JP2005311088A - Electromagnetic wave absorber - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electromagnetic wave absorber which can be applied to dedicated short range communication (DSRC) or the like. <P>SOLUTION: The electromagnetic wave absorber is formed by charging boron carbide powder of 5-70 vol.% whose boron (B) content is 80 mass% or more and 96 mass% or less, and/or, the electromagnetic wave absorber is formed by containing boron carbide powder of 5-70 vol.% whose boron (B) content is 80 mass% or more and 96 mass% or less. Preferably, the matrix material of the electromagnetic wave absorber is thermoplastic resin, and especially preferably, in the electromagnetic wave absorber, the real part of complex specific dielectric constant in frequency of 5-110 GHz measured by a free space method is 8 or more and the imaginary part is 1 or more, and the dielectric loss tangent is 0.1 or more and 0.5 or less. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

The present invention relates to an electromagnetic wave absorber. More specifically, a boron carbide powder containing a specific amount of boron and capable of efficiently absorbing electromagnetic waves, and an electromagnetic wave absorber using the same are provided.

In recent years, in the field of semiconductors or electronics, the frequency of electromagnetic waves used in communication equipment has been remarkably advanced, and electromagnetic waves in the gigahertz (GHz) band that vibrates more than 1 billion times per second are frequently used. It has become. In the field of Intelligent Transport Systems (ITS), a communication method called dedicated narrow area communication (DSRC) is used, and roadside devices (wireless devices installed on the roadside) and on-board devices (wireless devices mounted on vehicles). Wireless communication is also progressing between the two.

DSRC is currently put into practical use in the automatic toll collection system (ETC) on expressways, but will be used for electronic payments, operation management, logistics management, and various information provisions at gas stations, car ferries or service areas in the future. Application is expected. The electromagnetic waves used in these applications are high-frequency, so that high-power and high-density signal transport is possible, but if they are taken into other devices as noise, they cause information leakage, malfunction, and other various types of radio interference. There are concerns.

As a countermeasure, an electromagnetic shielding material or an electromagnetic wave absorber is used so that an electronic device or a communication device is not interfered by an electromagnetic wave entering from the outside, or an electromagnetic wave generated by these devices is not excessively leaked to the outside. . In particular, the electromagnetic wave absorber converts incident electromagnetic waves into thermal energy, and greatly attenuates the intensity of transmitted or reflected electromagnetic waves.

Conventionally, ferrite and carbon are mainly used as a material for an electromagnetic wave absorber. In many cases, these powders are dispersed or combined in a matrix of resin, rubber, paint, or the like and pasted or applied to a site where electromagnetic waves are to be absorbed.

However, since ferrite has a large specific gravity, when it is dispersed in the matrix, sedimentation is likely to occur due to the difference in specific gravity with the matrix, and it is difficult to form a uniform composite material. When a large amount is used in a communication device that involves movement or in an automobile, the main body becomes heavy, causing problems in the mobility of the communication device or automobile.

On the other hand, for carbon, the specific gravity is relatively small, so that the above-mentioned problem as seen in ferrite does not occur. However, since the powder is bulky, it is difficult to increase the filling amount into the matrix. There is a drawback that the electromagnetic wave absorption characteristics of the composite material become insufficient. In order to avoid this, although crystalline graphite with relatively good filling properties may be used as carbon, graphite particles are large in anisotropy and easily oriented in the matrix. The electromagnetic wave absorption performance of the material is impaired.

Furthermore, an electromagnetic wave absorber containing ferrite or carbon is suitable for absorbing electromagnetic waves in the MHz band or 1 to several GHz band, but application in the DSRC, for example, ETC and in-vehicle radar is being studied 5.8 to 76 GHz. However, there is a problem that electromagnetic waves in a high frequency band such as the above cannot be sufficiently absorbed.

