JP7144185B2 - Electromagnetic wave absorber and method for manufacturing electromagnetic wave absorber - Google Patents

Description

TECHNICAL FIELD The present invention relates to an electromagnetic wave absorber and a method for producing an electromagnetic wave absorber, and more particularly to an electromagnetic wave absorber that absorbs electromagnetic waves in the frequency band of 75 GHz to 80 GHz.

BACKGROUND ART In recent years, a collision avoidance support system has become popular as one of so-called advanced driver assistance systems (ADAS). A collision avoidance support system normally uses a millimeter-wave radar device to detect objects in front of and behind a vehicle (for example, other vehicles, pedestrians, obstacles, etc.). A millimeter-wave radar device includes an antenna that emits electromagnetic waves of a predetermined frequency (for example, 76 GHz) and an electromagnetic wave absorber that absorbs electromagnetic waves of that frequency, and irradiates electromagnetic waves with good directivity to an object. configured to allow Absorptivity refers to the property of neither transmitting nor reflecting electromagnetic waves of a predetermined frequency.

A resin composition as such an electromagnetic wave absorber is known from Patent Document 1, for example. In this device, particles as an absorbing material having an ability to absorb electromagnetic waves in a predetermined frequency band are dispersed in a first resin, and are isolated and dispersed in a second resin. In this case, conductive particles such as carbon particles are used as the particles, and a sea-island structure is formed in which the first resin in which such conductive particles are dispersed is the island phase and the second resin is the sea phase. The density of the conductive particles in the sea phase is high, and the density of the conductive particles in the sea phase is low.

The resin composition of the conventional example has the advantage of being able to absorb electromagnetic waves of GHz or higher over a wide band, but has a low absorption rate of about 40% for electromagnetic waves in a predetermined frequency band. Therefore, development of an electromagnetic wave absorber having a high absorption rate for electromagnetic waves in a predetermined frequency band is desired.

JP 2015-15373 A

In view of the above points, an object of the present invention is to provide an electromagnetic wave absorber having a high absorption rate for electromagnetic waves in the frequency band of 75 GHz to 80 GHz, and a method for manufacturing the electromagnetic wave absorber.

In order to solve the above problems, the electromagnetic wave absorber of the present invention is isolated and dispersed in a second resin in a state in which an absorbing material having absorbency for electromagnetic waves in a predetermined frequency band is dispersed in the first resin, The absorbent is a sintered body obtained by sintering carbon particles via silver fine particles , and the weight ratio of the silver fine particles to the weight of the first resin is in the range of 4 to 11% by weight, and the first The weight ratio of the first resin to the total weight of the resin and the second resin is in the range of 10 to 70% by weight, and the first resin is at least one selected from polyvinyl butyral and polyvinyl acetoacetal, The second resin is polymethyl methacrylate and has an absorption rate of 68% or more for electromagnetic waves in a frequency band of 75 GHz to 80 GHz .

According to the present invention, carbon particles are sintered with silver fine particles to form a low-resistance sintered body, and the sintered body is dispersed in the first resin. An electromagnetic wave absorber having a high absorption rate for electromagnetic waves in the frequency band of is obtained. According to the examples described later, it is confirmed that the absorption rate of electromagnetic waves in the frequency band of 75 GHz to 80 GHz is 68% or more, and by optimizing the weight ratio of the first resin, the absorption rate is 90% or more. Absorbance was confirmed. In addition, since the sintered body has the function of reflecting and scattering electromagnetic waves, it is possible to eliminate the need for a so-called backing layer made of metal, which is advantageous.

In the present invention, the carbon particles may be granular, powdery, spherical, rod-shaped, flat plate-shaped, fibrous, hollow, angular or massive. Examples include carbon black, acetylene black, carbon It can be selected from nanotubes, carbon nanofibers, graphene and fullerene. Further, the average particle size of the fine silver particles is preferably 1 nm to 100 nm. In this case, it is preferable to contain 4 to 11 parts by weight of fine silver particles with respect to 100 parts by weight of the first resin, and it is preferable to contain 4 to 11 parts by weight of carbon particles with respect to 100 parts by weight of the first resin.

