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

Electromagnetic wave absorber

[0001] The present disclosure relates to an electromagnetic wave absorber that absorbs electromagnetic waves, and in particular to an electromagnetic wave absorber that suppresses fluctuations in the frequency of the absorbed electromagnetic waves due to temperature changes and can effectively absorb electromagnetic waves of a desired frequency even in an environment where temperature changes occur.

[0002] Electromagnetic wave absorbing compositions that absorb electromagnetic waves are used to avoid the effects of leakage electromagnetic waves emitted to the outside from electric circuits, etc., and undesirably reflected electromagnetic waves. 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 paints that are applied to desired locations.

[0003] In recent years, research into technologies that utilize electromagnetic waves in the centimeter wave to millimeter wave bands and even higher frequency bands beyond the millimeter wave band has been progressing in mobile communications such as mobile phones, wireless LANs, automatic toll collection systems (ETC), etc. In response to this technological trend toward utilizing electromagnetic waves of higher frequencies, there is a growing demand for electromagnetic wave absorbers that can absorb unwanted electromagnetic waves, such as those capable of absorbing electromagnetic waves in the millimeter wave band of approximately 30 GHz to 300 GHz, or higher frequency bands.

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

[0005] JP 2019-145534 A

[0006] The electromagnetic wave absorption sheet described in Patent Document 1 contains, as a binder, hexagonal ferrite, whose coercive force increases when the ambient temperature rises, shifting the peak frequency of the absorbed electromagnetic waves to a higher value, and epsilon iron oxide, whose coercive force decreases when the ambient temperature rises, shifting the frequency of the absorbed electromagnetic waves to a lower value.This means that the effects of the frequency shifts in the electromagnetic waves absorbed 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 above-mentioned conventional electromagnetic wave absorbing sheets have had problems in realizing electromagnetic wave absorbing sheets at low cost, such as the need for epsilon iron oxide particles in addition to hexagonal ferrite, which increases material costs, and the need to uniformly mix and disperse hexagonal ferrite particles, which have a relatively large particle size and are flat, with epsilon iron oxide particles, which are spherical and have a small average particle size of 50 nm, in a binder.

[0008] In order to solve such conventional problems, the present disclosure aims to obtain an electromagnetic wave absorber that can effectively absorb electromagnetic waves at high frequencies above the millimeter wave band and can also effectively absorb electromagnetic waves at a desired frequency even when changes in the ambient temperature occur.

[0009] In order to solve the above problems, the electromagnetic wave absorber disclosed in the present application is an electromagnetic wave absorber that contains, in a binder, magnetoplumbite-type hexagonal ferrite that magnetically resonates in a frequency band of millimeter waves, and the magnetoplumbite-type hexagonal ferrite has some metal sites substituted with La, and is 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 is changed from 25°C to 105°C, the value of the frequency at which a positive peak occurs exhibits a positive rate of change, and the rate of the amount of change in the frequency at which a positive peak occurs when the temperature is changed 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.

[0010] The electromagnetic wave absorbing material disclosed in the present application has the above-mentioned configuration, and therefore its electromagnetic wave absorption characteristics change little with changes in ambient temperature, and it can effectively absorb electromagnetic waves of the desired frequency even in an environment where temperature changes occur.

[0011] Fig. 1 is a schematic diagram illustrating the general configuration of an electromagnetic wave absorbing sheet according to the present embodiment. Fig. 2 is a diagram illustrating the relationship between the frequency of electromagnetic waves and the value of the imaginary part of the complex relative magnetic permeability of an electromagnetic wave absorber in the electromagnetic wave absorbing sheet of Example 2. Fig. 3 is a diagram illustrating the relationship between the frequency of electromagnetic waves and the transmission attenuation per unit thickness in the electromagnetic wave absorbing sheet of Example 2. Fig. 4 is a diagram illustrating the relationship between the frequency of electromagnetic waves and the value of the imaginary part of the complex relative magnetic permeability of an electromagnetic wave absorber in the electromagnetic wave absorbing sheet of Comparative Example 1. Fig. 5 is a diagram illustrating the relationship between the frequency of electromagnetic waves and the transmission attenuation per unit thickness in the electromagnetic wave absorbing sheet of Comparative Example 1. Fig. 6 is a diagram illustrating the relationship between the amount of change in the value of the imaginary part of the complex relative magnetic permeability and the amount of increase in thickness required to ensure a required amount of electromagnetic wave absorption at 105°C.

[0012] The electromagnetic wave absorber disclosed in the present application is an electromagnetic wave absorber containing, in a binder, magnetoplumbite-type hexagonal ferrite that magnetically resonates in a frequency band of millimeter waves, wherein the magnetoplumbite-type hexagonal ferrite has some metal sites 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 is changed from 25°C to 105°C, the value of the frequency at which a positive peak occurs exhibits a positive rate of change, and the rate of the amount of change in the frequency at which a positive peak occurs when the temperature is changed from 25°C to 105°C relative to the value of the frequency at which a positive peak occurs at 25°C is 1.5% or less.

