JP2023005603A - Electromagnetic wave absorption material, electromagnetic wave absorber, and electronic element, electronic component or electronic apparatus including the electromagnetic wave absorber - Google Patents

Abstract

An object of the present invention is to provide an electromagnetic wave absorbing material and an electromagnetic wave absorber that utilize dielectric loss and can be put to practical use in a high frequency band. An electromagnetic wave absorbing material comprising an electrically insulating matrix and ferromagnetic metal powder dispersed in the electrically insulating matrix, wherein the content of the ferromagnetic metal powder is 50% by volume or more. , the ferromagnetic metal powder has an average aspect ratio of 5.0 or less, and the electromagnetic wave absorbing material has a dielectric loss tangent (tan δE) peak in a frequency band of 0.1 GHz to 18 GHz, and A material having a tan δE) value of 0.05 or more. [Selection drawing] Fig. 7

Description

TECHNICAL FIELD The present invention relates to an electromagnetic wave absorbing material, an electromagnetic wave absorber, and an electronic element, electronic component, or electronic device provided with the electromagnetic wave absorber.

2. Description of the Related Art With the rapid development of electronic information communication technology in recent years, the use of electromagnetic waves (radio waves) is rapidly increasing, and the electromagnetic waves used are becoming higher in frequency and wider in bandwidth. Specifically, in addition to systems in the quasi-microwave band represented by mobile phones (1.5 GHz, 2.0 GHz) and wireless LANs (2.45 GHz), high-speed wireless LANs (65 GHz) and anti-collision radars ( Introduction of new systems using radio waves in the millimeter wave band such as 76.5 GHz) is underway.

2. Description of the Related Art As the use of electromagnetic waves expands and increases in frequency, problems such as electromagnetic interference such as malfunction of electronic devices due to electromagnetic noise and electromagnetic interference such as adverse effects on the human body have come to the fore, and the demand for EMC measures is increasing. As one means of EMC countermeasures, there is known a technique of using an electromagnetic wave absorber (radio wave absorber) to absorb unnecessary electromagnetic waves and prevent their intrusion.

An example of the basic configuration of an electromagnetic wave absorber is shown in FIG. 1 using a schematic cross-sectional view. An electromagnetic wave absorber (1) is composed of an absorber body (2) and a metal film (3) that lines the absorber body (2). The absorber main body (2) is made of an electromagnetic wave absorbing material. An incident wave (4) incident on the absorber main body (2) is partly reflected (5) on the absorber surface and the rest penetrates inside the absorber. The intruding electromagnetic wave is reflected by the metal film (3), and part of it is radiated to the outside of the absorber as a reflected wave (6).

The electromagnetic wave attenuation mechanism is attenuation (interference attenuation) due to interference between the reflected wave (5) from the absorber surface and the reflected wave (6) from the absorber back surface, and attenuation due to absorption inside the absorber body (2) ( absorption attenuation). In the interference attenuation, when the thickness (d) of the absorber main body (2) and the wavelength (λ) of the electromagnetic wave have a specific relationship, the reflected wave (5) from the absorber surface and the reflected wave from the absorber back surface The phenomenon that (6) cancels each other is used. In the interference phenomenon, it is necessary to strictly control the thickness of the absorber body (2), and there is a problem that the absorption characteristics deteriorate according to the incident angle of the electromagnetic waves. Another problem is that the absorption band is narrow. On the other hand, the absorption attenuation utilizes the phenomenon that the absorber main body (2) absorbs the energy of an incident electromagnetic wave (electromagnetic wave energy), converts it into heat energy, and radiates it. The absorption phenomenon has the advantage that the thickness of the absorbent body (2) can be freely controlled to some extent and the absorption band can be widened.

In order to effectively utilize absorption attenuation, it is important to use absorbers (absorbent materials) that exhibit excellent absorption characteristics. Specifically, it is practically desired that the transmission attenuation factor of the absorber is 10 dB or more in the absorption band.

Magnetic loss, dielectric loss, or resistance loss of the material constituting the absorber is used for electromagnetic wave absorption. called electromagnetic wave absorber. Of these, magnetic electromagnetic wave absorbers are widely used because they exhibit excellent absorption characteristics.

A magnetic electromagnetic wave absorber utilizes the loss that occurs based on the magnetic resonance of a ferromagnetic material. That is, a ferromagnetic material has a magnetic moment mainly based on spin angular momentum of electrons (3d electrons) bound to atoms of magnetic elements (Fe, Ni, Co, etc.). Spontaneous magnetization occurs as a result of aligning the directions of the magnetic moments through the action of exchange interaction and the like. When a magnetic material is irradiated with electromagnetic waves, the direction of magnetization changes (magnetization oscillation) due to domain wall motion in a low frequency range and due to magnetization rotation in a high frequency range. As the frequency increases, magnetization fluctuations interfere with electromagnetic waves at a specific frequency, resulting in a phenomenon in which magnetization resonates, that is, magnetic resonance. At the frequency at which magnetic resonance occurs, the imaginary part (μ″) of magnetic permeability has a peak and the loss is maximum. Loss based on magnetic resonance is called magnetic loss.

Soft magnetic metals and ferrites (iron-based oxides) are conventionally known as materials for magnetic electromagnetic wave absorbers. Among them, soft magnetic metals have the advantage of being higher in saturation magnetization and magnetic permeability than ferrite, and are often used as absorber materials. For example, Patent Document 1 discloses a high-frequency electromagnetic wave absorber characterized by dispersing flat soft magnetic powder having a powder thickness of 3 μm or less in an insulating base material (Patent Document 1 claim 1). Further, in Patent Document 1, as the powder thickness is reduced, the peak frequency fr of μ″ (loss term of complex permeability) shifts to the high frequency side, and μ″ exceeds the limit line (S; snake line). It is described that high-frequency electromagnetic waves of 2 GHz or higher, which conventionally could not be absorbed, can be well absorbed ([0012] and [0014] of Patent Document 1).