In addition, silicon carbide (SiC) and boron carbide (B ₄ C) are known to have electromagnetic wave absorption characteristics as materials other than those described above, and SiC and B ₄ C microwave heating elements utilizing this property are also available. It has been proposed (see Patent Document 1).
JP-A-8-106980

SiC and B ₄ C have a lower specific gravity than ferrite, and the powder is not as bulky as carbon, so the filling property is good and the anisotropy is small. I don't have it. However, in the case of using electromagnetic waves with different frequencies such as DSRC, the electromagnetic wave absorber for mobile communication devices such as in-vehicle devices needs to satisfy the characteristic of comprehensively absorbing unnecessary electromagnetic waves of various frequencies. In fact, such an electromagnetic wave absorber has not been produced yet even if SiC or B ₄ C is used.

An object of the present invention is to solve the above-mentioned problems of conventional electromagnetic wave absorbers and to provide a novel electromagnetic wave absorber having excellent electromagnetic wave absorption characteristics.

The present inventor considered various boron carbide powders and electromagnetic wave absorbers using the same in view of the current state of the prior art, and boron carbide powders having a specific composition are excellent in electromagnetic wave absorption performance. The inventors have obtained the knowledge that the object of the present invention can be achieved when obtaining an absorber, and have achieved the present invention.

That is, the present invention is a boron carbide powder for an electromagnetic wave absorber, wherein the boron (B) content is 80 mass% or more and 96 mass% or less.

In addition, the present invention is an electromagnetic wave absorber characterized in that the boron carbide powder having a boron (B) content of 80% by mass or more and 96% by mass or less is filled in 5 to 70% by volume, and is a matrix material. An electromagnetic wave absorber comprising a boron carbide powder having a boron (B) content of 80% by mass or more and 96% by mass or less in an amount of 5 to 70% by volume, preferably a matrix material The electromagnetic wave absorber as described above, which is a thermoplastic resin.

Furthermore, the present invention relates to the above electromagnetic wave, wherein the real part of the complex relative permittivity at a frequency of 5 to 110 GHz measured by the free space method is 8 or more, the imaginary part is 1 or more, and the dielectric loss tangent is 0.1 or more and 0.5 or less. Absorber.

In addition, the present invention is an electromagnetic wave absorber for dedicated short-range communication (DSRC) characterized by comprising the above-mentioned electromagnetic wave absorber.

The boron carbide powder of the present invention has a composition in a specific range in which the boron (B) content is 80% by mass or more and 96% by mass or less, and is excellent in electromagnetic wave absorption characteristics at a frequency of 5 to 110 GHz. It is possible to easily provide the electromagnetic wave absorber of the present invention having excellent electromagnetic wave absorption characteristics described later.

Since the electromagnetic wave absorber of the present invention uses the boron carbide powder having the above specific composition range and has a specific space filling property, the real part of the complex relative dielectric constant in the frequency band of 5 to 110 GHz is 8 As described above, since the electromagnetic wave absorption characteristic having an imaginary part of 1 or more and a dielectric loss tangent of 0.1 or more and 0.5 or less can be achieved, is there.

Furthermore, since the electromagnetic wave absorber of the present invention contains the boron carbide powder having the above characteristics in a matrix material, an electromagnetic wave having the above electromagnetic wave absorption characteristics corresponding to various uses can be selected by appropriately selecting the matrix material. An absorber can be provided easily. In particular, when a thermoplastic resin is selected as the matrix material, conventionally known molding methods and processing methods applicable to thermoplastic resins can be applied, and shapes applicable to various applications can be easily attached. it can.

In addition, in the electromagnetic wave absorber of the present invention, for example, as described above, the real part of the complex relative permittivity in the frequency band of 5 to 110 GHz is 8 or more, the imaginary part is 1 or more, and the dielectric loss tangent is 0.1 or more and 0. Since it has an electromagnetic wave absorption characteristic of .5 or less, it can be practically used as an electromagnetic wave absorber for dedicated narrow area communication (DSRC).

The boron carbide powder of the present invention has a composition in a specific range in which the boron (B) content is 80% by mass or more and 96% by mass or less, and is excellent in electromagnetic wave absorption characteristics at a frequency of 5 to 110 GHz. As described above, when the inventor studied variously about the electromagnetic wave absorber using boron carbide, the inventors found that the boron carbide powder having the composition has excellent electromagnetic wave absorption characteristics at a frequency of 5 to 110 GHz. is there. Although this cause is not clear, it is estimated as follows.

