Electricity: electrical systems and devices – Electrostatic capacitors – Fixed capacitor
Reexamination Certificate
2002-10-23
2003-11-25
Dinkins, Anthony (Department: 2831)
Electricity: electrical systems and devices
Electrostatic capacitors
Fixed capacitor
C361S321500, C361S305000, C361S322000
Reexamination Certificate
active
06654227
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to ceramic electronic parts such as monolithic ceramic capacitors and monolithic positive temperature coefficient thermistors. More specifically, it relates to ceramic electronic parts including Ni inner electrodes and outer electrodes that are electrically connected to the Ni inner electrodes, contain Ag as a major component and Au and an inorganic binder as minor components.
2. Description of the Related Art
Inner electrodes of monolithic ceramic capacitors have been conventionally made of Ag, Ag—Pd, or another material. Conventional outer electrodes of these monolithic ceramic capacitors comprise a first electrode layer made of Ag that has excellent conductivity and can be baked at low temperatures, a second electrode layer made of Ni formed on the first electrode layer, and a third electrode layer made of Sn or solder formed as an outer layer to improve solderability.
However, these materials for the inner electrodes are expensive, and Ni, a less expensive base metal, has been increasingly used. However, Ni used as a material for the inner electrodes does not form a solid solution with the Ag of the first electrode layer of the outer electrodes and thereby hardly combines with the Ag constituting the outer electrodes, and the resulting monolithic ceramic capacitors may not yield a desired electric capacity.
As a possible solution to this problem, outer electrodes made of Cu that fully forms a solid solution with Ni has received attention. However, Cu is susceptible to oxidation, and a Cu-containing conductive paste must be baked in a reducing atmosphere to form the outer electrodes. In addition, the vehicle of the conductive paste is slowly decomposed during baking in such a reducing atmosphere due to a low oxygen concentration of the reducing atmosphere.
In the monolithic positive temperature coefficient thermistors, inner electrodes and ceramic sheets are concurrently fired in a reducing atmosphere, and outer electrodes are then formed by baking. By baking the outer electrodes in an air atmosphere, the monolithic positive temperature coefficient thermistors themselves are re-oxidized and thereby yield desired positive temperature characteristics of resistance. As the materials for the inner electrodes, Ni for attaining ohmic contact with n-type impurity semiconductors is preferably used. When Ni is used as the material for the inner electrodes, Ag, Al and Zn that can be baked in an air atmosphere can be used as materials for the outer electrodes.
However, when the inner electrodes comprise Ni and the outer electrodes comprises Ag alone, the inner electrodes do not form a solid solution with the outer electrodes, and they do not satisfactorily combine with each other. Specifically, the resulting positive temperature coefficient thermistors cannot significantly establish electrical connection between the inner electrodes and the outer electrodes.
Outer electrodes which comprise Al or Zn have low solderability. To improve solderability, a Ni plating layer can be formed by electroplating, but Al and Zn are eluted during electroplating.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide a ceramic electronic part including inner electrodes made of Ni, and outer electrodes that satisfactorily combine with the inner electrodes made of Ni, are formed by baking in an air atmosphere and contain Ag as a major component. Such ceramic electronic parts are used as multilayer ceramic electronic parts such as monolithic ceramic capacitors and monolithic positive temperature coefficient thermistors.
To achieve the above and other objects, the present invention provides, in one aspect, a ceramic electronic part including a ceramic assemblage containing laminated plural ceramic sheets; plural inner electrodes sandwiched between the ceramic sheets and containing Ni as a major component; and outer electrodes electrically connected to the inner electrodes, in which the outer electrodes contain Ag as a major component and Au and an inorganic oxide (an inorganic binder) as minor components.
The outer electrodes of the ceramic electronic parts of the present invention contain Ag as a major component and are formed by baking in an air atmosphere. If outer electrodes are made of Ag alone, such outer electrodes do not form a solid solution with inner electrodes made of Ni. However, in the aforementioned outer electrodes containing Ag as a major component and Au as a minor component, a solid solution can be formed between Ni and Ag by the medium of Au. Accordingly, the outer electrodes can satisfactorily combine with the inner electrodes. The term “major component” as used herein means a component that is contained in the largest amount in the electrodes in question.
The content of Au in the outer electrodes is preferably equal to or more than about 0.1 part by weight relative to 100 parts by weight of Ag. If the content of Au in the outer electrodes is less than about 0.1 part by weight, Ni constituting the inner electrodes may not form a solid solution with Ag constituting the outer electrodes, and an electrical contact resistance may be observed.
In contrast, if the content of Au is equal to or more than about 40 parts by weight relative to 100 parts by weight of Ag, the sintering properties of the outer electrode may be deteriorated and conductivity may be decreased.
The inorganic oxide can be lead borosilicate glass but is preferably lead-free glass from the viewpoint of toxic substance control. Such lead-free glass includes, for example, bismuth borate glass, bismuth borosilicate glass, zinc borosilicate glass and mixtures thereof.
The glass for use herein preferably has such a viscosity at high temperatures that its working point (log &eegr;[Pa.s]=4) is equal to or lower than about 600° C., since the baking temperature of Ag is from about 600° C. to 800° C., and it is preferred that the electrodes are formed by sintering a liquid phase.
The content of the inorganic oxide is preferably equal to or more than about 1% by volume and less than or equal to about 20% by volume based on the total solid contents. If the content is less than about 1% by volume, the electrodes may not have sufficient adhesive strength, and if it exceeds about 20% by volume, the electrodes may lose their conductivity.
The ceramic electronic parts include, for example, monolithic ceramic capacitors and monolithic positive temperature coefficient thermistors.
The present invention also provides, in another aspect, a method for manufacturing such ceramic electronic parts. The method includes the steps of preparing a ceramic green assemblage including plural ceramic green sheets and plural conductive paste sheets for inner electrodes, the ceramic green sheets including a powdery ceramic material, and the conductive paste sheets being sandwiched between the ceramic sheets and containing Ni as a major component; firing the ceramic green assemblage in a non-oxidizing atmosphere to thereby yield a ceramic sintered compact including ceramic sheets and inner electrodes sandwiched between the ceramic sheets; applying a conductive paste for outer electrodes to surfaces of the ceramic sintered compact where the inner electrodes are exposed, the conductive paste containing Ag as a major component and Au and an inorganic oxide as minor components; and baking the conductive paste for outer electrodes in an atmosphere containing oxygen to thereby yield outer electrodes conductively connected to the inner electrodes.
The present invention having the above configuration can provide ceramic electronic parts having Ni inner electrodes and outer electrodes that are formed by baking in an air atmosphere and are electrically satisfactorily connected to the inner electrodes.
REFERENCES:
patent: 5245309 (1993-09-01), Kawase et al.
patent: 5840382 (1998-11-01), Nishide et al.
patent: 6040755 (2000-03-01), Abe et al.
patent: 6362949 (2002-03-01), Takashima et al.
patent: 2002/0135972 (2002-09-01), Higuchi et al.
Hamada Kunihiko
Miki Takeshi
Noda Satoru
Dickstein Shapiro Morin & Oshinsky LLP.
Dinkins Anthony
Murata Manufacturing Co. Ltd.
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