Further, in Patent Document 2, a flat powder for an electromagnetic wave absorber characterized by comprising, in mass%, Si: 9 to 12%, Al: 1 to 5%, Cr: 1 to 5%, and the balance Fe and impurities and an electromagnetic wave absorber characterized by dispersing and kneading the flat powder in a flexible insulating material (claims 1 to 3 of Patent Document 2).

On the other hand, dielectric electromagnetic wave absorbers using dielectric loss are also known. For example, Patent Document 3 discloses applying a dielectric loss material containing a conductive material and an insulating material such as boron carbide, conductive carbon powder, and silicon carbide to an electromagnetic wave absorber (Patent Document Claims 1 to 10 of 3). Further, Patent Document 3 describes that the electromagnetic wave absorber is lightweight and exhibits excellent electromagnetic wave absorption characteristics with a wide absorption frequency band in the high frequency region ([0007] of Patent Document 3).

JP-A-11-087117 JP 2008-050644 A JP 2007-019287 A

Although such electromagnetic wave absorbers (magnetic electromagnetic wave absorbers) utilizing the magnetic loss of soft magnetic metals have been conventionally proposed, there is still room for improvement in terms of improving the electromagnetic wave absorption characteristics. That is, a theory called Snoek's limit is known for magnetism, and according to this theory, there is a relationship between the upper limit of the magnetic permeability of a magnetic material and the resonance frequency. For example, in materials with small magnetic anisotropy, the product of the upper limit of the magnetic permeability and the resonance frequency is constant. Therefore, the resonance frequency cannot be increased while maintaining a high magnetic permeability.

In Patent Documents 1 and 2, flat soft magnetic powder is used, and magnetic anisotropy caused by shape anisotropy is utilized to improve absorption characteristics. However, it is still difficult to break through Snake's limit and significantly improve the absorption characteristics at high frequencies. In addition, since the flat powder has poor packing properties, it is difficult to produce an absorber containing magnetic powder at a high packing rate. Therefore, there is a limit in improving the absorption characteristics and increasing the frequency. Although Patent Document 3 aims at a dielectric electromagnetic wave absorber, it is necessary to prepare a conductive ceramic material such as boron carbide. These ceramic materials require special techniques such as high-temperature, high-pressure synthesis for their production, which tends to lead to increased costs.

The inventors of the present invention have made intensive studies in view of such conventional circumstances. As a result, it was found that dielectric loss, which is different from magnetic loss, has a maximum value by controlling the shape and content of ferromagnetic metal powder in a material containing an electrically insulating matrix and ferromagnetic metal powder. Then, the inventors have found that by utilizing this dielectric loss, it is possible to obtain an electromagnetic wave absorbing material and an electromagnetic wave absorber that can be practically used in a high frequency band.

The present invention has been completed based on such findings, and aims to provide an electromagnetic wave absorbing material and an electromagnetic wave absorber that utilize dielectric loss and can be put to practical use in a high frequency band.

The present invention includes the following aspects (1) to (9). In the present specification, the expression "-" includes both numerical values. That is, "X to Y" is synonymous with "X or more and Y or less".

(1) An electromagnetic wave absorbing material comprising an electrically insulating matrix and ferromagnetic metal powder dispersed in the electrically insulating matrix,
The content ratio of the ferromagnetic metal powder is 50% by volume or more,
The ferromagnetic metal powder has an average aspect ratio of 5.0 or less,
The electromagnetic wave absorbing material has a dielectric loss tangent (tan δ _E ) peak in a frequency band of 0.1 GHz to 18 GHz, and a dielectric loss tangent (tan δ _E ) value at the peak is 0.05 or more.

(2) The above ( 1) electromagnetic wave absorbing material.

(3) The electromagnetic wave absorbing material according to (1) or (2) above, wherein the ferromagnetic metal powder has an average aspect ratio of 3.0 or less.

(4) The electromagnetic wave absorbing material according to any one of (1) to (3) above, wherein the ferromagnetic metal powder has a volume average particle diameter (D50) of 1.5 μm or more.

(5) The electromagnetic wave absorbing material according to any one of (1) to (4) above, wherein the electrically insulating matrix contains at least one of base oil, wax and rosin.

(6) An electromagnetic wave absorber comprising a molded body of the electromagnetic wave absorbing material according to any one of (1) to (5) above.

(7) The electromagnetic wave absorber according to (6) above, wherein the electromagnetic wave absorber has a thickness of 0.5 mm or more in the direction of incidence of the electromagnetic wave.

(8) The electromagnetic wave absorber according to (6) or (7) above, wherein the electromagnetic wave absorber has a region with a transmission attenuation rate of 10 dB or more at a thickness of 1 mm in a frequency band of 0.1 GHz or more and 18 GHz or less.

(9) An electronic element, electronic component, or electronic device comprising the electromagnetic wave absorber according to any one of (6) to (8) above.

INDUSTRIAL APPLICABILITY According to the present invention, an electromagnetic wave absorbing material and an electromagnetic wave absorber that utilize dielectric loss and can be put to practical use in a high frequency band are provided.

It is a conceptual diagram of an electromagnetic wave absorber. The transmission attenuation rate of electromagnetic wave absorbers (Examples 1 to 4) is shown. Figure 2 shows the particle size distribution of fillers; The transmission attenuation rate of electromagnetic wave absorbers (Examples 5 to 8) is shown. The transmission attenuation rate of electromagnetic wave absorbers (Examples 9 to 12) is shown. The transmission attenuation rate of electromagnetic wave absorbers (Examples 13 to 17) is shown. The transmission attenuation rate of the electromagnetic wave absorber (Example 6) is shown. The complex relative permittivity (real part, imaginary part) and the complex relative permeability (real part, imaginary part) of the electromagnetic wave absorber (Example 6) are shown. The transmission attenuation rate of the electromagnetic wave absorber (Example 18) is shown.