In the present invention, if the weight ratio of the first resin to the total weight of the first resin and the second resin is out of the range of 10 to 70% by weight, the absorbency may decrease. According to the examples described later, the electromagnetic wave absorption rate is 80% or more by setting the weight ratio of the first resin in the range of 20 to 40% by weight, and 90% (10 dB) or more by setting it to 30% by weight. was confirmed to have an electromagnetic wave absorption rate of

The method for producing an electromagnetic wave absorber of the present invention for producing the electromagnetic wave absorber includes steps of producing carbon particles to which silver fine particles are attached; mixing the carbon particles to which the silver fine particles are attached and a first resin, and A step of obtaining a mixture, a step of adding a second resin to the first mixture and mixing to obtain a second mixture, a step of molding the second mixture into a predetermined shape and drying, and a dried body obtained by drying is heated at a predetermined temperature to sinter the carbon particles with the silver fine particles.

In the present invention, the step of producing the carbon particles to which the silver fine particles are attached comprises dispersing the silver fine particles covered with a surfactant in a low-polar solvent to obtain a dispersion, and mixing the carbon particles into this dispersion. and a step of adding a polar solvent to the dispersion liquid in which the carbon particles are mixed to precipitate the carbon particles to which the surfactant-coated silver fine particles are attached. In this case, the low-polarity solvent is at least one liquid hydrocarbon selected from octane, nonane, decane, undecane, dodecane, tridecane, tetradecane, toluene, xylene, cyclododecane, cyclododecene, octylbenzene, and dodecylbenzene. Or they can be used in combination.

When the dried body is heated at a temperature of 180 to 300° C., at least one selected from fatty acids having 6 to 18 carbon atoms and aliphatic amines having 6 to 18 carbon atoms is used as the surfactant. is preferred. According to this, even when heated at a relatively low temperature of 180 to 300° C., the surfactant can be detached from the fine silver particles, and the surfactant can be prevented from remaining attached to the fine silver particles. .

The figure which shows typically the electromagnetic wave absorber of embodiment of this invention. Process drawing explaining the manufacturing method of the electromagnetic wave absorber of embodiment of this invention. (a) is a schematic diagram explaining step 1 shown in FIG. 2, (b) is a schematic diagram explaining step 3 shown in FIG. (a) is a graph showing the relationship between the weight ratio of the first resin and the electromagnetic wave absorption rate at 76 GHz, and (b) is a graph showing the relationship between the weight ratio of the first resin and the electromagnetic wave absorption rate at 77 GHz. and (c) is a graph showing the relationship between the weight ratio of the first resin and the electromagnetic wave absorption rate at 79 GHz. (a) and (b) are SEM images of the electromagnetic wave absorbing sheet produced in Example 2 of the present invention. FIG. 10 is a STEM image showing a sintered body dispersed in the electromagnetic wave absorbing sheet produced in Example 2 of the present invention. FIG. (a) is a graph showing the electromagnetic wave absorption rate obtained in Example 2 and Comparative Example 2 of the present invention, (b) is a graph showing the electromagnetic wave reflectance, and (c) is the electromagnetic wave transmittance. Graph showing.

Hereinafter, electromagnetic wave absorbers according to embodiments of the present invention will be described with reference to the drawings. As shown in FIG. 1, the electromagnetic wave absorber EA is isolated and dispersed in a second resin 12 in a state in which an absorbing material Ma, which absorbs electromagnetic waves in a predetermined frequency band, is dispersed in a first resin 11. It becomes As a result, a sea-island structure (phase separation structure) is formed in which the first resin 11 in which the absorbent material Ma is dispersed serves as an island phase, and the second resin 12 serves as a sea phase. Such a sea-island structure is obtained by mixing mutually incompatible resins as described later.

