Radio wave absorber

Abstract

The purpose of the present invention is to achieve high absorption performance in a wide band without increasing the number of layers or the thickness of each layer. A radio wave absorber 1 of the present invention has a magnetic layer 2 having a self-resonant frequency in the millimeter-wave band, and a reflective layer 3, and is characterized in that a plurality of plate-shaped pieces 4 containing a non-metallic magnetic material are arranged in the magnetic layer 2, and the plate-shaped pieces 4 are separated from one another by linear gaps (5a, 5b).

Description

Radio wave absorber

[0001] The present invention relates to a radio wave absorber having radio wave absorption characteristics in the millimeter wave band.

[0002] While the development of automotive autonomous driving technology is accelerating, millimeter wave radars are adopted as in-vehicle sensors. In autonomous driving that requires high safety and reliability, millimeter wave radars of various frequencies are used according to the usage, such as the 24 GHz band for rear millimeter wave radars and the 76.5 GHz band for front millimeter wave radars. In addition, sensors using millimeter wave radars are also being considered for use in the 60 GHz band for applications such as surveillance sensors to prevent people from being left unattended in the vehicle and vital sensors in the medical industry.

[0003] In order to improve the accuracy and prevent malfunction of such millimeter wave radars, there are cases where a radio wave absorber is used as a member that absorbs unnecessary radio wave leakage and reflection. Depending on the type of radar for which the radio wave absorber is used, the frequencies that need to be absorbed may cover multiple bands. For such radar applications, a radio wave absorber that exhibits high absorption performance over a wide band is required.

[0004] For example, Patent Document 1 discloses a laminate in which a reflector layer, a ferrite-containing layer structure having two or more ferrite-containing resin layers, a spacer layer, and a metal pattern layer formed in a geometric pattern are sequentially formed as a thin radio wave absorber having a wide absorption band.

[0005] Further, Patent Document 2 discloses a laminate in which a dielectric layer, a resistance layer is provided on one surface thereof, and a conductive layer is provided on the other surface, and the resistance layer and the conductive layer are thin films formed by sputtering indium tin oxide as a radio wave absorber that exhibits a high radio wave absorption amount over a wide band in the frequency band of 60 to 90 GHz.

[0006] Japanese Patent Application Laid-Open No. 2002-198682 Japanese Patent Application Laid-Open No. 2018-98367

[0007] Conventional radio wave absorbers require many layers, and forming so many layers is costly. In view of these circumstances, the present invention aims to provide a radio wave absorber that can have excellent absorption performance over a wide bandwidth even with a small number of constituent layers.

[0008] To solve the above problems, the present invention provides a radio wave absorber comprising a magnetic layer having a self-resonant frequency in the millimeter-wave band and a reflective layer, wherein the magnetic layer has a plurality of plate-like pieces containing a nonmetallic magnetic material and gaps provided between the plate-like pieces.

[0009] This invention can achieve high absorption performance over a wide bandwidth without increasing the number of layers constituting the radio wave absorber or the thickness of each layer, thus providing a thin, high-performance radio wave absorber.

[0010] This is a cross-sectional view showing an example of the radio wave absorber of the present invention. This is a plan view showing an example of the radio wave absorber of the present invention. This is a plan view showing another example of the arrangement of plate-shaped pieces. This is a cross-sectional view showing yet another example of the radio wave absorber of the present invention. This is a graph showing the radio wave absorption characteristics in the embodiment.

[0011] The radio wave absorber according to the present invention has a magnetic layer and a reflective layer. Figure 1 is a cross-sectional view showing a radio wave absorber 1, which is one embodiment of the radio wave absorber according to the present invention. The radio wave absorber 1 comprises a magnetic layer 2 and a reflective layer 3 laminated on the magnetic layer 2.

[0012] The magnetic layer 2 has a self-resonant frequency (SRF) in the millimeter-wave band (which may also be called the "self-resonance frequency") and comprises a plurality of plate-like pieces 4 and gaps 5 provided between the plate-like pieces 4. In other words, the magnetic layer 2 has a plurality of plate-like pieces 4 arranged at intervals on the same plane. To put it another way, the magnetic layer 2 has a plurality of regions (corresponding to the "plate-like pieces 4") that are separated from each other by gaps 5.