[0013] Here, "the ratio of the change in 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" means that the frequency at which the positive peak occurs at 25°C is f 25 , the value of the imaginary part of the complex relative permeability at that time is μ'' 25 , and 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 , respectively, the following equation (1) (f105 -f 25 ) / f 25 × 100 (1).

[0014] The metal sites contain at least one of barium (Ba), strontium (Sr), and calcium (Ca).

[0015] By doing so, the electromagnetic wave absorber disclosed in the present application has little 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, it is preferable that the value of the imaginary part of the complex relative permeability at a positive peak exhibits a negative rate of change when the temperature of the electromagnetic wave absorber changes from 25°C to 105°C, and that the rate of the amount of change in the value of the imaginary part of the complex relative permeability at a positive peak 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 a positive peak at 25°C is 15% or less.

[0017] The "proportion of the 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 defined as the frequency at which the positive peak occurs at 25°C. 25 , the value of the imaginary part of the complex relative permeability at that time is μ'' 25 , and 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 , respectively, the following equation (2) (μ'' 105 -μ'' 25 ) / μ'' 25 × 100 (2)

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

[0019] Furthermore, the La substitution amount (x) of the metal sites of the magnetoplumbite-type hexagonal ferrite is preferably 0.1 or more and 0.57 or less, which makes it possible to utilize the effect of reducing the amount of change in electromagnetic wave absorption characteristics with temperature change due to La substitution.

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

[0021] Here, "the absolute value of the ratio of the amount of change in the value of the complex relative magnetic permeability when the temperature of the electromagnetic wave absorber changes from 25°C to the value of the imaginary part of the complex relative magnetic permeability at 25°C, at the frequency at which a positive peak occurs 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 magnetic permeability of the electromagnetic wave absorber when the temperature of the electromagnetic wave absorber is 25°C" refers to the frequency at which a positive peak occurs at 25°C as f 25 , the value of the imaginary part of the complex relative permeability at that time is μ'' 25 , and the frequency f of the imaginary part of the complex relative permeability at 105 ° C. 25 The value at μ'' 105(f25) and then the following equation (3) (μ'' 105(f25) -μ'' 25 ) / μ'' 25 It is a numerical value expressed as (absolute value) × 100(3).

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

[0023] Furthermore, 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, it is preferable that the half-width when the temperature of the electromagnetic wave absorber is 25° C. and the half-width when the temperature of the electromagnetic wave absorber is 105° C. are both 8.5 GHz or more. By doing so, even when used as an electromagnetic wave absorber for a device related to an in-vehicle millimeter-wave radar of 79 GHz, which has a frequency width set to a wide 4 GHz, it is possible to ensure a practically sufficient electromagnetic wave absorption frequency band.

[0024] Furthermore, it is preferable that the value of the frequency at which a positive peak occurs 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 is 75 GHz or more and 80 GHz or less. By doing so, the electromagnetic wave absorber can be suitably used as an electromagnetic wave absorber adopted in on-board radar of automobiles, the market of which is expanding as a practical field.

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

[0026] It is also preferable that the electromagnetic wave absorber further contains, as a dielectric constant adjuster, conductive carbon black or at least one selected from 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 further that the overall thickness of the electromagnetic wave absorber is 1 mm or less.

[0028] It is also preferable that the content of the magnetoplumbite-type hexagonal ferrite in the electromagnetic wave absorber is 89 mass % or less.

[0029] In addition, in the standards of the AEC (Automotive Electronics Council), which is a standards organization for the reliability of automotive electronic components, the upper limit of the operating temperature range is set to 125°C in Grade 1 of the AEC-Q101 (integrated circuit) category, and therefore in this application 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 are specified. In other words, a small change in the electromagnetic wave absorption characteristics even when the temperature changes from 25°C to 105°C is a yardstick for improving the reliability of automotive electronic components.

[0030] The electromagnetic wave absorber disclosed in the present application will be described below by taking as an example an electromagnetic wave absorbing sheet formed in a sheet shape having a small thickness relative to its area.

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

[0032] Note that FIG. 1 is a diagram provided to facilitate understanding of the configuration of the electromagnetic wave absorbing sheet according to this embodiment, and the sizes and thicknesses of the components shown in the diagram are not shown in accordance with reality.

[0033] The electromagnetic wave absorbing sheet 1 exemplified in this embodiment includes magnetoplumbite hexagonal ferrite 1 a and a resin binder 1 b. In the magnetoplumbite hexagonal ferrite 1 a used in the electromagnetic wave absorbing sheet according to this embodiment, some of the metal sites such as strontium (Sr) and barium (Ba) are substituted with lanthanum (La).

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

[0035] [Magnetoplumbite-Type Hexagonal Ferrite] The electromagnetic wave absorbing sheet 1 according to this embodiment contains powder of magnetoplumbite-type hexagonal ferrite 1a in which some of the metal sites have been substituted with lanthanum (La) in a binder.