As the first resin 11, it is preferable to use one having an acetal group having a lone pair of electrons as a functional group. For example, at least one selected from polyvinyl butyral, polyvinyl acetoacetal, and the like can be used. The silver fine particles 22, which will be described later, are easily coordinated to the functional groups having such lone electron pairs, and the aggregation of the silver fine particles 22 in the first resin 11 can be suppressed. Dispersibility can be enhanced. As the second resin 12, it is preferable to use a material that is incompatible with the first resin 11. For example, polymethyl methacrylate can be used.

By the way, it has been found that when carbon particles are used as the absorbing material Ma dispersed in the first resin 11, the electromagnetic wave absorption rate is low. Therefore, in the present embodiment, the absorbent Ma dispersed in the first resin 11 is a sintered body obtained by sintering the carbon particles 21 with the silver fine particles 22 interposed therebetween. As the carbon particles 21, granular, powdery, spherical, rod-like, flat plate-like, fibrous, hollow, angular, or massive particles can be used, and examples thereof include carbon black, acetylene black, carbon nanotubes, and carbon nanofibers. , carbon filler, graphene and fullerene.

As the fine silver particles 22, those having an average particle diameter within the range of 1 nm to 100 nm can be used. If the average particle diameter is less than 1 nm, the specific surface area of the silver fine particles 22 becomes large, and the content of the surfactant 23 described later covering the surface of the silver fine particles 22 becomes too large. , the surfactant 23 may not be sufficiently desorbed, and if the average particle size exceeds 100 nm, the sintering of the carbon particles 21 through the silver fine particles 22 may become insufficient. The average particle size is a value obtained based on particle size analysis by the dynamic light scattering method of JISZ8828.

According to the above, when an electromagnetic wave is incident on the first resin 11 as an island phase, as a result of acceleration of all electrons in the absorber Ma, the carbon particles 21 dispersed in the first resin 11 ( A part of the electromagnetic wave is absorbed by the current flowing through the resistor) and converting the electromagnetic wave energy into heat energy. At this time, the carbon particles 21 are sintered through the silver fine particles 22 to form a low-resistance sintered body Ma. becomes easier to flow), and the electromagnetic wave absorption rate can be increased. The sintered body Ma not only absorbs electromagnetic waves, but also has the function of reflecting and scattering electromagnetic waves. In addition, the internal reflection and scattering between the solids (sintered body Ma) existing in the island phase 11 become electromagnetic waves radiated as a group, and other island phases (first resin) 11. Such absorption, reflection, and scattering of electromagnetic waves occur multiple times in the plurality of island phases 11 dispersed in the second resin 12 . As a result, the electromagnetic wave absorber EA can obtain a high absorption rate for electromagnetic waves in a predetermined frequency band. Furthermore, since the sintered body Ma has the function of reflecting and scattering electromagnetic waves as described above, for example, a so-called backing layer made of metal can be eliminated. Therefore, it is possible to prevent a decrease in absorptivity caused by a gap between the backing layer and the resin, which is advantageous.

Next, referring to FIG. 2, the method for manufacturing the electromagnetic wave absorber EA will be described by taking the case of manufacturing an electromagnetic wave absorbing sheet as an example. First, in step S1, the fine silver particles 22 whose surfaces are coated with the surfactant 23 are dispersed in the low-polar solvent 24 to prepare a dispersion liquid, and the carbon particles 21 are stirred and mixed in the prepared dispersion liquid ( See FIG. 3(a)). A known homogenizer can be used for stirring and mixing. It is also possible to prepare a dispersion in which the fine silver particles 22 coated with the surfactant 23 are previously dispersed in the low-polarity solvent 24, and to stir and mix the carbon particles 21 into this dispersion.

