Radio wave absorbent

Details Radio wave absorbent Radio wave absorbent

1999-05-21

2001-04-03

Koslow, C. Melissa (Department: 1755)

Compositions

Magnetic

Iron-oxygen compound containing

C252S062640, C252S062620, C341S001000, C307S091000, C361S818000, C455S300000

Reexamination Certificate

active

06210597

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a radio wave absorbent for use in a radio wave dark room, a radio wave absorptive wall, which is composed of a magnesium-zinc system ferrite.
BACKGROUND ART
Recently, with the progress of information communication technique or the prevalence of various electric apparatus, the influence of unnecessary electromagnetic noises exerted onto precision apparatus associated devices has raised problems. For the measurement of electromagnetic noises, a radio wave dark room (anechoic room) where there is no reflection of electromagnetic waves is used, and a radio wave absorbent is used in the inner wall of the radio wave dark room. Moreover, in order to prevent reception trouble from being caused by reflection of television waves by high-rise buildings or the like, the radio wave absorbent is used in the outer wall of the building or the like.
As a conventional radio wave absorbent, for example, used is a radio wave absorbent having a characteristic that a reflection attenuation in a frequency band of 90 MHz to 350 MHz is 20 dB or more. As the radio wave absorbent, for example, a radio wave absorbent obtained by sintering a nickel-zinc system ferrite material (Japanese Patent Application Laid-open Nos. 200303/1991, 129123/1993, 243023/1993, 84622/1994, and the like) and a radio wave absorbent obtained by sintering a magnesium-zinc system ferrite material (Japanese Patent Application Laid-open Nos. 72925/1989, 301524/1989, and the like) are exemplified.
However, the conventional radio wave absorbent which is obtained by sintering the nickel-zinc system ferrite material has a problem that a raw-material nickel is expensive.
On the other hand, the magnesium-zinc system ferrite material has a sintering temperature of around 1250 to 1300° C., which is higher than the sintering temperature of about 1100° C. of the nickel-zinc system ferrite material. Therefore, when the radio wave absorbent is industrially manufactured by sintering the magnesium-zinc system ferrite material, there is a problem that a high-temperature sintering furnace is necessary separately from a sintering furnace for the nickel-zinc system ferrite material.
Furthermore, the matching thickness in which all incoming radio waves are absorbed without causing reflection is important in the radio wave absorbent, but the radio wave absorbent obtained by sintering the conventional magnesium-zinc system ferrite material has a matching thickness of 8 mm or more, and is thicker than the radio wave absorbent obtained by sintering the nickel-zinc system ferrite material (matching thickness of 7 mm or less). The difference of about 1 mm in matching thickness exerts a remarkable influence on the total weight of the radio wave absorbent for use in the inner wall of the radio wave dark room or the outer wall of the building or the like, and the reduction of the matching thickness is constantly requested for in the radio wave absorbent.
The present invention has been developed in consideration of the circumstances described above, and an object thereof is to provide a radio wave absorbent which has a matching thickness of less than 8 mm and which can be obtained by sintering the material at a relatively low sintering temperature.
DISCLOSURE OF THE INVENTION
To attain this and other objects, the present invention provides a radio wave absorbent which is obtained by sintering a magnesium-zinc system ferrite material and which has the main component of a magnesium-zinc system ferrite containing 45 to 50 mol % of iron oxide, 7 to 19.7 mol % of magnesium oxide, 24 to 28.5 mol % of zinc oxide, 4 to 16 mol % of copper oxide, and 0.1 to 6 mol % of manganese oxide.
Moreover, in a preferable aspect, a reflection attenuation in a frequency band of 90 MHz to 350 MHz is 20 dB or more.
In another preferable aspect, a matching thickness is less than 8 mm.
In further preferable aspect, a sintering temperature is in the range of 950 to 1150° C.
According to the present invention, the radio wave absorbent contains main components of 45 to 50 mol % of iron oxide, 7 to 19.7 mol % of magnesium oxide, 24 to 28.5 mol % of zinc oxide, 4 to 16 mol % of copper oxide, and 0.1 to 6 mol % of manganese oxide, and sintering can be performed at a relatively low temperature of about 950 to 1150° C. The sintering temperature is remarkably lower than the sintering temperature of the conventional magnesium-zinc system ferrite material, and the sintering furnace for sintering the nickel-zinc system ferrite material can be used. Moreover, since the matching thickness is less than 8 mm, the total weight of the radio wave absorbent for use in the inner wall of the radio wave dark room or the outer wall of the building or the like is remarkably reduced as compared with that of the radio wave absorbent obtained by sintering the conventional magnesium-zinc system ferrite material. Furthermore, the radio wave absorbent of the present invention can reduce manufacture cost as compared with the radio wave absorbent obtained by sintering the conventional nickel-zinc system ferrite material.
BEST MODE FOR CARRYING OUT THE INVENTION
The embodiment of the present invention will be described below in detail.
The radio wave absorbent of the present invention is prepared by sintering a magnesium-zinc system ferrite material, and contains main components in the range of 45 to 50 mol % of iron oxide, 7 to 19.7 mol % of magnesium oxide, 24 to 28.5 mol % of zinc oxide, 4 to 16 mol % of copper oxide, and 0.1 to 6 mol % of manganese oxide.
In the composition region departing from the above-mentioned range, the matching thickness of the radio wave absorbent becomes 8 mm or more, &mgr;′ (the real number part of complex specific permeability) required for the radio wave absorbing characteristic in a low-frequency band is lowered, and further the peak width for the frequency of &mgr;″ (the imaginary number part of complex specific permeability) required for the radio wave absorbing characteristic becomes narrow. It is therefore difficult to set the reflection attenuation in the frequency band of 90 MHz to 350 MHz to at least 20 dB or more. Moreover, the matching thickness is enlarged.
Specifically, for example, when the content of iron oxide departs from the above-mentioned range, &mgr;′ is lowered. When the content of zinc oxide is less than the above-mentioned range, &mgr;′ is lowered. When the content of zinc oxide exceeds the above-mentioned range, Curie temperature is lowered, &mgr;″ is also lowered, and the matching thickness is enlarged. When the content of copper oxide is less than the above-mentioned range, it becomes difficult to sinter the material at a relatively low temperature of about 950 to 1150° C., in order to obtain the radio wave absorbent, and the matching thickness is further enlarged. On the other hand, when the content of copper oxide exceeds the above-mentioned range, &mgr;′ is lowered and the radio wave absorbing band is narrowed. Moreover, manganese oxide effectively promotes particle growth, enhances the initial permeability, lowers the resonance frequency of a magnetic wall, and expands the radio wave absorbing band toward the low-frequency side. When the content of manganese oxide exceeds the above-mentioned range, however, &mgr;′ is lowered, the specific permittivity is increased, and the radio wave absorbing characteristic is deteriorated.
In addition to the aforementioned components, the radio wave absorbent of the present invention may contain one or two or more of CaO, CoO, NiO, SiO
2
, TiO
2
, SnO
2
, MoO
3
, WO
3
, Bi
2
O
3
, In
2
O
3
, Cr
2
O
3
, Al
2
O
3
, Ta
2
O
5
, Nb
2
O
5
, V
2
O
5
, and the like in a ratio of 1% by weight or less.
As aforementioned, the radio wave absorbent of the present invention can be obtained by sintering in the atmospheric air at a relatively low temperature of about 950 to 1150° C. the magnesium-zinc system ferrite material whose composition after sintering is in the range described above, and its matching thickness becomes less than 8 mm.


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