Electricity: electrical systems and devices – Electrostatic capacitors – Fixed capacitor
Reexamination Certificate
2000-06-26
2001-09-18
Dinkins, Anthony (Department: 2831)
Electricity: electrical systems and devices
Electrostatic capacitors
Fixed capacitor
C361S321400, C361S321500
Reexamination Certificate
active
06292354
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a laminated ceramic capacitor and a method for manufacturing the same. In more detail, the present invention relates to a laminated ceramic capacitor using a base metal for inner electrodes in order to realize good electrostatic capacitance-temperature characteristics and large electrostatic capacitance, and a method for manufacturing the same.
2. Description of the Related Art
Barium titanate based ferroelectric ceramics in a laminated ceramic capacitor have been composed of grains comprising a core-shell structure and grains with a homogeneous composition. A shell layer is formed around the core in the grains comprising the core-shell structure. In the barium titanate based ferroelectric ceramics, grains of the barium titanate based ferroelectric ceramic forms the core, around which the shell layer is constructed of a dielectric material having a lower specific dielectric constant than that of barium titanate.
The grains with a homogeneous composition refers those merely composed of barium titanate, or those having no core-shell structure such as where the shell component in the core-shell structure is diffused and solubilized to form a homogeneous solid solution.
In manufacturing the laminated ceramic capacitor described above, starting materials in the ceramics such as TiO
2
and BaCO
3
are weighed and mixed, followed by calcination, as a core-forming material. The calcinated material obtained is ground into a calcinated powder. Materials for forming the shell portion such as an organic binder, a dispersing agent and water are mixed with the powder thereafter to obtain a ceramic slurry. A ceramic green sheet is obtained by molding the ceramic slurry into a sheet using, e.g., a doctor blade method. Then, inner electrodes are formed by printing a conductive paste containing a metal power such as Ag, Ag—Pd and Ni powder on the ceramic green sheet.
A plurality of the ceramic green sheets on which inner electrodes have been formed are laminated, followed by laminating an appropriate number of plain ceramic green sheets on the uppermost and lowermost layers of the laminate, to obtain a laminated body. A ceramic sintered body is obtained by firing the laminated body after pressing it along the direction of thickness. The laminated ceramic capacitor is manufactured by forming outer electrodes on both end faces of the sintered body obtained as described above.
It is believed that good temperature characteristics are obtained in the laminated ceramic capacitor comprising the grains having the core-shell structure and homogeneous grains.
Recently, miniaturization, large capacitance and low production cost have been required for the ceramic capacitor. Accordingly, the ceramic capacitor comprising the grains having the core-shell structure and grains having the homogeneous structure are also urgently required to be miniaturized, to have large capacitance and to be manufactured with a low production cost, besides having good temperature characteristics.
It is essential for miniaturizing a laminated ceramic capacitor with large capacitance to use a starting material having a high specific dielectric constant as the ceramic material, along with thinning the ceramic layers among the inner electrodes. Also, a base metal such as Ni is used for reducing the production cost.
Using Ni as the inner electrode contributes for reducing the production cost in the laminated ceramic capacitor using a ceramic sintered body comprising the grains having the core-shell structure and the grains having the homogeneous structure. However, attaining further miniaturization and larger capacitance was difficult in the dielectric ceramics having the barium titanate structure as hitherto described, because its specific dielectric constant is 4,000 at best.
Although use of a ceramic material having a lead based perovskite structure may be contemplated, the material may adversely affect the environment since it contains lead.
SUMMARY OF THE INVENTION
Accordingly, the object of the present invention is to provide a laminated ceramic capacitor comprising a core-shell structure in which a dielectric ceramic that can exhibit good electrostatic capacitance-temperature characteristic is used while using a base metal as an electrode material, in order to practically reduce the manufacturing cost besides attaining miniaturization and large capacitance of the ceramic capacitor.
In accordance one aspect, the present invention provides a laminated ceramic capacitor comprising: a ceramic sintered body comprising a reduction-resistant dielectric ceramic; a plurality of inner electrodes comprising a base metal disposed in the ceramic sintered body so as to be stacked along the direction of thickness via a ceramic layer; and a pair of outer electrodes formed on the outer surfaces of the ceramic sintered body and electrically connected to either of the inner electrodes, wherein grains having a core-shell structure and homogeneous grains are mixed together in the ceramic sintered body comprising the reduction-resistant dielectric ceramic, the grains having the core-shell structure and the grains having the homogeneous structure are mixed with an area ratio in the range of about 2:8 to 4:6 when an arbitrary cross section of the ceramic sintered body is observed.
The production cost may be reduced in the laminated ceramic capacitor according to the present invention since the inner electrodes are made of a base metal. Also, since the grains having the core-shell structure and the grains having the homogeneous structure are mixed with an area ratio in the range of about 2:8 to 4:6 when an arbitrary cross section of the ceramic sintered body is observed, the specific dielectric constant can be increased up to 4,500 or more as will be made clear in the examples to be described hereinafter, which is advantageous for attaining miniaturization and large capacitance. The D-characteristics of the static capacitance-temperature characteristics in the JIS standard are satisfied since the laminated ceramic capacitor comprises a core-shell structure.
The laminated ceramic capacitor according to the present invention is featured in that miniaturization and large capacitance can be attained while maintaining good static capacitance-temperature characteristics, by adjusting the mixing ratio between the grains having a core-shell structure and the grains having a homogeneous structure in the specified proportion described above.
A barium titanate based ceramic is preferably used as the reduction-resistant dielectric ceramic in the present invention.
In an another aspect, the present invention provides a method for manufacturing a laminated ceramic capacitor comprising the steps of: preparing a ceramic green sheet mainly comprising a reduction-resistant dielectric ceramic; forming inner electrodes by printing a base metal paste on one face of the ceramic green sheet; obtaining a laminated body by laminating a plurality of the ceramic green sheets on which the inner electrodes have been printed, and by further laminating plain ceramic green sheets on the uppermost and lowermost layers of the laminated ceramic green sheets, followed by pressing along the direction of thickness; obtaining a ceramic sintered body by firing the laminated body; and applying outer electrodes on the outer surfaces of the ceramic sintered body, wherein the firing temperature and firing time is selected so that the grains having a core-shell structure and the homogeneous grains are mixed with an area ratio in the range of about 2:8 to 4:6 in an arbitrary cross section of the ceramic sintered body.
The barium titanate based ceramic is also preferably used as the reduction-resistant dielectric ceramic in method for manufacturing the laminated ceramic capacitor according to the present invention.
REFERENCES:
patent: 4223369 (1980-09-01), Burn
patent: 5680291 (1997-10-01), Kim et al.
patent: 6101693 (2000-08-01), Kim et al.
patent: 6205015 (2001-03-01), Wada et al.
patent: 11-233363 (1999-08
Kawabata Kazuaki
Kobayashi Fumiyuki
Takagi Yoshikazu
Ueno Yasushi
Yoneda Yasunobu
Dinkins Anthony
Murata Manufacturing Co. Ltd.
Ostrolenk Faber Gerb & Soffen, LLP
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