KR20160067051A - Electromagnetic wave absorber and film forming paste - Google Patents

Abstract

The present invention relates to an electromagnetic wave absorber comprising a base member and an electromagnetic wave-absorbing film formed on the base member. The present invention is applicable to an absorber capable of absorbing a broad frequency band of electromagnetic waves. Even with the electromagnetic wave-absorbing film with a thickness of 1 mm or less, the electromagnetic wave absorber exhibits good absorption performance. The present invention also provides a film-forming paste suitably used to form an electromagnetic wave-absorbing film of an electromagnetic wave absorber. In an electromagnetic wave absorber including a base member and an electromagnetic wave-absorbing film formed on the base member, the electromagnetic wave absorbing film contains a specific epsilon iron oxide, thus having a relative dielectric constant of 6.5 to 65.

Description

ELECTROMAGNETIC WAVE ABSORBER AND FILM FORMING PASTE [0002]

The present invention relates to an electromagnetic wave absorber and, more particularly, to a radio wave absorber which absorbs radio waves and a film-forming paste which can be suitably used for forming an electromagnetic wave absorber in the radio wave absorber.

2. Description of the Related Art In various information communication systems such as a mobile phone, a wireless LAN, an ETC system, a highway traffic system, a road support system for automobile driving, and a satellite broadcast, the use of radio waves in a high frequency band is expanding. However, there is a fear that the use of radio waves in the high frequency band may cause malfunction or malfunction of the electronic device due to interference between the electronic components. As a measure against such a problem, a method of absorbing unnecessary radio waves by a radio wave absorber is taken.

Among the applications of radio waves in the high frequency band, research is being conducted on a driving support system for an automobile. In such a driving support system for a car, a radio wave of 76 GHz band is used in a vehicle-mounted radar for detecting an inter-vehicle distance and the like. It is predicted that the use of radio waves in a high frequency band of, for example, 100 GHz or more will be expanded in various applications, not limited to a driving support system of an automobile. Therefore, a radio wave absorber capable of satisfactorily absorbing radio waves in the 76 GHz band or higher frequency band is desired.

In order to meet such a demand, an electromagnetic wave absorber capable of absorbing electromagnetic waves in a wide range in a high frequency band, for example, a radio wave absorbing film containing magnetic crystals of iron oxide of epsilon-Fe ₂ O ₃ system (Refer to Patent Document 1).

Japanese Patent Laid-Open No. 2008-277726

2. Description of the Related Art [0002] Various electronic devices using radio waves in a high frequency band have been downsized. For this reason, it is desired that the electromagnetic wave absorber mounted on the electronic device be downsized and thinned. However, the radio wave absorbent described in Patent Document 1 satisfactorily absorbs radio waves in a wide frequency band. However, when the radio wave absorbing film is made thin as, for example, less than 1 mm in thickness, the radio wave absorbing performance may be impaired .

The present invention has been made in view of the above problems and is an electromagnetic wave absorber having a radio wave absorbing film formed on a base material and applicable to a radio wave of a wide frequency band, It is another object of the present invention to provide a radio wave absorber exhibiting good radio wave absorption characteristics. It is another object of the present invention to provide a film-forming paste suitably used for forming a radio wave absorbing film of the radio wave absorbent.

The inventors of the present invention found that the above problem can be solved by containing the specific epsilon-type iron oxide in the radio wave absorbing film and the relative dielectric constant of the radio wave absorbing film in the radio wave absorbing film having the electromagnetic wave absorbing film formed on the substrate to be 6.5 to 65, .

A first aspect of the present invention is a radio wave absorbent comprising a radio wave absorbing film formed on a substrate,

The radio wave absorber has a peak at which the radio wave absorption amount is maximum in the 60 to 270 GHz band,

Wherein the radio wave absorbing film comprises epsilon-type iron oxide,

As part of the Fe sites of the epsilon-type iron oxide is ε-Fe ₂ O ₃ crystal, and the crystal and the space group are identical and ε-Fe ₂ O ₃ crystal and ε-Fe ₂ O ₃ is substituted with an element M other than Fe And at least one selected from crystals represented by the formula? -M _x Fe _{2 - x} O ₃ and x is more than 0 and less than 2,

Wherein the electromagnetic wave absorber has a relative dielectric constant of 6.5 to 65.

A second aspect of the present invention is substituted with an element M other than the portion of the Fe sites of the ε-Fe ₂ O ₃ crystal, and the crystal and the space group are identical and ε-Fe ₂ O ₃ crystal and ε-Fe ₂ O ₃ is Fe And epsilon-type iron oxides represented by the formula 竜 -M _x Fe _{2 - x} O ₃ , wherein the x is at least one selected from crystals having an O exceeding 2,

Is a film-forming paste for imparting a film having a relative dielectric constant of 6.5 to 65.

According to the present invention, there is provided an electromagnetic wave absorber comprising a radio wave absorbing film formed on a base material, the electromagnetic wave absorber being applicable to a radio wave in a wide frequency band and capable of exhibiting good electromagnetic wave absorbing characteristics even when the electromagnetic wave absorbing film is a thin film having a thickness of less than 1 mm . According to the present invention, it is possible to provide a film-forming paste suitably used for forming a radio wave absorbing film of the radio wave absorber.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a diagram showing the relationship between a frequency and a reflection attenuation amount for the radio wave absorbers of Examples 1 and 2 and Comparative Example 1. FIG.
2 is a diagram showing the relationship between the frequency and the reflection attenuation amount for the radio wave absorber of the second and third embodiments.

