JPH06209181A - Radio wave absorber - Google Patents

Abstract

PURPOSE:To obtain radio wave absorber having a magnetic material layer whose permeability is large even in a high frequency band, by arranging pin- shaped soft ferrite sintered bodies vertically to a plate surface, and installing a magnetized permanent magnet on at least one surface of the plate. CONSTITUTION:Pin-shaped soft ferrite sintered bodies with 1-10mm diameter whose aspect ratio is 5 or larger are so arranged that the pin direction is vertical to the plane surface, and a ferrite plate 5 is formed by combining them with resin. Hard ferrite particle powder is kneaded in resin, and a layer of 2-30 mm in thickness is formed on the ferrite plate 5. Magnetized permanent magnets 4 are laminated on one side or on both sides or alternately laminated. A the pin-shaped soft ferrite sintered bodies used in the above process is nickel zinc spinel ferrite sintered bodies, or W type or Y type or Z type hexagonal system ferrite sintered bodies. The surface magnetic flux density of the permanent magnet is at least 100G. By a dielectric laminate using this, radio wave absorber is constituted.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a radio wave absorber, and more particularly to a radio wave absorber using ferrite.

[0002]

2. Description of the Related Art In order to prevent radio wave interference such as communication and broadcasting due to buildings, it has been conventionally practiced to construct a radio wave absorber by constructing a radio wave absorber on the wall surface. As a material for the radio wave absorber, a ferrite sintered body has been put to practical use in the frequency body of VHF to UHF.

[0003]

The electromagnetic wave absorber using the above-mentioned conventional ferrite sintered body has excellent electromagnetic wave absorption characteristics, but is poor in workability / formability and workability because it is a sintered body. There was a problem.

[0004]

The radio wave absorber of the present invention comprises:
The absorber has a multilayer structure in which a magnetic layer is provided on a metal layer and a dielectric layer is further provided on the magnetic layer. In the radio wave absorber of the present invention, needle-shaped soft ferrite sintered bodies having a diameter of 1 to 10 mm and an aspect ratio of 5 or more are arranged so that the needle-shaped direction is perpendicular to the plate surface, and these are made into resin. Is prepared by kneading a hard ferrite particle powder with a resin to form a plate having a thickness of 2 to 30 mm, and magnetized permanent magnets are laminated on one side or both sides or alternately.

The needle-shaped soft ferrite sintered body used here is a nickel zinc spinel ferrite sintered body, or a W type, Y containing alkaline earth metal ions.
-Type or Z-type hexagonal ferrite sintered body.

By using the bond magnetic body made of such needle-shaped magnetic body, it is possible to eliminate the problems of the radio wave absorber using the conventional ferrite sintered body.

[0007]

As shown in FIG. 2A, the magnetic layer 2 (thickness d
_m, the relative permeability μ = μ'-jμ ") dielectric layer 3 on the (thickness d _e, the relative dielectric constant ε = ε'-jε") Given what was provided, d _m, d _e In the frequency band where is sufficiently shorter than the wavelength, it can be treated as an equivalent circuit as shown in FIG.

In FIG. 1 (b), resistance, reactance,
Conductance and susceptance are expressed as follows.

Resistance: R = Z₀(Ω / c) μ ″ d_m Reactance: X = Z₀(Ω / c) μ'd_m  Conductance: G = (1 / Z₀) (Ω / c) ε ″ d
_e Susceptance: B = (1 / Z₀) (Ω / c) ε′d
_e Here, in the frequency band where the magnetic permeability of the magnetic material is μ ′> μ ″
Can ignore the resistance R of the equivalent circuit and only reactance
, The susceptance of the dielectric and the reactance of the magnetic
A parallel resonant circuit is formed by. If this is in resonance
The input impedance of this resonant circuit is infinite.
Is equivalent to an open wall, so the conductance of the dielectric is
To construct a Salisbury-type electromagnetic wave absorber.
It In the resonance state, B = 1 / X, so as a magnetic substance
Is μ ′> μ ″ and (ε′d_m) (Μ'd_e) = (C /
ω)²Conditions, that is, μ ′> μ ″, and thinning
Therefore, μ'is required to be large.