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

[0037] Here, the ratio shown in the general formula of the atomic ratio of the metal elements refers to the atomic ratio of the metal atoms contained in the single-phase crystal structure of the magnetoplumbite-type hexagonal ferrite magnetic powder, and each metal element is substituted at the A and Fe sites, which indicate metal sites, to constitute the magnetoplumbite-type crystal structure. Note that if each metal element mixed as a raw material during metal substitution is not substituted at each site of the magnetoplumbite-type hexagonal ferrite but remains outside of it in a different form, it is known that the frequency of the magnetic powder mixture finally obtained will be significantly 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 little change in electromagnetic wave absorption characteristics due to temperature changes. In particular, it can suppress changes in the frequency of the electromagnetic wave absorption peak due to temperature changes. In order to achieve the effect of being less susceptible to the above-mentioned temperature changes due to the substitution with La, it is more preferable that the substitution amount x of La shown in the general formula be 0.1 or more and 0.57 or less.

[0039] The content of magnetoplumbite-type hexagonal ferrite 1a in the electromagnetic wave absorbing sheet shown in this embodiment is preferably 89 mass % or less, more preferably 85.2 mass % or less, based on the total content including binder 1b and carbon black as a dielectric constant adjuster described later. As the content of magnetic material increases, the change in transmission attenuation, which indicates the rate of absorption of electromagnetic waves when temperature changes, increases. By setting the mass content of magnetoplumbite-type hexagonal ferrite 1a to 89 mass % or less, the rate of change in transmission attenuation for electromagnetic waves of a frequency that reaches an absorption peak at 25°C when the temperature of the electromagnetic wave absorbing sheet 1 changes from 25°C to 105°C can be set to 6.5% or less, thereby making it possible to provide 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 type selected from rubber, plastic, and thermoplastic elastomer as the resin binder 1b.

[0041] Examples of rubber binders that can be used 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] The plastic may be a thermosetting resin or a thermoplastic resin. Examples of the thermosetting resin include phenolic resin, urea resin, melamine resin, epoxy resin, unsaturated polyester resin, alkyd resin, silicone resin, and polyurethane. Examples of the thermoplastic resin 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 contain other materials in addition to the magnetoplumbite hexagonal ferrite 1a and the binder 1b.

[0045] By including a dielectric constant adjuster in the electromagnetic wave absorbing sheet 1, it becomes easy to adjust 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 makes it possible to improve the electromagnetic wave absorption performance due to the dielectric loss effect. Furthermore, by adjusting the dielectric constant, it is possible to match the characteristic impedance and suppress the reflection of electromagnetic waves at any thickness.

[0046] Examples of dielectric constant adjusters that can be used include carbon materials such as carbon black, carbon nanotubes, and carbon nanostructures, as well as barium titanate, conductive polymers, and metal wires. Carbon black is particularly preferred as a dielectric constant adjuster for the following reasons: first, it is less likely to develop electrical anisotropy in the TD / MD directions after sheet formation; second, it also functions as a rubber binder or plastic reinforcing material; and third, it is inexpensive and readily available. Furthermore, if the dielectric adjuster has an anisotropic shape, it will produce anisotropy in which the electromagnetic wave absorption characteristics change depending on the direction of incidence on the electromagnetic wave absorbing sheet. Therefore, granular carbon black is preferred as a dielectric constant adjuster over needle- or rod-shaped carbon nanotubes. More specifically, various conductive carbon blacks, such as furnace conductive carbon black, acetylene black, and ketjen black, can be suitably used.

[0047] Granular carbon black has a specific surface area of ​​30 to 2300 m 2 / g, and a specific surface area of ​​300 to 2000 m 2 / g, and further, a specific surface area of ​​800 to 1800 m 2 It is more preferable to use one having a molecular weight of 1 / g.

[0048] When carbon black is contained as a dielectric constant adjuster, its content is preferably less than 4.5%, and more preferably 3.0% or less, relative to the total mass of the electromagnetic wave absorbing sheet 1. If the electromagnetic wave absorbing sheet 1 contains 4.5% or more carbon black, the change in the peak frequency of the electromagnetic waves absorbed due to temperature changes increases, 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 with respect to temperature changes is small.

[0049] Various dispersants may also be included in the electromagnetic wave absorbing sheet 1. As the dispersant, compounds having a polar group such as a phosphate group, a sulfonic acid group, or a carboxy group may be used.

[0050] Examples of phosphoric acid compounds having a phosphoric acid group include arylphosphonic acids such as phenylphosphonic acid and phenylphosphonic acid dichloride, alkylphosphonic acids such as methylphosphonic acid, ethylphosphonic acid, octylphosphonic acid, and propylphosphonic acid, and polyfunctional phosphonic acids such as hydroxyethanediphosphonic acid and nitrotrismethylenephosphonic acid. Examples of compounds having a polar group such as a carboxy group include aliphatic carboxylic acids having 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 stearic acid.

[0051] The electromagnetic wave absorbing sheet 1 can be produced by a known kneading method using, for example, a batch kneader, a roll mill, a continuous kneader, or the like. If the material for the electromagnetic wave absorbing sheet 1 requires vulcanization, known methods using, for example, a hot press or a roto-cure (continuous vulcanizer) can be used as appropriate. If the material for the electromagnetic wave absorbing sheet 1 requires secondary vulcanization, known methods using, for example, a far-infrared vulcanization furnace (continuous) can be used as appropriate. If no vulcanizing agent is used, crosslinking can be performed using electron beam crosslinking or gamma ray crosslinking as appropriate. Other methods for forming the electromagnetic wave absorbing sheet 1 include injection molding, extrusion molding, rolling, and thermal crosslinking, which can be used when the binder is a rubber material. Furthermore, the electromagnetic wave absorbing sheet 1 can be formed by dispersing the material together 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 coating the dispersion on a predetermined substrate using a coating method such as comma coating, gravure coating, or die coating.