"Electromagnetic wave absorber"

The radio wave absorber according to the present invention comprises a radio wave absorbing film formed on a substrate. The radio wave absorber includes a predetermined epsilon-type iron oxide and has a relative dielectric constant of 6.5 to 65. The radio wave absorber according to the present invention has a peak at which the radio wave absorption amount is maximum in the 60 to 270 GHz band.

Hereinafter, the base material and the radio wave absorbing film will be described.

[materials]

The radio wave absorbent has a base material as a material for supporting the radio wave absorbing film. The material of the base material is not particularly limited, but a conductor is preferable from the viewpoint of the reflection characteristics of radio waves. The kind of the conductor is not particularly limited within the range not hindering the object of the present invention, and a metal is preferable. When the base material is made of a metal, aluminum, titanium, SUS, copper, brass, silver, gold, platinum, or the like is preferable as the material of the base material.

The shape of the substrate is not particularly limited and may be various shapes. In view of miniaturization of the radio wave absorbent, a plate-like base material is usually selected. The shape of the plate-shaped base material may have a curved surface or may be composed of only a flat surface. As the shape of the base material, it is preferable that the base material is a flat plate because of the ease of forming a radio wave absorbing film having a uniform film thickness.

When the base material is a plate-like material, the thickness of the base material is not particularly limited. From the viewpoint of miniaturization of the radio wave absorber, it is preferable that the thickness of the base material is 0.1 탆 to 5 cm.

[Radio wave absorbing film]

The radio wave absorbing film contains a specific epsilon type iron oxide. The relative dielectric constant of the radio wave absorbing film is a value within the above-mentioned predetermined range. By combining the radio wave absorbing film satisfying these conditions with the above-mentioned substrate, it is possible to apply to radio waves in a wide frequency band, and even when the radio wave absorbing film is a thin film having a film thickness of less than 1 mm, a radio wave absorbent exhibiting good radio wave absorbing property can be obtained have.

The thickness of the electromagnetic wave-absorbing film is not particularly limited within the range not hindering the object of the present invention. From the viewpoint of miniaturization of the radio wave absorbent, the thickness of the radio wave absorbent film is preferably less than 3 mm, more preferably 150 m or more and less than 3 mm, particularly preferably 200 m or more and less than 3 mm.

In addition, the film thickness of the electromagnetic wave-absorbing film which obtains the optimum electromagnetic wave absorption effect may vary depending on the composition of the material constituting the electromagnetic wave-absorbing film and the relative dielectric constant and the relative permeability of the electromagnetic wave-absorbing film. In this case, it is preferable to finely adjust the film thickness of the radio wave absorbing film to optimize the radio wave absorbing effect of the radio wave absorbing body.

Hereinafter, a method of adjusting essential and optional components included in the radio wave absorbing film and the relative dielectric constant and specific permeability of the radio wave absorbing film will be described.

<Epsilon type iron oxide>

As the epsilon-type iron oxide, specifically, the ε-Fe ₂ O ₃ crystal, and the crystal with a space group element M other than the part of Fe of the Fe sites of the ε-Fe ₂ equal to the O ₃ and ε-Fe ₂ O ₃ crystal And at least one selected from the crystals represented by the formula 竜 -M _x Fe _{2 - x} O ₃ , wherein x is more than 0 and less than 2, is used. Since such crystals of epsilon-type iron oxide are magnetic crystals, the crystals of the epsilon-type iron oxide may be referred to as &quot; magnetic crystals &quot; in the specification of the present application.

For the ε-Fe ₂ O ₃ crystal, a well-known one can be used. Determining the space group is ε-Fe ₂ O ₃ and the same, and ε-Fe type as being a part of the Fe sites of the ₂ O ₃ crystal substituted with the element M other than Fe ε-M _x Fe _{2 -} denoted by _x O ₃ wherein Crystals with x of more than 0 and less than 2 will be described later.

In the specification of the present application, 竜 -M _x Fe _{2 -x} O _{3 in} which a part of the Fe site of the ε-Fe ₂ O ₃ crystal is substituted with the substituting element M is also referred to as "M-substituted ε-Fe ₂ O ₃ ".

The particle diameter of the particles having the ε-Fe ₂ O ₃ crystal and / or the M-substituted ε-Fe ₂ O ₃ crystal in the magnetic phase is not particularly limited within the range not hindering the object of the present invention. For example, the particles having magnetic crystals of the epsilon-type iron oxide produced by the method described below have an average particle diameter measured from a TEM (transmission electron microscope) photograph in the range of 5 to 200 nm.

The coefficient of variation (standard deviation of particle diameter / average particle diameter) of particles having magnetic crystals of magnetic crystals of epsilon-type iron oxide produced by a method described later is in the range of less than 80%, and the particle diameter is relatively fine Particle group.

In the present invention, the powder of the magnetic particles of the epsilon-type iron oxide (that is, the particles having the ε-Fe ₂ O ₃ crystal and / or the M-substituted ε-Fe ₂ O ₃ crystal on the magnetic phase) . The &quot; magnetic phase &quot; referred to herein is a portion responsible for the magnetic properties of the powder.

