Radio wave absorber and method for manufacturing the same

Abstract

To provide a radio wave absorber having excellent radio wave absorption characteristics over a wide bandwidth up to 40 GHz.SOLUTION: The present invention relates to a radio wave absorber produced by firing a molded body containing ferrite, petalite, feldspar, and perlite, and optionally further containing dolomite and glaze, the molded body containing, in mass%, at least 70.0% and 85.0% or less of ferrite, at least 1.5% and 12.0% or less of petalite, at least 1.5% and 12.0% or less of feldspar, at least 1.5% and 12.0% or less of perlite, at least 0.0% and 8.0% or less of dolomite, and at least 0.0% and 9.0% or less of glaze.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] Electronic devices are required to have electromagnetic compatibility (EMC) to prevent electromagnetic waves generated by them from causing malfunctions in other devices, and conversely, to prevent electronic devices from malfunctioning due to external electromagnetic waves. EMC evaluation requires a measurement room called an anechoic chamber. The outer walls of the anechoic chamber are covered with metal plates to prevent external electromagnetic waves from entering the chamber and electromagnetic waves generated by measuring equipment inside the chamber from radiating outward. In addition, radio wave absorbers are installed inside the anechoic chamber to prevent unwanted electromagnetic wave reflections.

[0003] Traditionally, 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 expanded. This has led to a demand for radio wave absorbers with excellent radio wave absorption properties across a wide frequency band from 30 MHz to over 1 GHz. Accompanying this trend toward higher frequencies, the allowable frequency limits specified in the standards have been revised to 18 GHz or less. Furthermore, with the shift to an era of high-capacity, high-speed communications and the market introduction of 5G, the frequency band used by electronic devices has shifted to 28 GHz, in the millimeter wave range. Accordingly, radio wave absorbers are now required to have excellent radio wave absorption properties across a frequency band above 18 GHz and up to at least 40 GHz.

[0004] Patent Document 1 describes "a radio wave absorber characterized by being produced by firing a molded body containing ferrite powder, silicon carbide powder, pearlite 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 [Problem to be solved by the invention]

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

[0007] In view of the above problems, the present invention aims 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 problem]

[0008] As a result of intensive research to solve the above problems, the present inventors have discovered the following: By firing a molded body containing predetermined amounts of ferrite, petalite, feldspar, and pearlite, and optionally further containing predetermined amounts of dolomite and glaze, it is possible to obtain a radio wave absorber that has excellent radio wave absorption characteristics in a wide band up to 40 GHz.

[0009] That is, the gist and configuration of the present invention are 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, The molded body comprises, in mass %, Ferrite: 70.0% or more and 85.0% or less Petalite is 1.5% or more and 12.0% or less, Feldspar: 1.5% to 12.0% Pearlite: 1.5% to 12.0% Dolomite: 0.0% to 8.0% Glaze: 0.0% to 9.0% A radio wave absorber comprising:

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

[0012] [3] A step of mixing and molding raw materials 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 The molded body comprises, in mass %, Ferrite: 70.0% or more and 85.0% or less Petalite is 1.5% or more and 12.0% or less, Feldspar: 1.5% to 12.0% Pearlite: 1.5% to 12.0% Dolomite: 0.0% to 8.0% Glaze: 0.0% to 9.0% A method for manufacturing a radio wave absorber, comprising:

[0013] [4] The method for producing a radio wave absorber according to the above [3], wherein the molded body further contains, by mass %, 3.00% or less of hemp fiber. [Effects of the Invention]

[0014] According to the present invention, it is possible to provide a radio wave absorber having excellent radio wave absorption characteristics over a wide band up to 40 GHz, and a method for producing the same. [Brief explanation of the drawings]