[0052] [Other Configurations] In the electromagnetic wave absorber according to this embodiment, an adhesive layer can be provided on the surface opposite to the electromagnetic wave incident surface.

[0053] By providing an adhesive layer, the electromagnetic wave absorbing sheet can be easily attached to a predetermined location. The adhesive layer can be easily formed by applying an adhesive resin paste. Materials for the adhesive layer include acrylic adhesives, rubber adhesives, and silicone adhesives. The adhesive layer can also be made of adhesive tape such as acrylic adhesive tape or silicone adhesive tape.

[0054] A reflective layer may also be provided on the back side of the electromagnetic wave absorber when viewed from the electromagnetic wave incidence surface. By providing a reflective layer, it is possible to reliably block and absorb electromagnetic waves in high frequency bands above the millimeter wave band. The reflective layer can be made of metal foil, metal-deposited film, composite film made by laminating aluminum foil and polyester film, mesh-shaped 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. Examples of binders that can be used include rubber-based binders, thermoplastic resins, and thermosetting resins. By disposing the electromagnetic wave reflection suppression layer between the air layer and the electromagnetic wave absorber in this way, the relative permittivity decreases from the air layer side toward the electromagnetic wave absorbing layer, thereby reducing the difference between the relative permittivity of the air layer and the relative permittivity of the electromagnetic wave reflection suppression layer. This makes it possible to suppress reflection of electromagnetic waves incident from the electromagnetic wave reflection suppression layer side, thereby improving the electromagnetic wave absorption characteristics.

[0056] EXAMPLES Hereinafter, a specific example will be described in which an electromagnetic wave absorbing sheet was actually produced and its electromagnetic wave absorption characteristics were measured for 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]

[0059] (Preparation of Electromagnetic Wave Absorbing Sheet) The magnetoplumbite-type hexagonal ferrite contained in the electromagnetic wave absorbing sheet is a ferrite having the above-mentioned general formula "A (1-x) La x Fe (n-y) Al y The strontium ferrite used was one in which the Sr site was substituted with La, as shown in the particle diameter D 50 The average particle size was 3.1 μm, and the volume content was as shown in Table 1. In Table 1, the electromagnetic wave absorbing sheets shown as Comparative Example 1 and Comparative Example 2 used magnetoplumbite-type hexagonal ferrite in which the Sr site was not substituted with La.

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

[0061] To these materials, 2.5-dimethyl-2.5-di(t-butylperoxy)hexane (Shin-Etsu Chemical Co., Ltd., product name C-8A) as a vulcanizing agent and stearic acid (Kao Corporation, product name Lunac S-50V) and fatty acid amide (Fuso Chemical Co., Ltd., product name Plastron S) as processing aids were added, and the mixture was kneaded in a batch kneader at 35 rpm while running cold water to prevent the temperature from rising too much.The mixture was then calendered to a sheet thickness of 1.5 to 2.0 mm, vulcanized in a hot press at 160°C / 10 minutes, and then secondary vulcanized in a hot bath at 200°C / 4 hours to produce a 1.5 to 2.0 mm thick, 150 mm square sheet.

[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) The transmission attenuation of the electromagnetic wave absorbing sheet as the amount of electromagnetic wave absorption was measured using the free space method.

[0064] Measurements using the free space method were performed using a free space measurement device DPS24-01 (product name) manufactured by Keycom Corporation and a vector network analyzer MS46522B (product name) manufactured by Anritsu Corporation. Using two horn antennas, an input wave (millimeter wave) of a predetermined frequency was irradiated perpendicularly over a 100 mm diameter area onto a sample electromagnetic wave absorbing sheet fabricated from a transmitting / receiving antenna via a dielectric lens. The transmitted wave that passed through the sample was measured, and the intensity of the input wave was compared with the intensity of the transmitted wave to determine the transmission attenuation, which is the degree of attenuation, in dB. The obtained transmission attenuation (dB) was divided by the measured thickness of the electromagnetic wave absorbing sheet to obtain the transmission attenuation per mm of 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) at normal incidence. The vector network analyzer was subjected to TRL calibration in advance, and then gate processing in the time domain (gate settings: center 0 m, span 9 cm) was performed to remove signals due to multiple reflections.

[0065] The complex relative permittivity and complex relative permeability of the sample electromagnetic wave absorbing sheet were also measured by irradiating the sample with electromagnetic waves perpendicularly using the free space method in the same manner as above, and calculating the complex relative permittivity (real part, imaginary part) and complex relative permeability (real part, imaginary part) from the transmission characteristics (phase and amplitude) of the electromagnetic waves in the electromagnetic wave absorbing sheet.