As a result of intensive studies by the present inventors, FIG.
As shown in (a) and (b), needle-shaped soft ferrite sintered bodies having a diameter of 1 to 10 mm and an aspect ratio of 5 or more are arranged so that the needle-shaped direction is perpendicular to the plate surface, A plate 5 in which these are bonded with a resin is prepared, and hard ferrite particle powder is kneaded with the resin to form a thickness of 2 to 30 mm, and magnetized permanent magnets 4 are laminated on one side or both sides or alternately. The present invention has been completed by finding that the multi-layer bond magnetic material having the above structure satisfies the above-mentioned conditions.

The reason for this is still under consideration, but so far it is considered as follows. Now, consider a situation in which the plate surface of the plate-shaped magnetic body is placed vertically in the traveling direction of the electromagnetic wave as shown in FIG. Spontaneous magnetic moment M _{S of} magnetic material and vertical direction of plate surface (incident direction of electromagnetic wave)
Assuming that the angle formed by and is θ and the magnetic field of the electromagnetic wave is H, the magnetic field that induces the magnetic permeability is Hco, as shown in FIG.
represented by S.theta, when the magnetic permeability of the magnetic body when the theta = 0 and mu _e, permeability mu observed is given by the following equation.

Μ = (M−M _S ) / H = μ _e Hcos θ /
H = μ _e cos θ In conventional ferrite sintered bodies and ferrite green compacts, the spontaneous magnetic moments are oriented in random directions, so the collective average of cos θ is: <cos θ> = cos θ d (cos θ) / d ( cos
θ) = 1/2 That is, it is half of the magnetic permeability μ _e of the magnetic substance when θ = 0.

In the multi-layer bond magnetic material of the present invention, the shape anisotropy caused by orienting the acicular ferrite sintered body perpendicularly to the plate surface and the lamination of magnetized bond permanent magnets. By the static magnetic field, the magnetic moment is oriented perpendicular to the plate surface <cos θ>
= 1, and the magnetic permeability μ has a maximum value μ _e .

The resonance frequency ω _r of the natural resonance of the magnetic substance
Is ω _r = (gμ _B / h) H '= (gμ _B / h) [H _a +
(N _X −N _Z ) M _S ]. Where g is a Lande factor, μ _B is a Bohr magneton, h is a Planck's constant, H ′ is an anisotropic magnetic field, H _a is a crystalline magnetic anisotropic magnetic field, and N _X and N _Z are perpendicular to the traveling direction of the electromagnetic wave. , The demagnetizing factor in the parallel direction. In conventional ferrite sintered bodies and ferrite green compacts, since the shape used is plate-like, N _Z > N _X.
The resonance frequency of natural resonance tends to be low. In the multilayer bonded magnetic body of the present invention, by orienting the acicular ferrite sintered body perpendicular to the plate surface, N _Z > N _X , and the resonance frequency of natural resonance can shift to the high frequency side. .

As a result of repeated studies on the frequency characteristics of magnetic permeability, the present inventors have made a needle-shaped soft ferrite sintered body having a diameter of 1 to 10 mm and an aspect ratio of 5 or more into a plate having a needle-shaped direction. A sheet is prepared by arranging them so that they are perpendicular to the plane and binding them with resin, and kneading hard ferrite particle powder into the resin to form a sheet with a thickness of 2 to 30 mm, and magnetizing the permanent magnet on one side. Alternatively, in a multilayer bond magnetic material that is laminated on both sides or alternately, the spontaneous magnetic moment is aligned in the direction perpendicular to the plate surface due to the shape anisotropy by orienting the acicular ferrite and the static magnetic field from the permanent magnet. Therefore, the magnetic permeability is increased and the anisotropic magnetic field is increased, so that the resonance frequency is also increased, which allows the conventional ferrite sintering even in a high frequency band such as the MHz band. A large mu 'than ferrite powder compact, it was able to realize the material mu "is small.

In the acicular soft ferrite sintered body,
A desired μ can be obtained in the frequency band of 50 to 300 MHz by using a nickel-zinc spinel ferrite needle-shaped sintered body that has a high electric resistance, a large spontaneous magnetic moment, and can be fired in air. The acicular soft ferrite sintered body is a W-type, Y-type, or Z-type hexagonal ferrite sintered body containing alkaline earth metal ions having a large spontaneous magnetic moment and a large anisotropic magnetic field. By using, the desired μ can be obtained in the frequency band of 50 to 800 MHz. In particular, in the structure using the hexagonal ferrite sintered body, it is expected that the above-mentioned desired μ can be obtained even in a higher frequency band.

