JP6978346B2 - Noise suppression sheet - Google Patents

Description

The present invention relates to a noise suppression sheet capable of suppressing high frequency noise.

Patent Document 1 discloses a composite magnetic material capable of suppressing electromagnetic interference inside an electronic device.

The composite magnetic material disclosed in Patent Document 1 includes a first soft magnetic powder and a second soft magnetic powder bonded by an organic binder. The first soft magnetic powder has a flat shape having an aspect ratio of, for example, 10 or more. The second soft magnetic powder is sufficiently smaller than the first soft magnetic powder. The composite magnetic material formed as described above has, for example, a magnetic resonance frequency of about 30 to 40 MHz, and can suppress electromagnetic waves having a frequency of about 800 MHz. Therefore, this composite magnetic material can be used as a noise suppression sheet capable of suppressing high frequency noise of about 800 MHz to 2.4 GHz.

Japanese Unexamined Patent Publication No. 10-97913

With the increase in the amount of communication data by electronic devices such as mobile terminals, high-speed data communication using a higher frequency band is required. Specifically, data communication using a high frequency band exceeding 3 GHz is being studied. In order to use such a high frequency band, it is necessary to suppress noise in the high frequency band generated inside the electronic device due to data communication.

Therefore, an object of the present invention is to provide a noise suppression sheet capable of suppressing noise in a high frequency band exceeding 3 GHz.

The present invention comprises the first noise suppression sheet.
A noise suppression sheet with a sheet-like composite magnetic material.
The composite magnetic material contains a binder and a soft magnetic powder dispersed in the binder.
In all of the soft magnetic powders, the aspect ratio is less than 5, and the particle size when approximated to a true sphere is 300 nm or less.
The real component of the relative permeability of the composite magnetic material maintains a value larger than 1 up to the frequency band of 7 GHz or higher.
The imaginary component of the relative permeability of the composite magnetic material provides a noise suppression sheet that maintains a value of 0.5 or more up to a frequency band of 6 GHz or more.

Further, the present invention is a first noise suppression sheet as a second noise suppression sheet.
Provided is a noise suppression sheet having an aspect ratio of 3 or less in the soft magnetic powder.

Further, the present invention is a first or second noise suppression sheet as the third noise suppression sheet.
Provided is a noise suppression sheet having a particle size of 100 nm or less in the soft magnetic powder.

Further, according to the present invention, as the fourth noise suppression sheet, any one of the first to third noise suppression sheets is provided.
The soft magnetic powder provides a noise suppression sheet containing one or more kinds of elements selected from Fe, Ni and Co.

Further, the present invention is any one of the first to fourth noise suppression sheets as the fifth noise suppression sheet.
The binder provides a noise suppression sheet made of a polymer having a weight average molecular weight of 800,000 or more.

Further, the present invention is any one of the first to fifth noise suppression sheets as the sixth noise suppression sheet.
At least a part of the soft magnetic powder has a silica film covering the surface and has a silica film.
Provided is a noise suppression sheet having a thickness of 1 nm or more and 10 nm or less.

Further, the present invention is any one of the first to fifth noise suppression sheets as the seventh noise suppression sheet.
At least a part of the soft magnetic powder has a natural oxide film covering the surface.
Provided is a noise suppression sheet having a thickness of 1 nm or more and 2 nm or less of the natural oxide film.

Further, the present invention is any one of the first to seventh noise suppression sheets as the eighth noise suppression sheet.
Provided is a noise suppression sheet in which the volume occupancy of the soft magnetic powder in the composite magnetic material is 30 vol% or more and 40 vol% or less.

Further, according to the present invention, as the ninth noise suppression sheet, any one of the first to eighth noise suppression sheets is provided.
Provided is a noise suppression sheet having a saturation magnetization of 0.45 T or more and 0.98 T or less of the composite magnetic material.

Further, according to the present invention, as the tenth noise suppression sheet, any one of the first to ninth noise suppression sheets is provided.
The composite magnetic material provides a flexible noise suppression sheet.

Further, the present invention is any one of the first to tenth noise suppression sheets as the eleventh noise suppression sheet.
The composite magnetic material provides a noise suppression sheet having a tensile strength of less than 7 MPa.

Further, the present invention is any one of the first to eleventh noise suppression sheets as the twelfth noise suppression sheet.
The composite magnetic material provides a noise suppression sheet having a thickness of 0.1 mm or less.