Electromagnetic wave absorption characteristics are said to involve the electrical conductivity, dielectric properties and magnetic properties of the material, but boron carbide powder containing 80% by mass or more of boron (B) contains a large amount of boron. It is considered that the electrical conductivity and dielectric properties are different from B ₄ C, which is common as boron carbide, and as a result of changes in both properties, properties suitable for electromagnetic wave absorption are developed.

B ₄ C, which is common as boron carbide, contains 78.3 mass% of boron (B). The crystal structure is a rhombohedron, 8 regular icosahedrons with 12 B atoms located at the apexes are arranged at the apexes of the unit cell, and 3 carbon (C) atoms are located on the long axis at the center of the lattice. It has an arranged structure. Then, it has been clarified that boron carbide of various compositions exists such that B ₁₃ C ₂ is obtained by substituting one of the three C atoms in the B ₄ C unit cell with a B atom to obtain B ₁₃ C _2. Yes.

The boron carbide powder of the present invention has a boron (B) content of 80% by mass or more and 96% by mass or less, that is, it is sufficient if a part of C atoms is substituted with an appropriate amount of B atoms. Furthermore, regarding the lower limit value of the composition range, it is preferable that the composition has more B than B _4.5 C, that is, B / C (molar ratio) ≧ 4.5.

On the other hand, as for the upper limit of the composition range, B ₈ is boron carbide _C, and B 10 _C or B _{25 C} is known, since none of these can be used in the present invention, B ₂₅ C The boron (B) content corresponding to is preferably 96% by mass or less.

A general method for synthesizing boron carbide powder is to use a raw material in which a boron content such as boric acid and a carbon content such as petroleum coke are mixed using an arc furnace, a resistance heating furnace, a high-frequency heating furnace, or the like. In this method, the following reaction is caused by heating to a high temperature.

4H ₃ BO ₃ + 7C → B ₄ C + 6CO + 6H ₂ O

However, since most of the boron carbide produced by this method is B ₄ C, general boron carbide is B ₄ C.

In contrast to the above method, for example, a method of directly carbonizing metal boron (direct carbonization method) or a method of rapidly reducing and carbonizing boron oxide with a strongly reducing substance such as metal magnesium (thermite reaction method) is used. The method is a preferable production method because the B content is easily controlled to 80% by mass or more and 96% by mass or less. In these methods, it is sufficient to prepare in advance the ratio of B atoms to C atoms contained in the raw material so that B is 80% by mass or more and 96% by mass or less. On the other hand, the general method of reacting boric acid and carbon described above is because carbon also serves as a reducing agent, and since the reaction itself needs to be performed under high temperature conditions, the components of carbon and boron are volatilized. There is a drawback that it is difficult to control the composition of boron carbide.

In addition, the boron carbide powder obtained by the direct carbonization method or the thermite reaction method can be produced by (acid treatment), pulverization, and sieving as necessary.

The boron carbide powder of the present invention can be used by simply filling the plastic or inorganic container with the boron carbide powder, filling the gaps of a plurality of resinous sheets, or as detailed later. An electromagnetic wave absorber can be obtained by combining with other materials. In these applications, the filling rate in the space of the boron carbide powder of 5 to 70% by volume is preferably selected because of excellent electromagnetic wave characteristics in the frequency band of 5 to 110 GHz. If the content is less than 5% by volume, sufficient electromagnetic wave absorption performance may not be obtained, and if it exceeds 70% by volume, molding or shape retention becomes difficult. The filling rate in the space of the boron carbide powder is more preferably 20 to 60% by volume.

As an embodiment of the electromagnetic wave absorber of the present invention, for example, a composite material in which 5 to 70% by volume of the boron carbide powder having the above characteristics is blended in a matrix material such as a resin, rubber, or paint. In the composite material, the blending ratio of the boron carbide powder is preferably 20 to 60% by volume for the same reason as described above.