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

[0067] The complex relative permeability was measured by measuring the amplitude and phase of the transmitted electromagnetic wave (S21) at normal incidence in the frequency band from 60 GHz to 90 GHz, and the complex relative permeability (real part, imaginary part) was calculated from the phase, amplitude, and average relative permittivity from 55 GHz to 65 GHz using permeability calculation software from Keycom Corporation, in the same way as for the permittivity.

[0068] The half-widths of the imaginary part of the complex relative magnetic permeability (μ'') and the transmission attenuation were determined as the frequency ranges intermediate between the peak values ​​and the baselines of the imaginary part of the complex relative magnetic permeability (μ'') and the transmission attenuation, respectively, at 25°C and 105°C, with the average values ​​of the measured values ​​at 60 GHz and 90 GHz being used as the baselines.

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

[0070] Furthermore, the "peak value change rate (%)" is the value "(μ'')" of the above-mentioned formula (2). 105 -μ'' 25 ) / μ'' 25× 100", that is, in a curve showing the relationship between the frequency of the electromagnetic waves incident on the electromagnetic wave absorbing sheet and the value of the imaginary part of the complex relative magnetic permeability (μ''), the value of the imaginary part of the complex relative magnetic permeability (μ'') when the ambient temperature changes from 25°C to 105°C reaches its peak, i.e., it indicates a value obtained by dividing the magnitude of change in the value of the imaginary part of the complex relative magnetic permeability (μ'') at the frequency at which it reaches an extreme value, by the peak value of the imaginary part of the complex relative magnetic permeability (μ'') at 25°C.

[0071] Furthermore, the "permeability change rate (%)" is the value "(μ'')" of the above-mentioned formula (3). 105(f25) -μ'' 25 ) / μ'' 25 In the curve showing the relationship between the frequency of the electromagnetic wave incident on the electromagnetic wave absorbing sheet and the value of the imaginary part of the complex relative magnetic permeability (μ''), the value of the imaginary part of the complex relative magnetic permeability (μ'') peaks when the ambient temperature is 25°C, that is, the value of the imaginary part of the complex relative magnetic permeability (μ'') at the frequency where the value is an extreme value when the ambient temperature is changed from 25°C to 105°C is divided by the value at 25°C, and the absolute value is shown.

[0072] The "rate of change in attenuation (%)" indicates the value obtained by dividing the change in the maximum transmission attenuation value, where the transmission attenuation peaks, i.e., the extreme value, on a curve showing the relationship between the frequency of electromagnetic waves incident on an electromagnetic wave absorbing sheet and the value of transmission attenuation, when the ambient temperature changes from 25°C to 105°C, by the value of the maximum transmission attenuation at 25°C.

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

[0074] FIG. 2 is a diagram showing 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 of Example 2 shown in Table 1.

[0075] FIG. 3 is a diagram showing the relationship between the value of the transmission attenuation per mm of thickness and the change in frequency of the incident electromagnetic wave in the electromagnetic wave absorbing sheet of Example 2 shown in Table 1.

[0076] FIG. 4 is a diagram showing 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 of Comparative Example 1 shown in Table 1.

[0077] FIG. 5 is a diagram showing the relationship between the value of the transmission attenuation per mm of thickness and the change in frequency of the incident electromagnetic wave in the electromagnetic wave absorbing sheet of Comparative Example 1 shown in Table 1.

[0078] In a curve indicated by reference numeral 21 in Fig. 2 which shows the relationship between the frequency of the incident electromagnetic waves and the value of the imaginary part of the complex relative magnetic permeability (μ'') when the ambient temperature of the electromagnetic wave absorbing sheet of Example 2 is 25°C, there is a peak (extreme value) at a frequency of 75.15 GHz, and the value of the imaginary part of the complex relative magnetic permeability (μ'') at this time is 0.131. In addition, in a curve indicated by reference numeral 22 in Fig. 2 which shows the relationship between the frequency of the incident electromagnetic waves and the value of the imaginary part of the complex relative magnetic permeability (μ'') of the electromagnetic wave absorbing sheet of Example 2 when the ambient temperature is 105°C, there is a peak (extreme value) at a frequency of 75.68 GHz, and the value of the imaginary part of the complex relative magnetic permeability (μ'') at this time is 0.118. Therefore, when the ambient temperature changes from 25°C to 105°C, the amount of change in frequency at which the imaginary part of the complex relative permeability (μ'') peaks is 0.53 GHz, and the rate of change in 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 amount of change in the value of the imaginary part of the complex relative permeability (μ'') at its peak is 0.013, and the rate of change in peak value relative to the imaginary part of the complex relative permeability (μ'') at 25°C is 9.9%.

