JP2745805B2 - Manufacturing method of oxygen concentration sensor - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing an oxygen concentration sensor, and more particularly to a method for manufacturing an oxygen concentration sensor having an improved process of forming an electrode on a sensor element surface. .

[Conventional technology]

2. Description of the Related Art It has been widely practiced to detect an oxygen concentration in automobile exhaust gas using an oxygen concentration sensor and to control a ratio of air to fuel supplied to an engine based on this value. There are many types of oxygen concentration sensors, and in any case, the sensor element is exposed to high-temperature exhaust gas. Therefore, the sensor element is made of ceramic having high heat resistance, and an electrode made of a heat-resistant metal for extracting an electric signal is provided on the surface of the sensor element.

By the way, as a method of forming an electrode on the sensor element surface of the oxygen concentration sensor, (1) an electrode forming method by chemical plating, or (2) an electrode forming method by screen-printing and firing a conductive paste is mainly used. is there.
In the method (2), conductive pastes of various properties have been proposed. For example, Japanese Patent Application Laid-Open No. 63-120482 discloses a method in which a component having a fluxing action is added to a ceramic component constituting a sensor element. An electrode forming method using a conductive paste containing a ceramic material having the following composition is disclosed.

[Problems to be solved by the invention]

Each of the above two types of electrode forming methods has its own characteristics. That is, from the viewpoint of the manufacturing process, the method (1) has a disadvantage that it takes time to thicken the electrode (for example, it takes 24 hours to obtain an electrode having a thickness of 1 μm). or,
The method (2) has a drawback that it is difficult to obtain an electrode of 5 μm or less due to difficulties in printing technology and production of a conductive paste. Further, from the viewpoint of sensor characteristics, the electrode (plated electrode) of the sensor element manufactured by the method (1) has good responsiveness but low heat resistance, and conversely, the electrode of the sensor element manufactured by the method (2). The (paste electrode) has low response but good heat resistance. These are summarized in FIGS. 5 and 6.

FIG. 5 shows the relationship between the electrode thickness and the response of the sensor and the relationship between the electrode thickness and the heat resistance of the electrode. FIG. 6 shows the time variation of the sensor output. As can be seen from FIG. 6, when the response of the sensor is good, the sensor output accurately follows the fluctuation of the oxygen concentration in the exhaust gas, but when the response of the sensor is poor, the sensor output follows the fluctuation of the oxygen concentration in the exhaust gas. It can be seen that accurate detection of the oxygen concentration was not possible.

Therefore, if sufficient heat resistance can be obtained for practical use, the thinner the electrode, the better. As is clear from FIG. 5, when the electrode thickness is in the range of 1 to 5 μm, the responsiveness of the sensor and the heat resistance of the electrode are balanced, so that it is expected that the optimum electrode thickness is in this range. Is done. However, in the conventional method, an electrode having a thickness in the range of 1 to 5 μm cannot be easily and efficiently formed on the surface of the ceramic body on which the sensor element is to be formed.

The present invention is to solve the above-mentioned problems of the prior art. An object of the present invention is to provide a method for improving the process of forming an electrode on the surface of a sensor element so that an oxygen concentration sensor having excellent performance can be obtained easily and with good productivity.

[Means for solving the problem]

According to the method for manufacturing an oxygen concentration sensor of the present invention, the step of forming an electrode on the surface of the sensor element when manufacturing the oxygen concentration sensor includes the steps of:
Applying a conductive paste having a composition of 0 parts, 50 parts of a solvent, 5 parts of a plasticizer, 3 parts of a deflocculant and 12.5 parts of ceramic powder to the surface of a ceramic body on which a sensor element is to be formed by screen printing; Baking the sensor element to a thickness of 2 to 3 μm on the surface
m) forming an electrode layer.

As the conductive metal powder, a noble metal having a predetermined average particle size, for example, platinum, rhodium, iridium or the like can be used alone or in combination.

The binder, solvent, plasticizer, and peptizer are appropriately selected from those commonly used in this field.

As the ceramic powder, a powder made of a type of ceramic such that the electrode layer is firmly bonded to the sensor element after firing is selected. Therefore, a powder made of the same type of ceramic as the ceramic constituting the sensor element is preferable. In particular,
When zirconia or titania is used as the ceramic constituting the sensor element, it is preferable to use zirconia powder or titania powder as the ceramic powder.

The size and shape of the sensor element are not particularly limited as long as the method of the present invention can be applied. The shape of the sensor element may be, for example, a plate shape, a cylindrical shape with one end closed, or the like.

By screen printing, a conductive paste layer having a thickness such that an electrode layer having a thickness of 2 to 3 μm can be formed by subsequent burning is applied to the sensor element surface.

[Action]

In the method of the present invention, by selecting the composition of the conductive paste layer optimally, a thickness of 2
An electrode layer of about 3 μm can be formed.

Also, by using ceramic powder as one component of the conductive paste, the electrode layer is fired integrally with the sensor element.
Can be molded.

〔Example〕

 Hereinafter, embodiments of the present invention will be described.

I. Study of conductive paste composition Platinum (Pt) powder as conductive metal powder, polyvinyl butyral (PVB) as binder, butanol (BN) as solvent, dioctyl phthalate (DOP) as plasticizer, deflocculant The conductive paste was manufactured by using triolein (TO) as a ceramic powder and zirconia powder as a ceramic powder and changing the composition ratio. Next, this was screen-printed on a zirconia green sheet and fired under predetermined conditions to observe the state when the electrode layer was formed. The results are shown in Table 1 below.