[0015] [Figure 1] 1 is a graph showing the radio wave absorption characteristics in Examples 1 to 4 at frequencies of 12.4 to 40 GHz. [Figure 2] 1 is a graph showing the radio wave absorption characteristics in Examples No. 1 and Nos. 5 to 7 at frequencies of 12.4 to 40 GHz. [Figure 3] 1 is a graph showing the radio wave absorption characteristics at frequencies of 12.4 to 40 GHz in Examples No. 1 and Nos. 8 to 11. [Figure 4] 1 is a graph showing the radio wave absorption characteristics at frequencies of 12.4 to 40 GHz in Examples No. 1 and Nos. 12 to 14. [Figure 5] 1 is a graph showing the radio wave absorption characteristics at frequencies of 12.4 to 40 GHz in Examples No. 1 and Nos. 15 to 17. DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, an embodiment of a radio wave absorber according to the present invention will be described. Note that the embodiment described below is an example of a specific embodiment of the present invention, and the configuration of the present invention is not limited to this specific example.

[0017] The shape of the radio wave absorber according to one embodiment of the present invention is not particularly limited, but is preferably a shape in which the ratio of the volume of the radio wave absorber to the unit volume increases from the end on the electromagnetic wave arrival side to the other end. Examples of shapes include a wedge shape, a polygonal pyramid shape, and a cone shape, with a wedge shape or a pyramid shape (quadratic pyramid) being preferred. When the radio wave absorber is pyramidal, the base may be approximately 10 to 20 cm and the height may be approximately 5 to 10 cm. The radio wave absorber may also have a flat base, or may have a shape in which the pyramidal shape is provided on the base.

[0018] A radio wave absorber according to one 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 percentage of content is by mass.

[0019] The molded body contains ferrite. To obtain the desired radio wave absorption characteristics of a radio wave absorber obtained by firing the molded body, it is necessary for the molded body to contain ferrite. The ferrite material is not particularly limited, and one or more types selected from the group consisting of NiO / ZnO, LiO / ZnO, NiO / ZnO / CuO, MnO / ZnO, etc. can be used. From the viewpoint of obtaining good radio wave absorption characteristics over a wide frequency range, it is preferable to use NiO / ZnO or LiO / ZnO ferrite. Furthermore, the average particle size of the ferrite powder in the raw material stage can be approximately 35 to 45 μm, and the particle size distribution can be approximately 10 to 120 μm. The average particle size and particle size distribution of each raw material are values ​​measured by laser diffraction.

[0020] If the ferrite content of the compact is less than 70.0%, the desired radio wave absorption characteristics cannot be obtained. Therefore, the ferrite content is set to 70.0% or more. On the other hand, if the ferrite content of the compact exceeds 85.0%, the balance with the contents of other raw materials is lost, and the desired radio wave absorption characteristics may not be obtained. Therefore, the ferrite content is set to 85.0% or less.

[0021] The compact contains petalite. Petalite is a type of silicate mineral, and its chemical formula is LiAl(SiO 10) Note that spodumene (spodumene) or lithium aluminosilicate may be used as a substitute for petalite. When the compact contains petalite, the radio wave absorber can achieve effects such as improved radio wave absorption characteristics in the GHz band, ensured electrical conductivity, and shape retention of the sintered body. If the petalite content of the compact is less than 1.5%, the desired radio wave absorption characteristics in the GHz band cannot be obtained. Therefore, the petalite content is 1.5% or more, and preferably 2.0% or more. On the other hand, if the petalite content of the compact exceeds 12.0%, the balance with the contents of other raw materials is lost, and the desired radio wave absorption characteristics in the GHz band may not be obtained. Therefore, the petalite content is 12.0% or less, and preferably 10.0% or less. Petalite is preferably in the form of powder at the raw material stage, and the petalite powder can have an average particle size of 20 to 30 μm and a particle size distribution of about 5 to 70 μm.