In the noise suppression sheet according to the present invention, all of the soft magnetic powders contained in the composite magnetic material have a nano-sized substantially spherical shape. By including such a soft magnetic powder, the real number component (magnetic flux convergence component) in the specific magnetic permeability of the composite magnetic material can be maintained at a value larger than 1 up to the frequency band of 7 GHz or higher, and the specific magnetic permeability of the composite magnetic material can be maintained. The imaginary component (magnetic loss component) in the above can be maintained at a value of 0.5 or more up to a frequency band of 6 GHz or more. Further, the imaginary component (dielectric loss component) in the relative permittivity of the composite magnetic material can be lowered over a high frequency band exceeding 3 GHz. According to the above-mentioned characteristics, noise can be collected by a high magnetic flux convergence component, the collected noise can be taken deep inside the noise suppression sheet by a low dielectric loss component, and the captured noise can be suppressed by a magnetic loss component. That is, according to the present invention, it is possible to provide a noise suppression sheet capable of suppressing noise in a high frequency band exceeding 3 GHz.

It is a perspective view which shows typically the noise suppression sheet by embodiment of this invention. The noise incident on the main surface of the noise suppression sheet is schematically drawn by a broken line. It is sectional drawing which shows typically the noise suppression sheet of FIG. 1 along the line II-II. The noise incident on the main surface of the noise suppression sheet is schematically drawn by a broken line. Two of the soft magnetic powders are enlarged and schematically drawn. It is a figure which shows typically the internal space of the small electronic device to which the noise suppression sheet of FIG. 1 is attached. It is a graph which shows the frequency characteristic of the real number component of the relative magnetic permeability in each of the noise suppression sheets of Example 1, Example 2, Comparative Example 1 and Comparative Example 2. 6 is a graph showing the frequency characteristics of the imaginary component of the relative permeability in each of the noise suppression sheets of Example 1, Example 2, Comparative Example 1 and Comparative Example 2. 6 is a graph showing the frequency characteristics of the imaginary component of the relative permittivity in each of the noise suppression sheets of Example 1, Example 2, Comparative Example 1 and Comparative Example 2. It is a graph which shows the frequency characteristic of the real number component of the relative permittivity in each of the noise suppression sheets of Example 1, Example 2, Comparative Example 1 and Comparative Example 2.

As shown in FIG. 1, the noise suppression sheet 10 according to the embodiment of the present invention has a thin sheet shape. More specifically, the noise suppression sheet 10 of the present embodiment has a thickness of 0.1 mm or less. Referring to FIG. 3, the thin noise suppression sheet 10 can be used in a limited internal space (small space) 74 such as the inside of a small electronic device 70 such as a mobile phone. However, the present invention is not limited to this, and the noise suppression sheet 10 may be used inside a larger space. Further, the noise suppression sheet 10 may have a thickness according to the size of the space used.

Referring to FIG. 2, the noise suppression sheet 10 includes a sheet-shaped composite magnetic material 20. The composite magnetic material 20 contains a binder 30 made of an organic material and a soft magnetic powder 40 dispersed in the binder 30. In all of the soft magnetic powders 40, the aspect ratio is less than 5, and the particle size when approximated to a true sphere is 300 nm or less. The aspect ratio is the minor axis / major axis when the length of the line segment connecting the two most distant points of the soft magnetic powder 40 is the major axis and the longest diameter orthogonal to the major axis is the minor axis. The value. Such a composite magnetic material 20 can be manufactured, for example, as follows.

First, using a soft magnetic metal material, a nano-sized soft magnetic powder 40 having a substantially spherical shape is produced, for example, by a gas phase method (heat plasma method, flame method) or a liquid phase method (liquid phase reduction method). .. Next, the soft magnetic powder 40 is mixed with a sol-shaped binder 30 made of an organic material, and the mixture is stirred so that the soft magnetic powder 40 is uniformly dispersed in the binder 30 to prepare a slurry. Next, a sheet-shaped thin film is produced from the slurry by, for example, the doctor blade method. Next, the prepared thin film is cut into a plurality of sheets and laminated. Next, the laminated thin film is pressed in a direction orthogonal to the sheet surface. As a result, a sheet-like composite magnetic body 20 composed of a plurality of compressed thin films is formed. In the composite magnetic material 20, the substantially spherical nano-sized soft magnetic powder 40 is relatively densely and uniformly distributed in the binder 30 having a relatively high resistivity.