A specific embodiment of the present invention (hereinafter referred to as "this embodiment") will be described. The present invention is not limited to the following embodiments, and various modifications are possible within the scope of the present invention.

<<1. Electromagnetic wave absorption material >>
The electromagnetic wave absorbing material (hereinafter sometimes referred to as "material") of this embodiment includes an electrically insulating matrix and ferromagnetic metal powder dispersed in the electrically insulating matrix. Here, the electromagnetic wave absorbing material is a precursor for forming an electromagnetic wave absorber by molding it. For example, by molding an electromagnetic wave absorbing material, an electromagnetic wave absorber having a shape such as a sheet can be produced. In this specification, powder means an aggregate of a plurality of independent particles. A plurality of particles can also be said to constitute a powder.

<Ferromagnetic metal powder>
Ferromagnetic metal powder (hereinafter sometimes referred to as "metal powder") is dispersed and contained in an electrically insulating matrix. That is, the particles forming the metal powder are separated from each other via the component forming the matrix (matrix component). However, not all particles need to be spaced apart, and some particles may be in contact with each other.

The ferromagnetic metal powder is composed of ferromagnetic metal. Here, the ferromagnetic metal is a metal exhibiting ferromagnetism, that is, a metal having spontaneous magnetization at room temperature. Ferromagnetic metals also exhibit electrical conductivity. More specifically, ferromagnetic metals are iron (Fe), cobalt (Co), nickel (Ni), gadolinium (Gd), tetragonal ruthenium (Ru), and alloys containing these metals.

The material of the ferromagnetic metal powder is not limited as long as it is composed of ferromagnetic metal. The metal powder may consist of a single metal or may consist of an alloy. Here, the alloy is a concept that includes not only solid solutions but also intermetallic compounds. The ferromagnetic metal powder may be composed of only one kind of metal powder, or may contain a mixture of a plurality of kinds of metal powders. From the viewpoint of reducing conductor loss during non-resonance, the ferromagnetic metal powder preferably has a smaller electrical resistance. Preferably, the ferromagnetic metal powder has a volume resistivity of 10 ⁻⁵ Ωm or less.

Materials for the ferromagnetic metal powder (soft magnetic metal powder) include Fe—Ni alloy (permalloy), Fe—Ni—Mo alloy (super permalloy), Fe—Co alloy, Fe—Co—Ni alloy, Fe—Cr alloy, Fe--Cr--Al alloy, Fe--Cr--Si alloy, Fe--Si alloy, Fe--Si--Al alloy (sendust), Fe--Si--Cr alloy, Fe--Si--Cr--Ni alloy, Fe-based amorphous alloy, Alternatively, a Co-based amorphous alloy may be used. Preferably, the ferromagnetic metal powder is at least one selected from the group consisting of iron (Fe) powder, iron (Fe) alloy powder, nickel (Ni) powder, and nickel (Ni) alloy powder. These metal powders are readily available, inexpensive, and have high magnetic permeability. Therefore, as will be described later, magnetic properties can be effectively utilized.

In the electromagnetic wave absorbing material of this embodiment, the content of the ferromagnetic metal powder is 50% by volume or more. By including the ferromagnetic metal powder in the absorbing material at a high filling rate in this way, it is possible to impart electromagnetic wave absorbing properties based on dielectric loss to the material. Specifically, the imaginary part (ε″) of the dielectric constant becomes maximum at a specific frequency (dielectric loss peak frequency), and the absorption characteristic at that frequency becomes maximum.

The detailed reason why the electromagnetic wave absorbing material of the present embodiment develops electromagnetic wave absorbing properties based on dielectric loss is unknown. Although it should not be limitedly interpreted based on a specific theory, it is speculated as follows. That is, each particle constituting the metal powder is separated from each other via the matrix component. In other words, the matrix component intervenes between the particles. Since the matrix component has electrical insulation, a capacitance is generated in the matrix component interposed between the particles. Also, the higher the filling rate of the metal powder, the smaller the average distance between adjacent particles, that is, the thickness of the matrix component intervening between the particles. Therefore, the capacitance is increased, and as a result, the effective dielectric constant of the material as a whole is increased. A specific frequency is generated by the matrix exhibiting a dielectric relaxation phenomenon, or by causing a resonance phenomenon when the capacitance component (C component) of the matrix interacts with the resistance component (R component) and/or the inductive component (L component) of the metal powder. It is thought that the dielectric loss will increase in On the other hand, if the content of the metal powder is less than 50 volumes, the capacitive component will be small, and as a result, it is speculated that the electromagnetic wave absorption characteristics will be insufficient.

In order to improve the electromagnetic wave absorption characteristics, the higher the content of the ferromagnetic metal powder, the better. The content may be 55% by volume or more, 60% by volume or more, 65% by volume or more, 70% by volume or more, 75% by volume or more, or 80% by volume or more. On the other hand, if the content is excessively high, the probability that the particles forming the metal powder will come into contact with each other increases. When the particles come into contact with each other, the capacitive component of the matrix disappears. As a result, there is a risk that the electromagnetic wave absorption characteristics will rather deteriorate. The content may be 95% by volume or less, 90% by volume or less, or 85% by volume or less.