[0022] The molded body contains feldspar. Feldspar is a type of mineral whose main component is aluminosilicate. Aluminosilicate is a type of silicate that has a structure in which silicon and oxygen are connected in a three-dimensional network. 4+ Al 3+ The alkali metal ions (M +), and its chemical formula is xMO·yAl2O3·zSiO2·nH2O (x, y, z, and n are any integers). Examples of alkali metals M include Na, Ca, and K. Examples of feldspar include Kamado feldspar, Taishu feldspar, and Hiratsu feldspar. The inclusion of feldspar in the compact provides the electromagnetic wave absorber with improved electromagnetic wave absorption characteristics in the GHz band, ensured electrical conductivity, and shape retention of the sintered body. If the feldspar content of the compact is less than 1.5%, the desired electromagnetic wave absorption characteristics in the GHz band cannot be obtained. Therefore, the feldspar content is set to 1.5% or more, and preferably 1.8% or more. On the other hand, if the feldspar content of the compact exceeds 12.0%, the balance with the contents of other raw materials is lost, and the desired electromagnetic wave absorption characteristics in the GHz band may not be obtained. Therefore, the feldspar content is set to 12.0% or less, and preferably 10.0% or less. The feldspar is preferably in the form of powder at the raw material stage, and the average particle size of the feldspar powder can be set to about 10 to 20 μm, with a particle size distribution of about 5 to 40 μm.

[0023] The molded body contains perlite. Perlite is a siliceous volcanic rock composed mainly of amorphous silicon dioxide (SiO2) and a small amount of water. It is preferable to use obsidian-based perlite, which is obtained by heat-treating obsidian, a glassy igneous rock, at high temperatures. The inclusion of perlite in the molded body provides the electromagnetic wave absorber with improved electromagnetic wave absorption characteristics in the GHz band, ensured electrical conductivity, and shape retention of the sintered body. If the perlite content of the molded body is less than 1.5%, the desired electromagnetic wave absorption characteristics in the GHz band cannot be obtained. Therefore, the perlite content is set to 1.5% or more, preferably 2.0% or more, and more preferably 2.8% or more. On the other hand, if the perlite content of the molded body exceeds 12.0%, the balance with the contents of other raw materials is disrupted, and the desired electromagnetic wave absorption characteristics in the GHz band may not be obtained. Therefore, the perlite content is set to 12.0% or less, preferably 10.0% or less, and more preferably 8.0% or less. The perlite in the raw material stage is preferably in the form of powder, and the perlite powder can have an average particle size of 145 to 155 μm and a particle size distribution of about 20 to 460 μm.

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

[0025] In addition to the above, the molded body preferably contains a glaze. In the present invention, a glaze is a substance containing various oxides in a predetermined proportion and preferably has electrical conductivity. The glaze in the present invention may contain 40 to 50 mass% SiO2, 10 to 20 mass% Al2O3, 5 to 15 mass% ZnO, 5 to 15 mass% CaO, 5 to 15 mass% Fe2O3, 1 to 10 mass% Li2O, 1 to 5 mass% MnO, 1 to 5 mass% CoO, 1 to 5 mass% K2O, 1 mass% or less Na2O, and 1 mass% or less MgO. Since the glaze is an optional component in the present invention, the lower limit of the glaze content in the molded body is not particularly limited, and the glaze content may be 0.0%. However, by including a predetermined amount of glaze in the molded body, the radio wave absorber can achieve effects such as improved radio wave absorption characteristics in the GHz band, ensured electrical conductivity, and shape retention of the sintered body. For this reason, the glaze content is preferably 0.5% or more. However, if the glaze content of the molded body exceeds 9.0%, the desired radio wave absorption characteristics in the GHz band cannot be obtained. Therefore, if the molded body contains glaze, the glaze content should be 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, with a particle size distribution of approximately 1 to 40 μm.