Materials that can be used as a matrix are acrylic resin, polyethylene, polypropylene, bisphenol type epoxy resin, phenol novolac type epoxy resin, alicyclic type epoxy resin, heterocyclic type epoxy resin, glycidyl ester type epoxy resin, glycidyl amine type epoxy resin, Epoxy resins such as halogenated epoxy resins, polybenzimidazole, polybenzoxazole, polybenzthiazole, polyoxadiazole, polypyrazole, polyquinoxaline, polyquinazolinedione, polybenzoxazinone, polyindolone, polyquinazolone, polyindoxyl, silicon Resin, silicon-epoxy resin, phenol resin, melamine resin, urea resin, unsaturated polyester, polyamino bismaleimide, diallyl phthalate resin Fluorine resin, TPX resin (methyl pentene polymer “trade name made by Mitsui Petrochemical Co., Ltd.”), polyimide, polyamide imide, polyether imide, 66-nylon and MXD-nylon, amorphous nylon, polyamide, polybutylene terephthalate, polyethylene terephthalate, etc. Polyester, polyphenylene sulfide, modified polyphenylene ether, polyarylate, wholly aromatic polyester, polysulfone, liquid crystal polymer, polyether ether ketone, polyether sulfone, polycarbonate, maleimide modified resin, ABS resin, AAS (acrylonitrile, acrylic rubber, styrene) Resins, resins such as AES (acrylonitrile-ethylene-propylene-diene rubber-styrene) resin, butyl rubber, acrylic rubber, Elastomers such as tylene propylene rubber, silicone rubber, polyester elastomer, polybutadiene, chloroprene, natural rubber, polyisoprene, and as necessary, curing agents, curing accelerators, catalysts, vulcanizing agents, lubricants / release agents, stable Agents, light stabilizers, colorants, flame retardants, coupling agents, etc., but also glass such as soda glass, E glass, borosilicate glass, quartz glass, Inorganic materials such as clays such as clay, cement, alumina cement, mortar, and gypsum can also be used.

Among these matrix materials, it is easy to handle, film forming methods such as doctor blades, conventionally known forming methods and processing methods such as roll forming, extrusion forming, injection forming, press forming, etc. It is preferably selected because it can be applied in combination. Further, acrylic resin and xx resin are more preferably used as the thermoplastic resin because of good workability.

A predetermined amount of the boron carbide powder of the present invention is blended with the matrix material, mixed, and depending on the application, a molded product such as a film or a plate, or a liquid, paint, filler, etc. Used as a composite material.

When preparing the liquid composite material, mixing is possible by hand in the case of a small amount, but general mixing such as planetary mixer, hybrid mixer, Henschel mixer, Banbury mixer, kneader, ball mill, mixing roll, etc. A machine can be used.

In a preferred embodiment of the electromagnetic wave absorber of the present invention, the real part of the complex dielectric constant measured by the free space method is 8 or more, the imaginary part is 1 or more, and the dielectric loss tangent is 0.1. It has an electromagnetic wave absorption characteristic of 0.5 or less.

A material in which an electrically conductive powder such as boron carbide is dispersed in an insulating space or matrix material as in the present invention is called a dielectric electromagnetic wave absorber, and the dielectric properties of the entire material. Is involved in electromagnetic wave absorption characteristics.

Specifically, the dielectric properties include the real part (εr ′), the imaginary part (εr ″), and the ratio of the real part to the imaginary part of the complex relative permittivity (εr = εr′−jεr ″ [j is an imaginary unit]). The dielectric loss tangent of (tan δ = εr ″ / εr ′).

εr ′ corresponds to the wavelength compression effect of the electromagnetic wave absorber. The greater the εr ′, the greater the wavelength compression effect of the electromagnetic waves that pass through. If the wavelength compression effect is large, the thickness of the electromagnetic wave absorber at the frequency (matching frequency) at which the electromagnetic wave absorption effect is maximized (that is, the return loss rate is maximized) can be reduced.