[0079] The change in transmission attenuation per unit thickness of the electromagnetic wave absorbing sheet of Example 2 with respect to the change in frequency of the electromagnetic wave has a peak (extreme value) at a frequency of 75.64 GHz, and the value of the transmission attenuation at this time is −10.405 dB / mm, as shown in the curve indicated by reference numeral 31 in Fig. 3 which shows the relationship between the frequency of the incident electromagnetic wave and the value of the transmission attenuation when the ambient temperature is 25°C. Also, the curve indicated by reference numeral 32 in Fig. 3 which shows the relationship between the frequency of the incident electromagnetic wave and the value of the transmission attenuation when the ambient temperature of the electromagnetic wave absorbing sheet of Example 2 is 105°C has a peak (extreme value) at a frequency of 76.16 GHz, and the value of the 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 value of the transmission attenuation which reaches its peak at each temperature is 0.581 dB / mm, and the rate of change in transmission attenuation is 5.6% relative to the transmission attenuation at 25° C. Note that the sheet thickness required to obtain a transmission attenuation of 6 dB with the electromagnetic wave absorbing sheet, calculated from the transmission attenuation per mm at a frequency of 76.16 GHz at which the transmission attenuation reaches its peak at 25° C., is 0.577 mm, and the sheet thickness required to obtain a transmission attenuation of 6 dB when the ambient temperature reaches 105° C. at that frequency is 0.610 mm, and the rate of increase in thickness of the electromagnetic wave absorbing sheet required to obtain a transmission attenuation of 6 dB when the ambient temperature changes from 25° C. to 105° C. is 5.9%.

[0081] 4 and 5 show the relationship between the frequency of the 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 relationship with the transmission attenuation, for the electromagnetic wave absorbing sheet of Comparative Example 1, in which part of the Sr sites of the magnetoplumbite-type hexagonal ferrite contained in the electromagnetic wave absorbing sheet is not substituted with La.

[0082] In a curve indicated by reference numeral 41 in Fig. 4 , which shows the relationship between the frequency of the incident electromagnetic waves and the value of the imaginary part of the complex relative magnetic permeability (μ") of the electromagnetic wave absorbing sheet of Comparative Example 1 when the ambient temperature of the electromagnetic wave absorbing sheet is 25°C, there is a peak (extreme value) at a frequency of 76.16 GHz, and the value of the imaginary part of the complex relative magnetic permeability (μ") at this time is 0.302. In addition, in a curve indicated by reference numeral 42 in Fig. 4 , which shows the relationship between the frequency of the incident electromagnetic waves and the value of the imaginary part of the complex relative magnetic permeability (μ") of the electromagnetic wave absorbing sheet of Comparative Example 1 when the ambient temperature of the electromagnetic wave absorbing sheet is 105°C, there is a peak (extreme value) at a frequency of 78.15 GHz, and the value of the imaginary part of the relative magnetic permeability (μ") at this time is 0.254. Therefore, when the ambient temperature changes from 25°C to 105°C, the change in 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%. Furthermore, 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 its 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 relative to the change in frequency of the electromagnetic waves in the electromagnetic wave absorbing sheet of Comparative Example 1 has a peak (extreme value) at a frequency of 76.39 GHz, and the value of the transmission attenuation per unit thickness at this time is −10.100 dB / mm, as shown in the curve indicated by reference numeral 51 in Fig. 5 which shows the relationship between the frequency of the incident electromagnetic wave and the value of the transmission attenuation when the ambient temperature is 25°C. Furthermore, the curve indicated by reference numeral 52 in Fig. 5 which shows the relationship between the frequency of the incident electromagnetic wave and the value of the transmission attenuation when the ambient temperature of the electromagnetic wave absorbing sheet of Comparative Example 1 is 105°C, has a peak (extreme value) at a frequency of 78.58 GHz, and the value of the transmission attenuation 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 is 20.8% relative to the transmission attenuation at 25° C. Note that the sheet thickness required to obtain a transmission attenuation of 6 dB with the electromagnetic wave absorbing sheet, calculated from the transmission attenuation per mm at a frequency of 76.39 GHz at which the transmission attenuation peaks at 25° C., is 0.594 mm, and the sheet thickness required to obtain a transmission attenuation of 6 dB when the ambient temperature becomes 105° C. at that frequency is 0.750 mm, and the rate of increase in thickness of the electromagnetic wave absorbing sheet required to obtain a transmission attenuation of 6 dB when the ambient temperature changes from 25° C. to 105° C. is 26.3%.

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

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

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

[0089] In order 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 possible to increase the thickness of the electromagnetic wave absorbing sheet and thereby increase the amount of magnetoplumbite hexagonal ferrite contained in the electromagnetic wave absorbing sheet. By increasing the thickness of the electromagnetic wave absorbing sheet at the same volume ratio, the amount of magnetoplumbite hexagonal ferrite that absorbs electromagnetic waves increases, improving the electromagnetic wave absorption capacity of the electromagnetic wave absorbing sheet. As a result, a sheet can be designed that can effectively absorb electromagnetic waves of a predetermined frequency even when the electromagnetic wave absorption amount (absorption capacity) of the magnetoplumbite hexagonal ferrite that absorbs electromagnetic waves of a predetermined frequency decreases due to changes in the ambient temperature.