[0026] The molded body may further contain hemp fiber. In the present invention, hemp fiber is a type of plant fiber, and commercially available hemp fiber can be used. Since hemp fiber is an optional component in the present invention, the lower limit of the hemp fiber content in the molded body is not particularly limited, and the hemp fiber content may be 0.00%. However, hemp fiber plays a role in maintaining the shape of the molded body before firing. Therefore, the hemp fiber content in the molded body is preferably 0.10% or more, more preferably 0.20% or more, and even more preferably 0.50% or more. However, if the hemp fiber content is excessive, the filling of the slurry into the resin mold will be poor, and the density of the electromagnetic wave absorber after firing will be low, making it difficult to obtain suitable electromagnetic wave absorption characteristics. Therefore, the hemp fiber content is preferably 3.00% or less, more preferably 1.00% or less. Hemp fiber with a fiber length of approximately 5 to 10 mm and a fiber thickness of approximately 10 to 50 μm can be used. Note that paper pulp fiber, nylon fiber used as a concrete additive, and the like can also be used as substitutes for hemp fiber.

[0027] The radio wave absorber obtained by firing the molded body containing the above raw materials has excellent radio wave absorption characteristics. Here, excellent radio wave absorption characteristics means that when the radio wave absorption characteristics are measured, the reflection amount is −20 dB or less over 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 underside of the base of the radio wave absorber to form a composite radio wave absorber. The radio wave absorption characteristics are measured in the frequency band of 12.4 to 40 GHz using a reflection amount measuring device that uses a dielectric lens. The temperature range during measurement is preferably 20±10°C and the humidity range is 30±20%.

[0029] A method for producing a radio wave absorber according to one embodiment of the present invention includes the steps of mixing and molding raw materials to obtain a molded body, and firing the molded body to obtain a radio wave absorber. The raw materials include ferrite, petalite, feldspar, and perlite, and optionally further include dolomite and glaze. Preferably, the raw materials further include hemp fiber. The contents of each component in the raw materials are as described above.

[0030] The method for forming the raw material is not particularly limited, and as an example, it can be formed by the following method. First, a predetermined amount of petalite, feldspar, perlite, dolomite, conductive glaze, and hemp fiber that will become the ceramic substrate are weighed and placed in a container. 2 wt% of alginate (5% aqueous solution of Kimica Algin, manufactured by Kimica Co., Ltd.) as a binder, 10 wt% of polyvinyl alcohol (PVA, Cerna WF-804, manufactured by Chukyo Yushi Co., Ltd.), 2.8 wt% of stearic acid (Cellosol 920, manufactured by Chukyo Yushi Co., Ltd.), and 1 wt% of polyacrylate (SN Dispersant 5468, manufactured by San Nopco Co., Ltd.) as a dispersant were added relative to the solid content of the ceramic substrate and ferrite. 3.5 wt% of ion-exchanged water was added as moisture relative to the solid content of the ceramic substrate and ferrite, and the mixture was uniformly mixed with a stirrer. Ferrite powder was 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. Anionic surfactants or polycarboxylates may be used as the dispersant. The resulting slurry is molded into pyramidal or wedge shapes by slip casting using molds such as gypsum board or non-absorbent resin. A method of pressure molding a mixture of raw materials may also be used.

[0031] The molded body obtained as described above is fired to obtain the radio wave absorber of this embodiment. As firing conditions, the time required to raise the temperature to the firing temperature is 9 hours, the firing temperature (temperature of the atmosphere during firing) is preferably 1000 to 1200°C, and the firing time (holding time at the firing temperature) is preferably 2 to 3 hours. Furthermore, the firing atmosphere can be air.

[0032] For steps and conditions not described in the present invention, conventional methods can be used. [Example]

[0033] The raw materials used were LiO / ZnO ferrite (Li-Zn ferrite, manufactured by Toshoku Kogyo Co., Ltd.) with an average particle size of 35–45 μm and a particle size distribution of 10–120 μm, petalite (Petalite #200, manufactured by Maruto Co., Ltd.) with an average particle size of 20–30 μm and a particle size distribution of 5–70 μm, feldspar (Kamado Feldspar, Special Grade, manufactured by Kamado Industrial Cooperative Association) with an average particle size of 10–20 μm and a particle size distribution of 5–40 μm, perlite (Pacific Pearlite No. 5, manufactured by Pacific Materials Co., Ltd.) with an average particle size of 145–155 μm and a particle size distribution of 25–460 μm, dolomite (dolomite powder, manufactured by Tono Mining Cooperative Association) with an average particle size of 25–35 μm and a particle size distribution of 5–100 μm, and conductive glaze with an average particle size of 10–20 μm and a particle size distribution of 1–40 μm. The component composition of the glaze used is shown in Table 2. The hemp fiber used was a hemp fiber for plaster wall materials (crack prevention) manufactured by Kinki Wall Materials Industry Co., Ltd.