The composite magnetic material 20 of the noise suppression sheet 10 contains a magnetic material (soft magnetic powder 40), and has a magnetic flux convergence component, a magnetic loss component, and a dielectric loss component. The magnetic flux convergence component is a real component (μ') of the complex relative magnetic permeability (μr = μ'−jμ ″: hereinafter simply referred to as “specific magnetic permeability μr”) of the composite magnetic material 20, and the magnetic loss component is It is an imaginary component (μ ″) of the relative permeability μr. The dielectric loss component is the complex relative dielectric constant (εr = ε ′ −jε ″: hereinafter simply referred to as “specific dielectric constant εr”) of the composite magnetic material 20. It is an imaginary component (ε ″).

In the noise suppression sheet 10 according to the present embodiment, all of the soft magnetic powders 40 contained in the composite magnetic material 20 have a nano-sized substantially spherical shape. By including such a soft magnetic powder 40, the magnetic flux convergence component μ'of the composite magnetic body 20 can be maintained at a value larger than 1 up to a frequency band of 7 GHz or higher, and the magnetic loss component μ "of the composite magnetic body 20 can be maintained. , The value can be maintained at 0.5 or more up to the frequency band of 6 GHz or more. Further, the dielectric loss component ε ″ of the composite magnetic material 20 can be lowered over the high frequency band of 3 GHz or more.

For details, referring to Examples 1 and 2 of FIG. 4, the magnetic flux convergence component μ'of the composite magnetic material 20 is maintained at a value larger than 1 while gradually decreasing as a whole from 100 MHz to 7 GHz. Referring to Examples 1 and 2 of FIG. 5, the magnetic loss component μ ″ of the composite magnetic material 20 gradually increases as a whole from 100 MHz to 6 GHz and is maintained at a value of 0.5 or more in the frequency band of 2 GHz or more. Further , referring to Examples 1 and 2 of FIG. 6, the dielectric loss component ε ″ of the composite magnetic material 20 is maintained at a low value of 25 or less from 100 MHz to 7 GHz. In particular, the dielectric loss component ε ″ of the composite magnetic material 20 is maintained at a low value of 2 or less from 100 MHz to 9 GHz. On the other hand, referring to Examples 1 and 2 of FIG. 7, the relative dielectric constant of the composite magnetic material 20 is maintained. The real component ε'of the rate is maintained at an almost constant value (about 11 to 25) from 100 MHz to 7 GHz.

Referring to FIGS. 1 and 2, the noise suppression sheet 10 extends on a plane parallel to a horizontal plane (XY plane). Specifically, the noise suppression sheet 10 has an incident surface 12 and an opposite surface 14 parallel to the XY plane. The incident surface 12 and the opposite surface 14 are located on opposite sides of each other in a predetermined direction (Z direction). Referring to FIG. 3, the noise suppression sheet 10 is arranged in the vicinity of the antenna 78 of the small electronic device 70, for example. Specifically, the opposite surface 14 is attached to the inner surface of the housing 72 of the small electronic device 70. At this time, the incident surface 12 is exposed inside the small space 74 of the small electronic device 70, and the electromagnetic wave (noise) 80 generated from various electronic components 76 of the small electronic device 70 is incident on the incident surface 12. ..

In the present embodiment, the small space 74 of the small electronic device 70 is a space where 2πr / λ <1 when the distance of the electromagnetic wave 80 from the wave source is r and the wavelength of the electromagnetic wave 80 is λ. In such a small space 74, noise 80 in a high frequency band exceeding 3 GHz is likely to occur. Since the noise suppression sheet 10 of the present embodiment has a magnetic flux convergence component μ ′ larger than 1 (that is, larger than the magnetic flux convergence component μ ′ of the small space 74) even in this high frequency band, it is inside the small space 74. It is easy to collect the noise 80 in the high frequency band propagating.