[0013] Figure 2 is a plan view of the radio wave absorber 1. For the sake of explanation, in Figure 2, the plate-shaped individual pieces 4 are represented by the same hatching as in Figure 1.

[0014] Furthermore, the plate-shaped individual pieces 4 only need to have an area capable of exhibiting radio wave absorption performance, and their shape is not limited. As shown in Figure 2, plate-shaped individual pieces 4 of the same shape may be arranged in a continuous line, or plate-shaped individual pieces 4 of different shapes may be combined. Also, the magnetic layer 2 may be a combination of plate-shaped individual pieces of different shapes, and their arrangement may be regular or irregular.

[0015] In the configuration shown in Figure 2, identical, particularly square, plate-like pieces 4 are arranged in both the vertical and horizontal directions. That is, between the plate-like pieces 4, there are multiple parallel, linear gaps 5a (referred to as "horizontal gaps" for convenience) and multiple linear gaps 5b (referred to as "vertical gaps" for convenience) perpendicular to the gaps 5a.

[0016] Figures 3(a) to 3(e) show other examples of the arrangement of plate-shaped pieces in the planar direction of the reflective layer 3. The configuration in Figure 3(a) has only parallel vertical gaps 51 as gaps. That is, multiple rectangular plate-shaped pieces 4 are arranged parallel to each other. The configuration in Figure 3(b) has horizontal gaps 52a and vertical gaps 52b, as well as diagonal gaps 52c. That is, triangular plate-shaped pieces 42 are provided, obtained by dividing the plate-shaped piece 4 of Figure 2 in half along the diagonal. The configuration in Figure 3(c) has multiple circular plate-shaped pieces 4 arranged with gaps 53 between them. In the configuration in Figure 3(d), a straight horizontal gap 52a and a bent vertical gap 52b are provided between the plate-shaped pieces 44. The configuration in Figure 3(e) includes rectangular plate-shaped pieces 45a arranged in the same way as the plate-shaped pieces 4 in Figure 2, a linear horizontal gap 55a, a vertical gap 55b, and a plate-shaped piece 45b surrounding the outside.

[0017] In the configuration shown in Figure 2, as an example of a structure having multiple plate-like pieces and gaps provided between the plate-like pieces, the plate-like pieces 4 in the magnetic layer 2 are separated from each other by multiple linear gaps 5. However, the magnetic layer only needs to have two or more plate-like pieces, and there may be only one gap.

[0018] The magnetic layer preferably has two or more gaps. Furthermore, it is preferable that the gaps are linear, and that two or more linear gaps intersect.

[0019] For example, as shown in Figure 2, there is a case where a linear gap 5a separating the plate-shaped individual pieces 4 in the vertical direction intersects with a linear gap 5b separating the plate-shaped individual pieces 4 in the horizontal direction. The linear gaps may be perpendicular to each other or oblique to each other.

[0020] "Linear" can refer to a straight line, a curved line, a broken line, or any combination thereof.

[0021] Furthermore, it is preferable that the width of the "linear" gaps be approximately constant. "Approximately constant" means that the amount of change in width is 20% or less, or 10% or less.

[0022] Furthermore, it is preferable that the gap extends continuously from one side of the magnetic layer to the other side (Figures 2, 3(a) to (d)).

[0023] The radio wave absorber may be provided and used in a size required for the object to be installed, or it may be provided in an area larger than the required size and cut to the desired size. Preferably, the installed radio wave absorber contains two or more linear gaps. Furthermore, preferably, the installed radio wave absorber contains points where the linear gaps intersect.

[0024] Furthermore, the radio wave absorber preferably has a line symmetry in part, or more preferably in its entirety, in the arrangement of its plate-like pieces, i.e., in at least part of the shape of the gaps. The direction of the axis of symmetry is not limited.

