Electromagnetic wave absorber

Abstract

Provided is an electromagnetic wave absorber capable of effectively absorbing an electromagnetic wave of a high frequency of a millimeter wave band or above, and capable of effectively absorbing an electromagnetic wave of a desired frequency even when a surrounding temperature change occurs. The electromagnetic wave absorber comprises, in a binder 1b, a magnetoplumbite-type hexagonal ferrite 1a that magnetically resonates in a frequency band of a millimeter wave band. In the magnetoplumbite-type hexagonal ferrite, some of metal sites are replaced with La. In a graph showing the relationship between the frequency of an incoming electromagnetic wave and the value of the imaginary part of the complex relative permeability of the electromagnetic wave absorber, when the temperature of the electromagnetic wave absorber changes from 25°C to 105° C, the value of the frequency at which a positive peak occurs shows a positive rate of change, and the ratio of the amount of change in the frequency at which a positive peak occurs when the temperature changes from 25°C to 105° C to the value of the frequency at which a positive peak occurs at 25°C is 1.5% or less.

Description

[Technical Field]

[0001] This disclosure relates to an electromagnetic wave absorber that absorbs electromagnetic waves, and more particularly to an electromagnetic wave absorber that suppresses fluctuations in the frequency of absorbed electromagnetic waves due to temperature changes and can effectively absorb electromagnetic waves of a desired frequency even in environments with temperature fluctuations. [Background technology]

[0002] Electromagnetic wave absorbing compositions are used to avoid the effects of leaked electromagnetic waves emitted to the outside from electrical circuits and other sources, as well as unwanted reflected electromagnetic waves. These electromagnetic wave absorbing compositions are molded into predetermined shapes as electromagnetic wave absorbing members such as block-shaped electromagnetic wave absorbers and sheet-shaped electromagnetic wave absorbing sheets, and are also known to be used as electromagnetic wave absorbing coatings that are applied to desired locations.

[0003] In recent years, research has progressed on technologies that utilize electromagnetic waves in the centimeter-wave to millimeter-wave bands, and even higher frequency bands beyond the millimeter-wave band, for use in mobile communications such as mobile phones, wireless LANs, and automated toll collection systems (ETC). In response to this trend of utilizing higher frequency electromagnetic waves, there is a growing demand for electromagnetic wave absorbers that can absorb electromagnetic waves in the millimeter-wave band, approximately 30 GHz to 300 GHz, or even higher.

[0004] As an electromagnetic wave absorber (sheet) that absorbs electromagnetic waves in high frequency bands above the millimeter wave band, a material has been proposed that contains epsilon iron oxide and hexagonal ferrite in a resin binder in order to suppress changes in the frequency of absorbed electromagnetic waves when the ambient temperature changes (see Patent Document 1). [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2019-145534 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] The electromagnetic wave absorbing sheet described in Patent Document 1 contains, in its binder, hexagonal ferrite whose coercivity increases and the peak frequency of absorbed electromagnetic waves shifts to a higher value when the ambient temperature rises, and epsilon iron oxide whose coercivity decreases and the frequency of absorbed electromagnetic waves changes to a lower value when the ambient temperature rises. As a result, the effects of the frequency shift of absorbed electromagnetic waves when the ambient temperature changes cancel each other out, allowing the sheet to exhibit good electromagnetic wave absorption characteristics over a wide temperature range.

[0007] However, the conventional electromagnetic wave absorbing sheets described above have several challenges in realizing electromagnetic wave absorbing sheets at a low cost. These include the need for epsilon iron oxide particles in addition to hexagonal ferrite, which increases material costs, and the need to uniformly mix and disperse the relatively large, flat hexagonal ferrite particles and the spherical epsilon iron oxide particles with an average particle size of 50 nm within the binder.

[0008] This disclosure aims to solve the conventional problems by providing an electromagnetic wave absorber that can effectively absorb electromagnetic waves of high frequencies above the millimeter wave band, and can also effectively absorb electromagnetic waves of a desired frequency even when ambient temperature changes occur. [Means for solving the problem]

[0009] In order to solve the above problems, the electromagnetic wave absorber disclosed in the present application is an electromagnetic wave absorber including a magnetoplumbite-type hexagonal ferrite that resonates magnetically in the millimeter-wave band frequency range in a binder, wherein a part of the metal sites of the magnetoplumbite-type hexagonal ferrite is substituted with La, and in a graph showing the relationship between the frequency of incident electromagnetic waves and the value of the imaginary part of the complex relative permeability of the electromagnetic wave absorber, when the temperature of the electromagnetic wave absorber changes from 25°C to 105°C, the value of the frequency at which a positive peak occurs shows a positive rate of change, and the ratio of the change amount of the frequency at which a positive peak occurs when changing from 25°C to 105°C to the value of the frequency at which a positive peak occurs at 25°C is 1.5% or less.

Effect of the Invention

[0010] By having the above configuration, the electromagnetic wave absorber disclosed in the present application has a small change in electromagnetic wave absorption characteristics due to changes in ambient temperature, and can well absorb electromagnetic waves of a desired frequency even in an environment where a temperature change occurs.

Brief Description of the Drawings

[0011] [Figure 1] It is a schematic diagram for explaining the schematic configuration of the electromagnetic wave absorption sheet according to the present embodiment. [Figure 2] It is a diagram showing the relationship between the frequency of electromagnetic waves of the electromagnetic wave absorption sheet of Example 2 and the value of the imaginary part of the complex relative permeability of the electromagnetic wave absorber. [Figure 3] It is a diagram showing the relationship between the frequency of electromagnetic waves of the electromagnetic wave absorption sheet of Example 2 and the transmission attenuation amount per unit thickness. [Figure 4] It is a diagram showing the relationship between the frequency of electromagnetic waves of the electromagnetic wave absorption sheet of Comparative Example 1 and the value of the imaginary part of the complex relative permeability of the electromagnetic wave absorber. [Figure 5] It is a diagram showing the relationship between the frequency of electromagnetic waves of the electromagnetic wave absorption sheet of Comparative Example 1 and the transmission attenuation amount per unit thickness. [Figure 6] It is a diagram showing the relationship between the change amount of the value of the imaginary part of the complex relative permeability and the increase amount of the thickness required to ensure the required electromagnetic wave absorption amount at 105°C. [Modes for carrying out the invention]

[0012] The electromagnetic wave absorber disclosed herein is an electromagnetic wave absorber containing a magnetoplanbite-type hexagonal ferrite that magnetically resonates in the millimeter-wave frequency band within a binder, wherein a portion of the metal sites in the magnetoplanbite-type hexagonal ferrite are replaced with La, and in a graph showing the relationship between the frequency of the incident electromagnetic wave and the value of the imaginary part of the complex relative permeability of the electromagnetic wave absorber, when the temperature of the electromagnetic wave absorber changes from 25°C to 105°C, the value of the frequency at which it has a positive peak shows a positive rate of change, and the ratio of the change in the frequency at which it has a positive peak when the temperature changes from 25°C to 105°C to the value of the frequency at which it has a positive peak at 25°C is 1.5% or less.