The noise 80 collected on the noise suppression sheet 10 is incident on the incident surface 12. Since the composite magnetic material 20 has a low dielectric loss component ε ″ even in a high frequency band, reflection of noise 80 on the incident surface 12 can be suppressed. Further, the composite magnetic material 20 of the present embodiment can also be used as a dielectric material. In order to function, there is a relationship of σ = 2π · ε ″ · f between the frequency f of the dielectric loss component ε ″ and the noise 80 and the conductivity σ. The penetration depth (that is, the skin depth) δ of the electromagnetic wave 80 is expressed by the equation δ = √ {1 / (π ・ μ ′ ・ σ ・ f)}. Therefore, δ = √ {1 / ( 2π ²・ μ ′ ・ ε ″ ・ f ² )}, and the skin depth δ becomes smaller in inverse proportion to the frequency f of the electromagnetic wave 80. However, the skin depth δ at a predetermined frequency f can be increased by reducing the coefficient ε ″. Since the dielectric loss component ε ″ of the composite magnetic material 20 is low, the skin depth is large. Therefore, the noise 80 incident on the incident surface 12 penetrates deep inside the noise suppression sheet 10.

Since the composite magnetic material 20 has a magnetic loss component μ ″ larger than 0 (0.5 or more) even in the high frequency band, the noise 80 that has penetrated deep inside the composite magnetic material 20 is attenuated. The 80 is reflected by the housing 72, passes through the inside of the composite magnetic body 20 while being further attenuated, and is radiated from the incident surface 12 into the small space 74. The noise 80 radiated from the incident surface 12 is large. Therefore, even if the noise 80 is incident on the antenna 78, the antenna 78 is hardly affected. That is, the communication quality of the small electronic device 70 is hardly deteriorated.

As described above, according to the characteristics of the composite magnetic material 20 of the present embodiment, the noise 80 in the high frequency band exceeding 3 GHz is collected by the high magnetic flux convergence component μ ′, and the collected noise 80 is used as the low dielectric loss component ε ″. The noise 80 taken in can be suppressed by the magnetic loss component μ ". That is, according to the present invention, it is possible to provide a noise suppression sheet capable of suppressing noise 80 in a high frequency band exceeding 3 GHz.

For example, the wavelength λ of the noise 80 having a frequency of 3 GHz is 10 cm, and the distance r = λ / 2π from the wave source defining the near field is less than 16 mm. That is, the small space 74 is in the vicinity of the high impedance field where the wave impedance is the wave source of the minute dipole antenna (electric field component) and the low impedance field where the minute loop antenna (magnetic field component) is the wave source. Therefore, it is difficult to match the impedance of the small space 74 with the impedance of the incident surface 12 of the noise suppression sheet 10.

On the other hand, in most cases, the source (wave source) of the noise 80 in the small space 74 is a small loop antenna (magnetic field component). The noise suppression sheet 10 according to the present embodiment collects the noise 80 of the magnetic field component by the magnetic flux convergence component μ ′ and suppresses it by the magnetic loss component μ ″. In addition, according to the present embodiment, the noise of the magnetic field component is suppressed. A larger noise suppression effect can be obtained by increasing the penetration depth δ of 80. The noise suppression sheet 10 according to the present embodiment does not match the interface impedance of the incident surface 12 with the impedance of the small space 74. The noise 80 propagating in the near field can be suppressed.

With reference to FIG. 2, in the present embodiment, various features such as the material of the binder 30, the shape, composition, and volume filling rate of the soft magnetic powder 40 are not particularly limited. However, as described below, by setting these features in a preferable range, the effect of the present embodiment can be enhanced.

From the viewpoint of reducing the dielectric loss component ε ″ of the composite magnetic material 20 as much as possible, the soft magnetic powder 40 preferably has a shape as close to a true spherical shape as possible. More specifically, the aspect ratio of the soft magnetic powder 40. Is preferably 3 or less. In this way, by making the shape of the soft magnetic powder 40 closer to a true spherical shape, the demagnetizing coefficient on the three axes (X-axis, Y-axis and Z-axis) is isotropic. Therefore, the specific magnetic permeability μr in the plane parallel to the sheet surface (XY plane) of the composite magnetic material 20 becomes low. When the specific magnetic permeability μr becomes low, the specific magnetic permeability μr, the magnetic resonance frequency fr, and the saturation magnetization Ms The magnetic resonance frequency fr is increased by the relational expression μr · fr∝Ms. As a result, ferromagnetic resonance can be exhibited in the high frequency band, and a large magnetic loss component μ ″ can be obtained.