[0017]

EXAMPLES Next, examples of the present invention will be described.

As Example 1, a needle-shaped (Ni, Zn) -ferrite sintered body (Ni _0.3 Zn _0.7 Fe ₂ O ₄ ) having a diameter of 2 mm and an aspect ratio of 8 was used, and the needle-shaped direction was on the plate surface. They are arranged vertically, and these are bonded with an epoxy resin, and for measurement, the outer diameter is 38.5 mm and the inner diameter is 17.0.
A ring-shaped (Ni, Zn) -ferrite bond magnetic body having a thickness of 9.5 mm and a thickness of 9.5 mm was prepared.

First, 24 g of EVA resin and 242 g of Ba-ferrite (BaFe ₁₂ O ₁₉ ) particle powder are added in an amount of 60 to 60%.
The kneader was sufficiently kneaded at a temperature of 65 ° C.
After roll-molding this, it was magnetized in a magnetic field of 10 KOe and had an outer diameter of 38.5 mm, an inner diameter of 17.0 mm and a thickness of 3 m.
It was punched into a ring shape of m to prepare a Ba-ferrite bond permanent magnet (surface magnetic flux density ~ 180G).

As shown in FIG. 1 (b), the above-mentioned (Ni, Zn) -ferrite bond magnetic material is sandwiched between two pieces of Ba-ferrite bond permanent magnets so that the N pole and the S pole face each other. did. The frequency band of the multi-layer bond magnetic body having this sandwich structure is 50-
The input impedance at 800 MHz was measured using a network analyzer, and the magnetic permeability was calculated.
The results are shown in Table 1.

[0021]

[Table 1]

As Example 2, a needle-shaped (Ni, Zn) -ferrite sintered body (Ni _0.3 Zn _0.7 Fe ₂ O ₄ ) having a diameter of 2 mm and an aspect ratio of 8 was placed in the plate surface with the needle-shaped direction. They were arranged so as to be vertical, and they were bonded with an epoxy resin to prepare a ring-shaped (Ni, Zn) -ferrite bond magnetic body having an outer diameter of 38.5 mm, an inner diameter of 17.0 mm and a thickness of 9.5 mm.

Specifically, 24 g of EVA resin and 242 g of Ba-ferrite (BaFe ₁₂ O ₁₉ ) particle powder are added in an amount of 60 to 60%.
The kneader was sufficiently kneaded at a temperature of 65 ° C.
After roll-molding this, it was magnetized in a magnetic field of 10 KOe and had an outer diameter of 38.5 mm, an inner diameter of 17.0 mm and a thickness of 6 m.
It was punched into a ring shape of m to prepare a Ba-ferrite bond permanent magnet (surface magnetic flux density ~ 180G). Then, as shown in FIG. 1A, a frequency band of 50 to 800 MHz was obtained for one of the above-mentioned (Ni, Zn) -ferrite bond magnetic body and one of the Ba-ferrite bond permanent magnets superposed on each other. The transmittance in the above was calculated by the same method. The results are shown in Table 2.

[0024]

[Table 2]

As Example 3, a needle-shaped (Ni, Zn) -ferrite sintered body (Ni _0.3 Zn _0.7 Fe ₂ O ₄ ) having a diameter of 2 mm and an aspect ratio of 8 was used, and the needle-shaped direction was the plate surface. They were arranged so as to be vertical, and they were bonded with an epoxy resin to prepare a ring-shaped (Ni, Zn) -ferrite bond magnetic body having an outer diameter of 38.5 mm, an inner diameter of 17.0 mm and a thickness of 5.5 mm.

In particular, 24 g of EVA resin and 242 g of Ba-ferrite (BaFe ₁₂ O ₁₉ ) particle powder are added in an amount of 60 to 60%.
The kneader was sufficiently kneaded at a temperature of 65 ° C.
After roll-molding this, it was magnetized in a magnetic field of 10 KOe and had an outer diameter of 38.5 mm, an inner diameter of 17.0 mm and a thickness of 3 m.
It was punched into a ring shape of m to prepare a Ba-ferrite bond permanent magnet (surface magnetic flux density ~ 180G).