[0013] Here, "the ratio of the change in the frequency of the positive peak when the temperature changes from 25°C to 105°C to the value of the frequency of the positive peak at 25°C" means that the frequency of the positive peak at 25°C is f 25 The value of the imaginary part of the complex relative permeability at that time is μ'' 25 Furthermore, the frequency at which the positive peak occurs at 105°C is f 105 The value of the imaginary part of the complex relative permeability at that time is μ'' 105 When these conditions are met, the following equation (1) (f 105 -f 25 ) / f 25 ×100 (1) It is a numerical value that can be expressed as follows.

[0014] Furthermore, the above-mentioned metal site includes at least one of barium (Ba), strontium (Sr), and calcium (Ca).

[0015] In this manner, the electromagnetic wave absorber disclosed in this application exhibits minimal change in electromagnetic wave absorption characteristics due to changes in ambient temperature, and can effectively absorb electromagnetic waves of a desired frequency even when the ambient temperature changes.

[0016] In the electromagnetic wave absorber disclosed in the present application, in a graph showing the relationship between the frequency of the incident electromagnetic wave and the value of the imaginary part of the complex relative permeability of the electromagnetic wave absorber, the value of the imaginary part of the complex relative permeability at the positive peak shows a negative change rate when the temperature of the electromagnetic wave absorber changes from 25°C to 105°C, and the ratio of the change amount of the value of the imaginary part of the complex relative permeability at the positive peak when changing from 25°C to 105°C to the value of the imaginary part of the complex relative permeability at the positive peak at 25°C is preferably 15% or less.

[0017] Note that the "ratio of the change amount of the value of the imaginary part of the complex relative permeability at the positive peak when changing from 25°C to 105°C to the value of the imaginary part of the complex relative permeability at the positive peak at 25°C" means that the frequency at the positive peak at 25°C is f 25 and the value of the imaginary part of the complex relative permeability at that time is μ'' 25 also, the frequency at the positive peak at 105°C is f 105 and the value of the imaginary part of the complex relative permeability at that time is μ'' 105 respectively, and when the following formula (2) (μ'' 105 -μ'' 25 ) / μ'' 25 ×100 (2) represents the numerical value.

[0018] By doing so, the change in the value of the imaginary part of the complex relative permeability due to temperature change can be reduced, so that electromagnetic waves of a desired frequency can be well absorbed even when the ambient temperature changes.

[0019] Also, the substitution amount (x) of La in the metal site of the magnetoplumbite-type hexagonal ferrite is preferably 0.1 or more and 0.57 or less. By doing so, the effect of reducing the change amount of the electromagnetic wave absorption characteristics with respect to temperature change due to La substitution can be utilized.

[0020] Furthermore, when the temperature of the electromagnetic wave absorber is 25°C, it is preferable that the absolute value of the ratio of the change in the value of the imaginary part of the complex relative permeability when the temperature of the electromagnetic wave absorber changes from 25°C to 105°C to the value of the imaginary part of the complex relative permeability at 25°C is 14% or less at the frequency at which there is a positive peak in the graph showing the relationship between the frequency of the incident electromagnetic wave and the value of the imaginary part of the complex relative permeability of the electromagnetic wave absorber.

[0021] Here, "the absolute value of the ratio of the change in the complex relative permeability value when the temperature of the electromagnetic wave absorber changes from 25°C to 105°C, relative to the value of the complex relative permeability imaginary part at 25°C, at the frequency at which there is a positive peak in the graph showing the relationship between the frequency of the incident electromagnetic wave and the value of the imaginary part of the complex relative permeability of the electromagnetic wave absorber when the temperature of the electromagnetic wave absorber changes from 25°C to 105°C" means that the frequency at which there is a positive peak at 25°C is f 25 The value of the imaginary part of the complex relative permeability at that time is μ'' 25 Also, the frequency f of the imaginary part of the complex relative permeability at 105°C 25 The value in μ'' 105(f25) When these conditions are met, the following equation (3) (μ'' 105(f25) -μ'' 25 ) / μ'' 25 (Absolute value) × 100 (3) It is a numerical value that can be expressed as follows.

[0022] By doing so, when the temperature changes from 25°C to 105°C, the decrease in the imaginary part of the complex relative permeability at the peak frequency at 25°C is reduced, and the change in electromagnetic wave absorption characteristics due to temperature is minimized. In addition, the rate of increase in thickness when improving electromagnetic wave absorption characteristics by increasing the thickness of the electromagnetic wave absorber can be reduced.

[0023] Furthermore, it is preferable that, in the graph showing the relationship between the frequency of the incident electromagnetic wave and the value of the imaginary part of the complex relative permeability of the electromagnetic wave absorber, both the full width at half maximum (FMAX) when the temperature of the electromagnetic wave absorber is 25°C and the FMAX when the temperature of the electromagnetic wave absorber is 105°C are 8.5 GHz or higher. By doing so, even when used as an electromagnetic wave absorber in a device related to a 79 GHz automotive millimeter-wave radar with a wide frequency bandwidth of 4 GHz, a practically sufficient electromagnetic wave absorption frequency band can be secured.

[0024] Furthermore, it is preferable that the frequency at which the positive peak occurs in the graph showing the relationship between the frequency of the incident electromagnetic wave and the value of the imaginary part of the complex relative permeability of the electromagnetic wave absorber is between 75 GHz and 80 GHz. This allows the electromagnetic wave absorber to be suitably used in automotive radar systems, a field where the market is expanding as a practical application.

[0025] As the binder for the electromagnetic wave absorber, at least one of rubber, plastic, or thermoplastic elastomer can be suitably used.

[0026] Furthermore, it is preferable that the electromagnetic wave absorber further contains conductive carbon black as a dielectric constant adjusting agent, or at least one of the group consisting of furnace conductive carbon black, acetylene black, and Ketjen black.

[0027] Furthermore, it is preferable that the electromagnetic wave absorber includes at least an electromagnetic wave absorbing layer and a layer having an electromagnetic wave surface reflection suppression function, and is in the form of a sheet with an overall thickness of 2 mm or less, and moreover, that the overall thickness of the electromagnetic wave absorber is 1 mm or less.

[0028] Furthermore, it is preferable that the content of the magnetoprumbite-type hexagonal ferrite in the electromagnetic wave absorber is 89% by mass or less.

[0029] Furthermore, in the AEC (Automotive Electronics Council) standards, which are the standards body for the reliability of automotive electronic components, the upper limit of the operating temperature range is set at 125°C in Grade 1 of the AEC-Q101 (integrated circuits) category. Therefore, in this application, we have specified the absorption characteristics of the electromagnetic wave absorber in the temperature range from room temperature of 25°C to the upper limit of the operating temperature range of 125°C. In other words, a small change in electromagnetic wave absorption characteristics even when the temperature changes from 25°C to 105°C is a guideline for improving the reliability of automotive electronic components.

[0030] The electromagnetic wave absorber disclosed in this application will be described below using an electromagnetic wave absorbing sheet, which is formed in a sheet shape with a thin thickness relative to its area, as an example.

[0031] (Embodiment) [Electromagnetic wave absorbing sheet] Figure 1 is a cross-sectional view illustrating the configuration of the electromagnetic wave absorbing sheet 1 according to this embodiment.

[0032] Note that Figure 1 is a diagram provided to facilitate understanding of the configuration of the electromagnetic wave absorbing sheet according to this embodiment, and the size and thickness of the components shown in the figure are not necessarily accurate representations of reality.