By making the particle size of the soft magnetic powder 40 smaller than the skin depth, it is possible to suppress a decrease in the magnetic flux convergence component μ'due to the eddy current. In addition, since the particle shape of the soft magnetic powder 40 is substantially spherical and isotropic, the demagnetic field coefficient in the noise suppression sheet surface is as large as 1/3 and μ'is small, so that the equation δ = √ {1 / (2π ²・ μ ′ ・ ε ″ ・ f ² )} reduces the skin depth δ at high frequencies. This makes it possible to increase the magnetic resonance frequency fr. As described above, the magnetic resonance frequency. By increasing fr, ferromagnetic resonance loss can be obtained in the high frequency band. Therefore, it is preferable that the soft magnetic powder 40 has a particle size as small as possible and is spherical. More specifically, the soft magnetic powder. The particle size at 40 is preferably 100 nm or less.

The soft magnetic powder 40 can be made from various magnetic materials. For example, the soft magnetic powder 40 may contain one or more elements selected from Fe, Ni and Co. For example, the soft magnetic powder 40 is made from Fe-Ni alloys and Fe-Co alloys that do not contain Mg, Al, Si, Ca, Zr, Ti, Hf, Zn, Mn, lanthanoids, Ba and Sr at all except for unavoidable impurities. Can be made. According to the above composition, the skin depth of the composite magnetic material 20 with respect to the noise 80 of 1 GHz is about 300 nm, which is equal to or larger than the particle size.

By producing the soft magnetic powder 40 with the above composition, the saturated magnetostriction λs of ^{the soft magnetic powder 40 can be 20 × 10 -6} or more, and the saturation magnetization Ms of the soft magnetic powder 40 can be 1.5 T or more and It can be 2.45T. By dispersing a large amount of the soft magnetic powder 40 in the binder 30, the saturation magnetization Ms of the composite magnetic material 20 becomes 0.45 T or more and 0.98 T or less. Further, by setting the saturation magnetization Ms of the soft magnetic powder 40 to 1.7 T or more and 2.45 T, the saturation magnetization Ms of the composite magnetic material 20 can be set to 0.51 T or more and 0.98 T or less. .. By increasing the saturation magnetization Ms of the soft magnetic powder 40 in this way, the magnetic resonance frequency fr can be increased and the magnetic flux convergence component μ'in the high frequency band exceeding 3 GHz can be made larger than 1. From the viewpoint of increasing the saturation magnetization Ms, the volume occupancy of the soft magnetic powder 40 in the composite magnetic material 20 is preferably 30 vol% or more, and from the viewpoint of easily producing the composite magnetic material 20, 40 vol% or less. Is preferable.

As the particle size of the soft magnetic powder 40 becomes smaller, the surface area (specific surface area) with respect to the volume increases. The specific surface area of the soft magnetic powder 40 according to this embodiment is about 8 to ^{18 m 2 / g.} From the viewpoint of covering a large number of soft magnetic powders 40 having a large specific surface area to form a sheet-like composite magnetic body 20 without causing cracks or the like, the binder 30 is a polymer having a large weight average molecular weight (that is, a coating). It is preferably a polymer having a large area). More specifically, the binder 30 is preferably made of a polymer having a weight average molecular weight of 800,000 or more, and more preferably made of a polymer having a weight average molecular weight of 1,000,000 or more. The composite magnetic material 20 provided with such a binder 30 has a tensile strength of less than 7 MPa.

More specifically, the binder 30 is, for example, a (meth) acrylic polymer such as acrylic rubber or an alkyl acrylate copolymer, acrylonitrile-butadiene rubber (NBR), silicone rubber, polyurethane, polyethylene, ethylene / propylene rubber ( Various materials such as olefin rubber such as EPM), ethylene propylene diene rubber (EPDM), polyphenylene oxide (PPO), and bismastriazine resin may be formed alone or in combination of two or more.

According to this embodiment, a large number of substantially spherical soft magnetic powders 40 are dispersed in a binder 30 made of a dielectric. Therefore, in the plane parallel to the sheet plane (XY plane) of the composite magnetic material 20, a large number of capacitors having the dielectric sandwiched between the metal as the conductor are formed and connected in series. According to this embodiment, the dielectric loss component ε ″ in the XY plane (that is, the dielectric loss component ε ″ in the amplitude direction of the noise 80 passing through the inside of the composite magnetic material 20) can be further reduced by this structure.