"Having ε-Fe ₂ O ₃ crystal and / or M-substituted ε-Fe ₂ O ₃ crystal on the magnetic phase" means that the magnetic phase is an ε-Fe ₂ O ₃ crystal and / or an M-substituted ε-Fe ₂ O ₃ crystal , And includes cases in which impurity magnetic crystals that are unavoidable in manufacturing are mixed on the magnetic body.

The magnetic crystals of epsilon-type iron oxide impurity crystals of iron oxide to be different from the ε-Fe ₂ O ₃ crystal with a space group _{(specifically, α-Fe 2 O 3,} γ-Fe 2 O 3, FeO, and Fe ₃ O ₄ , and a crystal in which a part of Fe in these crystals is substituted with another element).

When the magnetic crystal of the epsilon-type iron oxide includes an impurity crystal, it is preferable that the magnetic crystal of? -Fe ₂ O ₃ and / or M-substituted? -Fe ₂ O ₃ is a main phase. That is preferable, or more in a magnetic crystal of the epsilon iron oxide constituting the radio wave absorbing material, ε-Fe ₂ O ₃ and / or M-substituted 50% by mole to the ε-Fe ₂ O ₃ molar ratio as the magnetic ratio of the crystal compound of Do.

The abundance ratio of crystals can be obtained by an analysis by a lead belt method based on an X-ray diffraction pattern. A non-magnetic compound such as silica (SiO ₂ ) formed in the sol-gel process may adhere to the periphery of the magnetic phase.

(M-substituted ε-Fe ₂ O ₃ )

As long as the condition and the space group are the same as? -Fe ₂ O ₃ and the condition that part of the Fe site of the? -Fe ₂ O ₃ crystal is replaced with the element M other than Fe is satisfied, M-substituted? -Fe ₂ O The kind of the element M in ₃ is not particularly limited. The M-substituted? -Fe ₂ O ₃ may contain a plurality of elements M other than Fe.

Preferable examples of the element M include In, Ga, Al, Sc, Cr, Sm, Yb, Ce, Ru, Rh, Ti, Co, Ni, Mn, Zn, Zr and Y. Of these, In, Ga, Al and Rh are preferable. When M is Al, in the composition represented _by ? -M _x Fe _{2 - x} O ₃ , it is preferable that x is within a range of, for example, not less than 0 and less than 0.8. When M is Ga, x is preferably in the range of 0 to less than 0.8, for example. When M is In, x is preferably in the range of 0 to less than 0.3, for example. When M is Rh, x is preferably in the range of 0 to less than 0.3, for example.

According to the present invention, there is provided a radio wave absorber having a peak at which the radio wave absorption amount becomes maximum in the 60 to 270 GHz band, preferably 60 to 230 GHz band. The frequency at which the radio wave absorption amount becomes maximum can be adjusted by adjusting at least one of the kind of the element M and the substitution amount in the M-substituted? -Fe ₂ O ₃ .

Such M-substituted? -Fe ₂ O ₃ magnetic crystals can be synthesized, for example, by a combination of a reversed micelle method and a sol-gel method, which will be described later, and a firing step. Further, M-substituted ε-Fe ₂ O ₃ magnetic crystals can be synthesized by a direct synthesis method and a sol-gel method, which are disclosed in Japanese Patent Laid-Open Publication No. 2008-174405, and a firing step.

Specifically,

Jian Jin, Shinichi Ohkoshi and Kazuhito Hashimoto, ADVANCED MATERIALS 2004, 16, No.1, January 5, p.48-51,

Shin-ichi Ohkoshi, Shunsuke Sakurai, Jian Jin, Kazuhito Hashimoto, JOURNAL OF APPLIED PHYSICS, 97, 10K312 (2005)

Shunsuke Sakurai, Jian Jin, Kazuhito Hashimoto and Shinichi Ohkoshi, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, Vol.74, No.7, July, 2005, p.1946-1949,

The reverse micelle method described in, for example, &quot; Asuka Namai &quot;, Shunsuke Sakurai, Makoto Nakajima, Tohru Suemoto, Kazuyuki Matsumoto, Masahiro Goto, Shinya Sasaki, and Shinichi Ohkoshi, Journal of the American Chemical Society, Vol. 131, p.1170-1173, M-substituted? -Fe ₂ O ₃ magnetic crystals can be obtained by a process combining a sol-gel process and a firing process.

In the reversed micelle method, two kinds of micellar solutions containing a surfactant, that is, micellar solution I (raw micellar) and micellar solution II (neutralizing micellar) are mixed to proceed the precipitation reaction of iron hydroxide in micelles. Next, silica coating is performed on the surface of the iron hydroxide fine particles produced in the micelle by the sol-gel method. The iron hydroxide fine particles having a silica coating layer are separated from the liquid and then subjected to a heat treatment at a predetermined temperature (within a range of 700 to 1300 占 폚) in an atmospheric environment. Fine particles of ε-Fe ₂ O ₃ crystal can be obtained by this heat treatment.

More specifically, M-substituted? -Fe ₂ O ₃ magnetic crystals are produced, for example, as follows.