[0033] The electromagnetic wave absorbing sheet 1 illustrated in this embodiment includes a magnetoprumbite-type hexagonal ferrite 1a and a resin binder 1b. In the electromagnetic wave absorbing sheet according to this embodiment, the magnetoprumbite-type hexagonal ferrite 1a has some of the metal sites, such as strontium (Sr) and barium (Ba), replaced with lanthanum (La).

[0034] Although shown as included in binder 1b in Figure 1, it contains a dispersant for effectively dispersing magnetoplumbite-type hexagonal ferrite 1a in the resin binder 1b.

[0035] [Magnetoplumbite-type hexagonal ferrite] The electromagnetic wave absorbing sheet 1 according to this embodiment contains a binder containing a powder of magnetoprumbite-type hexagonal ferrite 1a in which some of the metal sites are replaced with lanthanum (La).

[0036] Magnetoplumbite-type hexagonal ferrite 1a, in which some of the metal sites are replaced by lanthanum and iron (Fe) sites are replaced by aluminum (Al), has the general formula: A (1-x) La x Fe (n-y) Al y Represented as such, A, which indicates a metallic site, is one or more selected from the group consisting of Sr (strontium), Ba (barium), and Ca (calcium), and satisfies the requirements 0.01 ≤ x ≤ 0.57, 1.00 ≤ y ≤ 2.20, and 11.00 ≤ n ≤ 12.50.

[0037] Here, the ratios shown in the general formula for the atomic ratio of the metal elements refer to the atomic ratio of metal atoms contained in the single-phase crystal structure of the magnetoplumbite-type hexagonal ferrite magnetic powder, where each metal element is substituted into the A and Fe sites, which represent metal sites as they constitute the magnetoplumbite-type crystal structure. It is known that if the metal elements mixed as raw materials during metal substitution do not substitute into the sites of the magnetoplumbite-type hexagonal ferrite but remain outside in a different form, the resulting magnetic powder mixture will have a frequency far outside the desired absorption frequency range.

[0038] Magnetoplumbite-type hexagonal ferrite 1a, in which some of the metal sites are substituted with La, is preferred because it exhibits small changes in electromagnetic wave absorption properties due to temperature changes. In particular, it can suppress changes in the frequency of the electromagnetic wave absorption peak due to temperature changes. To achieve the above effect of being less susceptible to temperature changes due to La substitution, the amount of La substitution x shown in the general formula above is more preferably 0.1 or more and 0.57 or less.

[0039] In this embodiment, the content of magnetoplumbite-type hexagonal ferrite 1a in the electromagnetic wave absorbing sheet is preferably 89% by mass or less, and more preferably 85.2% by mass or less, relative to the total amount including the binder 1b and carbon black as a dielectric constant adjuster described later. When the content of magnetic material increases, the change in transmission attenuation, which indicates the rate at which electromagnetic waves are absorbed when a temperature change occurs, increases. By setting the mass content of magnetoplumbite-type hexagonal ferrite 1a to 89% by mass or less, when the temperature of the electromagnetic wave absorbing sheet 1 changes from 25°C to 105°C, the rate of change in transmission attenuation for electromagnetic waves at the frequency at which absorption peaked at 25°C can be set to 6.5% or less, making it possible to create an electromagnetic wave absorbing sheet 1 with a small decrease in transmission attenuation due to temperature changes.

[0040] [binder] In the electromagnetic wave absorbing sheet 1 according to this embodiment, it is preferable to use at least one selected from rubber, plastic, and thermoplastic elastomer as the resin binder 1b.

[0041] Rubber binder Examples of suitable rubbers include natural rubber (NR), isoprene rubber (IR), butadiene rubber (BR), styrene-butadiene rubber (SBR), butyl rubber (IIR), nitrile rubber (NBR), ethylene-propylene rubber (EPDM), chloroprene rubber (CR), acrylic rubber (ACM), chlorosulfonated polyethylene rubber (CSR), urethane rubber (PUR), silicone rubber (Q), fluororubber (FKM), ethylene-vinyl acetate rubber (EVA), epichlorohydrin rubber (CO), polysulfide rubber (T), and urethane rubber (U).

[0042] As plastics, thermosetting resins and thermoplastic resins can be used. Examples of thermosetting resins include phenolic resins, urea resins, melamine resins, epoxy resins, unsaturated polyester resins, alkyd resins, silicone resins, and polyurethanes. Examples of thermoplastic resins include polyethylene, polypropylene, polystyrene, ABS resin, methyl methacrylate resin, polyvinyl chloride, polyamide, polyethylene terephthalate, polybutylene terephthalate, and polycarbonate.

[0043] Examples of thermoplastic elastomers that can be used include styrene-based thermoplastic elastomers (SIS, styrene-isoprene copolymer, SBS, styrene-butadiene copolymer), olefin-based thermoplastic elastomers, urethane-based thermoplastic elastomers, and polyester-based thermoplastic elastomers.

[0044] [Other materials] The electromagnetic wave absorbing sheet 1 according to this embodiment may include components other than magnetoprumbite-type hexagonal ferrite 1a and binder 1b.

[0045] By including a dielectric constant adjusting agent in the electromagnetic wave absorbing sheet 1, it becomes easy to set the dielectric constant of the electromagnetic wave absorbing sheet 1 to a desired value. Increasing the dielectric constant of the electromagnetic wave absorbing sheet 1 improves the electromagnetic wave absorption performance due to the dielectric loss effect. Furthermore, by adjusting the dielectric constant, it is possible to match it with the characteristic impedance and suppress the reflection of electromagnetic waves at any thickness.

[0046] As dielectric constant modifiers, carbon materials such as carbon black, carbon nanotubes, and carbon nanostructures, as well as barium titanate, conductive polymers, and metal wires can be used. In particular, carbon black has two advantages: firstly, it is less prone to electrical anisotropy in the TD / MD direction after sheet molding, and secondly, Rubber binder It is preferable as a dielectric constant modifier because it functions as a reinforcing material for plastics, and thirdly because it is inexpensive and readily available. Dielectric constant modifier If the shape of the dielectric constant modifier exhibits anisotropy, it will cause anisotropy in which the electromagnetic wave absorption characteristics change depending on the direction in which the electromagnetic wave is incident on the electromagnetic wave absorption sheet. Therefore, it is preferable to use granular carbon black as the dielectric constant modifier rather than using needle-shaped or rod-shaped materials such as carbon nanotubes. More specifically, various conductive carbon blacks such as furnace conductive carbon black, acetylene black, and Ketjen black can be suitably used.

[0047] As granular carbon black, the specific surface area is 30-2300 m². 2 It can be used if it is 300-2000 m² in size. 2 Preferably, it should be a material with a specific surface area of ​​800-1800 m². 2 It is preferable to use the one that is / g.

[0048] When carbon black is included as a dielectric constant adjuster, its content is preferably less than 4.5% of the total mass of the electromagnetic wave absorbing sheet, and more preferably 3.0% or less. If carbon black is included at 4.5% or more, the change in the peak frequency of absorbed electromagnetic waves due to temperature changes in the electromagnetic wave absorbing sheet 1 becomes large, and the rate of change in the frequency at which the transmission attenuation peaks when the temperature of the electromagnetic wave absorbing sheet 1 changes from 25°C to 105°C becomes greater than 1.5%, making it difficult to realize an electromagnetic wave absorbing sheet in which the change in the transmission attenuation rate at a predetermined frequency is small with respect to temperature changes.

