Electricity: electrical systems and devices – Electrostatic capacitors – Fixed capacitor
Reexamination Certificate
2001-02-15
2003-05-13
Reichard, Dean A. (Department: 2831)
Electricity: electrical systems and devices
Electrostatic capacitors
Fixed capacitor
C361S308100, C361S305000
Reexamination Certificate
active
06563690
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a multilayer ceramic capacitor and a method for the manufacture thereof; and, more particularly, to a multilayer ceramic capacitor having good electrical characteristics without a short failure between internal electrodes made of a base metal such as Ni or Cu, and a method for the manufacture thereof.
BACKGROUND OF THE INVENTION
There is shown in
FIG. 1
, a cross sectional view of a prior art multilayer ceramic capacitor. As shown, the multilayer ceramic capacitor includes a reoxidized ceramic body
10
, a pair of external electrodes
12
installed at two opposite sides of the reoxidized ceramic body
10
, a plurality of ceramic dielectric layers
16
stacked on top of another, and a plurality of internal electrodes
14
each of which is provided on top of said each ceramic dielectric layer
16
.
The respective internal electrodes
14
and the ceramic dielectric layers
16
are alternately stacked as an integral body. Further, two internal electrodes
14
adjacent to each other are electrically connected to two corresponding external electrodes
12
.
The ceramic dielectric layers
16
are made of, e.g., a barium titanate-based dielectric material, and the internal and the external electrodes
14
,
12
are made of, e.g., a conductive paste. The conductive paste is obtained by mixing Ni or Cu metal powder as a major constituent with a ceramic powder having an identical composition as the ceramic dielectric layers
16
, and sintering the mixture.
An exemplary process for forming the multilayer ceramic capacitor is described below.
First, the barium titanate-based ceramic powder is mixed with an organic binder and water to form a ceramic slurry. Thereafter, ceramic green sheets having a thin sheet shape are formed from the slurry by using, e.g., a doctor blade.
Then, internal electrode patterns made of the conductive paste are printed on the top surfaces of portions of the ceramic green sheets by using a screen print method and are dried, wherein the ceramic green sheets having the internal electrode patterns printed thereon are referred to as “printed ceramic green sheets”, and the ceramic green sheets having no internal electrode patterns thereon are referred to as “bare ceramic green sheets”. The conductive paste may be prepared by dispersing Ni metal powder into a mixture having an organic binder such as ethyl cellulose resin, unsaturated polyester resin or butyral resin, a solvent such as &agr;-terpineol or toluene, and a vehicle containing a surfactant.
Next, the printed ceramic green sheets are stacked and then the bare ceramic green sheets are stacked against the outermost sheets of the stacked ceramic green sheets to serve as a protecting layer, thereby forming a laminated ceramic body. Subsequently, the laminated ceramic body is pressed in a direction in which the sheets are stacked, and diced into chip-shaped multilayered ceramic bodies.
Thereafter, the chip-shaped multilayered ceramic bodies are loaded into a furnace to undergo a firing process. The multilayered ceramic bodies are calcined in an oxidative atmosphere at a temperature of about 600° C. to remove the organic binder contained therein, and then in a non-oxidative atmosphere, e.g., a reductive and/or a neutral atmosphere, are heated at a higher temperature than about 1000° C. to sinter the ceramic green sheets and the internal electrode patterns incorporated therein.
After lowering the temperature of the furnace from about 1000° C. to about 600° C. and changing the non-oxidative atmosphere thereof to the oxidative atmosphere, the sintered ceramic bodies are reoxidized to form reoxidazed ceramic bodies. The reoxidization process is performed so as to improve the insulation of the ceramic layers which may have been deteriorated during the sintering process carried out in the non-oxidative atmosphere.
Finally, after the paste for the external electrodes is applied on two opposite sides of each of the reoxidized ceramic bodies, annealing is carried out on the reoxidized ceramic bodies to install the external electrodes thereon, thereby forming the multilayer ceramic capacitor.
In such a method for manufacturing the multilayer ceramic capacitor, while the reoxidization process is carried out in the sintered ceramic body, sufficient oxygen is not fed into a central portion(K) surrounded by a broken line(see
FIG. 1
) which may in turn hamper the ceramic layers within the central portion of the sintered ceramic body from being reoxidized sufficiently. This may deteriorate an insulation resistance(“IR”) of the ceramic layers within the central portion, to thereby cause an early failure of the multilayer ceramic capacitor.
In order to solve the above problem, attempts have been made to add an extra amount of the acceptor; however, such would lower the dielectric constant or the life time of the capacitor.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide a multilayer ceramic capacitor having a central portion capable of being easily oxidized during an oxidization process and a method for the manufacture thereof.
In accordance with one aspect of the present invention, there is provided a multilayer ceramic capacitor comprising:
a multilayer ceramic body including alternately stacked ceramic layers and internal electrodes, the ceramic layers having an acceptor therein, the acceptor representing one or more metallic elements for facilitating a reoxidation process of the multilayer ceramic body; and
a pair of external electrodes installed at two opposite sides of the multilayer ceramic body,
wherein the acceptor concentration in the ceramic layers stacked at a central portion of the multilayer ceramic body is higher than that in the remaining ceramic layers of the multilayer ceramic body.
In accordance with another aspect of the present invention, there is provided a method for manufacturing a multilayer ceramic capacitor comprising the steps of:
forming a laminated ceramic body, the laminated ceramic body including alternately stacked ceramic green sheets and internal electrodes;
sintering the laminated ceramic body in a non-oxidative atmosphere to form a sintered ceramic body; and
reoxidizing the sintered ceramic body in an oxidative atmosphere to form a reoxidized ceramic body,
wherein the ceramic green sheets have an acceptor therein, the acceptor representing one or more metallic elements for facilitating the reoxidizing step, and the acceptor concentration in the ceramic green sheets at a central portion of the laminated ceramic body is greater than that in the remaining ceramic green sheets of the laminated ceramic body.
In accordance with still another aspect of the present invention, there is provided a method for manufacturing a multilayer ceramic capacitor comprising the steps of:
forming a laminated ceramic body, the laminated ceramic body including alternately stacked ceramic green sheets and internal electrodes;
sintering the laminated ceramic body in a non-oxidative atmosphere to form a sintered ceramic body; and
reoxidizing the sintered ceramic body in an oxidative atmosphere to form a reoxidized ceramic body,
wherein the internal electrodes have an acceptor therein, the acceptor representing one or more metallic elements for facilitating the reoxidizing step, and the acceptor concentration in the internal electrodes at a central portion of the laminated ceramic body is greater than that in the remaining internal electrodes of the laminated ceramic body.
REFERENCES:
patent: 4241378 (1980-12-01), Dorrian
patent: 5812363 (1998-09-01), Kuroda et al.
patent: 6078494 (2000-06-01), Hansen
patent: 6134098 (2000-10-01), Kuroda et al.
patent: 6229686 (2001-05-01), Shimahara et al.
Chazono Hirokazu
Kishi Hiroshi
Shizuno Hisamitsu
Katten Muchin Zavis & Rosenman
Reichard Dean A.
Taiyo Yuden Co. Ltd.
Thomas Eric W
LandOfFree
Multilayer ceramic capacitor and method for the manufacture... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Multilayer ceramic capacitor and method for the manufacture..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Multilayer ceramic capacitor and method for the manufacture... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3023737