εr ″ corresponds to dielectric loss. The larger εr ″, the greater the loss associated with dielectric polarization. Since the loss corresponds to the absorption of the electromagnetic wave, the larger εr ″, the better the electromagnetic wave can be absorbed.

From the above, it can be seen that the larger εr ′ and εr ″ are, the better the electromagnetic wave absorber can be obtained. However, in order to actually absorb the electromagnetic wave, the electromagnetic wave must enter the absorber. In order for an electromagnetic wave to enter the absorber, the complex relative dielectric constant (εr), the wavelength (λ) of the electromagnetic wave, and the thickness (d) of the electromagnetic wave absorber must approach a certain condition called a non-reflection condition. Therefore, εr ′ and εr ″, which are elements of εr, must satisfy a certain relationship.

The non-reflective condition is expressed by the following equation in the case of normal incidence, for example.
1 = (εr) ^{−1 / 2} * tanh (j * (2πd / λ) * (εr) ^1/2 )

The constant relationship that εr ′ and εr ″ satisfy in order to approach the non-reflection condition can be expressed by a dielectric loss tangent (tan δ = εr ″ / εr ′). In the electromagnetic wave absorber comprising the boron carbide powder of the present invention, εr ′ is a value (8 or more) at which a wavelength compression effect can be obtained, and εr ″ is a value at which sufficient dielectric loss (= electromagnetic wave absorption) occurs (1 And tan δ = εr ″ / εr ′ is a value close to the non-reflective condition (0.1 or more and 0.5 or less).

The electromagnetic wave absorber of the present invention satisfies these three requirements simultaneously in a wide frequency range of 5 to 110 GHz, and has not been conventionally found.

In addition, in the electromagnetic wave absorber that uses the dielectric properties of the material, the amount of electromagnetic wave absorption (that is, the return loss of electromagnetic wave) can be changed by changing the shape of the electromagnetic wave absorber and the filling amount of boron carbide powder. The frequency (matching frequency) at which can be maximized can be changed.

In other words, at the position where the electromagnetic wave absorber is provided, for example, the electromagnetic wave absorber having a different amount of boron carbide powder, or an electromagnetic wave absorber having different thicknesses (especially, depth in the direction of propagation of radio waves) is used. This makes it possible to produce an electromagnetic wave absorber or a composite of electromagnetic wave absorbers having a plurality of matching frequencies in a frequency band of 5 to 110 GHz and suitable for absorbing and removing various types of unnecessary electromagnetic waves. Become.

Moreover, since the electromagnetic wave absorber of the present invention has excellent electromagnetic wave absorption characteristics as described above, it is suitable as an electromagnetic wave absorber for dedicated narrow area communication (DSRC) that requires such characteristics.

(Example 1) Boron powder (Starck, Grade I) and carbon powder (Lyon Ketjen EC) were mixed at a molar ratio of 13: 2, and then 1400 ° C in an argon atmosphere using a resistance heating furnace. For 2 hours. The obtained lump was pulverized in a mortar, and then powder was obtained through a sieve mesh having an opening of 160 μm. This powder was confirmed to be B ₁₃ C ₂ by powder X-ray diffraction. As a result of elemental analysis, 85% by mass of boron (B) was contained.

100 parts by mass of an acrylic emulsion (FX-851 manufactured by High Pressure Gas Industry, 55% resin content) and a dispersant (San Nopco SN Dispersant 2060) 2 so that the boron carbide powder is 40% by volume with respect to the resin content. After adding to 0.2 parts by mass of a mass part and an antifoaming agent (San Nopco SN deformer 314) to a liquid matrix, the mixture was mixed using a hybrid mixer (Keyence HM-500) to prepare a slurry. Next, this slurry was formed into a 1 mm thick sheet, and then solidified by heating at 70 ° C. for 3 hours to obtain a composite of boron carbide powder and an acrylic resin.

As a result of measuring the complex relative dielectric constant (εr′−jεr ″) of the composite by a free space method using a network analyzer, the complex relative dielectric constant in the 5-110 GHz band is εr ′ = 9 to 25, εr ′. '= 2 to 5 and the dielectric loss tangent (tan δ) was 0.1 to 0.5.