[0049] Furthermore, the electromagnetic wave absorbing sheet 1 may contain various dispersants. As dispersants, compounds having polar groups such as phosphate groups, sulfonic acid groups, and carboxyl groups can be used.

[0050] Phosphate compounds having a phosphate group can include arylphosphonic acids such as phenylphosphonic acid and phenylphosphonic acid dichloride, alkylphosphonic acids such as methylphosphonic acid, ethylphosphonic acid, octylphosphonic acid, and propylphosphonic acid, or polyfunctional phosphonic acids such as hydroxyethanediphosphonic acid and nitrotrismethylenephosphonic acid. Furthermore, as compounds having polar groups such as a carboxyl group, aliphatic carboxylic acids with 12 to 18 carbon atoms, such as caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, behenic acid, oleic acid, elaidic acid, linoleic acid, linolenic acid, and stearolic acid, can be used.

[0051] The electromagnetic wave absorbing sheet 1 can be manufactured by known kneading methods, such as using a batch kneader, roll mill, or continuous kneader. If vulcanization is required for the material of the electromagnetic wave absorbing sheet 1, known methods such as a heat press or a funnel cure (continuous vulcanizing gas) can be used as appropriate. Furthermore, if secondary vulcanization is required for the material of the electromagnetic wave absorbing sheet 1, known methods such as a far-infrared vulcanizing furnace (continuous) can be used as appropriate. If a vulcanizing agent is not used, crosslinking can be performed using electron beam crosslinking or gamma ray crosslinking as appropriate. In addition, other methods for forming the electromagnetic wave absorbing sheet 1 include injection molding, extrusion molding, rolling, and a thermal crosslinking method that can be used when the binder is a rubber material. Furthermore, the sheet can be dispersed with a binder material such as rubber or plastic in various organic solvents, alcohol, or water using a ball mill or bead mill, and then coated onto a predetermined substrate by coating methods such as comma coating, gravure coating, or die coating.

[0052] [Other configurations] Furthermore, in the electromagnetic wave absorber according to this embodiment, an adhesive layer can be provided on the surface opposite to the surface on which the electromagnetic waves are incident.

[0053] By providing an adhesive layer, the electromagnetic wave absorbing sheet can be easily attached to a designated location. The adhesive layer can be easily formed by applying an adhesive resin paste. Acrylic adhesives, rubber adhesives, silicone adhesives, etc., can be used as the material for the adhesive layer. In addition, adhesive tapes such as acrylic adhesive tapes and silicone adhesive tapes can be used for the adhesive layer.

[0054] Furthermore, a reflective layer may be provided on the back side of the electromagnetic wave absorber when viewed from the electromagnetic wave incident surface. By providing a reflective layer, shielding and absorption of electromagnetic waves in high frequency bands above the millimeter wave band can be reliably achieved. The reflective layer can be made of metal foil, metal vapor-deposited film, composite film made by laminating aluminum foil and polyester film, mesh-like conductor, silver nanowire (Ag-NW), conductive polymer film, etc.

[0055] Furthermore, an electromagnetic wave reflection suppression layer may be provided on the electromagnetic wave incident surface side of the electromagnetic wave absorber. The electromagnetic wave reflection suppression layer contains a binder, and the proportion of the binder is greater than the proportion of the binder contained in the electromagnetic wave absorber. Rubber-based binders, thermoplastic resins, and thermosetting resins can be used as the binder. By placing the electromagnetic wave reflection suppression layer between the air layer and the electromagnetic wave absorber in this way, the relative permittivity decreases sequentially from the air layer side toward the electromagnetic wave absorber layer, and the difference between the relative permittivity of the air layer and the relative permittivity of the electromagnetic wave reflection suppression layer becomes smaller. As a result, the reflection of electromagnetic waves incident from the electromagnetic wave reflection suppression layer side can be suppressed, thereby improving the electromagnetic wave absorption characteristics.

[0056] (Examples) The following describes a specific example of actually fabricating an electromagnetic wave absorbing sheet and measuring its electromagnetic wave absorption characteristics, based on the electromagnetic wave absorbing sheet described in this embodiment.

[0057] Table 1 shows the structure of each sheet and a list of the measurement results.

[0058] [Table 1]

[0059] (Fabrication of electromagnetic wave absorbing sheets) As for the magnetoprumbite-type hexagonal ferrite included in the electromagnetic wave absorbing sheet, the general formula "A" mentioned above is used. (1-x) La x Fe (n-y) Al y Strontium ferrite in which the Sr site is replaced with La, represented as , was used. Note that the particle size D 50 The thickness was 3.1 μm, and the volume content was as shown in Table 1. In Table 1, the electromagnetic wave absorbing sheets described as Comparative Example 1 and Comparative Example 2 used magnetoplanbite-type hexagonal ferrite in which the Sr sites were not substituted with La.

[0060] The binder used was silicone rubber (product name "KE-951KU" manufactured by Shin-Etsu Chemical Co., Ltd.).

[0061] To these materials, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane (Shin-Etsu Chemical Co., Ltd., "C-8A" product name) was added as a vulcanizing agent, and stearic acid (Kao Corporation, "Lunaq S-50V" product name) and fatty acid amide (Fuso Chemical Industry Co., Ltd., "Plastron S" product name) were added as processing aids. The mixture was kneaded in a batch-type kneader at a rotation speed of 35 rpm while flowing cold water to prevent the temperature from rising too high. The mixture was then formed into sheets with a thickness of 1.5 to 2.0 mm using a calender, vulcanized at 160°C / 10 min using a hot press, and then subjected to secondary vulcanization in a hot bath at 200°C / 4 hr to produce sheets with a thickness of 1.5 to 2.0 mm and a square area of ​​150 mm.

[0062] The weight ratio of the binder was 97.1%, the weight ratio of the vulcanizing agent was 0.7%, the weight ratio of the processing aid was 0.8%, and the weight ratio of the fatty acid amide was 1.5%.

[0063] (Measurement method) For the electromagnetic wave absorbing sheet, the transmission attenuation, which is the amount of electromagnetic wave absorbed, was measured using the free-space method.

[0064] The free-space method measurement was performed using a free-space measuring device DPS24-01 (product name) manufactured by Keycom Co., Ltd. and a vector network analyzer MS46522B (product name) manufactured by Anritsu Corporation. Using two horn antennas, a predetermined frequency input wave (millimeter wave) was perpendicularly irradiated from the transmitting and receiving antennas through a dielectric lens onto a sample electromagnetic wave absorbing sheet in a 100 mm diameter area. The transmitted wave that passed through the sample was measured, and the intensity of the input wave and the intensity of the transmitted wave were compared to determine the transmission attenuation in dB. The obtained transmission attenuation (dB) was then divided by the measured thickness of the electromagnetic wave absorbing sheet to obtain the transmission attenuation per 1 mm thickness (dB / mm). The frequency range was 60 GHz to 90 GHz, and 801 measurement points were set up to measure the electromagnetic wave transmission attenuation (S21) under normal incidence. Furthermore, the vector network analyzer underwent TRL calibration beforehand, and then time-domain gate processing (gate settings: center 0m, span 9cm) was performed to remove signals caused by multiple reflections.