In the electromagnetic wave absorbing material of this embodiment, the ferromagnetic metal powder has an average aspect ratio of 5.0 or less. Here, the average aspect ratio is the average value of the aspect ratios of multiple particles forming the metal powder, and can be obtained by observation with a scanning electron microscope (SEM). The aspect ratio is the ratio of the major axis diameter to the minor axis diameter of each particle (major axis diameter/minor axis diameter). Higher sphericity of the particles allows for better electromagnetic wave absorption properties. A powder with a small average aspect ratio is preferable because each particle constituting the powder has a high degree of sphericity. When metal powder with a small aspect ratio is used, the friction acting between metal powder particles is small, and the metal powder is densely packed during material production. Therefore, an electromagnetic wave absorbing material with a high metal powder filling rate can be obtained. In contrast, flat powders with large magnetic anisotropy are used in conventional magnetic electromagnetic wave absorbing materials. Such flat powder has a large aspect ratio, and it is difficult to increase the filling rate. Since the electromagnetic wave absorbing material of the present embodiment utilizes electromagnetic wave absorbing properties based on dielectric loss, it is possible to use metal powders with good filling properties and a small aspect ratio. The average aspect ratio may be 4.0 or less, 3.0 or less, 2.5 or less, 2.0 or less, or 1.5 or less.

In the electromagnetic wave absorbing material of this embodiment, the ferromagnetic metal powder is preferably soft magnetic metal powder. Using ferromagnetic metal powder (soft magnetic metal powder) in this way has various advantages. For example, reflected waves on the surface of the electromagnetic wave absorber can be suppressed, and electromagnetic wave absorption can be performed efficiently. That is, as shown in FIG. 1, an incident wave (4) incident on the absorber main body (2) is partly reflected by the absorber surface and the rest penetrates into the absorber. Reflected waves (5) on the surface of the absorber do not contribute to absorption inside the absorber. Therefore, suppressing the reflected wave (5) is effective in improving the absorption characteristics.

In this respect, the reflected wave can be suppressed by matching the characteristic impedance of the electromagnetic wave absorber with the characteristic impedance of the external environment. The characteristic impedance Z of the electromagnetic wave absorber and the characteristic impedance _Z0 of the external environment are expressed as functions of the dielectric constant and magnetic permeability of the electromagnetic wave absorber and the external environment (air), as shown in the following equations (1) and (2). be done. In the following equations (1) and (2), μ and ε are the magnetic permeability and dielectric constant of the electromagnetic wave absorber, respectively, and μ ₀ and ε ₀ are the magnetic permeability of the external environment (air) (1.256× 10 ⁻⁶ H/m) and dielectric constant (8.854×10 ⁻¹² F/m).

Ferromagnetic metal powder generally has high magnetization and low coercive force. Therefore, the magnetic permeability (μ) is high. By using the ferromagnetic metal powder, the high dielectric constant (ε) of the matrix component is offset, and as a result, the characteristic impedance Z of the electromagnetic wave absorber can be easily matched with the characteristic impedance _Z0 of the external environment. That is, reflection on the absorber surface is suppressed.

Further, by using ferromagnetic metal powder (soft magnetic metal powder), not only the dielectric loss but also the magnetic loss inherent in the ferromagnetic metal powder can be utilized for electromagnetic wave absorption. In other words, since ferromagnetic metal powder has ferromagnetic properties, it has a maximum value of the imaginary part (μ″) of magnetic permeability at a specific frequency. This frequency is called magnetic resonance frequency or magnetic loss peak frequency. The magnetic loss is maximized at the magnetic loss peak frequency, and as a result, electromagnetic wave absorption characteristics based on the magnetic loss appear. By aligning the magnetic loss peak frequency with the dielectric loss peak frequency, both dielectric loss and magnetic loss can be utilized in the same frequency range. Therefore, it is possible to further improve the electromagnetic wave absorption characteristics. Alternatively, by shifting the magnetic loss peak frequency from the dielectric loss peak frequency, it is possible to widen the absorption band. In particular, the electromagnetic wave absorbing material of this embodiment has a high filling rate of the ferromagnetic metal powder. Therefore, the magnetic loss effect based on the ferromagnetic metal powder can be effectively utilized.

The ferromagnetic metal powder preferably has a volume average particle size (D50) of 1.5 μm or more. By increasing the particle size, the electromagnetic wave absorption characteristics can be enhanced. Although the detailed reason is unknown, it is speculated that it may be caused by the skin effect. D50 is more preferably 2.0 μm or more, still more preferably 5.0 μm or more, and particularly preferably 8.0 μm or more. The upper limit of D50 is not limited. However, if the D50 is too large, there is a risk that a homogeneous composition will not be obtained when applying the electromagnetic wave absorbing material to a composition such as a sheet, grease or paint. D50 is preferably 20.0 μm or less, more preferably 15.0 μm or less. The volume average particle diameter (D50) can be calculated as a 50% cumulative diameter in the particle size distribution obtained by obtaining the volume particle size distribution by a method such as a laser diffraction scattering method.

The particle size distribution of ferromagnetic metal powder is not limited. For example, it may exhibit a multimodal particle size distribution, or it may exhibit a unimodal particle size distribution. However, if the filling rate is the same, it is possible to obtain higher electromagnetic wave absorption characteristics by using a metal powder exhibiting a unimodal particle size distribution than by using a metal powder exhibiting a multimodal particle size distribution. can. Although the detailed reason is unknown, it is speculated that it may be related to the loss due to the generation of eddy current due to contact between particles. In addition, it is speculated that the probability of contact between particles is high in a multimodal particle size distribution, and that the capacitive component in the matrix is likely to disappear. A metal powder exhibiting a unimodal particle size distribution can be obtained by using a single type of metal powder with a uniform particle size distribution.

<Electrically insulating matrix>
An electrically insulating matrix (hereinafter sometimes referred to as "matrix") serves as a base material (substrate) of an electromagnetic wave absorbing material. The material of the matrix is not limited as long as it has electrical insulation. It may be composed of any of solids, liquids, gels and mixtures thereof. Examples include insulating materials such as organic compounds, resins, rubbers, glass, and ceramics. In this specification, an electrically insulating material refers to a material having a volume resistance (specific resistance) of 10 ⁵ Ωcm or more.