According to the present embodiment, the composite magnetic material 20 does not contain an inorganic filler or a carbon-based filler. Therefore, the composite magnetic material 20 has high flexibility and can be attached to various places. However, the present invention is not limited to this. For example, the composite magnetic material 20 may contain one or more flame retardants such as melamine cyanurate, red phosphorus, aluminum hydroxide, magnesium hydroxide, and phosphazene as long as the flexibility is not significantly reduced. .. These flame retardants may be dispersed in the binder 30.

An oxide film of a base element may be formed on the surface of the soft magnetic powder 40. In other words, at least a part of the soft magnetic powder 40 may have a natural oxide film 46 covering the surface. However, from the viewpoint of preventing a decrease in the volume occupancy of the magnetic material in the composite magnetic material 20, the thickness of the natural oxide film 46 should be thin. More specifically, the thickness of the natural oxide film 46 is preferably 1 nm or more and 2 nm or less.

Silicon element (Si) is preliminarily charged in the mother alloy on the surface of the soft magnetic powder 40 and heat-treated in an atmosphere where the oxygen partial pressure is adjusted. From silicon oxide by a thermal oxidation method, a sol-gel method using alkoxysilane, or the like. A high resistance film may be formed. In other words, at least a part of the soft magnetic powder 40 may have a silica film 48 covering the surface. By covering the surface of the soft magnetic powder 40 with the silica film 48, the dielectric loss component ε ″ of the composite magnetic material 20 can be further lowered. In addition, the surface resistivity of the noise suppression sheet 10 becomes large, so that the noise suppression sheet 10 The thickness of the silica film 48 is preferably thick from the viewpoint of obtaining the above-mentioned effects, while the viewpoint of preventing a decrease in the volume occupancy of the magnetic material in the composite magnetic material 20. Therefore, the thickness of the silica film 48 should be thin. More specifically, the thickness of the silica film 48 is preferably 1 nm or more and 10 nm or less.

The noise suppression sheet 10 of the present embodiment includes only the composite magnetic material 20. However, the present invention is not limited to this. For example, the incident surface 12 may be covered with an insulating film. That is, the noise suppression sheet 10 may include a layer such as an insulating film.

Hereinafter, the effects of the present invention will be described in more detail with reference to specific examples and comparative examples.

(Noise suppression sheet of Example 1)
A soft magnetic powder made of an Fe—Ni alloy and having a substantially spherical shape was produced by a vapor phase method. The soft magnetic powder contained 50% by mass of Ni, and the surface of the soft magnetic powder was covered with a natural oxide film. The aspect ratio of the soft magnetic powder was about 1 to 1.5, and the particle size of the soft magnetic powder was 100 nm or less. The specific surface area of the soft magnetic powder was 8.3 m ² / g. The soft magnetic powder was dispersed in toluene in which the binder was dissolved to prepare a magnetic paint. As the binder, an alkyl acrylate copolymer having a carboxylic acid as a special functional group was used. The weight average molecular weight (Mw) of the binder was about 1 million. A green sheet was formed from a magnetic paint by the doctor blade method. The green sheet was cut into multiple pieces. A plurality of green sheets were laminated and heat-cooled and pressed to prepare a noise suppression sheet of Example 1 made of a sheet-shaped composite magnetic material.

The saturation magnetization Ms of Example 1 was measured by a sample vibration type magnetometer. The saturation magnetization Ms of Example 1 was 0.53T. Further, by this measurement, it was found that the volume occupancy of the soft magnetic powder contained in Example 1 was 35 vol%.

(Noise suppression sheet of Example 2)
A soft magnetic powder made of an Fe—Co alloy and having a substantially spherical shape was produced by a vapor phase method (thermal plasma method). The soft magnetic powder contained 30% by mass of Ni, and the surface of the soft magnetic powder was covered with a natural oxide film. The aspect ratio of the soft magnetic powder was about 1 to 1.5, and the particle size of the soft magnetic powder was 100 nm or less. The specific surface area of the soft magnetic powder was 17.7 m ² / g. The soft magnetic powder was dispersed in toluene in which the binder was dissolved to prepare a magnetic paint. As the binder, an alkyl acrylate copolymer having a carboxylic acid as a special functional group was used. The weight average molecular weight (Mw) of the binder was about 1 million. A green sheet was formed from a magnetic paint by the doctor blade method. The green sheet was cut into multiple pieces. A plurality of green sheets were laminated and heat-cooled and pressed to prepare a noise suppression sheet of Example 2 made of a sheet-shaped composite magnetic material.