[0065] The complex relative permittivity and complex relative permeability of the sample electromagnetic wave absorbing sheet were also measured using the free-space method, similar to the method described above. Electromagnetic waves were irradiated perpendicularly to the sample, and the complex relative permittivity (real and imaginary parts) and complex relative permeability (real and imaginary parts) were calculated from the electromagnetic wave transmission characteristics (phase and amplitude) of the electromagnetic wave absorbing sheet, respectively.

[0066] The complex relative permittivity was measured by measuring the amplitude and phase of the transmitted electromagnetic wave (S21) under normal incidence in the frequency band from 55 GHz to 65 GHz, and the average complex relative permittivity (real part, imaginary part) was calculated using frequency variation method software from Keycom Co., Ltd.

[0067] For the measurement of complex relative permeability, the amplitude and phase of the transmitted electromagnetic wave (S21) under normal incidence were measured in the frequency band from 60 GHz to 90 GHz. Similar to the dielectric constant, the complex relative permeability (real and imaginary parts) was calculated from the phase, amplitude, and average relative permittivity from 55 GHz to 65 GHz using permeability calculation software from Keycom Co., Ltd.

[0068] The imaginary part of the complex relative permeability (μ'') and the full width at half maximum of the transmission attenuation were determined using the average values ​​of measurements taken at 60 GHz and 90 GHz at 25°C and 105°C as the baseline, and the frequency range was defined as the midpoint between the peak values ​​and the baseline for each of the imaginary part of the complex relative permeability (μ'') and the transmission attenuation.

[0069] (Measurement results) In Table 1, the "Percentage change of peak frequency (%)" is the value of equation (1) above, "(f 105 -f 25 ) / f 25 "×100" represents the value obtained by dividing the magnitude of the change in the frequency at which the value of the imaginary part (μ'') of the complex relative permeability peaks (i.e., reaches an extreme value) when the ambient temperature changes from 25°C to 105°C, in the curve showing the relationship between the frequency of electromagnetic waves incident on the electromagnetic wave absorbing sheet and the value of the imaginary part (μ'') of the complex relative permeability, by the value of the peak frequency at 25°C.

[0070] Furthermore, the "percentage change in peak value (%)" is the value "(μ''" from equation (2) above. 105 -μ'' 25 ) / μ'' 25 "×100" represents the value obtained by dividing the magnitude of the change in the value of the imaginary part (μ'') of the imaginary part (μ'') at the frequency where the value of the imaginary part (μ'')

[0071] Furthermore, the "permeability change rate (%)" is the value of equation (3) above, "(μ'' 105(f25) -μ'' 25 ) / μ'' 25"(Absolute Value) × 100" is a curve showing the relationship between the frequency of electromagnetic waves incident on the electromagnetic wave absorbing sheet and the value of the imaginary part (μ'') of the complex relative permeability. The value of the imaginary part (μ'') of the complex relative permeability at the frequency where it peaks, i.e., reaches an extreme value, when the ambient temperature changes from 25°C to 105°C. The absolute value of this value is obtained by dividing the magnitude of the change in the value of the imaginary part (μ'') at this frequency by the value at 25°C.

[0072] The "attenuation rate change (%)" is calculated by dividing the change in the maximum transmission attenuation value (the value at which the transmission attenuation peaks, i.e., reaches its extreme value) when the ambient temperature changes from 25°C to 105°C, as shown in the curve illustrating the relationship between the frequency of electromagnetic waves incident on the electromagnetic wave absorption sheet and the transmission attenuation value, by the maximum transmission attenuation value at 25°C.

[0073] Furthermore, the "thickness increase rate (%)" is a value that indicates how much the thickness of the electromagnetic wave absorbing sheet required to achieve a transmission attenuation of 6 dB at the electromagnetic wave absorption peak frequency has increased from the thickness required at an ambient temperature of 25°C to the thickness required at an ambient temperature of 105°C.

[0074] Figure 2 shows the relationship between the frequency of the incident electromagnetic wave and the value of the imaginary part (μ'') of the complex relative permeability of the electromagnetic wave absorbing sheet in Example 2 shown in Table 1.

[0075] Figure 3 shows the relationship between the transmission attenuation per 1 mm thickness and the change in the frequency of the incident electromagnetic wave in the electromagnetic wave absorbing sheet of Example 2, also shown in Table 1.

[0076] Figure 4 shows the relationship between the frequency of the incident electromagnetic wave and the value of the imaginary part (μ'') of the relative permeability of the electromagnetic wave absorbing sheet in Comparative Example 1 shown in Table 1.

[0077] Figure 5 shows the relationship between the transmission attenuation per 1 mm thickness and the change in the frequency of the incident electromagnetic wave in the electromagnetic wave absorbing sheet of Comparative Example 1, also shown in Table 1.

[0078] In Figure 2, the curve shown as reference numeral 21, which illustrates the relationship between the frequency of incident electromagnetic waves and the value of the imaginary part (μ'') of the complex relative permeability when the ambient temperature of the electromagnetic wave absorbing sheet of Example 2 is 25°C, shows a peak (extremum) at a frequency of 75.15 GHz, where the value of the imaginary part (μ'') of the complex relative permeability is 0.131. Similarly, in Figure 2, the curve shown as reference numeral 22, which illustrates the relationship between the frequency of incident electromagnetic waves and the value of the imaginary part (μ'') of the complex relative permeability of the electromagnetic wave absorbing sheet when the ambient temperature of the electromagnetic wave absorbing sheet of Example 2 is 105°C, shows a peak (extremum) at a frequency of 75.68 GHz, where the value of the imaginary part (μ'') of the complex relative permeability is 0.118. Therefore, when the ambient temperature changes from 25°C to 105°C, the change in the frequency at which the imaginary part (μ'') of the complex relative permeability peaks is 0.53 GHz, and the rate of change of peak frequency relative to the peak frequency at 25°C is 0.7%. Furthermore, when the ambient temperature changes from 25°C to 105°C, the change in the value of the imaginary part (μ'') of the complex relative permeability at its peak is 0.013, and the rate of change of the peak value relative to the imaginary part (μ'') of the complex relative permeability at 25°C is 9.9%.

[0079] The change in transmission attenuation per unit thickness of the electromagnetic wave absorbing sheet of this Example 2 with respect to changes in electromagnetic wave frequency is shown in Figure 3 as a curve showing the relationship between the frequency of incident electromagnetic waves and the value of transmission attenuation when the ambient temperature is 25°C, indicated by reference numeral 31. There is a peak (extreme value) at a frequency of 75.64 GHz, and the value of transmission attenuation at this time is -10.405 dB / mm. Furthermore, in the curve showing the relationship between the frequency of incident electromagnetic waves and the value of transmission attenuation when the ambient temperature of the electromagnetic wave absorbing sheet of Example 2 is 105°C, indicated by reference numeral 32 in Figure 3, there is a peak (extreme value) at a frequency of 76.16 GHz, and the value of transmission attenuation at this time is -9.824 dB / mm.

[0080] Therefore, when the ambient temperature changes from 25°C to 105°C, the change in the peak transmission attenuation value at each temperature is 0.581 dB / mm, and the rate of change in transmission attenuation relative to the transmission attenuation at 25°C is 5.6%. Furthermore, the thickness of the electromagnetic wave absorbing sheet required to obtain a transmission attenuation of 6 dB, calculated from the transmission attenuation per 1 mm at the frequency of 76.16 GHz where the transmission attenuation peaks at 25°C, is 0.577 mm. When the ambient temperature at the same frequency is 105°C, the thickness of the sheet required to obtain a transmission attenuation of 6 dB is 0.610 mm. Thus, when the ambient temperature changes from 25°C to 105°C, the increase in the thickness of the electromagnetic wave absorbing sheet required to obtain a transmission attenuation of 6 dB is 5.9%.