The content of the electrically insulating matrix is preferably 5% by volume or more and 50% by volume or less. Since the content of the metal powder is 50% by volume or more, the content of the matrix is limited to 50% by volume or less. The content of the matrix may be 45% by volume or less, 40% by volume or less, 35% by volume or less, 30% by volume or less, 25% by volume or less, or 20% by volume or less.

The electrically insulating matrix preferably comprises at least one of base oil, wax and rosin. Any one of base oil, wax, and rosin may be included, or a plurality thereof may be included in combination. It may contain all of base oil, wax and rosin.

The electrically insulating matrix may contain a base oil. The base oil is characterized by high wettability with respect to metal particles and excellent electrical insulation. Therefore, the base oil can be used to provide a thin electrically insulating coating on the particle surface. Moreover, the base oil has a characteristic that the film formed on the surface of the particles is difficult to break because of its moderate viscosity. Therefore, by using the base oil, it is possible to prevent direct contact between the particles even if the filling rate of the metal powder is increased.

The type of base oil is not limited. Any of mineral oil, synthetic oil, vegetable oil and animal oil may be used. Mineral oils may be both paraffinic and naphthenic. As the synthetic oil, hydrocarbon-based, ester-based, ether-based, silicone-based, fluorine-based, and the like can be used. Ester-based materials are preferred, and polyether esters are particularly preferred.

The base oil can form an electrically insulating matrix with other ingredients. The content of the base oil contained in the absorbent material is preferably 8.0% by volume or more and 40.0% by volume or less, more preferably 10.0% by volume or more and 35.0% by volume or less. By setting the content ratio within the above range, it becomes possible to more effectively prevent direct contact between particles constituting the metal powder.

The electrically insulating matrix may contain wax. Wax has the effect of imparting lubricity and functions as a binder. That is, by using wax, the metal powder slides well, and the filling properties are improved. In addition, since the wax solidifies at room temperature after molding the absorbent material, the binding force of the metal powder is increased, which has the effect of improving the shape retention. From the viewpoint of improving the dispersibility of the metal powder, the wax preferably has a high acid value. The acid value of the wax is preferably 20 mg/g or more, more preferably 40 mg/g or more. Although the type of wax is not limited, synthetic wax, vegetable wax, and/or petroleum wax are preferred.

Waxes can form an electrically insulating matrix with other ingredients. The wax content in the absorbent material is preferably 2.0% by volume or more and 15.0% by volume or less, more preferably 4.0% by volume or more and 10.0% by volume or less. By setting the content ratio within the above range, appropriate lubricity and shape retention can be imparted to the absorbent material.

The electrically insulating matrix may comprise rosin. Rosin has the effect of imparting viscosity and functions as a binder. That is, the use of rosin has the effect of making the viscosity of the absorbent material moderate and improving the shape retention. From the viewpoint of improving the dispersibility of the metal powder, the rosin preferably has a high acid value. The acid value of rosin is preferably 100 mg/g or more, more preferably 120 mg/g or more. Although the type of rosin is not limited, tall rosin, gum rosin, and/or wood rosin are preferred.

Rosin can form an electrically insulating matrix with other ingredients. The content of rosin contained in the absorbent material is preferably 5.0% by volume or more and 20.0% by volume or less, more preferably 8.0% by volume or more and 15.0% by volume or less. By setting the content ratio within the above range, appropriate viscosity and shape retention can be imparted to the absorbent material.

The electrically insulating matrix may optionally contain a dispersant. A dispersant has the effect of increasing the affinity between the metal powder and the base oil. Therefore, by adding a dispersant, the dispersibility of the metal powder can be improved. Specifically, even when the filling rate of the metal powder is high, the base oil permeates narrow gaps in the metal powder, which has the effect of suppressing direct contact between particles.

The dispersant may be appropriately selected according to the surface conditions of the acid groups and bases of the powder. Metal powders often have a basic surface, and the dispersant used therefor preferably has a high acid value. When a dispersant having a high acid value is used, the effect of improving the dispersibility of the metal powder becomes even more pronounced. The acid value of the dispersant is preferably 5.0 mg/g or more, more preferably 20.0 mg/g or more. Although the type of dispersant is not limited, polyether carboxylic acid, polyether phosphate, and/or higher fatty acid polyester are preferred. The content of the dispersant contained in the absorbent material is preferably 0.1% by volume or more and 3.0% by volume or less, more preferably 0.5% by volume or more and 2.0% by volume or less. By setting the content ratio within the above range, it is possible to sufficiently exhibit the effect of improving the dispersibility.

The electrically insulating matrix may contain other ingredients besides base oils, waxes, rosins, and dispersants. Other ingredients include rust inhibitors, metal deactivators, rheology control agents and/or antioxidants. Other components may be appropriately selected depending on the application.

The electromagnetic wave absorbing material of this embodiment has a maximum value of dielectric loss in a high frequency band. For example, the dielectric loss is maximized in the frequency bands of ultra-high frequency (30 MHz-300 MHz), ultra-high frequency (300 MHz-1 GHz), sub-microwave (1 GHz-3 GHz), and/or microwave (3 GHz and above). More specifically, the electromagnetic wave absorbing material can have a dielectric loss tangent (tan δ _E ) peak in the frequency band of 0.1 GHz to 18 GHz. In addition, the dielectric loss tangent (tan δ _E ) value (maximum value) at this peak is increased to 0.05 or more, 0.10 or more, 0.15 or more, 0.20 or more, 0.25 or more, or 0.30 or more is possible.

<<2. Manufacture of electromagnetic wave absorbing material>>
As long as the electromagnetic wave absorbing material of this embodiment satisfies the requirements described above, the manufacturing method is not limited. For example, it can be produced by kneading a matrix-constituting component or its precursor with ferromagnetic metal powder. Kneading may be performed using a known device such as a planetary mill, a revolutionary mixer, and/or three rolls.