A frequency band of 50 is obtained by alternately stacking the above-mentioned two (Ni, Zn) -ferrite bond magnetic bodies and three Ba-ferrite bond permanent magnets.
Magnetic permeability at ˜800 MHz was calculated by the same method. The results are shown in Table 3.

[0028]

[Table 3]

As Example 4, a needle-shaped Co _0.6 Zn _1.4 Z-hexagonal ferrite sintered body (Ba ₃ Co _0.6 Zn _1.4 Fe ₂₄ O ₄₁ ) having a diameter of 2 mm and an aspect ratio of 8 was formed in the needle-shaped direction. Are arranged so that they are perpendicular to the plate surface, and these are bonded with an epoxy resin to form an outer diameter of 38.5 mm and an inner diameter of 1
A ring-shaped sample having a thickness of 7.0 mm and a thickness of 9.5 mm was prepared.

In particular, 60 g of EVA resin 24 g and Ba-ferrite (BaFe ₁₂ O ₁₉ ) particle powder 242 g
The kneader was sufficiently kneaded at a temperature of 65 ° C.
After roll-molding this, it was magnetized in a magnetic field of 10 KOe and had an outer diameter of 38.5 mm, an inner diameter of 17.0 mm and a thickness of 3 m.
It was punched into a ring shape of m to prepare a Ba-ferrite bond permanent magnet (surface magnetic flux density ~ 180G).

The above-mentioned (Co, Zn) -Z-hexagonal ferrite bonded magnetic material was sandwiched between two pieces of this Ba-ferrite bonded permanent magnet so that the N pole and the S pole faced each other. The frequency band of the multi-layer bond magnetic body having this sandwich structure is 50 to 800 MHz.
The magnetic permeability in was calculated by the same method. The results are shown in Table 4.
Shown in.

[0032]

[Table 4]

Next, as Comparative Example 1, (Ni, Zn) -ferrite powder having the same composition as in Example 1 (average particle size 1.2)
(mm) was kneaded with EVA resin, and then a ring-shaped sample having an outer diameter of 38.5 mm, an inner diameter of 17.0 mm and a thickness of 9.5 mm was prepared using a mold. Then, the input impedance in the frequency band of 50 to 800 MHz of the bond magnetic body having the above structure was measured using a network analyzer, and the magnetic permeability was calculated. The results are shown in Table 5.

[0034]

[Table 5]

As Comparative Example 2, Co _0.6 Zn _1.4 Z-hexagonal ferrite powder (average particle size 1.2 mm) having the same composition as in Example 4 was kneaded with EVA resin, and then a die was used to give an outer diameter of 38. 0.5 mm, inner diameter 17.0 mm, thickness 9.5
A ring-shaped sample of mm was prepared. Then, the input impedance in the frequency band of 50 to 800 MHz of the bond magnetic body having the above structure was measured using a network analyzer, and the magnetic permeability was calculated. The results are shown in Table 6.

[0036]

[Table 6]

The conventional nickel-zinc spinel ferrite-bonded magnetic material has the transmittance characteristics as shown in Comparative Example 1, and the frequency at which μ ′ = μ ″ is estimated to be about 140 MHz. Μ ′> μ ″ only in the frequency band, and a Salisbury-type radio wave absorber laminated with a dielectric using this function only in a frequency body of 140 MHz or less. On the other hand, a multilayered bonded magnetic material obtained by sandwiching a bonded magnetic material in which a nickel zinc spinel ferrite needle-shaped sintered body is oriented perpendicular to the plate surface with a magnetized Ba-ferrite bond permanent magnet has a magnetic permeability of As shown in the characteristics of Example 1, μ ′ =
The frequency of μ ″ is significantly increased so that it is estimated to be about 600 MHz, and μ ′> μ ″ in the frequency band of 600 MHz or less. That is, the frequency band in which the Salisbury-type electromagnetic absorber, which is laminated with the dielectric using this multilayer-type bonded magnetic substance, functions can be expanded to 600 MHz. Similarly, in the multilayer bond magnetic material of Example 2, about 60
0 MHz, about 600 for the multilayer bond magnetic material of Example 3
Since μ ′> μ ″ is established up to the frequency band of MHz, the frequency band in which the above-mentioned Salisbury type electromagnetic wave absorber functions can be greatly expanded to the high frequency side.