First, iron (III) nitrate as a iron source is added to a water phase of micelle solution I having n-octane as an oil phase and M nitrate (Al (III) hydrate of aluminum nitrate (III), hydrated gallium (III) n hydrate of Ga in the case of In and indium (III) trihydrate of nitrate in the case of Ga) and a surfactant (for example, cetyltrimethylammonium bromide) .

To the aqueous phase of the micelle solution I, an appropriate amount of a nitrate of alkaline earth metal (Ba, Sr, Ca, etc.) may be dissolved. This nitrate acts as a shape control agent. When an alkaline earth metal is present in the liquid, particles of M-substituted ε-Fe ₂ O ₃ magnetic crystals in a rod shape are finally obtained. In the absence of the shape control agent, particles of M-substituted ε-Fe ₂ O ₃ magnetic crystals close to spherical are obtained.

The alkaline earth metal added as the shape control agent may remain in the surface layer portion of the produced M-substituted ε-Fe ₂ O ₃ magnetic crystal. To M-substituted ε-Fe ₂ O ₃ by weight of the alkaline earth metal in the magnetic crystals are M-substituted ε-Fe ₂ O ₃ by weight of the substituted element M in the magnetic crystals and, with respect to the sum of Fe by weight and 20% by weight or less And more preferably 10% by weight or less.

An aqueous ammonia solution is used for the aqueous phase of the micelle solution II having n-octane as an oil phase.

After the micelle solutions I and II are mixed, a sol-gel method is applied. That is, stirring is continued while dropping silane (for example, tetraethylorthosilane) into a mixed solution of micellar solution to cause the formation reaction of iron hydroxide or iron hydroxide containing element M in the micelles. As a result, the surface of the fine iron hydroxide precipitation particles produced in the micelle is coated with silica produced by the hydrolysis of silane.

Then, the silica-coated M element-containing iron hydroxide particles are separated from the liquid, washed and dried, charged into the furnace, and heated in air at 700 to 1300 ° C, preferably 900 to 1200 ° C (Calcination) at a temperature range of preferably 950 to 1150 占 폚.

By this heat treatment, the oxidation reaction proceeds in the silica coating, and the fine particles of the fine iron element-containing iron hydroxide are changed into fine M substituted ε-Fe ₂ O ₃ particles.

At the time of the oxidation reaction, in the presence of the silica-coated α-Fe ₂ O ₃ or γ-Fe ₂ O as well as the determination of the _3, ε-Fe ₂ O ₃ and the space group is the same M-substituted ε-Fe ₂ O ₃ crystal And serves to prevent sintering of the particles. Further, if an appropriate amount of alkaline earth metal coexists, the shape of the particles tends to grow into a rod shape.

As described above, the M-substituted? -Fe ₂ O ₃ magnetic crystals can be more advantageously economically advantageously obtained by the step of combining the direct synthesis method and the sol-gel method disclosed in Japanese Patent Laying-Open No. 2008-174405 and the firing step Can be synthesized.

Briefly, a neutralizing agent such as ammonia water is added to a water solvent in which a salt of a trivalent iron salt and a substituting element M (Ga, Al, etc.) is first dissolved in a stirred state to form a hydroxide of iron Optionally substituted) may be formed.

Then, a sol-gel method is applied to form a coating layer of silica on the surface of the precursor particles. When the silica-coated particles are separated from the liquid and then subjected to a heat treatment (firing) at a predetermined temperature, fine particles of M-substituted ε-Fe ₂ O ₃ magnetic crystals are obtained.

In the synthesis of the M-substituted? -Fe ₂ O ₃ as described above, iron oxide crystals (impurity crystals) which make the space group different from the? -Fe ₂ O ₃ crystal may be produced. There are α-Fe ₂ O ₃ and γ-Fe ₂ O ₃ as the most common polymorphism with a composition of Fe ₂ O _{3 and} a different crystal structure. Other iron oxides include FeO and Fe ₃ O ₄ .

The inclusion of such an impurity crystal is not desirable in bringing the characteristics of the M-substituted ε-Fe ₂ O ₃ crystal as high as possible, but it is permissible within a range that does not impair the effect of the present invention.

The coercive force Hc of the M-substituted ε-Fe ₂ O ₃ magnetic crystal changes depending on the substitution amount by the substitutional element M. That is, by adjusting the degree of substitution by the M-substituted ε-Fe ₂ O ₃ substitution element M in the magnetic crystal, it is possible to adjust the M-substituted ε-Fe ₂ O ₃ the coercive force Hc of the magnetic crystal.

Specifically, for example, when Al or Ga is used as the substitution element M, the coercive force Hc of the M-substituted ε-Fe ₂ O ₃ magnetic crystal decreases as the substitution amount increases. On the other hand, when Rh or the like is used as the substitutional element M, the coercive force Hc of the M-substituted ε-Fe ₂ O ₃ magnetic crystal increases as the substitution amount increases.

Ga, Al, In and Rh are preferable as the substitution element M in that it is easy to adjust the coercive force Hc of the M-substituted? -Fe ₂ O ₃ magnetic crystal depending on the substitution amount by the substitution element M.

As the coercive force Hc is lowered, the peak frequency at which the radio wave absorption amount of the epsilon-type iron oxide is maximized also shifts to the low-frequency side or the high-frequency side. That is, the frequency of the peak of the radio wave absorption amount can be controlled by the replacement amount of the M element.