[0025] The width of the linear gap 5 is preferably designed according to the self-resonant frequency of the magnetic layer 2 (plate-shaped individual pieces 4). The inventors believe that when the width of the linear gap is 1.5 to 2.5 times the wavelength corresponding to the self-resonant frequency, it exhibits higher absorption performance over a wider bandwidth. The gap width is more preferably 1.8 to 2.2 times, and even more preferably 1.9 to 2.1 times.

[0026] The plate-shaped individual pieces 4 constituting the magnetic layer 2 include a material having a self-resonant frequency in the millimeter-wave band. The material having a self-resonant frequency in the millimeter-wave band is preferably a non-metallic magnetic material. Examples of non-metallic magnetic materials include ferromagnetic iron oxide-based materials such as magnetoplanbite-type ferrite, spinel ferrite, and ε-iron oxide.

[0027] In particular, ferromagnetic materials exhibit high absorption performance at specific self-resonant frequencies, determined by their shape, crystal structure, and atomic arrangement, which contribute to their magnetic anisotropy. However, it is not easy to broaden the bandwidth of a radio wave absorber using a single material. For example, increasing the thickness of a magnetic layer made of a single material improves absorption performance, but does not significantly broaden the bandwidth. Conventionally, broadening the bandwidth of radio wave absorbers using ferromagnetic materials has been achieved by using ferromagnetic materials with different compositions or by using ferromagnetic materials in combination with other materials. In contrast, by having plate-like fragments and gaps in the magnetic layer, high absorption performance over a wide bandwidth can be achieved without increasing the number of layers or the thickness of each layer. Therefore, a thin, high-performance radio wave absorber can be provided.

[0028] The reason why the above effects are obtained by arranging the plate-like pieces at intervals can be considered as follows, but the present invention is not limited thereto. Compared to the case where layers made of magnetic material are continuous without gaps, the presence of gaps within the magnetic layer causes changes in the resonance conditions and / or impedance matching. Such changes alter the electromagnetic field distribution within the magnetic layer, resulting in improved radio wave absorption performance over a wide bandwidth.

[0029] The gap may be an air void, or a material with material constants similar to those of air may be present within the gap.

[0030] Furthermore, the radio wave absorber may contain multiple types of materials having a self-resonant frequency in the millimeter-wave band so that it has absorption characteristics in different frequency bands. In this case, multiple materials may be contained in a single layer, or multiple layers using different materials may be laminated together.

[0031] The plate-shaped pieces 4 in the magnetic layer are preferably sintered bodies of a non-metallic magnetic material. If they are sintered bodies, the density of the non-metallic magnetic material can be increased compared to a molded body obtained by kneading a binder with powder of a non-metallic magnetic material. Therefore, a radio wave absorber in which the plate-shaped pieces are sintered bodies can have better radio wave absorption characteristics in the millimeter-wave band.

[0032] Magnetoplanbite-type ferrites are particularly preferred as nonmetallic magnetic materials because they possess high crystalline magnetic anisotropy and can exhibit magnetic resonance in the millimeter-wave band. In magnetoplanbite-type ferrites, it is preferable that a portion of the Fe is substituted with one or more elements from among Ti, Mn, Zn, Co, Ni, Cu, Sn, Zr, Al, etc. When a portion of the Fe is substituted with these elements, the crystalline magnetic anisotropy and saturation magnetization change, allowing the self-resonant frequency to be controlled.

[0033] Specifically, the magnetoplumbite-type ferrite is preferably barium ferrite or strontium ferrite. Barium ferrite has a self-resonant frequency in the band below 40 GHz, while strontium ferrite has a self-resonant frequency in the band above 60 GHz. Furthermore, by substituting some of the constituent elements in these ferrites with the various elements mentioned above, the frequency band exhibiting the self-resonant frequency can be shifted.

[0034] The reflective layer 3 is preferably made of a conductive material. Examples of conductive materials include metals, such as aluminum and copper. The conductivity of the reflective layer 3 is 1 × 10⁻⁶. ３ It is preferable that the ratio is s / m or higher.