Further, the conventional W-type, Y-type, and Z-type hexagonal ferrite bond magnetic bodies containing alkaline earth metal ions have magnetic permeability characteristics as shown in Comparative Example 2, and μ ′
= Μ ″ is a frequency of 800 MHz or more, and the absolute value of μ ′ is 3.8 at a frequency of 800 MHz. Using this, a Salisbury type electromagnetic wave absorber laminated with a dielectric is sufficient in the frequency band of 800 MHz or less. Although it functions, the bond magnetic substance becomes considerably thick due to the small absolute value of μ '. On the other hand, W type, Y type, and Z type containing alkaline earth metal ions The hexagonal ferrite needle-shaped sintered body of No. 1 was sandwiched with a magnetized Ba-ferrite bond permanent magnet to laminate a bond magnetic body whose needle-like direction was perpendicular to the plate surface, and As shown in the characteristics of Example 4, μ ′ =
The frequency of μ ″ is 800 MHz or more, and the absolute value of μ ′ is 5.2 at a frequency of 800 MHz. That is, a salisbury type electromagnetic wave absorber laminated with a dielectric using this multilayer bond magnetic material has a frequency of 800 MHz. It can be used in the following frequency bands, and since the absolute value of μ'is large, the magnetic layer can be thinned.

The magnetic permeability characteristics of the magnetic material having a wide frequency band of μ ′> μ ″ and a large μ ′ in the frequency band are obtained by providing a laminated body of a dielectric layer and a magnetic layer on a metal layer. This is a characteristic basically required for the magnetic layer used for the salisbury type electromagnetic wave absorber.As described above, according to the present invention, the frequency band in which μ ′> μ ″ is wide toward the high frequency side and In this frequency band, a bonded magnetic material sheet having a magnetic permeability characteristic with a large μ'is obtained.

[0040]

As described above, in the multilayer bonded magnetic body of the present invention, the frequency band of μ '> μ "is widened to the high frequency side, and the large μ'is transparent in those frequency bands. Since it has magnetic susceptibility characteristics, by using it to construct a Salisbury-type electromagnetic wave absorber by laminating it with a dielectric, the workability and formability are greatly improved compared to a ferrite sintered body. Thus, it is possible to obtain an electromagnetic wave absorber that can be easily applied to the wall surface of a building or the like.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an embodiment of the present invention.

FIG. 2 is a diagram showing a basic configuration of an electromagnetic wave absorber and an equivalent circuit diagram.

FIG. 3 is a diagram showing a configuration of a radio wave absorber and a magnetic moment for explaining the operation of the present invention.

[Explanation of symbols]

 1 Metal Layer 2 Magnetic Layer 3 Dielectric Layer 4 Permanent Magnet 5 Soft Ferrite Plate

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Keiki Shimizu 1-4-2, Mita, Minato-ku, Tokyo NEC Electric Engineering Co., Ltd. (72) Inokue Yamamoto 7 Shinagawa Yokokawa, Nishi-ku, Hiroshima City, Hiroshima -1 Toda Kogyo Co., Ltd. (72) Inventor Tatsuya Nakamura 7-1 Yokogawa Shinmachi, Nishi-ku, Hiroshima City, Hiroshima Prefecture Toda Kogyo Co., Ltd.

Claims (4)

[Claims] 1. A ferrite plate is prepared by arranging needle-shaped soft ferrite sintered bodies so that the needle-shaped direction is perpendicular to the plate surface, and bonding them with a resin to prepare hard ferrite particles. A radio wave absorber characterized in that permanent magnets made of powder and resin are superposed on one side or both sides, or ferrite plates and permanent magnets are alternately laminated. 2. The radio wave absorber according to claim 1, wherein the soft ferrite sintered body is nickel zinc spinel ferrite. 3. The electromagnetic wave absorber according to claim 1, wherein the soft ferrite sintered body is W-type, Y-type, or Z-type hexagonal ferrite containing alkaline earth metal ions. 4. The surface magnetic flux density of the permanent magnet is at least 10.
The electromagnetic wave absorber according to claim 1, which is 0G.