[0081] Figures 4 and 5 show the relationship between the frequency of incident electromagnetic waves and the value of the imaginary part (μ'') of the complex relative permeability of the electromagnetic wave absorbing sheet, as well as the transmission attenuation, for Comparative Example 1, in which some of the Sr sites of the magnetoplumbite-type hexagonal ferrite contained in the electromagnetic wave absorbing sheet are not substituted with La.

[0082] In Figure 4, indicated by reference numeral 41, the curve showing the relationship between the frequency of incident electromagnetic waves and the value of the complex relative permeability imaginary part (μ'') of the electromagnetic wave absorbing sheet of Comparative Example 1 when the ambient temperature is 25°C shows a peak (extremum) at a frequency of 76.16 GHz, where the value of the complex relative permeability imaginary part (μ'') is 0.302. Furthermore, in Figure 4, indicated by reference numeral 42, the curve showing the relationship between the frequency of incident electromagnetic waves and the value of the complex relative permeability imaginary part (μ'') of the electromagnetic wave absorbing sheet of Comparative Example 1 when the ambient temperature is 105°C shows a peak (extremum) at a frequency of 78.15 GHz, where the value of the relative permeability imaginary part (μ'') is 0.254. Therefore, when the ambient temperature changes from 25°C to 105°C, the change in the frequency at which the imaginary part of the complex relative permeability (μ'') peaks is 1.99 GHz, and the rate of change in peak frequency relative to the peak frequency at 25°C is 2.6%. Also, when the ambient temperature changes from 25°C to 105°C, the change in the value of the imaginary part of the complex relative permeability (μ'') at peak is 0.048, and the rate of change in peak value relative to the imaginary part of the relative permeability (μ'') at 25°C is 15.9%.

[0083] Furthermore, the change in transmission attenuation per unit thickness of the electromagnetic wave absorbing sheet of Comparative Example 1 with respect to changes in electromagnetic wave frequency is shown in Figure 5 as a curve showing the relationship between the frequency of incident electromagnetic waves and the transmission attenuation value when the ambient temperature is 25°C (indicated by reference numeral 51). There is a peak (extreme value) at a frequency of 76.39 GHz, and the transmission attenuation value per unit thickness at this time is -10.100 dB / mm. Also, in the curve showing the relationship between the frequency of incident electromagnetic waves and the transmission attenuation value of the electromagnetic wave absorbing sheet of Comparative Example 1 when the ambient temperature is 105°C (indicated by reference numeral 52 in Figure 5), there is a peak (extreme value) at a frequency of 78.58 GHz, and the transmission attenuation value per unit thickness at this time is -8.001 dB / mm.

[0084] Therefore, when the ambient temperature changes from 25°C to 105°C, the change in the peak transmission attenuation value is 2.099 dB / mm, and the rate of change in attenuation relative to the transmission attenuation at 25°C is 20.8%. Furthermore, the thickness of the electromagnetic wave absorbing sheet required to obtain a transmission attenuation of 6 dB, calculated from the transmission attenuation per 1 mm at the frequency of 76.39 GHz where the transmission attenuation peaks at 25°C, is 0.594 mm. When the ambient temperature at the same frequency is 105°C, the thickness of the sheet required to obtain a transmission attenuation of 6 dB is 0.750 mm. Thus, when the ambient temperature changes from 25°C to 105°C, the increase in the thickness of the electromagnetic wave absorbing sheet required to obtain a transmission attenuation of 6 dB is 26.3%.

[0085] As shown in Table 1, in the magnetoplumbite-type hexagonal ferrite sheets in which some of the metal sites are replaced with La (Examples 1 to 10), the change in transmission attenuation per unit thickness at the peak frequency at 25°C is 7.5% or less, indicating that good electromagnetic wave absorption characteristics are observed even when the ambient temperature changes from 25°C to 105°C.

[0086] In contrast, in electromagnetic wave absorbing sheets where some of the metal sites of magnetoplumbite-type hexagonal ferrite are not substituted with La (Comparative Example 1, Comparative Example 2), the change in transmission attenuation per unit thickness at the peak frequency at 25°C is 20% or more, indicating that good electromagnetic wave absorption characteristics cannot be observed when the ambient temperature changes from 25°C to 105°C.

[0087] Furthermore, even in the case of a magnetoplumbite-type hexagonal ferrite sheet in which some of the metal sites are replaced with La, the electromagnetic wave absorbing sheet shown as Comparative Example 3 shows that the ratio of the change in the imaginary part (μ'') of the complex relative permeability when the ambient temperature changes from 25°C to 105°C to the value of the imaginary part (μ'') of the complex relative permeability at the peak value at 25°C exceeds 1.5%, with 1.9%. The change in transmission attenuation per unit thickness at the peak frequency at 25°C is large at 12.5%, indicating that good electromagnetic wave absorption characteristics cannot be observed when the ambient temperature changes from 25°C to 105°C.

[0088] From the above considerations, it can be seen that the requirements for obtaining an electromagnetic wave absorbing sheet that can effectively absorb electromagnetic waves of a desired frequency even when the ambient temperature changes are that some of the metal sites of the magnetoplumbite-type hexagonal ferrite contained in the electromagnetic wave absorbing sheet are replaced with La, and that the ratio of the change in the imaginary part (μ'') of relative permeability when the ambient temperature changes from 25°C to 105°C to the value of the imaginary part (μ'') of relative permeability at the peak value at 25°C (permeability change rate (%)) is 1.5% or less.

[0089] Furthermore, as a measure to obtain an electromagnetic wave absorbing sheet that can effectively absorb electromagnetic waves of a predetermined frequency even when the ambient temperature changes, it is conceivable to increase the thickness of the electromagnetic wave absorbing sheet and thereby increase the amount of magnetoplanbite-type hexagonal ferrite contained in the sheet. By increasing the thickness of the electromagnetic wave absorbing sheet at the same volume ratio, the amount of magnetoplanbite-type hexagonal ferrite that absorbs electromagnetic waves increases, improving the electromagnetic wave absorption capacity of the electromagnetic wave absorbing sheet. As a result, even if the amount of electromagnetic wave absorption (absorption capacity) of the magnetoplanbite-type hexagonal ferrite that absorbs electromagnetic waves of a predetermined frequency decreases due to changes in ambient temperature, it is possible to design a sheet that can effectively absorb electromagnetic waves of a predetermined frequency.

[0090] The inventors assumed that the electromagnetic wave absorption sheet should have a transmission attenuation of 6 dB or more, i.e., absorb half of the incident electromagnetic waves, as a guideline for the transmission attenuation (absolute value) of electromagnetic waves. They determined the change in transmission attenuation when the ambient temperature was 105 °C for an electromagnetic wave absorption sheet whose transmission attenuation peaked at an ambient temperature of 25 °C, and measured the relationship between this change and the increase in thickness required to create an electromagnetic wave absorption sheet that would still achieve a transmission attenuation of 6 dB at the absorption peak frequency at 25 °C, even at 105 °C.

[0091] Figure 6 shows the relationship between the change in the imaginary part of the complex relative permeability and the increase in thickness required to ensure the necessary amount of electromagnetic wave absorption at 105°C.

