Coating processes – Electrical product produced
Reexamination Certificate
2002-04-24
2004-03-16
Barr, Michael (Department: 1762)
Coating processes
Electrical product produced
C427S430100, C427S376100, C427S376200, C427S419100, C427S240000
Reexamination Certificate
active
06706317
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method for manufacturing a sensing element of a gas sensor. The gas sensor is generally installed in an exhaust gas passage of an internal combustion engine for combustion control or emission control of the internal combustion engine.
A gas sensing element is necessary to control the combustion of an internal combustion engine. The gas sensing element has a cup-shaped solid electrolytic body having a reference gas chamber formed therein. An inside electrode is provided on an inner surface of the solid electrolytic body. An outside electrode is provided on an outer surface of the solid electrolytic body. A porous layer is provided to cover the outside electrode.
When the gas sensing element is manufactured, a solid electrolytic body having a predetermined shape is first prepared. The inside electrode and the outside electrode are respectively provided on the inner and outer surfaces of the solid electrolytic body. Next, the solid electrolytic body is dipped into a slurry to coat a slurry film on the outside electrode. Then, the slurry film is sintered to form the porous layer.
The dipping method of the solid electrolytic body includes a step of dipping the solid electrolytic body into a slurry stored in a dipping tank. To perform the dipping treatment of slurry uniformly and effectively, the dipping tank has large size sufficient for dipping a plurality of solid electrolytic bodies into the slurry at a time. An appropriate jig is prepared to hang the plurality of solid electrolytic bodies so as to be positioned above the dipping tank. The jig is then lowered toward the dipping tank to dip the plurality of solid electrolytic bodies into the slurry stored in the dipping tank.
The dipping tank is equipped with a stirrer provided on the bottom of this dipping tank. The stirrer rotates in the slurry to cause a flow of slurry in a circumferential direction.
It is also possible to use an independent dipping tank for separately dipping each solid electrolytic body into a slurry stored in this independent dipping tank. Namely, the number of independent dipping tanks is equal to the number of solid electrolytic bodies to be dipped simultaneously. By lowering the jig hanging the plurality of solid electrolytic bodies into respective dipping tanks, each solid electrolytic body is dipped into the slurry stored in the corresponding dipping tank.
Each solid electrolytic body is rotatable about its own axis when hung down from the jig. When the solid electrolytic body is dipped into the slurry, the solid electrolytic body rotates in the slurry. It is however possible to stop the rotation of the solid electrolytic body when the solid electrolytic body is dipped into the slurry.
It is possible to remove the stirrer when the solid electrolytic body rotates about its own axis when hung down from the jig.
However, according to the conventional dipping method for the solid electrolytic body, the slurry concentration and the slurry component in the dipping tank tends to be nonuniform when a slurry having high viscosity is used. The condition of the slurry film coated on the outside electrode becomes different in each solid electrolytic body.
In other words, performance and characteristics of the porous layer is not constant according to the conventional dipping method. The manufactured gas sensors will have performances different from each other.
SUMMARY OF THE INVENTION
In view of the problems of the conventional dipping method, the present invention has an object to provide a method for manufacturing a gas sensing element having a uniform porous layer.
In order to accomplish the above and other related objects, the present invention provides a first method for manufacturing a gas sensing element which has a cup-shaped solid electrolytic body having a reference gas chamber formed therein, an inside electrode provided on an inner surface of the solid electrolytic body, an outside electrode provided on an outer surface of the solid electrolytic body, and a porous layer formed so as to cover the outside electrode. The first manufacturing method comprises a step of forming the solid electrolytic body, a step of providing the inside electrode on a predetermined portion of the inner surface of the solid electrolytic body and providing the outside electrode on a predetermined portion of the outer surface of the solid electrolytic body, and a step of dipping the solid electrolytic body into a slurry which is prepared for forming the porous layer by using a dipping apparatus.
The dipping apparatus used in the first manufacturing method comprises a dipping tank for storing the slurry which is prepared for forming the porous layer, a viscosity adjusting tank equipped with a viscosity sensor for measuring a viscosity of the slurry and a viscosity adjusting mechanism for adjusting the viscosity of the slurry based on a sensing value of the viscosity sensor, a fluid passage for connecting the dipping tank and the viscosity adjusting tank, and a circulating pump provided in the fluid passage for forcibly circulating the slurry between the dipping tank and the viscosity adjusting tank.
The dipping step of the first manufacturing method includes a step of dipping the solid electrolytic body into the slurry stored in the dipping tank while the slurry is forcibly circulated between the dipping tank and the viscosity adjusting tank by the circulating pump, a step of forming a slurry film on a predetermined portion of the solid electrolytic body through this dipping treatment, and a step of sintering the slurry film to form the porous layer.
The dipping apparatus of the first manufacturing method comprises the viscosity adjusting tank. The slurry is forcibly circulated between the viscosity adjusting tank and the dipping tank. Thus, the slurry is always stirred. The concentration and viscosity of the slurry used for dipping solid electrolytic bodies can be always kept to a uniform and constant value.
It becomes possible to accurately equalize the condition of a slurry film coated on each solid electrolytic body. Hence, performance and characteristics of the porous layer are constant in each solid electrolytic body. The manufactured gas sensors show the same performances.
The porous layer of the gas sensing element of this invention functions as trap layer.
The outside electrode needs to be exposed to a measured gas atmosphere during detection of gas concentration. When the measured gas contains poisonous or harmful substances, the trap layer is provided to protect (i.e., cover) the outside electrode and the gas sensing element. The trap layer traps the poisonous or harmful substances and assures accurate detection of gas concentration.
Furthermore, it is possible to provide an additional layer on the porous layer. Furthermore, it is possible to provide a second trap layer on the porous layer.
Furthermore, when the outside electrode is provided on the solid electrolytic body and the additional layer is provided on the outside electrode, it is preferable to provide the porous layer so as to cover the additional layer according to the manufacturing method of the present invention. The additional layer is, for example, a protective layer or the like as shown in the later-described embodiments.
In general, the porous layer of the present invention is provided to cover the entire surface of the outside electrode. However, it is possible to provide the porous layer so as to partly cover the outside electrode.
Furthermore, it is possible to provide the porous layer so as to cover other portion of the solid electrolytic solid electrolytic body other than the outside electrode.
Furthermore, the dipping apparatus used in the first manufacturing method is equipped with the viscosity adjusting tank. The viscosity adjusting tank has a function of maintaining the viscosity (i.e., concentration) of slurry stored in the dipping tank at a constant value. Accordingly, it becomes possible to prepare uniform slurry preferable for the dipping treatment.
The viscosity adjustment
Barr Michael
Denso Corporation
Nixon & Vanderhye PC
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