[0035] Furthermore, the radio wave absorber may be provided with a protective layer on one or both sides. The protective layer may be an adhesive layer for attaching it to an article or the like. The adhesive layer is provided so that the radio wave absorber can be installed in accordance with the direction in which the radio waves are incident. For example, as shown in Figure 4, the adhesive layer 7, reflective layer 3, magnetic layer 2, and protective layer 6 may be stacked in this order. Alternatively, the positions of the adhesive layer 7 and the protective layer 6 may be swapped.

[0036] Such radio wave absorbers can be manufactured, for example, as follows:

[0037] The plate-shaped pieces constituting the magnetic layer can be manufactured by known methods. For example, a plate-shaped magnetic material can be obtained as a molded body formed by molding a mixture of a non-metallic magnetic material powder and a binder resin into a plate shape. Alternatively, a plate-shaped magnetic material can be obtained by molding a mixture of magnetic powder and a binder resin into a plate shape and then sintering it. The plate-shaped magnetic material can be cut into plate-shaped pieces of desired shape and size by methods such as laser processing, if necessary.

[0038] As the reflective layer, metal plates, metal foils, metal vapor-deposited films, metal-plated substrates, etc., can be used.

[0039] To laminate a magnetic layer and a reflective layer, for example, a magnetic layer can be laminated on top of the reflective layer, with plate-shaped pieces pre-arranged on the substrate via an adhesive. Alternatively, the magnetic layer can be formed by directly arranging plate-shaped pieces on top of the reflective layer via an adhesive.

[0040] Another method for forming a magnetic layer is to remove the magnetic material from a plate-shaped magnetic material attached to a substrate or reflective layer, specifically the portion where a linear gap is to be created.

[0041] The protective layer is formed, for example, by applying a resin to the magnetic layer or reflective layer, or by attaching a resin sheet to the magnetic layer or reflective layer with an adhesive. The adhesive may be either a double-sided tape or a liquid adhesive. The adhesive layer is provided on the outermost surface of the radio wave absorber, for example, by applying a resin that will serve as the adhesive layer or by attaching double-sided tape.

[0042] The present disclosure includes the following technologies. (1) A radio wave absorber including a magnetic layer having a self-resonant frequency in the millimeter wave band and a reflective layer, wherein the magnetic layer has a plurality of plate-like pieces containing a non-metallic magnetic material and gaps provided between the plate-like pieces. (2) The radio wave absorber according to (1), wherein the non-metallic magnetic material is an iron oxide-based material. (3) The radio wave absorber according to (1) or (2), wherein the non-metallic magnetic material is a magnetoplumbite-type ferrite. (4) The radio wave absorber according to any one of (1) to (3), wherein the plate-like pieces containing the non-metallic magnetic material are sintered bodies. (5) The radio wave absorber according to any one of (1) to (4), wherein two or more gaps are provided. (6) The radio wave absorber according to any one of (1) to (5), wherein the width of the gap is 2 ± 0.5 times the wavelength corresponding to the self-resonant frequency. (7) The radio wave absorber according to any one of (1) to (6), wherein the conductivity of the reflective layer is 1 × 10 ３ s / m or more.

[0043] <Radio wave absorbing material> The material having a self-resonant frequency in the millimeter wave band used in this example and the comparative example is a magnetoplumbite-type Al-substituted strontium ferrite (hereinafter, may be simply referred to as "ferrite").

[0044] <Sample preparation> [Example 1] (1) Formation of plate-like pieces Ferrite was sintered by a known method to obtain a plate-like magnetic body. The plate-like magnetic body was 125 mm square and had a thickness of 600 μm. The plate-like magnetic body was cut by a laser to obtain plate-like pieces of 24 mm square.

[0045] (2) Formation of the magnetic layer Twenty-five plate-like pieces were attached to a base material PET with a thickness of 100 μm via an adhesive layer to obtain a magnetic layer. The plate-like pieces were regularly arranged in 5 rows vertically and 5 rows horizontally, separated by linear gaps with a width of 5 mm, and the linear gaps were arranged so as to form a lattice.

[0046] (3) Reflective layer A 2-mm-thick aluminum metal plate was used as the reflective layer.