[0034] In each example, the raw materials were mixed in the proportions shown in Table 1, to which a binder, a dispersant, and 3.5% by mass of ion-exchanged water were added, and the mixture was mixed in 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 from the slurry. As a result, a molded body was obtained in the shape of a flat plate (base: 100mm x 100mm, thickness: 15mm) on which two pyramids (base: 47mm x 47mm, height: 70mm) were arranged vertically and horizontally (total of four pyramids). The molded body was removed from the mold and fired under conditions of holding at 1100°C in air for two hours to obtain a radio wave absorber. The obtained molded body was fired under conditions of holding at 1100°C in air for two hours to obtain a radio wave absorber.

[0035] [Table 1]

[0036] [Table 2]

[0037] The radio wave absorption characteristics of the obtained radio wave absorbers were measured using the method described above. The measurement results are shown in Figures 1 to 5. Figure 1 is a graph showing the radio wave absorption characteristics of Nos. 1 to 4 at frequencies of 12.4 to 40 GHz, visualizing the relationship between the glaze content and the radio wave absorption characteristics. Figure 2 is a graph showing the radio wave absorption characteristics of Nos. 1 and 5 to 7 at frequencies of 12.4 to 40 GHz, visualizing the relationship between the petalite content and the radio wave absorption characteristics. Figure 3 is a graph showing the radio wave absorption characteristics of Nos. 1 and 8 to 11 at frequencies of 12.4 to 40 GHz, visualizing the relationship between the feldspar content and the radio wave absorption characteristics. Figure 4 is a graph showing the radio wave absorption characteristics of Nos. 1 and 12 to 14 at frequencies of 12.4 to 40 GHz, visualizing the relationship between the dolomite content 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 No. 1 and Nos. 15 to 17, visualizing the relationship between the perlite content and the radio wave absorption characteristics. In all examples, the temperature during measurement was within the range of 20±10°C, and the humidity was within the range of 30±20%.

[0038] 1 to 5, in the examples in which the raw material composition satisfies the range specified in the present invention, the reflection amount is −20 dB or less, i.e., a return loss of 20 dB or more can be achieved 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 in which the raw material composition does not satisfy the range specified in the present invention, there are frequency bands in the frequency band of 12.4 to 40 GHz in which a return loss of 20 dB or more cannot be achieved. [Industrial Applicability]

[0039] According to the present invention, it is possible to provide a radio wave absorber having excellent radio wave absorption characteristics over a wide band up to 40 GHz, and a method for producing 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, The molded body comprises, in mass %, Ferrite is 70.0% or more and 85.0% or less, Petalite is 1.5% or more and 12.0% or less, Feldspar: 1.5% to 12.0% Pearlite is 1.5% or more and 12.0% or less, Dolomite is 0.0% or more and 8.0% or less, and Glaze: 0.0% to 9.0% A radio wave absorber comprising:

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

3. A step of mixing and molding raw materials including ferrite, petalite, feldspar, and perlite, and optionally further including dolomite and glaze, to obtain a molded body; a step of firing the molded body to obtain a radio wave absorber; and The molded body comprises, in mass %, Ferrite is 70.0% or more and 85.0% or less, Petalite is 1.5% or more and 12.0% or less, Feldspar: 1.5% to 12.0% Pearlite is 1.5% or more and 12.0% or less, Dolomite is 1.5% or more and 8.0% or less, and Glaze: 0.0% to 9.0% A method for manufacturing a radio wave absorber, comprising:

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

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