[0090] Assuming that the inventors were designing an electromagnetic wave absorbing sheet with a transmission attenuation (absolute value) of 6 dB or more as a guideline for the transmission attenuation of electromagnetic waves, i.e., one that absorbs half of the incident electromagnetic waves, they grasped the change in transmission attenuation of an electromagnetic wave absorbing sheet whose transmission attenuation peaks at an ambient temperature of 25°C when the ambient temperature is raised to 105°C as the amount of change in the value of the imaginary part (μ'') of the relative permeability of the magnetoplumbite-type hexagonal ferrite, and measured the relationship with the percentage increase in thickness required to obtain an electromagnetic wave absorbing sheet that would obtain a transmission attenuation of 6 dB even at 105°C at the absorption peak frequency at 25°C.

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

[0092] FIG. 6 is a plot of the "peak frequency change rate (%)" and the "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 Fig. 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 increase in the thickness of the electromagnetic wave absorbing sheet required to absorb electromagnetic waves of that frequency at 105°C remains at 8% or less, with almost no change. On the other hand, if the change in the frequency at which the imaginary part of the relative permeability (μ'') peaks exceeds 1.5% when the ambient temperature changes from 25°C to 105°C, the increase in the required thickness increases sharply. Therefore, even when using a measure to increase the absolute value of the electromagnetic wave absorption amount by the electromagnetic wave absorbing sheet and obtain an electromagnetic wave absorbing sheet with good electromagnetic wave absorption properties even when the ambient temperature changes by increasing the thickness of the electromagnetic wave absorbing sheet, it has been confirmed that a change in the frequency at which the imaginary part of the relative permeability (μ'') peaks of 1.5% or less when the ambient temperature changes from 25°C to 105°C is a condition for obtaining an electromagnetic wave absorbing sheet with good electromagnetic wave absorption properties.

[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 indicates 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 reaches a positive peak at 25°C is 15% or less.

[0095] Even when the operating temperature of the 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 magnetic permeability is small, the change in the electromagnetic wave absorption characteristics will be small. Therefore, in particular, by setting the ratio of the change in the value of the imaginary part of the complex relative magnetic permeability when the temperature changes from 25°C to 105°C to the value of the imaginary part of the complex relative magnetic permeability that reaches a positive peak at 25°C to 15% or less, good electromagnetic wave absorption characteristics can be exhibited over a wide temperature range. If the change in the extreme value of the imaginary part of the complex relative magnetic permeability exceeds 15%, the change in the electromagnetic wave absorption characteristics due to temperature will be large, and good electromagnetic wave absorption characteristics cannot be exhibited over a wide temperature range.

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

[0097] By setting the absolute value of the change in the imaginary part of the complex relative permeability at the peak frequency when the temperature is 25°C when the ambient temperature changes from 25°C to 105°C to 14% or less relative to the value at 25°C, 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 becomes small, and the change in the electromagnetic wave absorption characteristics due to temperature becomes small. In contrast, if the change is greater than 14%, the change in the electromagnetic wave absorption characteristics due to temperature becomes large, and it becomes impossible to exhibit good electromagnetic wave absorption characteristics over a wide temperature range.

[0098] Furthermore, in the electromagnetic wave absorbing sheet according to this embodiment, 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, it is preferable that the half-value width when the temperature of the electromagnetic wave absorber is 25°C and the half-value width when the temperature of the electromagnetic wave absorber is 105°C are both 8.5 GHz or more.

[0099] When the half-width of the electromagnetic wave absorber is 8.5 GHz or more at temperatures of both 25°C and 105°C, it is possible to absorb electromagnetic waves in a wide frequency band. As a result, even if the electromagnetic wave absorption characteristics of the electromagnetic wave absorber change due to temperature changes, good electromagnetic wave absorption characteristics can be maintained. If the half-width is less than 8.5 GHz, the electromagnetic wave absorption characteristics are significantly affected by temperature changes.

[0100] Furthermore, as the thickness of the electromagnetic wave absorbing sheet increases, the flexibility of the electromagnetic wave absorbing sheet decreases, making it difficult to position the electromagnetic wave absorbing sheet in the desired location. In particular, when the electromagnetic wave absorbing sheet is used to absorb unwanted electromagnetic waves from radar devices installed in automobiles, the adoption rate of which has rapidly increased due to recent technological advances, it is required to accurately position the electromagnetic wave absorbing sheet in an increasingly narrow space. In such cases, increasing the thickness of the electromagnetic wave absorbing sheet is undesirable, including the problem of increased manufacturing costs.

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

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

[0103] Furthermore, it was also confirmed that as the content ratio of carbon black as a permittivity adjuster contained in the electromagnetic wave absorbing sheet increases, the change in the frequency of the electromagnetic waves at which the value of the imaginary part of the complex relative magnetic permeability (μ'') of the magnetoplumbite-type hexagonal ferrite peaks when the ambient temperature changes from 25°C to 105°C increases. Therefore, by setting the content ratio of the permittivity adjuster contained in the electromagnetic wave absorbing sheet to 4.0% or less, it becomes possible to set the amount of change in the frequency at which the imaginary part of the complex relative magnetic permeability (μ'') peaks to 1.5% or less, which is the preferred condition.