The saturation magnetization Ms of Example 2 was measured by a sample vibration type magnetometer. The saturation magnetization Ms of Example 2 was 0.66T. Further, by this measurement, it was found that the volume occupancy of the soft magnetic powder contained in Example 2 was 35 vol%.

(Noise suppression sheet of Comparative Example 1)
A soft magnetic powder having a substantially spherical shape, which was made of an Fe—Si—Cr alloy containing 3.5% by mass of Si and 4.5% by mass of Cr, was produced by a vapor phase method (thermal plasma method). The soft magnetic powder was flattened using a ball mill to prepare a flat-shaped soft magnetic powder. The aspect ratio of the soft magnetic powder was 46, and the particle size of the soft magnetic powder was 23 μm. Using the prepared soft magnetic powder, a noise suppression sheet of Comparative Example 1 made of a sheet-shaped composite magnetic material was prepared by the same method as in Examples 1 and 2.

The saturation magnetization Ms of Comparative Example 1 was measured by a sample vibration type magnetometer. The saturation magnetization Ms of Comparative Example 1 was 0.85T. Further, by this measurement, it was found that the volume occupancy of the soft magnetic powder contained in Comparative Example 1 was 50 vol%.

(Noise suppression sheet of Comparative Example 2)
A commercially available carbonyl iron powder having a substantially spherical shape was prepared. The particle size of the carbonyl iron powder was 4 μm (= 4000 nm), and the specific surface area of the carbonyl iron powder was 0.3 m ² / g. The carbonyl iron powder was dispersed in toluene in which the binder was dissolved to prepare a magnetic paint. As the binder, an alkyl acrylate copolymer having a carboxylic acid as a special functional group was used. The weight average molecular weight (Mw) of the binder was about 1 million. A green sheet was formed from a magnetic paint by the doctor blade method. The green sheet was cut into multiple pieces. A plurality of green sheets were laminated and heat-cooled and pressed to produce a noise suppression sheet of Comparative Example 2 made of a sheet-shaped composite magnetic material.

The saturation magnetization Ms of Comparative Example 2 was measured by a sample vibration type magnetometer. The saturation magnetization Ms of Comparative Example 2 was 0.78T. Further, by this measurement, it was found that the volume occupancy of the soft magnetic powder contained in Comparative Example 2 was 39 vol%.

(Measurement of complex magnetic permeability and complex permittivity)
For each of Example 1, Example 2, Comparative Example 1 and Comparative Example 2, the relative permeability μr (complex relative permeability) and the relative permittivity εr (complex relative permittivity) in the plane parallel to the sheet were measured. .. This measurement was made by the coaxial tube method using a network analyzer. The measurement results are shown in FIGS. 4 to 7, respectively.

Referring to FIGS. 4 and 5, Comparative Example 1 has a high magnetic loss component μ ″, while the magnetic flux convergence component μ ′ rapidly decreases as the frequency increases beyond 2 GHz. The magnetic flux convergence of Comparative Example 1 The component μ'is smaller than the magnetic flux convergence component μ'of Examples 1 and 2 in the frequency band exceeding 6 GHz, and a sufficient noise suppression effect cannot be obtained. The particle size of Comparative Example 2 is the depth of the skin. Therefore, the magnetic flux convergence component μ ′ gradually decreases due to the magnetic loss due to the eddy current, and becomes smaller than that of Examples 1 and 2 in the high frequency band of 3 GHz to 6 GHz, and is 1 or less at 7 GHz. Will be.

Example 1 magnetically resonates at 180 MHz. The magnetic flux convergence component μ'of Example 1 is 1 or less at 7 GHz, as in Comparative Example 2. Further, the magnetic loss component μ ″ of Example 1 peaks at two frequencies in a high frequency band exceeding 3 GHz. The magnetic loss component μ ″ of Example 1 is different from Comparative Example 2 in a frequency band exceeding 6 GHz. It is almost the same. However, the magnetic loss component μ ″ of Example 1 gradually increases from 200 MHz. Therefore, the low-pass filter performance of Example 1 is higher than that of Comparative Example 2. In Example 2, magnetic resonance occurs at 1.1 GHz. The magnetic flux convergence component μ ′ of Example 2 is larger than 1 even at 9 GHz. Further, the magnetic loss component μ ″ of Example 2 peaks at two frequencies in the high frequency band exceeding 3 GHz. The magnetic loss component μ ″ of Example 2 is substantially the same as that of Comparative Example 2 at 9 GHz, and tends to increase even in a frequency band exceeding 9 GHz and become larger than that of Comparative Example 2.