As is evident from Table 1, polyvinyl butyral (PV
B) [Binder] and dioctyl phthalate (DOP)
[Plasticizer] has a large influence on the formation of the electrode layer, and if it deviates from the appropriate amount, bleeding occurs after printing and cracking occurs after firing. Therefore, the composition of the conductive paste was evaluated according to Table II.
The composition was determined.

II.Formation of electrode layer 100 parts of zirconia powder by weight ratio, binder 1
1 part, 70 parts of a solvent, 5 parts of a plasticizer, and 3 parts of a deflocculant were mixed, stirred for 50 hours by a ball mill, and then formed into a sheet having a thickness of 200 μm by a doctor blade device.
1) A zirconia green sheet 1 shown in [perspective view] and FIG. 1 (b-1) [cross-sectional view] was obtained. Next, a conductive paste was screen-printed on the surface of the zirconia green sheet 1 to form a conductive paste layer 2 having a thickness of 5 μm as shown in FIGS. 1 (a-2) and 1 (b-2). Here, the conductive paste having the composition of the evaluation II of the above I was used. The conductive paste preferably has a viscosity of 10,000 to 20,000 cps. Further, for example, Pt-Rh powder may be used instead of Pt powder. FIG. 2 shows the relationship between the ratio of the binder to the conductive metal powder (% by weight) and the ratio of the fired thickness to the printed thickness (% by weight). From this, it can be seen that when the binder ratio is 60% by weight, the thickness of the conductive paste layer 2 after firing is 40% of the thickness at the time of printing.

The zirconia green sheet 1 having a conductive paste formed on the surface is heated to a maximum of 420 ° C. and degreased.
C. for 1 hour, a large amount of binder in the conductive paste layer 2 disappears as shown by the arrow in FIG. 1 (b-2 '), and the thickness decreases by the thickness of the contracted portion 2'. Fig. 1 (a
3) and a 2 μm-thick electrode layer 3 shown in FIG. 1 (b-3). Similarly, on the other surface of the zirconia green sheet 1, the electrode layer 3 and the pair of electrode layers 3 'on the front and back are also provided.
(Not shown).

III. Manufacture of Oxygen Concentration Sensor FIG. 3 shows a schematic configuration diagram of an embodiment of the sensor element of the oxygen concentration sensor manufactured using the zirconia green sheet 1 having the electrode layers 3, 8 'formed on the surface. This sensor element is of a stacked type, in which 4 is a protective layer, 5 is an air duct layer, and 6 is a lower end layer.

FIG. 4 shows an oxygen concentration sensor manufactured by the method of the present invention (product of the present invention, electrode layer thickness 2 μm) and an oxygen concentration sensor manufactured by the conventional method (conventional product, paste electrode, electrode layer thickness 5 μm). The above shows the time variation of the sensor output. The product of the present invention has a good response since the thickness of the electrode layer is optimally selected as compared with the conventional product. Further, in the product of the present invention, since the electrode layer is integrally fired and molded with the zirconia green sheet, the heat resistance of the electrode layer is higher than that of the conventional product.

(Effect of the Invention) According to the method of manufacturing an oxygen concentration sensor of the present invention, a conductive paste whose composition is optimally selected is applied by screen printing to the surface of a ceramic body on which a sensor element is to be formed, and then the sensor element is fired. Since the electrode layer having a thickness of 2 to 3 μm is formed on the surface, an oxygen concentration sensor having good responsiveness and high heat resistance can be easily obtained.

In addition, the method of the present invention can produce a thin electrode in a small number of steps, so that productivity is improved, and the amount of expensive noble metal used is small, which is effective in reducing the production cost.

Further, the method of the present invention can be widely applied to the manufacture of various types of oxygen concentration sensors such as a limiting current type and a semiconductor type using various types of sensor elements such as a stacked type and a cylindrical type having one end closed. Can be.

[Brief description of the drawings]

FIG. 1 is an explanatory view of a step of forming an electrode on a sensor element surface in a method of manufacturing an oxygen concentration sensor according to the present invention, and FIG. 2 is a relation between a ratio of a binder to a conductive metal powder and a ratio of a fired thickness to a printed thickness. FIG. 3 is a schematic configuration diagram of an embodiment of a sensor element of an oxygen concentration sensor manufactured by the method of the present invention. FIG. 4 is an oxygen concentration sensor manufactured by the method of the present invention and manufactured by a conventional method. FIG. 5 is a diagram showing the time variation of the sensor output of the oxygen concentration sensor. FIG. 5 is a diagram showing the relationship between the electrode thickness and the response of the oxygen concentration sensor and the relationship between the electrode thickness and the heat resistance of the electrode. FIG. 6 is a diagram showing the time variation of the sensor output of the oxygen concentration sensor having the paste electrode and the plating electrode. In the figure, 1... Zirconia green sheet 2... Conductive paste layer 2 ′... Shrunk portion 3 3 ′... Electrode layer 4... Protective layer 5.

Claims (1)

(57) [Claims] 1. The process of forming an electrode on the surface of a sensor element when manufacturing an oxygen concentration sensor comprises the steps of:
Applying a conductive paste having a composition of 5 parts, 50 parts of a solvent, 5 parts of a plasticizer, 3 parts of a deflocculant and 12.5 parts of a ceramic powder to a surface of a ceramic body on which a sensor element is to be formed by screen printing; Sinter the sensor element to a thickness of 2-3 μm on the surface
Forming an electrode layer of the above.