Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Including a second component containing structurally defined...
Reexamination Certificate
2002-02-13
2004-08-10
Kiliman, Leszek B (Department: 1773)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Including a second component containing structurally defined...
C428S328000, C428S403000, C428S407000, C428S634000
Reexamination Certificate
active
06773800
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to an electromagnetic wave absorbent wherein magnetic powders are dispersed in an insulative resin as a bonding agent, and a method for producing magnetic powders for the electromagnetic wave absorbent.
For making functions of electronic machinery or communication apparatus stable, the electromagnetic wave absorbent is used in order to absorb electric waves to be external disturbance outside of the apparatus or electric waves escaping from the interior thereof for preventing noises or hindrance of electric waves.
Related art electromagnetic wave absorbents include irregular magnetic powders such as spinel or hexagonal ferrite sintered substances, which are dispersed in an insulative resin as a bonding agent.
Main applications for the electromagnetic wave absorbent include mobile communication machinery and other devices using a frequency band from para-microwave to microwave, such as portable telephones or PHS (personal handy-phone system) or casings of machinery.
In the electromagnetic wave absorbent, material parameters based on the electromagnetic wave absorbing properties have a complex dielectric constant and a complex permeability in a high frequency, and in the electromagnetic wave absorbent using the magnetic powders, a magnetic loss portion &mgr;″ being an imaginary number component of the complex permeability &mgr;=&mgr;′−j&mgr;″ plays a role in the electromagnetic wave absorbing properties.
The spinel ferrite based material has in general the complex permeability as shown in FIG.
4
A. That is, when a frequency f increases a certain value, a real number &mgr;′ of the permeability &mgr; having been almost constant at that time rapidly goes down, and &mgr;″ takes a maximal value in a resonance frequency fr being a higher frequency zone than &mgr;′. The larger the maximal value of this &mgr;″ is, the larger the energy loss generates, and the good electromagnetic absorbing properties are shown.
However, as seen in
FIG. 4B
, the higher resonance frequency (ferrite A<ferrite B<ferrite C) the spinel ferrite based material has, the smaller maximal value &mgr;″ has. Therefore, a high permeability cannot be obtained in the high frequency particularly in such as a GHz zone, and therefore a good electromagnetic wave absorbing effect cannot be expected.
This is called as “snoek's critical line” shown with a two-dotted line in the same, and a product of the resonance frequency and the permeability is constant in a formula (1).
[Formula 1:]
fr
=
γ
3
πμ
0
Is
(
1
)
(In the formula, fr is a resonance frequency, &mgr;′ is a real number, &ggr; is gyromagnetic constant, &mgr;
0
is a permeability of vacuum, and Is is saturation magnetization.)
In contrast, since the hexagonal ferrite sintered substance has a small magnetic anisotropy of an in-plane, the permeability is large. Moreover, the anisotropic energy is large to direct magnetization in a plane-orthogonal direction. Therefore, the resonance occurs at a higher frequency than that of the spinel ferrite sintered substance.
Namely, in the hexagonal ferrite sintered substance, the product of the resonance frequency and the permeability is expressed with a formula (2).
[Formula 2]:
fr
(
μ
′
-
1
)
=
γ
Is
3
πμ
0
H
A2
H
A1
(
2
)
(In the formula, fr is resonance frequency, &mgr;′ is real number, &ggr; is gyromagnetic constant, &mgr;
0
is permeability of vacuum, Is is saturation magnetization, HA
1
is the magnetic anisotropy for directing the magnetic moment in the in-plane direction, and HA
2
is the magnetic anisotropy for directing the magnetic moment in the plane-orthogonal direction.) Since HA
2
/HA
1
in the formula is 1 or more, the high permeability can be maintained until a high frequency band exceeding “snoek's critical line”.
However, the saturation magnetization of the hexagonal ferrite is around 0.5 T, and so the above-mentioned effect has been limited.
Therefore, the magnetic powders, which comprise a metallic soft magnetic material being a thickness around “skin depth” and being a flat shape of an aspect ratio (diameter/thickness) being 10 or higher, have been recognized as a material having a large magnetic loss portion &mgr;″, which show a good electromagnetic wave absorption.
The thickness of “skin depth” is expressed with a formula (3).
[Formula 3:]
(
skin
depth
)
=
ρ
π
f
μ
(
3
)
(p: electric resistivity, &mgr;: magnetic permeability, f: frequency).
However, even if flat magnetic powders are used, the electromagnetic wave absorbent having an enough absorption effect is not always obtained in the present situation.
Therefore, in the related art, the demand for the high electromagnetic wave absorbing effect has been satisfied by increasing the rate of magnetic powders in the electromagnetic wave absorbent. However, the known electromagnetic wave absorbent has not complied with the recent demands for more intensively absorbing the electromagnetic wave in specific frequency bands depending on a further advanced higher output of the machinery.
As the ratio of the magnetic powders in the electromagnetic wave absorbent is increased, the ratio of the resin as the bonding agent is relatively less. The electromagnetic wave absorbent makes strength or formability less owing to the relative decrease of the ratio of the resin. Therefore, the increasing method of the rate of the magnetic powders has been limited.
For solving the above-mentioned problems, inventors carried out analyses on shapes and structure of magnetic powder, and found the following facts.
The present flat magnetic powders are generally produced by subjecting spherical raw powders made by, e.g., an atomizing process to mechanically breaking, elongating and tearing processes with a ball mill. In this method, even if the spherical raw powders are regulated almost in a uniform size, large dispersions occur in the sizes or shapes of produced magnetic powders, since strength to be loaded on the raw powders in subsequent breaking, elongating and tearing processes is different per each of powders. Therefore, the magnetic powders especially have large dispersions of plane shapes and thickness as to respective magnetic powders. Further, even though the sizes of the magnetic powders are classified and regulated in a certain range, dispersions of the plane shape and the thickness are large and the thickness of any portion of each magnetic powders are irregular. Therefore, the frequency properties are standardized between the magnetic powders, if the dispersions are large. In other words the frequency property does not have an acute peak of a specific frequency, but has a broad distribution over a wide frequency band. Therefore, the absorption effect of the magnetic powders is lowered in the specific frequency. Further, when the magnetic powders are dispersed into the resin, a waste of space occurs due to their irregularity in shape. Therefore, the known magnetic powders cannot obtain a high electromagnetic wave absorbing effect.
When the structure of the magnetic powders is considered, Ni—Fe alloy shows a most excellent soft magnetic property among metallic soft magnetic materials. This alloy exhibits the highest soft magnetic property when it is of a solid solution under a non-equilibrium condition at room temperatures. However, as in Ni—Fe alloy, an intermetallic compound Ni
3
Fe having the low soft magnetic property is under an equilibrium condition at room temperatures, the related art of the flat magnetic powder subjected to dissolution and cooling processes has a structure including the intermetallic compound. Therefore, from this structure, the high electromagnetic wave absorbing effect cannot be provided, either.
On the other hand, for solving the above-mentioned problems, it is proposed in JP-A-2001-60790 to use disc like magnetic powders having circular plan
Hosoe Akihisa
Inazawa Shinji
Nitta Koji
Okayama Katsumi
Toyoda Junichi
Kiliman Leszek B
McDermott Will & Emery LLP
Sumitomo Electric Industries Ltd.
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