Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
2002-04-05
2003-09-23
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S357000, C428S402000, C428S900000, C428S702000, C252S062560, C252S062590, C252S062620, C252S062630, C252S062640
Reexamination Certificate
active
06623879
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to soft-magnetic hexagonal ferrite composite particles, and a green sheet using the same and a soft-magnetic hexagonal ferrite sintered ceramics. More particularly, the present invention relates to soft-magnetic hexagonal ferrite composite particles comprising soft-magnetic hexagonal ferrite particles, and barium carbonate particles and/or strontium carbonate particles; a green sheet using the soft-magnetic hexagonal ferrite composite particles; and a soft-magnetic hexagonal ferrite sintered ceramics using the soft-magnetic hexagonal ferrite composite particles which is produced by forming the soft-magnetic hexagonal ferrite composite particles into a compact and then sintering the compact, and which exhibits a high sintering density and a high volume resistivity as well as such frequency characteristics that an imaginary part of permeability thereof is not more than 1 at 400 MHz and becomes large at a frequency near several GHz, while a real part of permeability is kept substantially constant without lowering the real part in a frequency range of from low frequency to several hundreds MHz.
As well known in the arts, soft-magnetic cubic spinel ferrite sintered ceramics show a high imaginary part of permeability in a frequency range of several hundreds MHz, and, therefore, have been used as impedance elements for attenuating noises over a frequency range of several hundreds MHz due to magnetic loss thereof caused in such a frequency range, or electromagnetic wave absorbers for absorbing electromagnetic waves.
In addition, the soft-magnetic cubic spinel ferrite sintered ceramics show a constant real part of permeability over a frequency range of from low frequency to several tens MHz and, therefore, have also been widely used as inductor elements owing to a high inductance thereof.
With the recent development and progress of mobile communication systems such as portable telephones and PHS as well as high-speed digital devices for indoor use such as wireless LAN, personal computers and game devices, it has been rapidly attempted to apply signals having a frequency range of several hundreds MHz to these devices. However, the use of such signal frequencies in these devices has caused a significant problem that noises are produced near several GHz as harmonic thereof. Therefore, it has been strongly required to provide impedance elements and electromagnetic waver absorbers capable of attenuating and absorbing noises and electromagnetic waves having a higher frequency range near several GHz without any adverse influence on signals or electromagnetic waves having a frequency range of several hundreds MHz. In order to meet these requirements, it is necessary to not only reduce an imaginary part of permeability in a frequency range of several hundreds MHz, but also increase the imaginary part of permeability in a frequency range near several GHz.
Further, in order to use frequencies of several hundreds MHz as signals, it is required to provide an inductor element capable of exhibiting a constant high inductance in a frequency range of from low frequency to several hundreds MHz. To meet this requirement, it is also required that the real part of permeability of the inductor element is kept substantially constant without lowering in a frequency range of from low frequency to several hundreds MHz.
However, it is known that the soft-magnetic cubic spinel ferrite sintered ceramics have a so-called Snoek's limit, so that it is not possible to reduce the imaginary part of permeability thereof in a frequency range of several hundreds MHz. Therefore, when the conventional soft-magnetic cubic spinel ferrite sintered ceramics are used as impedance elements or electromagnetic wave absorbers in electronic devices using signals having a frequency range of several hundreds MHz, there arises such a problem that the signal frequencies of several hundreds MHz required for operating the electronic devices are disadvantageously attenuated or absorbed because of magnetic loss thereof.
Also, the real part of permeability of the conventional sintered ceramics is reduced in a frequency range beyond several hundreds MHz according to the Snoek's limit. As a result, there is caused such a problem that inductor elements using the conventional soft-magnetic cubic spinel ferrite sintered ceramics tend to be inapplicable to the electronic devices using signals having a frequency range of several hundreds MHz.
On the other hand, there have been proposed soft-magnetic hexagonal ferrite sintered ceramics having a crystal structure of Z-type ferrite, Y-type ferrite or W-type ferrite which can exhibit a small imaginary part of permeability in a frequency range of several hundreds MHz and a large imaginary part of permeability in a frequency range near several GHz exceeding the Snoek's limit. Specifically, when the soft-magnetic hexagonal ferrite sintered ceramics are used as impedance elements or electromagnetic wave absorbers, it is possible to use signals having a frequency range of several hundreds MHz, and it is also expected to attenuate and absorb noises near several GHz which are produced as harmonic of these signal frequencies.
Further, there have been proposed soft-magnetic hexagonal ferrite sintered ceramics capable of keeping a real part of permeability thereof substantially constant in a frequency range of several hundreds MHz without lowering. Namely, when the soft-magnetic hexagonal ferrite sintered ceramics are used as inductor elements, it is possible to use signals having a frequency range of several hundreds MHz.
However, the soft-magnetic hexagonal ferrite sintered ceramics have such a disadvantage that the sintering density thereof is as low as about 4.9×10
3
kg/m
3
at most, thereby rendering the sintered ceramics practically unusable. As to this fact, Japanese Patent Application Laid-Open (KOKAI) No. 2001-39718 describes that “hexagonal ferrite has a low sintering density despite excellent permeability in a higher frequency range, resulting in insufficient mechanical strength of the obtained sintered ceramics, and it has been, therefore, difficult to use the hexagonal ferrite as surface mounting parts of electronic devices”.
Since the soft-magnetic cubic spinel ferrite sintered ceramics presently used have a sintering density of not less than 5.0×10
3
kg/m
3
, it has been strongly required that the soft-magnetic hexagonal ferrite sintered ceramics also exhibit a high sintering density substantially identical to that of the soft-magnetic cubic spinel ferrite sintered ceramics. Also, the sintering density and permeability of the soft-magnetic hexagonal ferrite sintered ceramics have a specific close relationship to each other. Therefore, when the sintering density becomes lower, it may be difficult to exhibit a good permeability inherent to the soft-magnetic hexagonal ferrite sintered ceramics.
In addition, the soft-magnetic hexagonal ferrite sintered ceramics have such a disadvantage that the volume resistivity thereof is as low as 1×10
5
&OHgr;m at most, resulting in defective insulation. As to this fact, the above Japanese Patent Application Laid-Open (KOKAI) No. 2001-39718 describes that “hexagonal ferrite . . . . In addition, since the hexagonal ferrite has a low resistivity as compared to spinel ferrite, it may be required to take sufficient measures for insulation upon the production of coils, resulting in troublesome production process”.
Since the soft-magnetic cubic spinel ferrite sintered ceramics (Ni-Zn-based sintered ceramics) presently used have a volume resistivity of 1×10
6
&OHgr;m or higher, the soft-magnetic hexagonal ferrite sintered ceramics have also been strongly required to have the substantially same high volume resistivity as that of the soft-magnetic cubic spinel ferrite sintered ceramics.
Further, the soft-magnetic hexagonal ferrite sintered ceramics have been required to not only have enhanced sintering density and volume resistivity, but also exhibit a small imaginary part o
Okano Yoji
Sugita Norio
Yoshida Takayuki
Blackwell-Rudasill G. A.
Jones Deborah
Nixon & Vanderhye P.C.
Toda Kogyo Corporation
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