Here, the mixing ratio of the silver fine particles 22 is preferably set in the range of 4 to 11 parts by weight with respect to 100 parts by weight of the first resin (11) added later. It is preferable to set the amount in the range of 4 to 11 parts by weight with respect to 100 parts by weight of 1 resin (11). Outside these ranges, the electromagnetic wave absorptivity may decrease. As the surfactant 23, it is preferable to use at least one selected from fatty acids having 6 to 18 carbon atoms and aliphatic amines having 6 to 18 carbon atoms. Examples of fatty acids having 6 to 18 carbon atoms include hexanoic acid, 2-ethylbutyric acid and neohexanoic acid having 6 carbon atoms; heptanoic acid, 2-methylhexanoic acid and cyclohexanecarboxylic acid having 7 carbon atoms; Octanoic acid, neooctanoic acid, 2-ethylhexanoic acid; C9 nonanoic acid; C10 neodecanoic acid, decanoic acid; C11 undecanoic acid; C12 neododecanoic acid, dodecanoic acid; tetradecanoic acid having 16 carbon atoms; palmitic acid having 18 carbon atoms; and stearic acid having 18 carbon atoms, oleic acid, linoleic acid, and linolenic acid. Examples of aliphatic amines having 6 to 18 carbon atoms include hexylamine, cyclohexylamine and aniline having 6 carbon atoms; heptylamine having 7 carbon atoms; octylamine and 2-ethylhexylamine having 8 carbon atoms; decylamine having 10 carbon atoms; dodecylamine having 12 carbon atoms; tetradodecylamine having 14 carbon atoms; and stearylamine and oleylamine having 18 carbon atoms. . Fatty acids and aliphatic amines having less than 6 carbon atoms may reduce the dispersibility of the fine silver particles 22 in the low-polar solvent 24, whereas fatty acids and aliphatic amines having 19 or more carbon atoms may cause relatively If the heat treatment is performed at a low temperature, the surfactant 23 (fatty acid or aliphatic amine) may not be sufficiently desorbed from the surface of the fine silver particles 22, and the resistance of the sintered body Ma may increase. As the low-polarity solvent 24, for example, at least one selected from octane, nonane, decane, undecane, dodecane, tridecane, tetradecane, toluene, xylene, octylbenzene, dodecylbenzene, decalin, tetralin, cyclododecane, cyclohexylbenzene and cyclododecene. Seeds can be used singly or in combination.

After that, the polar solvent 25 is added (step S2), and after sufficiently stirring, the mixture is allowed to stand still for a predetermined time (for example, 2 to 12 hours). As a result, as shown in FIG. 3B, the carbon particles 21 to which the silver fine particles 22 are attached settle (step S3). As the polar solvent 25, for example, at least one selected from alcohols such as methanol, ethanol and propanol, and ketones such as acetone, methyl ethyl ketone, methyl isopropyl ketone and methyl isobutyl ketone can be used. By removing the supernatant liquid by decantation or the like (step S4), carbon particles 21 to which silver microparticles 22 are attached are produced. The steps from the addition of the polar solvent 25 (step S2) to the removal of the supernatant (step S4) may be repeated multiple times.

The first resin 11 is added to the carbon particles 21 to which the silver fine particles 22 thus produced are adhered (step S5), and sufficiently stirred and mixed to obtain a first mixture (step S6). A known homogenizer can be used for this stirring and mixing. As the first resin 11, a resin dissolved in advance in a solvent can be used.

The second resin 12 is added to the first mixture obtained in step S6 (step S7) and sufficiently stirred and mixed to obtain a second mixture (step S8). A known homogenizer can be used for this stirring and mixing. As the second resin, a resin dissolved in advance in a solvent or a powdery resin can be used.

The second mixture obtained in step S8 is flowed down into a predetermined container to be shaped, and dried for a predetermined time (for example, 6 to 12 hours) to obtain a dried product (step S9). The temperature during drying can be set in the range of 60 to 100°C. Finally, the dried body is heat-pressed at a temperature of 180-300° C. and a pressure of 5-15 MPa (step S10). As a result, the carbon particles 21 are sintered with the fine silver particles 22, and the surfactant 23 is detached from the fine silver particles 22 to obtain an electromagnetic wave absorbing sheet. In step S10, it is not always necessary to pressurize, but by pressurizing, the distance between the carbon particles 21 becomes shorter and sintering becomes easier.