[0104] As described above, the electromagnetic wave absorbing sheet according to this embodiment contains magnetoplumbite-type hexagonal ferrite in which some of the metal sites are substituted with La, and the frequency value at which a positive peak occurs 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 exhibits a positive rate of change when the temperature of the electromagnetic wave absorber changes from 25°C to 105°C, and the rate of the change in frequency at which a positive peak occurs when the temperature changes from 25°C to 105°C to the frequency value at which a positive peak occurs at 25°C is 1.5% or less, thereby making it possible to provide an electromagnetic wave absorbing sheet that can effectively absorb electromagnetic waves of a desired frequency even when the ambient temperature changes.

[0105] In the above embodiment, an electromagnetic wave absorbing sheet having a thin thickness relative to its area is exemplified, but the electromagnetic wave absorber disclosed in the present application can be realized as a so-called block-shaped electromagnetic wave absorber having a constant thickness relative to its area, and by satisfying the same requirements as the electromagnetic wave absorbing sheet described above, it is possible to realize an electromagnetic wave absorber that changes little in the amount of electromagnetic wave absorption with temperature changes and can effectively absorb electromagnetic waves of a desired frequency.

[0106] The electromagnetic wave absorber disclosed in the present application contains magnetoplumbite-type hexagonal ferrite in which some of the metal sites are substituted with La, and the frequency value at which a positive peak occurs 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 exhibits a positive rate of change when the temperature of the electromagnetic wave absorber changes from 25°C to 105°C, and the rate of the change in frequency at which a positive peak occurs when the temperature of the electromagnetic wave absorber changes from 25°C to 105°C to the frequency value at which a positive peak occurs at 25°C is 1.5% or less, thereby realizing an electromagnetic wave absorber that can effectively absorb electromagnetic waves of a desired frequency even when the ambient temperature changes. Therefore, the electromagnetic wave absorber is useful, for example, as an electromagnetic wave absorber that absorbs unwanted electromagnetic waves from an on-board radar installed in an automobile.

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

[0108] 1 Electromagnetic wave absorbing sheet (electromagnetic wave absorber) 1a Magnetoplumbite hexagonal ferrite 1b Binder

Claims

1. An electromagnetic wave absorber comprising a magnetoplumbite-type hexagonal ferrite in a binder that magnetically resonates in the millimeter wave frequency band, wherein the magnetoplumbite-type hexagonal ferrite has some metal sites substituted with La, and wherein 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 is changed from 25°C to 105°C, the frequency value at which a positive peak occurs exhibits a positive rate of change, and the rate of change in the frequency at which a positive peak occurs when the temperature is changed from 25°C to 105°C relative to the frequency value at which a positive peak occurs at 25°C is 1.5% or less.

2. An electromagnetic wave absorber as described in claim 1, wherein, in a graph showing the relationship between the frequency of the 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 exhibits a negative rate of change when the temperature of the electromagnetic wave absorber changes from 25°C to 105°C, and the rate of the amount of change in the value of the imaginary part of the complex relative permeability at the positive peak when changed from 25°C to 105°C to the value of the imaginary part of the complex relative permeability which reaches a positive peak at 25°C is 15% or less.

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

4. An electromagnetic wave absorber according to any one of claims 1 to 3, wherein, at a frequency which produces a positive peak in a graph showing the relationship between the frequency of the incident electromagnetic waves and the value of the imaginary part of the complex relative magnetic permeability of the electromagnetic wave absorber when the temperature of the electromagnetic wave absorber is 25°C, the absolute value of the ratio of the amount of change in the value of the imaginary part of the complex relative magnetic permeability when the temperature of the electromagnetic wave absorber is changed from 25°C to 105°C to the value of the imaginary part of the complex relative magnetic permeability at 25°C is 14% or less.

5. An electromagnetic wave absorber according to any one of claims 1 to 4, wherein in a graph showing the relationship between the frequency of the incident electromagnetic waves and the value of the imaginary part of the complex relative permeability of the electromagnetic wave absorber, the half-value width when the temperature of the electromagnetic wave absorber is 25°C and the half-value width when the temperature of the electromagnetic wave absorber is 105°C are both 8.5 GHz or more.

6. An electromagnetic wave absorber according to any one of claims 1 to 5, 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 frequency value at which a positive peak occurs is 75 GHz or more and 80 GHz or less.

7. The electromagnetic wave absorber according to any one of claims 1 to 6, wherein the binder of the electromagnetic wave absorber is at least one of rubber, plastic, and thermoplastic elastomer.

8. The electromagnetic wave absorber according to any one of claims 1 to 7, further comprising conductive carbon black as a dielectric constant adjuster.

9. The electromagnetic wave absorber according to any one of claims 1 to 7, further comprising at least one selected from the group consisting of furnace conductive carbon black, acetylene black, and ketjen black as a dielectric constant adjuster.

10. An electromagnetic wave absorber according to any one of claims 1 to 9, comprising at least an electromagnetic wave absorbing layer and a layer having an electromagnetic wave surface reflection suppressing function, and being in the form of a sheet with an overall thickness of 2 mm or less.

11. The electromagnetic wave absorber according to claim 10, wherein the total thickness of the electromagnetic wave absorber is 1 mm or less.

12. An electromagnetic wave absorber according to any one of claims 1 to 11, wherein the magnetoplumbite-type hexagonal ferrite content of said electromagnetic wave absorber is 89 mass % or less.