In the case of a radio wave absorber generally used, if the incident angle or frequency of the radio wave deviates from the designed value, the absorption amount becomes almost zero. On the other hand, when the epsilon-type iron oxide is used, even if the value is slightly out of range, it exhibits radio wave absorption in a wide frequency range and a propagation angle of incidence. Therefore, according to the present invention, it is possible to provide a radio wave absorber capable of absorbing radio waves in a wide frequency band.

The particle diameter of the epsilon-type iron oxide can be controlled by, for example, adjusting the heat treatment (firing) temperature in the above process.

According to a combination of the above-mentioned reverse micelle method and sol-gel method or a combination of a direct synthesis method and a sol-gel method disclosed in Japanese Patent Laid-Open Publication No. 2008-174405, the average particle size measured from a TEM (transmission electron microscope) As the diameter, it is possible to synthesize particles of epsilon-type iron oxide having a particle diameter in the range of 5 to 200 nm. The average particle diameter of the epsilon-type iron oxide is more preferably 10 nm or more, and more preferably 20 nm or more.

Further, when the average particle diameter, which is the number average particle diameter, is determined, when the particles of the epsilon-type iron oxide are in the rod phase, the average particle diameter is calculated as the diameter of the particle in the long axis direction of the particles observed on the TEM image. The number of particles to be measured when obtaining the average particle diameter is not particularly limited as long as it is a sufficiently large number corresponding to the calculation of the average value, but it is preferably 300 or more.

In addition, a silica coating coated on the surface of the iron hydroxide fine particles by the sol-gel method may exist on the surface of the M-substituted ε-Fe ₂ O ₃ magnetic crystal after the heat treatment (calcination). When a non-magnetic compound such as silica is present on the surface of the crystal, the handling property, durability, weather resistance, etc. of the magnetic crystal are improved.

Suitable examples of the non-magnetic compound include, in addition to silica, heat-resistant compounds such as alumina and zirconia.

However, if the amount of the non-magnetic compound to be adhered is too large, the particles may agglomerate violently, which is not preferable.

If the non-magnetic compound is silica, M-substituted ε-Fe ₂ O ₃ Si by weight of the magnetic crystals with respect to the weight and the sum of Fe by weight of the M-substituted ε-Fe ₂ O ₃ substitution element of the magnetic crystal M , Preferably not more than 100% by weight.

Some or most of the silica attached to the M-substituted ε-Fe ₂ O ₃ magnetic crystal can be removed by immersion in an alkali solution. The amount of silica adhered can be adjusted in any amount by this method.

The content of the epsilon-type iron oxide in the material constituting the electromagnetic wave absorbing film is not particularly limited within the range not hindering the object of the present invention. Typically, the content of the epsilon-type iron oxide is preferably 30% by weight or more, more preferably 40% by weight or more, particularly preferably 60% by weight or more and 60 to 91% by weight based on the weight of the material constituting the electromagnetic wave absorbing film % Is most preferable.

<Method of adjusting relative dielectric constant>

The electromagnetic wave absorbing film has a relative dielectric constant of 6.5 to 65, preferably 10 to 50, and more preferably 15 to 30. The method of adjusting the relative dielectric constant of the radio wave absorbing film is not particularly limited. As a method of adjusting the relative permittivity of the electromagnetic wave absorbing film, a method of containing a dielectric powder in the electromagnetic wave absorbing film and adjusting the content of the dielectric powder can be mentioned.

Suitable examples of the dielectric material include barium titanate, strontium titanate, calcium titanate, magnesium titanate, bismuth titanate, zirconium titanate, zinc titanate, and titanium dioxide. The electromagnetic wave absorbing film may contain a plurality of types of dielectric powders in combination.

The particle diameter of the dielectric powder used for adjustment of the dielectric constant of the electromagnetic wave absorbing film is not particularly limited within the range not hindering the object of the present invention. The average particle diameter of the dielectric powder is preferably 1 to 100 nm, more preferably 5 to 50 nm. Here, the average particle diameter of the dielectric powder is the number average diameter of the primary particles of the dielectric powder observed by an electron microscope.

When the relative dielectric constant of the electromagnetic wave absorbing film is adjusted by using the dielectric powder, the amount of the dielectric powder to be used is not particularly limited as long as the relative dielectric constant of the electromagnetic wave absorbing film is within a predetermined range. The amount of the dielectric powder to be used is typically preferably 0 to 20% by weight, more preferably 5 to 10% by weight based on the weight of the material constituting the electromagnetic wave absorbing film.

Incorporation of carbon nanotubes into the radio wave absorbing film can adjust the relative dielectric constant of the radio wave absorbing film. It is preferable to incorporate the carbon nanotubes in the radio wave absorbing film in the point that it is easy to obtain the radio wave absorbent excellent in the radio wave absorbing ability. The carbon nanotubes may be used in combination with the above dielectric powder.

The mixing amount of the carbon nanotubes relative to the material constituting the electromagnetic wave absorbing film is not particularly limited as long as the relative dielectric constant of the electromagnetic wave absorbing film is within a predetermined range. However, since the carbon nanotubes are also conductive materials, if the amount of the carbon nanotubes used is excessive, the radio wave absorption characteristics of the radio wave absorber may be impaired.