[0092] Figure 6 plots the "peak frequency change rate (%)" and "thickness increase rate (%)" for the electromagnetic wave absorbing sheets of Examples 1 to 10 and Comparative Examples 1 to 3 shown in Table 1.

[0093] As shown in Figure 6, when the ambient temperature changes from 25°C to 105°C, if the change in the frequency at which the imaginary part of the relative permeability (μ'') peaks is 1.5% or less, the percentage increase in the thickness of the electromagnetic wave absorbing sheet required to absorb electromagnetic waves of that frequency at 105°C remains almost unchanged at 8% or less. However, when the ambient temperature changes from 25°C to 105°C, if the change in the frequency at which the imaginary part of the relative permeability (μ'') peaks exceeds 1.5%, the increase in the required thickness increases rapidly. Therefore, even when using a strategy to increase the absolute value of electromagnetic wave absorption by increasing the thickness of the electromagnetic wave absorbing sheet and obtain an electromagnetic wave absorbing sheet with good electromagnetic wave absorption characteristics even when the ambient temperature changes, it was confirmed that a condition for obtaining an electromagnetic wave absorbing sheet with good electromagnetic wave absorption characteristics is that the change in the frequency at which the imaginary part of the relative permeability (μ'') peaks is 1.5% or less when the ambient temperature changes from 25°C to 105°C.

[0094] Furthermore, in the electromagnetic wave absorbing sheet according to this embodiment, it is preferable that the value of the imaginary part of the complex relative permeability shows a rate of change when the temperature changes from 25°C to 105°C, and that the ratio of the amount of change in the value of the imaginary part of the complex relative permeability when the temperature changes from 25°C to 105°C to the value of the imaginary part of the complex relative permeability that has a positive peak at 25°C is 15% or less.

[0095] Even when the operating temperature of an electromagnetic wave absorbing sheet changes from 25°C to 105°C, if the change in the extreme value of the imaginary part of the complex relative permeability is small, the change in electromagnetic wave absorption characteristics will also be small. For this reason, in particular, by keeping the ratio of the change in the value of the imaginary part of the complex relative permeability when the temperature changes from 25°C to 105°C to the value of the imaginary part of the complex relative permeability that has a positive peak at 25°C to 15% or less, good electromagnetic wave absorption characteristics can be achieved over a wide temperature range. If the change in the extreme value of the imaginary part of the complex relative permeability exceeds 15%, the change in electromagnetic wave absorption characteristics due to temperature becomes large, and good electromagnetic wave absorption characteristics cannot be achieved over a wide temperature range.

[0096] Furthermore, at the frequency where there is a positive peak at 25°C, it is preferable that the absolute value of the ratio of the change in the imaginary part of the complex relative permeability when the temperature of the electromagnetic wave absorbing sheet changes from 25°C to 105°C to the imaginary part of the complex relative permeability at 25°C is 14% or less.

[0097] At the peak frequency when the temperature is 25°C, if the absolute value of the change in the imaginary part of the complex relative permeability when the ambient temperature changes from 25°C to 105°C is kept to 14% or less compared to the value at 25°C, then when the temperature changes from 25°C to 105°C, the decrease in the imaginary part of the complex relative permeability at the peak frequency at 25°C will be small, and the change in electromagnetic wave absorption characteristics due to temperature will be small. Conversely, if the amount of change is greater than 14%, the change in electromagnetic wave absorption characteristics due to temperature will be large, and it will not be possible to exhibit good electromagnetic wave absorption characteristics over a wide temperature range.

[0098] Furthermore, in the electromagnetic wave absorbing sheet according to this embodiment, it is preferable that, in the graph showing the relationship between the frequency of the incident electromagnetic wave and the value of the imaginary part of the complex relative permeability of the electromagnetic wave absorber, both the full width at half maximum when the temperature of the electromagnetic wave absorber is 25°C and the full width at half maximum when the temperature of the electromagnetic wave absorber is 105°C are 8.5 GHz or higher.

[0099] The electromagnetic wave absorber can absorb electromagnetic waves across a wide frequency band because its full width at half maximum (FMAX) is 8.5 GHz or higher at both 25°C and 105°C. As a result, even if the electromagnetic wave absorption characteristics of the absorber change due to temperature changes, good electromagnetic wave absorption characteristics can be maintained. If the FMAX is smaller than 8.5 GHz, the electromagnetic wave absorption characteristics will be more significantly affected by temperature changes.

[0100] Furthermore, increasing the thickness of the electromagnetic wave absorbing sheet reduces its flexibility, making it difficult to position the sheet precisely where desired. This is particularly true when used to absorb unwanted electromagnetic waves from radar systems mounted on automobiles, a use that has seen a rapid increase in adoption due to recent technological advancements. In such cases, precise placement of the electromagnetic wave absorbing sheet in a confined space is required. In these situations, increasing the thickness of the electromagnetic wave absorbing sheet is undesirable, including the increased manufacturing cost.

[0101] From this perspective, the thickness of the electromagnetic wave absorbing sheet is preferably 2 mm or less, and preferably 1 mm or less. Furthermore, since the frequency of electromagnetic waves used in radar mounted on automobiles is between 75 GHz and 80 GHz, the value of the imaginary part of the complex relative permeability of the electromagnetic wave absorbing sheet is preferably between 75 GHz and 80 GHz.

[0102] Furthermore, it was confirmed that the change in the frequency of electromagnetic waves at which the imaginary part (μ'') of the complex relative permeability of magnetoplumbite-type hexagonal ferrite peaks when the ambient temperature changes from 25°C to 105°C tends to increase when the weight ratio of magnetoplumbite-type hexagonal ferrite exceeds 80%. Therefore, by keeping the weight ratio of magnetoplumbite-type hexagonal ferrite at a maximum of 90% or less, the change in the frequency at which the imaginary part (μ'') of the complex relative permeability peaks when the ambient temperature changes from 25°C to 105°C can be kept below the desirable condition of 1.5%.

[0103] Furthermore, it was confirmed that as the proportion of carbon black as a dielectric constant modifier in the electromagnetic wave absorbing sheet increases, the change in the frequency of electromagnetic waves at which the imaginary part (μ'') of the complex relative permeability of magnetoplumbite-type hexagonal ferrite peaks when the ambient temperature changes from 25°C to 105°C also increases. Therefore, by setting the proportion of the dielectric constant modifier in the electromagnetic wave absorbing sheet to 4.0% or less, the change in the frequency at which the imaginary part (μ'') of the complex relative permeability peaks can be kept at the desirable condition of 1.5% or less.

[0104] As described above, the electromagnetic wave absorbing sheet according to this embodiment includes magnetoplumbite-type hexagonal ferrite in which a portion of the metal sites are replaced with La, and the value of the frequency at which the positive peak occurs in the graph showing the relationship between the frequency of the incident electromagnetic wave and the value of the imaginary part of the complex relative permeability of the electromagnetic wave absorber shows a positive rate of change when the temperature of the electromagnetic wave absorber changes from 25°C to 105°C, and the ratio of the amount of change in the frequency at which the positive peak occurs when the temperature changes from 25°C to 105°C to the value of the frequency at which the positive peak occurs at 25°C is 1.5% or less, thereby making it possible to make an electromagnetic wave absorbing sheet that can absorb electromagnetic waves of a desired frequency well even when the ambient temperature changes.