[0047] (4) Production of the radio wave absorber The reflective layer and the magnetic layer were laminated by adhering the base material PET and the reflective layer via an adhesive layer to obtain a radio wave absorber.

[0048] [Example 2] An electromagnetic wave absorber was obtained in the same manner as in Example 1, except that the width of the linear gaps in the magnetic layer was set to 8 mm.

[0049] [Example 3] An electromagnetic wave absorber was obtained in the same manner as in Example 1, except that the width of the linear gaps in the magnetic layer was set to 10 mm.

[0050] [Comparative Example] The width of the linear gaps in the magnetic layer was 0 mm, that is, 25 plate-like pieces were arranged in a grid of 5 vertically and 5 horizontally without any gaps to obtain a radio wave absorber.

[0051] <Measurement of Self-Resonant Frequency> The self-resonant frequency of the sintered ferrite plate-shaped magnetic material was measured as follows. Using the free-space method, the S-parameters of the plate-shaped magnetic material were measured using a vector network analyzer (Anritsu ME7838A), and the self-resonant frequency was determined from the S11 peak. The self-resonant frequency of the plate-shaped magnetic material used in this embodiment was 74.5 GHz. The wavelength corresponding to the self-resonant frequency was calculated as the wavelength in air, based on the speed of light (3 × 10⁻¹⁰). ８ The wavelength was calculated by dividing the (m / s) value by the self-resonant frequency. In other words, the wavelength corresponding to the self-resonant frequency of the plate-shaped magnetic material used in this embodiment was approximately 4.0 mm.

[0052] <Measurement of Conductivity> The conductivity of the reflective layer was determined by calculating the resistivity using a four-terminal method with a low resistivity meter (Loresta GP MCP-T610 manufactured by Nitto Seiko Analytech Co., Ltd.) and then converting it to conductivity.

[0053] <Measurement of Absorption Characteristics> The absorption characteristics of the radio wave absorber are expressed as S11, which is the S-parameter of the radio wave absorber measured using the free-space method with a vector network analyzer (Anritsu ME7838A). The absorption characteristics of each sample are shown in Figure 5.

[0054] Compared to the radio wave absorber shown in the comparative example, in which the magnetic layer is not separated by linear gaps, the radio wave absorbers of Examples 1 to 3 exhibit superior radio wave absorption characteristics near the self-resonant frequency of the magnetic layer material used. In particular, Example 2, in which the linear gaps were designed to be approximately twice the wavelength corresponding to the self-resonant frequency, showed higher absorption over a wider bandwidth near the self-resonant frequency.

[0055] This invention can achieve high absorption performance over a wide bandwidth without increasing the thickness of the constituent layers or each layer of the radio wave absorber, and can provide a low-cost, thin, and high-performance radio wave absorber.

[0056] 1. Radio wave absorber 2. Magnetic layer 3. Reflective layer 4. Plate-shaped individual pieces 5. Gaps between 5, 5a, and 5b 6. Protective layer 7. Adhesive layer

Claims

1. A radio wave absorber comprising a magnetic layer having a self-resonant frequency in the millimeter-wave band and a reflective layer, wherein the magnetic layer comprises a plurality of plate-like pieces containing a nonmetallic magnetic material and gaps provided between the plate-like pieces.

2. The radio wave absorber according to claim 1, wherein the nonmetallic magnetic material is an iron oxide-based material.

3. The radio wave absorber according to claim 1, wherein the nonmetallic magnetic material is a magnetoplanbite-type ferrite.

4. The radio wave absorber according to claim 1, wherein the plate-shaped individual pieces containing the nonmetallic magnetic material are sintered bodies.

5. The radio wave absorber according to claim 1, wherein two or more of the aforementioned gaps are provided.

6. The radio wave absorber according to claim 1, wherein the width of the gap is 1.5 to 2.5 times the wavelength corresponding to the self-resonant frequency.

7. The conductivity of the reflective layer is 1 × 10 ３ The radio wave absorber according to claim 1, wherein the frequency is s / m or higher.

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