Hereinafter, examples that embody the embodiments of the present invention will be described by taking an electromagnetic wave absorbing sheet as an example.

(Example 1)
Granular carbon particles (manufactured by Cabot under the trade name “Vurcan XC-72R”) were blended with 11 parts by weight of fine silver particles (manufactured by Ulvac under the trade name of “Ag nano metal ink L-Ag1T”), and a small amount ( 100 parts by weight) of toluene was added, and the mixture was sufficiently stirred and mixed at 25° C. with a homogenizer (for 10 minutes or longer). Acetone five times the weight of toluene (500 parts by weight) was added to the solution, and the mixture was sufficiently stirred. After the stirring was stopped, the mixture was allowed to stand for 12 hours to allow the carbon particles with the silver fine particles to settle. After removing the supernatant liquid, 10 parts by weight of polyvinyl butyral (trade name "polyvinyl butyral 630" manufactured by Wako Pure Chemical Industries, Ltd.) as the first resin dissolved in methyl ethyl ketone in advance (converted to parts by weight of the first resin only) was added. , sufficiently (10 minutes or longer) with a homogenizer to obtain a first mixture. To this first mixture, 90 parts by weight (converted to parts by weight of the second resin only) of polymethyl methacrylate (trade name "LG2" manufactured by Sumitomo Chemical Co., Ltd.) as a second resin dissolved in methyl ethyl ketone in advance was added, and the mixture was homogenized with a homogenizer. A second mixture was obtained by sufficiently stirring and mixing (10 minutes or more). This second mixture was poured down into a film container of 150 mm×150 mm×50 mm and dried at 60° C. for 12 hours to obtain a dried product. This dried body was heat-pressed at 200° C. and 10 MPa for 1 hour to prepare an electromagnetic wave absorbing sheet of 150 mm×150 mm×1 mm. In Example 1, the weight ratios of the first resin and the second resin are 10% by weight and 90% by weight.

(Example 2)
In Example 2, 30 parts by weight of polyvinyl butyral (trade name "polyvinyl butyral 630" manufactured by Wako Pure Chemical Industries, Ltd.) is added as the first resin, and polymethyl methacrylate (product of Sumitomo Chemical Co., Ltd.) is added as the second resin. An electromagnetic wave absorbing sheet was produced in the same manner as in Example 1 above, except that 70 parts by weight of LG2 was added.

(Example 3)
In Example 3, 50 parts by weight of polyvinyl butyral (trade name "Polyvinyl butyral 630" manufactured by Wako Pure Chemical Industries) is added as the first resin, and polymethyl methacrylate (trade name manufactured by Sumitomo Chemical Co., Ltd.) is added as the second resin. An electromagnetic wave absorbing sheet was produced in the same manner as in Example 1 above, except that 50 parts by weight of LG2 was added.

Next, a comparative example for the above example will be described.

(Comparative example 1)
In this Comparative Example 1, 100 parts by weight of polyvinyl butyral (trade name “Polyvinyl butyral 630” manufactured by Wako Pure Chemical Industries) as the first resin is added, and polymethyl methacrylate (trade name manufactured by Sumitomo Chemical Co., Ltd.) is added as the second resin. An electromagnetic wave-absorbing sheet was produced in the same manner as in Example 1 above, except that (0 weight parts) was not added.

(Comparative example 2)
In Comparative Example 2, silver fine particles (trade name “Ag nano metal ink L-Ag1T” manufactured by ULVAC) are not added (0 parts by weight), that is, carbon particles 21 are not sintered. An electromagnetic wave absorbing sheet was produced in the same manner as in Example 2 above.