The amount of the carbon nanotubes to be used is typically 0 to 20% by weight, preferably 1 to 10% by weight, based on the weight of the material constituting the electromagnetic wave absorbing film.

<Non-magnetic permeability adjustment method>

The specific permeability of the electromagnetic wave absorbing film is not particularly limited, but is preferably 1.0 to 1.5. The method of adjusting the specific permeability of the electromagnetic wave absorptive film is not particularly limited. As a method for adjusting the specific permeability of the electromagnetic wave absorbing film, there may be mentioned a method of adjusting the substitution amount by the substitution element M in the epsilon type iron oxide or a method of adjusting the content of the epsilon type iron oxide in the electromagnetic wave absorbing film.

<Polymer>

The electromagnetic wave absorptive film may contain a polymer in order to facilitate the formation of a radio wave absorbing film having uniformly dispersed epsilon type iron oxide or the like in the film and having a uniform film thickness. In the case where the electromagnetic wave-absorbing film contains a polymer, components such as epsilon-type iron oxide can be easily dispersed in a matrix made of a polymer. In the case where the electromagnetic wave absorbing film is formed by using a film forming paste to be described later, the film forming paste contains a polymer, whereby the film forming property of the film forming paste is improved.

The kind of polymer does not impair the object of the present invention and is not particularly limited as long as it is capable of forming a radio wave absorbing film. The polymer may be an elastic material such as elastomer or rubber. The polymer may be a thermoplastic resin or a curable resin. When the polymer is a curable resin, the curable resin may be a photo-curable resin or a thermosetting resin.

Suitable examples of the polymer in the case of a thermoplastic resin include polyacetal resin, polyamide resin, polycarbonate resin, polyester resin (polybutylene terephthalate, polyethylene terephthalate, polyarylate and the like), FR-AS resin, FR-ABS resin , AS resin, ABS resin, polyphenylene oxide resin, polyphenylene sulfide resin, polysulfone resin, polyether sulfone resin, polyether ether ketone resin, fluorine resin, polyimide resin, polyamideimide resin, Polybutadiene resin, polybenzimidazole resin, BT resin, polymethylpentene, ultrahigh molecular weight polyethylene, FR-polypropylene, cellulose resin, (meth) acrylic resin (Polymethyl methacrylate and the like), and polystyrene.

Suitable examples of the polymer in the case of a thermosetting resin include a phenol resin, a melamine resin, an epoxy resin, and an alkyd resin. As the photo-curing resin, various vinyl monomers and resins obtained by photo-curing monomers having unsaturated bonds such as various (meth) acrylate esters can be used.

Suitable examples of the polymer being an elastic material include olefin elastomers, styrene elastomers, polyamide elastomers, polyester elastomers, and polyurethane elastomers.

In the case of forming a radio wave absorbing film using a film-forming paste to be described later, the film-forming paste may contain a dispersion medium and a polymer. In this case, it is preferable that the polymer is soluble in the dispersion medium in terms of the applicability of the paste and the ease of uniformly dispersing the epsilon-type iron oxide or the like in the polymer.

In the case where the material constituting the electromagnetic wave absorbing film contains a polymer, the content of the polymer is not particularly limited within the range not hindering the object of the present invention. The content of the polymer is typically preferably 5 to 30% by weight, more preferably 10 to 25% by weight based on the weight of the material constituting the electromagnetic wave absorbing film.

<Dispersant>

The electromagnetic wave absorptive film may contain a dispersing agent for the purpose of dispersing the epsilon-type iron oxide or the substance added for adjusting the relative dielectric constant and the specific permeability well in the film. A method of blending a dispersing agent in a material constituting the electromagnetic wave absorptive film is not particularly limited. The dispersing agent may be uniformly mixed with the epsilon-type iron oxide or polymer. When the material constituting the radio wave absorbing film includes a polymer, the dispersing agent may be incorporated in the polymer. The epsilon-type iron oxide, which has been previously treated with a dispersant, or a substance added to adjust the relative dielectric constant and the specific permeability may be added to the material constituting the electromagnetic wave absorbing film.

The kind of the dispersing agent is not particularly limited within the range not hindering the object of the present invention. A dispersant can be selected among various dispersants conventionally used for dispersion of various inorganic fine particles and organic fine particles.

Suitable examples of the dispersing agent include a silane coupling agent, a titanate coupling agent, a zirconate coupling agent, and an aluminate coupling agent.

The content of the dispersing agent is not particularly limited within the range not hindering the object of the present invention. The content of the dispersant is preferably 0.1 to 30% by weight, more preferably 1 to 15% by weight, and particularly preferably 1 to 10% by weight based on the weight of the material constituting the electromagnetic wave absorbing film.

&Lt; Other components >

The material constituting the electromagnetic wave absorptive film may contain various additives other than the above-mentioned components insofar as the object of the present invention is not impaired. Examples of the additive that can be included in the material constituting the electromagnetic wave absorbing film include a colorant, an antioxidant, an ultraviolet absorber, a flame retardant, a flame retarding aid, a plasticizer, and a surfactant. These additives are used in view of the amounts in which they are conventionally used, as long as they do not impair the object of the present invention.

By combining the above-described base material and the radio wave absorbing film, the radio wave absorbent of the present invention is formed.