<<3. Electromagnetic wave absorber>>
The electromagnetic wave absorber of this embodiment includes a molded body of the electromagnetic wave absorbing material described above. That is, it is produced by molding an electromagnetic wave absorber. A molding method is not limited. For example, the electromagnetic wave absorbing material may be formed into a sheet by rolling, a doctor blade method, or the like. Alternatively, a technique of coating an electromagnetic wave absorbing material on a substrate to form a coating film may be used. The technique is not limited as long as a molded article can be obtained.

The electromagnetic wave absorber exhibits better absorption characteristics as its thickness increases. The thickness in the electromagnetic wave incidence direction is preferably 0.5 mm or more, more preferably 1.0 mm or more, and even more preferably 1.5 mm or more. The thickness in the electromagnetic wave incident direction is the thickness between the front and back sides of the electromagnetic wave absorber (electromagnetic wave absorbing material molded body), and the front and back sides refer to the plane facing the direction of incidence of the electromagnetic wave and the opposite side thereof. It is about the aspect to do.

The electromagnetic wave absorber may be provided with a member other than the molded body of the electromagnetic wave absorbing material. For example, an impedance matching layer or a surface protective layer may be provided on the surface. Also, a reflecting member may be provided on the back surface. As an impedance matching layer, a layer in which magnetic powder or dielectric powder is dispersed in resin is exemplified. As the surface protective layer, a layer made of resin or glass is exemplified. A film-like, foil-like, or net-like metal member can be used as the reflecting member.

The use of the electromagnetic wave absorber is not limited, and it may be applied to known uses. For example, it can be applied to electronic elements, electronic parts, or electronic equipment that have an electromagnetic wave absorber. It may also be applied to construction members such as outer walls and inner walls of buildings. The mode of use is not limited, either, and a known mode may be used. For example, a sheet-like or coated electromagnetic wave absorber may be attached to the outer surface or inner surface of the electronic device housing. Alternatively, it may be provided on the outer surface of the package of the electronic component. Furthermore, an embodiment in which an electromagnetic wave absorber is provided inside the electronic element or electronic component may be adopted.

The electromagnetic wave absorber of this embodiment exhibits electromagnetic wave absorption characteristics based on dielectric loss in a high frequency band. For example, in the frequency bands of ultra-short waves (30 MHz-300 MHz), ultra-short waves (300 MHz-1 GHz), quasi-microwaves (1 GHz-3 GHz), and/or microwaves (3 GHz or more), it exhibits electromagnetic wave absorption characteristics that can be put to practical use. More specifically, the electromagnetic wave absorber has a transmission attenuation rate of 10 dB or more, 15 dB or more, 20 dB or more, 25 dB or more, or 30 dB or more in a frequency band of 0.1 GHz or more and 18 GHz or less. can have

According to this embodiment, it is possible to obtain an electromagnetic wave absorbing material and an electromagnetic wave absorber that utilize dielectric loss and can be put to practical use in a high frequency band. Such electromagnetic wave absorbing materials and electromagnetic wave absorbers have not been known in the past. For example, Patent Literature 1 and Patent Literature 2 propose obtaining absorption characteristics exceeding Snake's limit by flattening the soft magnetic powder contained in the electromagnetic wave absorber. However, the electromagnetic wave absorbers disclosed in these documents utilize only the magnetic loss generated by ferromagnetic soft magnetic powder, and the electromagnetic wave absorber of the present embodiment utilizing dielectric loss does not absorb electromagnetic waves. Mechanism is different. In addition, the soft magnetic flat powder has a large friction between particles, making it difficult to achieve high filling.

Although Patent Document 3 proposes applying a dielectric loss material to an electromagnetic wave absorber, conductive materials such as boron carbide are used as the dielectric loss material, and these are not metal powders. In fact, Patent Document 3 discloses a comparative example sample (Comparative Example 2) containing 45% by volume of aluminum powder, which is a metal powder, but it is said that Comparative Example 2 does not exhibit absorption characteristics ( Table 2 of Patent Document 3).

The present invention will be described in more detail using the following examples and comparative examples. However, the invention is not limited to the following examples.

[Experimental example A]
In Experimental Example A, FeSiCr alloy powder was used as a filler (ferromagnetic metal powder), and electromagnetic wave absorbers containing fillers in various proportions were produced. Then, the effect of the filler content was investigated.

(1) Preparation of electromagnetic wave absorber [Examples 1 to 4]
Filler, base oil, dispersant, wax, and rosin were provided as raw materials. FeSiCr powder A (D50: 8.0 μm) shown in Table 1 below was used as the filler. As raw materials (base oil, dispersant, wax, and rosin) other than fillers, those shown in Table 2 below were used.

The particle size distribution shown in Table 1 below was measured by a laser diffraction scattering method (according to JIS R 1629:1997). The average aspect ratio was obtained by observing the metal powder with an SEM, measuring the minor axis diameter and major axis diameter of each of 50 particles in a plurality of fields of view to determine the aspect ratio, and calculating the number average. Furthermore, the tap density was measured by performing tapping 300 times with a stroke of 3 cm using a shaking specific gravity meter (Kuramochi Scientific Instruments Seisakusho KRS-409).

Next, the prepared raw materials (filler, base oil, dispersant, wax, rosin) are weighed, and kneaded using a rotation and revolution mixer (Thinky Co., Ltd., Awatori Mixer, model ART-930 Twin). and obtained a kneaded product. Thus, an electromagnetic wave absorbing material was produced. Weighing was carried out so as to obtain the composition shown in Table 3 below. The kneading treatment was performed for 4 minutes at a rotational speed of 1350 rpm.

After that, the obtained kneaded material was sandwiched between films and rolled by using hot rolling rolls (Model IMC-1A04, Imoto Seisakusho Co., Ltd.) so that the film thickness became a predetermined value to obtain a sheet. Hot rolling was performed at a temperature of 100°C. The obtained sheet had a thickness of 1 mm. This sheet was used for evaluation as an electromagnetic wave absorber.