(Example 2) Magnesium oxide (MgO) powder and boric acid (H ₃ BO ₃ ) powder were mixed at a molar ratio of 3: 2, and then heated in the atmosphere at 850 ° C for 2 hours. The obtained lump was pulverized in a mortar, and then powder was obtained through a sieve mesh having an opening of 160 μm. This powder was confirmed to be magnesium borate (3MgO · B ₂ O ₃ ) by a powder X-ray diffraction method. This powder was mixed with metal magnesium (Mg) powder and the same carbon (C) powder as in Example 1 so that the molar ratio was 3MgO.B ₂ O ₃ : Mg: C = 2: 6: 1, and argon was added. Heated in atmosphere at 1400 ° C. for 2 hours. The obtained lump was pulverized in a mortar, and then powder was obtained through a sieve mesh having an opening of 160 μm. This powder was confirmed to be a mixture of magnesium oxide (MgO), magnesium borate (3MgO.B ₂ O ₃ ) and boron carbide (B ₄ C or B ₁₃ C ₂ ) by a powder X-ray diffraction method.

Further, the powder was treated with hydrochloric acid (HCl) to remove MgO and 3MgO.B ₂ O ₃ , and then powder X-ray diffraction measurement was performed again. As a result, the lattice constant of the product was 0.5612 nm, and B It was confirmed that the boron carbide powder was composed of an intermediate crystal lattice of ₄ C (lattice constant 0.5600 nm) and B ₁₃ C ₂ (lattice constant 0.5633 nm). As a result of further elemental analysis, it contained 81% by mass of boron.

1 mass% flame retardant, 0.5 mass% silane coupling agent and 0.5 mass% in silicone rubber dissolved in toluene so that this powder is 50 volume% based on the rubber (elastomer) content A slurry was obtained by dispersing in a matrix prepared by adding the above vulcanizing agent.

The slurry was formed into a 1 mm thick sheet using a doctor blade film forming machine, heated at 80 ° C. for 1 hour to volatilize toluene, and press vulcanized at a temperature of 170 ° C. and a pressure of 9.8 MPa for 10 minutes. Then, secondary vulcanization was performed at 200 ° C. under atmospheric pressure for 5 hours to obtain a composite of boron carbide powder and silicone rubber. When the dielectric properties of the composite at 5 to 110 GHz were measured in the same manner as in Example 1, the complex relative dielectric constant was εr ′ = 12 to 30, εr ″ = 3 to 8, and the dielectric loss tangent (tan δ) was 0. .2 to 0.5.

(Example 3) (1) A composite of boron carbide powder and acrylic resin of Example 1, (2) A composite obtained in the same manner as in Example 1 except that the thickness was 0.5 mm, and (3 ) A composite obtained in the same manner as in Example 1 except that the filling amount of boron carbide powder was set to 30% by volume was obtained from the front side in the order of (2), (3), and (1). After the acrylic emulsion was applied, the laminate was laminated, and heated again at 70 ° C. for 3 hours while applying a load of 10 g / cm ² to bond and integrate the layers to obtain a composite. When the dielectric properties of the composite at 5 to 110 GHz were measured in the same manner as in Example 1, the complex dielectric constant was εr ′ = 10 to 28, εr ″ = 3 to 6, and the dielectric loss tangent (tan δ) was 0. .2 to 0.4.

When the electromagnetic wave absorption characteristic (reflection loss rate) of this composite at 5 to 110 GHz was measured by the free space method, matching frequencies showing a remarkable return loss of 15 decibels or higher were confirmed at three locations of 18 GHz, 40 GHz, and 65 GHz. It was done.

(Example 4) A parabolic antenna for microwave transmission is installed at a position of 4 meters on a paved road surface, imitating an automatic toll collection system (ETC) on a highway that is a kind of dedicated narrow area communication (DSRC), A network analyzer connected to this antenna transmits a microwave composed of 5.8 GHz right-handed circularly polarized light and makes it incident on the paved road surface at an angle of 45 °, and the reflected wave is installed at a position 1 meter above the paved road surface. Received with a parabolic antenna.