[0105] In the above embodiment, an electromagnetic wave absorbing sheet with a thin thickness relative to its area was used as an example. However, the electromagnetic wave absorber disclosed in this application can be realized as a so-called block-shaped electromagnetic wave absorber having a certain thickness relative to its area. By having the same requirements as the electromagnetic wave absorbing sheet described above, it is possible to realize an electromagnetic wave absorber that exhibits little change in the amount of electromagnetic wave absorbed with respect to temperature changes and can effectively absorb electromagnetic waves of a desired frequency. [Industrial applicability]

[0106] The electromagnetic wave absorber disclosed in this application includes magnetoplanbite-type hexagonal ferrite in which some of the metal sites are replaced with La. The value of the frequency at which the positive peak occurs in the graph showing the relationship between the frequency of the incident electromagnetic wave and the value of the imaginary part of the complex relative permeability of the electromagnetic wave absorber shows a positive rate of change when the temperature of the electromagnetic wave absorber changes from 25°C to 105°C, and the ratio of the change in the frequency at which the positive peak occurs when the temperature changes from 25°C to 105°C to the value of the frequency at which the positive peak occurs at 25°C is 1.5% or less. This makes it possible to realize an electromagnetic wave absorber that can absorb electromagnetic waves of a desired frequency well even when the ambient temperature changes. For this reason, it is useful, for example, as an electromagnetic wave absorber for absorbing unwanted electromagnetic waves in an on-board radar mounted on an automobile.

[0107] Furthermore, the electromagnetic wave absorbing sheet disclosed in this application can contribute to achieving Goal 9 (Build resilient infrastructure, promote inclusive and sustainable industrialization and foster innovation) of the 17 Sustainable Development Goals (SDGs) established by the United Nations. [Explanation of Symbols]

[0108] 1. Electromagnetic wave absorbing sheet (electromagnetic wave absorber) 1a Magnetopranbite-type hexagonal ferrite 1b Binder

Claims

1. An electromagnetic wave absorber containing magnetoplanbite-type hexagonal ferrite that resonates magnetically in the millimeter-wave frequency band within a binder, The aforementioned magnetoprumbite-type hexagonal ferrite has some of its metallic sites replaced by La. An electromagnetic wave absorber characterized in that, in a graph showing the relationship between the frequency of incident electromagnetic waves and the value of the imaginary part of the complex relative permeability of the electromagnetic wave absorber, when the temperature of the electromagnetic wave absorber changes from 25°C to 105°C, the value of the frequency at which it has a positive peak shows a positive rate of change, and the ratio of the amount of change in the frequency at which it has a positive peak when the temperature changes from 25°C to 105°C to the value of the frequency at which it has a positive peak at 25°C is 1.5% or less.

2. An electromagnetic wave absorber comprising a binder containing magnetoplanbite-type hexagonal ferrite that magnetically resonates in the millimeter-wave frequency band, The aforementioned magnetoprumbite-type hexagonal ferrite has some of its metallic sites replaced by La. An electromagnetic wave absorber characterized in that, in a graph showing the relationship between the frequency of incident electromagnetic waves and the value of the imaginary part of the complex relative permeability of the electromagnetic wave absorber, the value of the imaginary part of the complex relative permeability at the positive peak shows a negative rate of change when the temperature of the electromagnetic wave absorber changes from 25°C to 105°C, and the ratio of the amount of change in the value of the imaginary part of the complex relative permeability at the positive peak when the temperature changes from 25°C to 105°C to the value of the imaginary part of the complex relative permeability at 25°C which has a positive peak is 15% or less.

3. The electromagnetic wave absorber according to claim 1, wherein, in a graph showing the relationship between the frequency of the incident electromagnetic wave and the value of the imaginary part of the complex relative permeability of the electromagnetic wave absorber, the value of the imaginary part of the complex relative permeability at the positive peak shows a negative rate of change when the temperature of the electromagnetic wave absorber changes from 25°C to 105°C, and the ratio of the amount of change in the value of the imaginary part of the complex relative permeability at the positive peak when the temperature changes from 25°C to 105°C to the value of the imaginary part of the complex relative permeability at the positive peak at 25°C is 15% or less.

4. The electromagnetic wave absorber according to claim 1 or 2, wherein the amount of La substitution (x) at the metal sites of the magnetoprumbite-type hexagonal ferrite is 0.1 or more and 0.57 or less.

5. The electromagnetic wave absorber according to claim 1 or 2, wherein, when the temperature of the electromagnetic wave absorber is 25°C, at the frequency at which there is a positive peak in the graph showing the relationship between the frequency of the incident electromagnetic wave and the value of the imaginary part of the complex relative permeability of the electromagnetic wave absorber, the absolute value of the ratio of the change in the value of the imaginary part of the complex relative permeability when the temperature of the electromagnetic wave absorber changes from 25°C to 105°C to the value of the imaginary part of the complex relative permeability at 25°C is 14% or less.

6. The electromagnetic wave absorber according to claim 1 or 2, wherein, in a graph showing the relationship between the frequency of the incident electromagnetic wave and the value of the imaginary part of the complex relative permeability of the electromagnetic wave absorber, the full width at half maximum when the temperature of the electromagnetic wave absorber is 25°C and the full width at half maximum when the temperature of the electromagnetic wave absorber is 105°C are both 8.5 GHz or higher.

7. The electromagnetic wave absorber according to claim 1 or 2, wherein, in a graph showing the relationship between the frequency of the incident electromagnetic wave and the value of the imaginary part of the complex relative permeability of the electromagnetic wave absorber, the value of the frequency at which the positive peak occurs is 75 GHz or more and 80 GHz or less.

8. The electromagnetic wave absorber according to claim 1 or 2, wherein the binder of the electromagnetic wave absorber is at least one of rubber, plastic, and thermoplastic elastomer.

9. The electromagnetic wave absorber according to claim 1 or 2, further comprising conductive carbon black as a dielectric constant adjusting agent.

10. The electromagnetic wave absorber according to claim 1 or 2, further comprising at least one from the group consisting of furnace conductive carbon black, acetylene black, and Ketjen black as a dielectric constant adjusting agent.

11. The electromagnetic wave absorber according to claim 1 or 2, wherein the electromagnetic wave absorber comprises at least an electromagnetic wave absorbing layer and a layer having an electromagnetic wave surface reflection suppression function, and is in the form of a sheet with an overall thickness of 2 mm or less.

12. The electromagnetic wave absorber according to claim 11, wherein the overall thickness of the electromagnetic wave absorber is 1 mm or less.

13. The electromagnetic wave absorber according to claim 1 or 2, wherein the content of the magnetoplumbite-type hexagonal ferrite in the electromagnetic wave absorber is 89% by mass or less.

14. The electromagnetic wave absorber according to claim 1 or 2, wherein the content of the magnetoplumbite-type hexagonal ferrite in the electromagnetic wave absorber is 85.2% by mass or less.

15. The electromagnetic wave absorber according to claim 1 or 2, wherein the electromagnetic wave absorber further contains carbon black as a dielectric constant adjuster, and the amount of carbon black is less than 4.5% of the total mass of the electromagnetic wave absorbing sheet.

16. The electromagnetic wave absorber according to claim 1 or 2, wherein the electromagnetic wave absorber contains granular carbon black as a dielectric constant modifier.

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