(2) Evaluation The electromagnetic wave absorption characteristics of the samples of Examples 1 to 4 were evaluated by transmission attenuation measurement using a microstrip line. Specifically, a sheet-shaped electromagnetic wave absorber was cut into a size of 10 cm×5 cm to prepare a measurement sample. Next, the measurement sample was placed on a microstrip line (Keycom Co., Model TF-6C), and a load (500 g) was applied thereon. A high-frequency signal from a network analyzer was applied to the microstrip line under load, and the reflection characteristics were measured. Then, using the S parameters (S11, S21), the transmission attenuation factor _RTP was obtained according to the following equation (3).

(3) Evaluation Results FIG. 2 shows the transmission attenuation factors of the samples obtained in Examples 1 to 4 using the FeSiCr alloy powder as the filler. In Examples 1 to 4, an increase in the transmission attenuation rate was observed in the frequency range near 6 GHz. Therefore, it was found that it exhibits electromagnetic wave absorption characteristics in this frequency band. It was also found that the higher the filler ratio, the greater the attenuation rate. For example, in Examples 1 to 3 with a filler ratio of 50% by volume or more, the maximum attenuation rate was 20 dB or more. In particular, in Examples 1 and 2, in which the filler ratio was 65% by volume or more, the maximum attenuation rate was as high as 50 dB or more. On the other hand, in Example 4 with a filler ratio of 30% by volume, the maximum attenuation rate was as low as about 5 dB. From this, it was found that a practical electromagnetic wave absorber can be realized by increasing the filler ratio to 50% by volume or more.

[Experimental example B]
In Experimental Example B, two types of FeSiCr alloy powders with different particle sizes were used as fillers (ferromagnetic metal powders). Then, electromagnetic wave absorbers containing these two types of fillers in various ratios were produced, and the effect of the particle size of the fillers was investigated.

(1) Production of electromagnetic wave absorber [Examples 5 to 12]
Filler, base oil, dispersant, wax, and rosin were provided as raw materials. As fillers, FeSiCr alloy powder A (D50: 8.0 μm) and FeSiCr alloy powder B (D50: 1.9 μm) shown in Table 1 were used singly or in combination. In addition, materials shown in Table 2 above were used as materials (base oil, dispersant, wax, and rosin) other than the filler.

Next, an electromagnetic wave absorber was produced in the same manner as in Experimental Example A. Weighing was carried out so as to obtain the composition shown in Table 4 below. The film thickness of the sheet was set to 1 mm.

(2) Evaluation The particle size distribution of the fillers (FeSiCr alloy powder A+FeSiCr alloy powder B) used in Examples 5 to 12 was measured by a laser diffraction scattering method (according to JIS R 1629:1997). In addition, the electromagnetic wave absorption characteristics were evaluated in the same manner as in Experimental Example A.

(3) Evaluation results Fig. 3 shows the particle size distribution of the filler. Both FeSiCr alloy powder A and FeSiCr alloy powder B exhibited a substantially unimodal particle size distribution, but their mixed powder exhibited a multimodal particle size distribution.

FIG. 4 shows the transmission attenuation factors of the samples obtained in Examples 5 to 8 (filler filling rate: 65.00% by volume). FIG. 5 shows the transmission attenuation factors of the samples obtained in Examples 9 to 12 (filler filling rate: 68.42% by volume). At both the filler filling rates of 65.00% by volume and 68.42% by volume, the peak value of the transmission attenuation rate was higher as the ratio of the filler with a larger particle size was larger.

[Experimental example C]
In Experimental Example C, electromagnetic wave absorbers containing FeSiCr alloy powder as a filler (ferromagnetic metal powder) were produced with various film thicknesses, and the effect of the film thickness (sheet thickness) was examined.

(1) Production of electromagnetic wave absorber [Examples 13 to 17]
Filler, base oil, dispersant, wax, and rosin were provided as raw materials. FeSiCr alloy powder A (D50: 8.0 μm) shown in Table 1 above was used as the filler. In addition, materials shown in Table 2 above were used as materials (base oil, dispersant, wax, and rosin) other than the filler.

Next, an electromagnetic wave absorber was produced in the same manner as in Experimental Example A. Weighing was carried out so as to obtain the composition shown in Table 5 below. Also, the sheet thickness was adjusted to 0.1 to 1.5 mm by changing the hot rolling conditions.

(2) Evaluation For Examples 13 to 17, the electromagnetic wave absorption characteristics were evaluated in the same manner as in Experimental Example A.

(3) Evaluation Results FIG. 6 shows the transmission attenuation factors of the samples (sheet thickness: 0.1 to 1.5 mm) obtained in Examples 13 to 17. By setting the sheet thickness to 0.5 mm or more, the peach value of the transmission attenuation rate increased.

[Experimental example D]
(1) Evaluation In Experimental Example D, the sample obtained in Example 6 (FeSiCr alloy powder, filling rate 65% by volume) was used to measure the electromagnetic wave absorption characteristics in the near field in the frequency band of 3 to 18 GHz. Specifically, using a network analyzer and a jig, IEC standard No. : Measured according to the noise suppression sheet evaluation method specified in IEC62333-1 and IEC62333-2.

(2) Evaluation Results FIG. 7 shows the transmission attenuation factor in the frequency band of 3 to 18 GHz. A peak of the transmission attenuation factor was observed near 12 GHz, and the peak value exceeded 30 dB. It was found that the electromagnetic wave absorber of this embodiment exhibits high absorption performance even in a high frequency range of 10 GHz or higher.