Next, the electromagnetic wave absorption rate in the frequency bands of 76 GHz, 77 GHz and 79 GHz of the electromagnetic wave absorbing sheets produced in Examples 1 to 3 and Comparative Example 1 was measured by a known free space method/S parameter method. FIG. 4 shows the results of plotting the measured electromagnetic wave absorptivity. The horizontal axis of FIG. 4 is the weight ratio of the first resin to the total weight of the first resin and the second resin. According to this, the weight ratio of the first resin is preferably in the range of 10-70% by weight, more preferably 20-40% by weight, and most preferably 30% by weight. An electromagnetic wave absorption rate of 68% or more can be obtained by setting the weight ratio of the first resin to 10 to 70% by weight, and an electromagnetic wave absorption rate of 80% or more can be obtained by setting the weight ratio of the first resin to 20 to 40% by weight. , 30% by weight, an electromagnetic wave absorption rate of 90% (10 dB) or more can be obtained.

A cross section of the electromagnetic wave absorbing sheet produced in Example 2 was prepared by an ion polishing (IP) method, and the cross section was observed with a scanning electron microscope (SEM). FIG. 5 shows the backscattered electron image. As shown in FIG. 5(a), a sea-island structure (phase-separated structure) having island phases 11 made of polyvinyl butyral and sea phases 12 made of polymethyl methacrylate was obtained, and the diameter of the spherical island phases ranged from several μm to 100 μm. It was confirmed that island facies of various sizes and shapes were dispersed in the sea facies. Furthermore, for the part of the electron image shown in FIG. Line) images were observed, and it was confirmed that there was no clear difference in carbon concentration between the sea phase 12 and the island phase 11 . It is considered that this is because both the resins forming the sea phase 12 and the island phase 11 contain a large amount of carbon. On the other hand, it was confirmed that the sea phase 12 has a higher oxygen concentration than the island phase 11 and the island phase 11 has a higher silver concentration than the sea phase 12 . Furthermore, in the backscattered electron image shown in FIG. 5( a ), the island phase 11 is brighter than the sea phase 12 . Further, the sample having the cross section prepared above was observed with a scanning transmission electron microscope (STEM), and the STEM image is shown in FIG. According to this, it was confirmed that the fine silver particles 22 adhered to the carbon particles 21 to constitute the sintered body Ma. From the above, it was found that the sintered body Ma was dispersed in the island phase 11 .

Next, the electromagnetic wave absorption rate, electromagnetic wave reflectance, and electromagnetic wave transmittance in the frequency band of 75 GHz to 80 GHz of the electromagnetic wave absorbing sheets produced in Example 2 and Comparative Example 2 were measured by the free space method/S parameter method. The measured electromagnetic wave absorptivity, electromagnetic wave reflectance, and electromagnetic wave transmittance are shown in FIG. In this measurement method, the reflection coefficient S ₁₁ and the transmission coefficient S ₂₁ are obtained when the sample is irradiated with an electromagnetic wave of a predetermined frequency, and the electromagnetic wave absorption rate and the electromagnetic wave reflection are calculated using the following equations (1) to (3). It is a method to obtain the rate and electromagnetic wave transmittance.
Electromagnetic absorption rate=1-|S ₁₁ | ² -|S ₂₁ | ² (1)
Electromagnetic reflectance=|S ₁₁ | ² (2)
Electromagnetic wave transmittance=|S ₂₁ | ² (3)