Referring to FIGS. 6 and 7, Comparative Example 1 has a large dielectric loss component ε ″ and a real number component ε ′ of a relative permittivity εr, and largely reflects noise in a high frequency band exceeding 3 GHz. In Examples 1 and 2, the dielectric loss component ε ″ and the real number component ε ′ of the relative permittivity εr are about 1/5 of that of Comparative Example 1, and noise is suppressed even in a high frequency band exceeding 3 GHz. Noise can be introduced inside the sheet.

As can be understood from FIGS. 4 to 7, Noise 1 and Example 2 can suppress noise to a higher frequency than Comparative Example 1 and Comparative Example 2. That is, in order to obtain the noise absorption effect of the magnetic loss component μ ″ in the small space of a small electronic device, the magnetic flux convergence component μ ′ is maintained at a value larger than 1 in the high frequency band exceeding 3 GHz, and the dielectric loss The present invention of reducing the component ε ″ is effective.

10 Noise suppression sheet 12 Incident surface 14 Opposite surface 20 Composite magnetic material 30 Binder 40 Soft magnetic powder 46 Natural oxide film 48 Silica film 70 Small electronic equipment 72 Housing 74 Internal space (small space)
76 Electronic components 78 Antenna 80 Electromagnetic waves (noise)

Claims (13)

A noise suppression sheet with a sheet-like composite magnetic material.
The composite magnetic material contains a binder and a soft magnetic powder dispersed in the binder.
In all of the soft magnetic powders, the aspect ratio is less than 5, and the particle size when approximated to a true sphere is 100 nm or less.
The real component of the relative permeability of the composite magnetic material maintains a value larger than 1 up to a frequency band of at least 7 GHz.
The real component of the relative permeability of the composite magnetic material is less than 3 in the frequency band of 1 GHz.
The imaginary component of the relative permeability of the composite magnetic material maintains a value of 0.5 or more from 2 GHz to at least 6 GHz in the frequency band.
A noise suppression sheet in which the imaginary component of the relative permittivity of the composite magnetic material maintains a value of 25 or less up to a frequency band of at least 7 GHz. The noise suppression sheet according to claim 1.
A noise suppression sheet in which the imaginary component of the relative permittivity of the composite magnetic material maintains a value of 2 or less up to a frequency band of at least 9 GHz. The noise suppression sheet according to claim 1 or 2.
A noise suppression sheet in which the real number component of the relative permittivity of the composite magnetic material maintains a value of 11 to 25 up to a frequency band of at least 7 GHz. The noise suppression sheet according to any one of claims 1 to 3.
A noise suppression sheet having an aspect ratio of 3 or less in the soft magnetic powder. The noise suppression sheet according to any one of claims 1 to 4.
The soft magnetic powder is a noise suppression sheet containing one or more elements selected from Fe, Ni and Co. The noise suppression sheet according to any one of claims 1 to 5.
The binder is a noise suppression sheet made of a polymer having a weight average molecular weight of 800,000 or more. The noise suppression sheet according to any one of claims 1 to 6.
At least a part of the soft magnetic powder has a silica film covering the surface and has a silica film.
A noise suppression sheet having a thickness of 1 nm or more and 10 nm or less. The noise suppression sheet according to any one of claims 1 to 6.
At least a part of the soft magnetic powder has a natural oxide film covering the surface.
A noise suppression sheet having a thickness of 1 nm or more and 2 nm or less of the natural oxide film. The noise suppression sheet according to any one of claims 1 to 8.
A noise suppression sheet in which the volume occupancy of the soft magnetic powder in the composite magnetic material is 30 vol% or more and 40 vol% or less. The noise suppression sheet according to any one of claims 1 to 9.
A noise suppression sheet having a saturation magnetization of 0.45 T or more and 0.98 T or less of the composite magnetic material. The noise suppression sheet according to any one of claims 1 to 10.
The composite magnetic material is a flexible noise suppression sheet. The noise suppression sheet according to any one of claims 1 to 11.
The composite magnetic material is a noise suppression sheet having a tensile strength of less than 7 MPa. The noise suppression sheet according to any one of claims 1 to 12.
The composite magnetic material is a noise suppression sheet having a thickness of 0.1 mm or less.

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