<< Film Forming Paste >>

The film-forming paste contains the above-mentioned epsilon-type iron oxide for the radio wave absorbing film. The film-forming paste may contain a substance, a polymer and other components added to the electromagnetic wave-absorbing film for the purpose of adjusting the relative dielectric constant or the specific permeability as described above. When the polymer is a curable resin, the film-forming paste includes a compound which is a precursor of the curable resin. In this case, the film-forming paste contains a curing agent, a curing accelerator, a polymerization initiator and the like if necessary.

The composition of the film-forming paste is determined so that the relative dielectric constant of the electromagnetic wave-absorptive film formed by using the paste is within the above-mentioned predetermined range.

The film-forming paste usually includes a dispersion medium. However, when the film-forming paste contains a precursor of a liquid curable resin such as a liquid epoxy compound, a dispersion medium is not necessarily required.

As the dispersion medium, an aqueous solution of water, an organic solvent, and an organic solvent may be used. As the dispersion medium, an organic solvent is preferable because it is easy to dissolve the organic component, the latent heat of vaporization is low and the removal by drying is easy.

Suitable examples of the organic solvent used as the dispersion medium include ketones such as diethyl ketone, methyl butyl ketone, dipropyl ketone, and cyclohexanone; alcohols such as n-pentanol, 4-methyl-2-pentanol, cyclohexanol and diacetone alcohol; Ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, propylene glycol monomethyl ether, propylene glycol monoethyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol dimethyl ether, Ether-based alcohols such as diethylene glycol diethyl ether; Saturated aliphatic monocarboxylic acid alkyl esters such as n-butyl acetate and amyl acetate; Lactic acid esters such as ethyl lactate and n-butyl lactate; Methyl cellosolve acetate, ethyl cellosolve acetate, propylene glycol monomethyl ether acetate, propylene glycol monoethyl ether acetate, ethyl-3-ethoxypropionate, 2-methoxybutyl acetate, 3-methoxybutyl acetate, Methyl-3-methoxybutyl acetate, 3-ethyl-3-methoxybutyl acetate, 2-ethoxybutyl acetate, 4-methoxybutylacetate, 2- And ether esters such as ethoxybutyl acetate, 4-propoxybutyl acetate, and 2-methoxypentyl acetate. These may be used alone or in combination of two or more.

The solid concentration of the film-forming paste is a method of applying a film-forming paste, but is appropriately adjusted in accordance with the film thickness of the radio wave absorbing film. Typically, the solid concentration of the film-forming paste is preferably 3 to 60% by weight, more preferably 10 to 50% by weight. The solid content concentration of the paste is calculated as the weight of the solid content of the total of the weight of the component not dissolved in the dispersion medium and the weight of the component dissolved in the dispersion medium.

&Quot; Method of manufacturing electromagnetic wave absorber &quot;

The method of manufacturing the radio wave absorbent is not particularly limited. The electromagnetic wave absorber may be formed by separately forming a base material and a radio wave absorbing film and then joining them to each other to form a radio wave absorbent film directly on the surface of the base material.

When the base material and the electromagnetic wave absorbing film are formed separately and then the two are bonded, the bonding method is not particularly limited. As the bonding method, a method of bonding the electromagnetic wave absorbing film and the base material together using an adhesive as required can be mentioned. When the electromagnetic wave-absorbing film is a thick film having a thickness of, for example, 1 mm or more, the base material and the electromagnetic wave-absorbing film may be bonded using a screw such as a screw or a screw.

When the component constituting the electromagnetic wave-absorbing film includes a thermoplastic polymer, a mixture of essential or optional components contained in the electromagnetic wave-absorbing film is used to form a radio wave absorbing film by a known method such as extrusion molding, injection molding, Can be manufactured. In this case, the base material may be used as an insert material, and a radio wave absorbent body in which the base material and the radio wave absorbing film are integrated can be formed by a known insert molding method.

While these methods have advantages of high production efficiency, they have a disadvantage in that it is difficult to manufacture a thin film electromagnetic wave absorbing film having a film thickness of less than 1 mm.

From this point of view, the above-mentioned method using the film-forming paste is particularly preferable in that it is possible to form the radio wave absorbing film with high efficiency and to form the radio wave absorbing film directly on the base material without particularly limiting the film thickness.

Hereinafter, a method of manufacturing a radio wave absorber by coating a film-forming paste on a substrate to form a radio wave absorbing film will be described.

The method of applying the film forming paste on the substrate is not particularly limited as long as it can form a radio wave absorbing film having a desired film thickness. Examples of the application method include spray coating, dip coating, roll coating, curtain coating, spin coating, screen printing, doctor blade, and applicator.

By the above method, the formed film is dried and the dispersion medium is removed to form a radio wave absorbing film. The film thickness of the coating film is properly adjusted so that the film thickness of the radio wave absorbing film obtained after drying becomes a desired film thickness.

When the film-forming paste contains a compound of photopolymerization or thermosetting, the film may be exposed or heated to form a radio wave-absorbing film.

[ Example ]

Hereinafter, the present invention will be described in more detail with reference to Examples of the present invention, but the present invention is not limited to the following Examples.

