1 and 15 to 17, and visualizes the relationship between the content of pearlite and the radio wave absorption characteristics. In any of the examples, the temperature during measurement was in the range of 20 ± 10°C, and the humidity was in the range of 30 ± 20%.

[0038] As shown in FIGS. 1 to 5, in the examples where the composition of the raw material satisfies the specified range of the present invention, the reflection amount can be -20 dB or less, that is, a reflection attenuation amount of 20 dB or more can be realized over the entire frequency band of 12.4 to 40 GHz, and the radio wave absorption characteristics are excellent. On the other hand, in the examples where the composition of the raw material does not satisfy the specified range of the present invention, there is a frequency band in the frequency band of 12.4 to 40 GHz where a reflection attenuation amount of 20 dB or more cannot be realized.

Industrial Applicability

[0039] According to the present invention, it is possible to provide a radio wave absorber having excellent radio wave absorption characteristics in a wide frequency band up to 40 GHz and a method for manufacturing the same.

Claims

1. A radio wave absorber obtained by firing a molded body containing ferrite, petalite, feldspar, and perlite, and optionally further containing dolomite and glaze, wherein the molded body contains, by mass%, ferrite in an amount of 70.0% or more and 85.0% or less, petalite in an amount of 1.5% or more and 12.0% or less, feldspar in an amount of 1.5% or more and 12.0% or less, perlite in an amount of 1.5% or more and 12.0% or less, dolomite in an amount of 0.0% or more and 8.0% or less, and glaze in an amount of 0.0% or more and 9.0% or less characterized radio wave absorber.

2. The radio wave absorber according to claim 1, wherein the molded body further contains, by mass%, hemp fiber in an amount of 3.00% or less.

3. A method for manufacturing a radio wave absorber, comprising: a step of mixing and molding a raw material containing ferrite, petalite, feldspar, and perlite, and optionally further containing dolomite and glaze, to obtain a molded body; a step of firing the molded body to obtain a radio wave absorber; and wherein the molded body contains, by mass%, ferrite in an amount of 70.0% or more and 85.0% or less, petalite in an amount of 1.5% or more and 12.0% or less, feldspar in an amount of 1.5% or more and 12.0% or less, perlite in an amount of 1.5% or more and 12.0% or less, dolomite in an amount of 1.5% or more and 8.0% or less, and glaze in an amount of 0.0% or more and 9.0% or less characterized method for manufacturing a radio wave absorber.

4. The method for manufacturing a radio wave absorber according to claim 3, wherein the molded body further contains, by mass%, hemp fiber in an amount of 3.00% or less.

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