As shown in FIG. 7A, it was confirmed that the electromagnetic wave absorbing sheet of Example 2 exhibits an electromagnetic wave absorbing rate of 90% (10 dB) or higher, which is higher than that of Comparative Example 2. As is clear from the above formulas (1) to (3), the electromagnetic wave absorption rate means the ratio of electromagnetic waves that are neither reflected nor transmitted. As shown in FIGS. 7B and 7C, it was confirmed that the comparative example 2 had higher reflectance and transmittance than the example 2. FIG. The reason for this is considered as follows. That is, when the absorbent is composed only of carbon particles as in Comparative Example 2, it can be considered as an equivalent circuit in which the resistance of the carbon particles themselves and the capacitance (capacitor) between the carbon particles are intricately coupled. . It is generally known that the impedance of a capacitor is inversely proportional to the frequency of electromagnetic waves. When the frequency of the electromagnetic wave is as low as 75 GHz to 80 GHz, it becomes difficult for current to flow through the carbon particles, which are resistors, and the electromagnetic wave is difficult to be absorbed, resulting in high reflectance and transmittance. On the other hand, as in Example 2, the absorber Ma is composed of a low-resistance sintered body in which carbon particles are sintered through silver fine particles to increase the conductivity, so that the carbon particles 21 that are resistors Since the current flows through the resistor, the energy of the electromagnetic wave is converted into heat energy, so that the reflectance and the transmittance can be lowered.

It should be noted that the present invention is not limited to the above embodiments. In the above embodiment, the electromagnetic wave absorber EA is formed into a sheet-shaped electromagnetic wave absorbing sheet, but the shape of the electromagnetic wave absorber EA is arbitrary. It can be anything.

Further, in the above-described embodiment, the case where the granular carbon particles 21 are sintered through the silver fine particles 22 to form the sintered body Ma has been described. The shape may be arbitrary.

Further, in the above-described embodiment, the case where the sintered bodies Ma are dispersed in the first resin 11 serving as the island phase has been described, but the sintered bodies Ma may be dispersed in the second resin 12 serving as the sea phase.

EA... Electromagnetic wave absorber, Ma... Absorbent, sintered body 11... First resin, island phase 12... Second resin, sea phase 21... Carbon particles, 22... Fine silver particles, 23... Surfactant, 24 ... low polarity solvent, 25 ... polar solvent.

Claims (4)

An electromagnetic wave absorber that is isolated and dispersed in a second resin in a state in which an absorbing material that absorbs electromagnetic waves in a predetermined frequency band is dispersed in the first resin,
The absorbent is a sintered body obtained by sintering carbon particles via silver fine particles ,
The weight ratio of the silver fine particles to the weight of the first resin is in the range of 4 to 11% by weight,
The weight ratio of the first resin to the total weight of the first resin and the second resin is in the range of 10 to 70% by weight,
The first resin is at least one selected from polyvinyl butyral and polyvinyl acetoacetal, the second resin is polymethyl methacrylate,
An electromagnetic wave absorber characterized by having an absorption rate of 68% or more for electromagnetic waves in a frequency band of 75 GHz to 80 GHz . A method for manufacturing an electromagnetic wave absorber for manufacturing the electromagnetic wave absorber according to claim 1 ,
a step of producing carbon particles to which silver fine particles are attached;
a step of mixing the carbon particles to which the silver fine particles are attached and a first resin to obtain a first mixture;
adding a second resin to the first mixture and mixing to obtain a second mixture;
molding the second mixture into a predetermined shape and drying;
A method for producing an electromagnetic wave absorber, comprising a step of heating a dried body obtained by drying at a predetermined temperature to sinter the carbon particles with the silver fine particles. The step of producing carbon particles to which the silver fine particles are attached includes:
a step of dispersing fine silver particles coated with a surfactant in a low-polar solvent to obtain a dispersion, and mixing carbon particles into the dispersion;
3. The method for producing an electromagnetic wave absorber according to claim 2 , further comprising the step of adding a polar solvent to the dispersion liquid containing the carbon particles and causing the carbon particles to settle, to which the silver fine particles coated with a surfactant are adhered. . The method for producing an electromagnetic wave absorber according to claim 3 , wherein the dried body is heated at a temperature of 180 to 300 ° C.,
A method for producing an electromagnetic wave absorber, wherein the surfactant is at least one selected from fatty acids having 6 to 18 carbon atoms and aliphatic amines having 6 to 18 carbon atoms.

Images