[Examples 1 to 5 and Comparative Examples 1 to 3]

(Preparation of film-forming paste)

Epsilon type iron oxide, carbon nanotube (CNT), barium titanate, resin, and dispersant were added to taphenol so as to have the compositions shown in Table 1, and the components were uniformly dissolved or dispersed to obtain a film-forming paste. The solid concentration of the film-forming paste was adjusted to 40 wt%.

In Examples and Comparative Examples, 竜 -Ga _{0 . 45} Fe _{1 . 55} O ₃ was used. The average particle diameter of the epsilon-type iron oxide was 20 to 30 nm.

As the carbon nanotube (CNT), a multilayer carbon nanotube having a long diameter of 150 nm was used.

Barium titanate having an average particle diameter of 10 nm was used.

As the resin, cellulose (methyl cellulose) was used.

As a dispersant, a mixture of di (isopropyloxy) di (isostearoyloxy) titanium and vinyltrimethoxysilane in a weight ratio of 1: 1 was used.

(Production of radio wave absorber)

As the substrate, a substrate having a thickness of 2 mm and made of aluminum was used. On the substrate, the film-forming paste prepared by the above-described method was slit-coated to form a coating film. The film thickness of the coated film was adjusted so that the film thickness of the electromagnetic wave absorbing film was the film thickness described in Table 1. [ Then, the formed coating film was dried to obtain radio wave absorbers of each of Examples and Comparative Examples. Table 1 shows the film thickness, relative dielectric constant and specific magnetic permeability of the radio wave absorbing film obtained for the obtained radio wave absorbing body.

Electromagnetic wave absorption film composition (% by weight) Film thickness
(탆) Relative dielectric constant Specific permeability Epsilon type
Iron oxide CNT Titanic acid
barium Suzy Dispersant Example 1 68.0 2.0 1.4 23.8 4.8 230 17.2 1.1 Example 2 64.9 3.9 1.3 22.7 7.1 225 25.8 1.1 Example 3 66.2 4.0 0 23.2 6.6 220 26.0 1.1 Example 4 66.2 4.0 0 23.2 6.6 182 21.3 1.1 Example 5 64.5 5.3 0 23.0 7.2 194 32.1 1.1 Comparative Example 1 68.9 0 1.4 24.7 4.8 230 6.3 1.1 Comparative Example 2 53.3 13.3 1.3 22.0 10.0 210 69.1 1.0 Comparative Example 3 27.5 27.5 1.0 18.1 25.9 190 85.3 1.0

For the radio wave absorbers of Examples 1 and 2 and Comparative Example 1, radio wave absorption characteristics at 45 to 95 GHz were measured by a free space method using a vector network analyzer. The measurement results are shown in Fig.

The radio wave absorption characteristics of the radio wave absorbers of Example 3, Comparative Example 2 and Comparative Example 3 were also measured in the same frequency band of 45 to 95 GHz. The results of measurement of the radio wave absorber of Example 3 are shown in Fig. 2 together with the results of measurement of the radio wave absorber of Example 2. Fig.

As a result of the measurement, it was found that the radio wave absorbers of Comparative Examples 2 and 3 did not absorb radio waves only to a very small extent.

1 and 2 show that the electromagnetic wave absorbers of Examples 1 to 3 including the electromagnetic wave absorbing film containing the epsilon-type iron oxide and having the specific dielectric constant within the predetermined range are excellent in electromagnetic wave propagation characteristics Absorbing property.

On the other hand, in Comparative Examples 1 to 3, even in the case of including the epsilon-type iron oxide having excellent radio wave absorption characteristics, in the radio wave absorber having the electromagnetic wave absorptive film whose relative dielectric constant of the electromagnetic wave absorptive film is not within the predetermined range, It can be seen that almost no radio waves can be absorbed.

Claims (7)

A radio wave absorbent comprising a radio wave absorbing film formed on a substrate,
The radio wave absorber has a peak at which the radio wave absorption amount is maximum in the 60 to 270 GHz band,
Wherein the radio wave absorbing film comprises epsilon-type iron oxide,
As part of the Fe sites of the epsilon-type iron oxide is ε-Fe ₂ O ₃ crystal, and the crystal and the space group are identical and ε-Fe ₂ O ₃ crystal and ε-Fe ₂ O ₃ is substituted with an element M other than Fe At least one selected from crystals represented by the formula 竜 -M _x Fe _{2 - x} O ₃ , wherein x is more than 0 and less than 2,
Wherein the electromagnetic wave absorbing film has a relative dielectric constant of 6.5 to 65. The method according to claim 1,
Wherein the substrate is made of a conductor. The method according to claim 1 or 2,
Wherein the radio wave absorbing film comprises carbon nanotubes. The method according to claim 1 or 2,
Wherein the substrate is a flat plate. ε-Fe ₂ O ₃ crystal, and the crystal and the space group is ε-Fe equal to the ₂ O _3, and expression as a part of the Fe sites of the ε-Fe ₂ O ₃ crystal substituted with the element M other than Fe ε-M _x And epsilon-type iron oxide represented _by Fe _{2 - x} O ₃ , wherein x is at least one selected from crystals having an O exceeding 2,
A film forming paste for imparting a film having a relative dielectric constant of 6.5 to 65. The method of claim 5,
Further comprising a carbon nanotube. A film formed by using the film-forming paste according to claim 5 or 6.

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