[Experimental example E]
(1) Evaluation In Experimental Example E, the sample obtained in Example 6 (FeSiCr alloy powder, filling rate of 65% by volume) was subjected to complex relative permittivity (εr = _εr′ − _jεr ″) and complex relative permeability _. Measurements of the magnetic susceptibility (μ _r =μ _r '-jμ _r '') were performed. where ε _r ' and ε _r '' are the real and imaginary parts of the complex relative permittivity, respectively, and μ _r ' and μ _r '' are the real and imaginary parts of the complex relative permeability, respectively.

<Dielectric constant>
The complex dielectric constant (εr= _εr′ − _jεr ″) of the electromagnetic wave absorber was measured by the coaxial tube S-parameter method. Specifically, a ring-shaped sample (with an inner diameter of 3.06 to 3.10 mm and an outer diameter of 6.93 to 6.96 mm) was prepared and set in a coaxial tube holder. Then, it was measured using a vector network analyzer (Anritsu Corporation, MS46122A). Also, the dielectric loss tangent (tan δ _E ) was calculated according to the following equation (4) using the real part (ε _r ′) and the imaginary part (ε _r ″) of the dielectric constant.

<Permeability>
The complex relative magnetic permeability (μ _r =μ _r '−jμ _r '') of the electromagnetic wave absorber was measured by the S-parameter reflection method. Specifically, a measurement sample (outer diameter 6.93 to 6.96 mm, inner diameter 3.06 to 3.10 mm, film thickness 2 mm) was produced by punching out a sheet-like electromagnetic wave absorber into a doughnut-shaped disk. Next, the measurement sample was set on a jig, and frequency sweep was performed using a vector network analyzer to measure the complex relative permeability. Also, the loss factor (tan δ _M ) was calculated according to the following equation (5) using the real part (μ _r ') and the imaginary part (μ _r '') of the relative permeability.

(2) Results The complex relative permittivity and the complex relative permeability of the sample of Example 6 (FeSiCr alloy powder, filling rate 65% by volume) are shown in FIGS. 8(a) and 8(b), respectively. The real part of the relative permittivity (ε _r ′) decreased near 6 GHz. In addition, the imaginary part of the dielectric constant (ε _r ″) and the dielectric loss tangent (tan δ _E ) peaked in this frequency band, indicating that the dielectric loss increases in this frequency band. On the other hand, the imaginary part of the magnetic permeability decreased on the lower frequency side. Therefore, it was found that the magnetic loss occurs in the lower frequency region than the frequency band (near 6 GHz) where the dielectric loss peak is observed. From this, it was found that by using FeSiCr alloy powder, which is a ferromagnetic metal powder, as a filler, both dielectric loss and magnetic loss can be utilized for absorption characteristics.

[Experimental example F]
In Experimental Example F, an electromagnetic wave absorber was produced using a mixed powder of two types of nickel (Ni) powders with different particle sizes as a filler (ferromagnetic metal powder).

(1) Preparation of electromagnetic wave absorber [Example 18]
Filler, base oil, dispersant, wax, and rosin were provided as raw materials. As fillers, Ni powder A (D50: 4.0 μm) and Ni powder B (D50: 0.7 μm) shown in Table 1 above were used. In addition, materials shown in Table 2 above were used as materials (base oil, dispersant, wax, and rosin) other than the filler.

Next, an electromagnetic wave absorber was produced in the same manner as in Experimental Example A. Weighing was carried out so as to obtain the composition shown in Table 6 below. The film thickness of the sheet was set to 1 mm.

(2) Evaluation Regarding Example 18, the electromagnetic wave absorption characteristics were evaluated in the same manner as in Experimental Example A.

(3) Evaluation Results The transmission attenuation factors of the samples obtained in Example 18 are shown in FIG. The transmission attenuation rate had a peak in the frequency range around 5 to 6 GHz, and the peak value exceeded 40 dB. It was found that good absorption characteristics can be obtained even when Ni powder is used as the ferromagnetic metal powder.

REFERENCE SIGNS LIST 1 electromagnetic wave absorber 2 absorber body 3 metal film 4 incident wave 5 reflected wave 6 reflected wave

Claims (9)

An electromagnetic wave absorbing material comprising an electrically insulating matrix and ferromagnetic metal powder dispersed in the electrically insulating matrix,
The content ratio of the ferromagnetic metal powder is 50% by volume or more,
The ferromagnetic metal powder has an average aspect ratio of 5.0 or less,
The electromagnetic wave absorbing material has a dielectric loss tangent (tan δ _E ) peak in a frequency band of 0.1 GHz to 18 GHz, and a dielectric loss tangent (tan δ _E ) value at the peak is 0.05 or more. 2. The ferromagnetic metal powder according to claim 1, wherein said ferromagnetic metal powder is at least one selected from the group consisting of iron (Fe) powder, iron (Fe) alloy powder, nickel (Ni) powder, and nickel (Ni) alloy powder. of electromagnetic wave absorbing material. 3. The electromagnetic wave absorbing material according to claim 1, wherein said ferromagnetic metal powder has an average aspect ratio of 3.0 or less. The electromagnetic wave absorbing material according to any one of claims 1 to 3, wherein the ferromagnetic metal powder has a volume average particle diameter (D50) of 1.5 µm or more. The electromagnetic wave absorbing material according to any one of claims 1 to 4, wherein the electrically insulating matrix comprises at least one of base oil, wax and rosin. An electromagnetic wave absorber comprising a molded body of the electromagnetic wave absorbing material according to any one of claims 1 to 5. 7. The electromagnetic wave absorber according to claim 6, wherein said electromagnetic wave absorber has a thickness of 0.5 mm or more in the electromagnetic wave incidence direction. 8. The electromagnetic wave absorber according to claim 6, wherein said electromagnetic wave absorber has a region with a transmission attenuation rate of 10 dB or more at a thickness of 1 mm in a frequency band of 0.1 GHz or more and 18 GHz or less. An electronic element, electronic component, or electronic device comprising the electromagnetic wave absorber according to any one of claims 6 to 8.

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