Next, the sheet-shaped composite produced in the same manner as in Example 1 except that the thickness was 2.5 mm, was cut into a square having a side of 0.5 m and placed at the microwave incident position on the pavement surface, The microwave was transmitted and received in the same manner as described above. Assuming that the intensity of the reflected wave from the pavement road surface is P ₀ , the intensity of the reflected wave from the composite is P ₁ , and the intensity ratio is -20 log ₁₀ (P ₁ / P ₀ ), the result is 25 dB. It was found that the body has applicable performance as an electromagnetic wave absorber for preventing multiple reflections of unwanted radio waves in ETC.

(Comparative example 1) After mixing boric acid powder and petroleum coke powder, it heated at 2200 degreeC for 5 hours using the high frequency type resistance heating furnace. The obtained lump was pulverized with a ball mill made of iron balls, sieved to a particle size of 45 μm or less using a sieve mesh, further washed with an aqueous nitric acid solution to remove iron, filtered and dried to obtain a powder. This powder was confirmed to be B ₄ C by powder X-ray diffraction. As a result of elemental analysis, 78% by mass of boron was contained.

Then the B _{4 C} powder was mixed with an acrylic emulsion or the like to be 40 vol% in the same manner as in Example 1, to prepare a slurry, further molded into a sheet shape of 1mm thick, solidified, B ₄ A composite of C powder and acrylic resin was obtained.

As a result of measuring the complex relative permittivity (εr′−jεr ″) of the composite by a free space method using a network analyzer, the complex relative permittivity in the 5-110 GHz band is εr ′ = 8-30, εr ′. '= 2-9 and the dielectric loss tangent (tan δ) was 0.01-0.8.

Since the boron carbide powder of the present invention has a specific range of boron (B) content and is excellent in electromagnetic wave absorption characteristics, various forms of electromagnetic wave absorbers can be easily obtained using this. Very useful on top.

Since the electromagnetic wave absorber of the present invention is spatially filled with boron carbide powder having the above-described characteristics, it includes various forms of members excellent in electromagnetic wave absorption characteristics, and is particularly preferable. In the form, since the real part of the complex relative permittivity at a frequency of 5 to 110 GHz is 8 or more, the imaginary part is 1 or more, and the dielectric loss tangent is 0.1 or more and 0.5 or less. For example, it can be used as an unnecessary electromagnetic wave absorber for dedicated narrow area communication (DSRC) such as an automobile toll collection system (ETC), an in-vehicle radar, etc., information home appliances, wireless LAN, ultra high bandwidth wireless (UWB), mobile phone There is a feature that it can also be used as an absorbing material for unnecessary electromagnetic waves, etc. for preventing ghost generation at the time of base station or television reception. In particular, an electromagnetic wave absorber formed by blending a specific amount of the specific boron carbide powder into various matrix materials such as thermoplastic resins is widely used in exterior materials, wall materials, curtains, etc. for houses. Has availability.

Claims (6)

A boron carbide powder for an electromagnetic wave absorber, wherein the boron (B) content is 80% by mass or more and 96% by mass or less. An electromagnetic wave absorber comprising a boron carbide powder having a boron (B) content of 80% by mass to 96% by mass filled to 5 to 70% by volume. An electromagnetic wave absorber comprising 5 to 70% by volume of boron carbide powder having a boron (B) content of 80% by mass to 96% by mass in a matrix material. The electromagnetic wave absorber according to claim 3, wherein the matrix material is a thermoplastic resin. The real part of the complex relative permittivity at a frequency of 5 to 110 GHz measured by the free space method is 8 or more, the imaginary part is 1 or more, and the dielectric loss tangent is 0.1 or more and 0.5 or less. Item 5. An electromagnetic wave absorber according to Item 4. An electromagnetic wave absorber for dedicated short-range communication (DSRC), comprising the electromagnetic wave absorber according to any one of claims 2 to 4.