JP3020688B2 - Oxygen sensor element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an oxygen concentration detector and, more particularly, to platinum and ceramics developed for the purpose of improving the adhesion of an electrode to a solid electrolyte and preventing sintering of the electrode. The present invention relates to an oxygen concentration detector using a cermet electrode.

[0002]

2. Description of the Related Art Conventionally, as an oxygen sensor for detecting the concentration of oxygen in exhaust gas generated from an internal combustion engine for automobiles, a pair of electrodes is provided on a zirconia porcelain, which is an oxygen ion conductive solid electrolyte, to provide an oxygen concentration battery. There is known an oxygen sensor configured to detect the oxygen concentration using the principle.

[0003]

In such an oxygen sensor, a method of forming an electrode by attaching a platinum thin film to zirconia porcelain by a technique such as electroless plating or sputtering is known. The electrodes attached to the zirconia porcelain in this way have excellent activity at low temperatures, but have the drawback of poor adhesion to porcelain and poor durability.

[0004] As another method of attaching electrodes, there is a method of baking a cermet material at a high temperature to increase the adhesive strength with a solid electrolyte. However, this method has a drawback that the electrode is sintered at a high temperature, so that the sintering of the electrode proceeds and the activation as the electrode is reduced. Further, the electrode attached in this manner has poor low-temperature activity, and may not operate sufficiently as a sensor when used under conditions where the element temperature is low. Furthermore, the control point of the air-fuel ratio of the engine is
The determination is made using such an oxygen sensor.However, if the oxygen sensor with the cermet electrode attached by baking at high temperature is used for a long time in the exhaust gas, the activity of the cermet electrode is gradually increased. There is also a disadvantage that the control point of the air-fuel ratio may change with the passage of time.

SUMMARY OF THE INVENTION An object of the present invention is to provide an oxygen concentration detector having a cermet electrode having a high activity and a high adhesion to a substrate made of a solid electrolyte, overcoming the above-mentioned drawbacks. is there.

[0006]

In order to solve the above problems, an oxygen concentration detector of the present invention comprises a substrate made of an oxygen ion conductive solid electrolyte and a pair of cermet electrodes provided on the substrate. In the equipped oxygen concentration detector,
At least one surface of the cermet electrode is provided with a platinum layer formed by electrolytic plating.

As described above, in the oxygen concentration detector of the present invention, since the platinum layer is formed on the surface of the cermet electrode by, for example, electrolytic plating or the like, the substrate made of the solid electrolyte of the cermet electrode can be used. The activity of the electrode can be enhanced by the platinum layer formed on the electrode surface while maintaining the strong adhesive force of the electrode. Therefore, it is possible to provide an oxygen concentration detector having both low-temperature activity and durability.

In the present invention, a stabilized zirconia or a partially stabilized zirconia material obtained by mixing zirconia with a predetermined stabilizer is used as an oxygen ion conductive solid electrolyte which is a material of an element body constituting an oxygen concentration detector. It is preferably used. As the stabilizer, for example, yttria (Y
₂ O ₃ ), calcia (CaO), magnesia (Mg
O), ytterbia (Yb ₂ O ₃ ) and the like can be suitably used.

As is well known, for example, sintering aids such as kaolin and SiO ₂ and Al ₂ O ₃ are appropriately blended.

Using a material selected from the above-mentioned solid electrolyte materials, a bottomed circular or plate-like molded body serving as the main body of the oxygen sensor element is formed by a pressure molding method such as a known rubber pressing method. Alternatively, it is formed using a method combining a thick film method and a multilayer laminating method. An electrode is formed on the surface of the formed body prior to firing. The electrodes form a measurement electrode that is exposed to the gas to be measured and a reference electrode that is exposed to a reference gas having a predetermined reference oxygen concentration. These electrodes, for example, platinum, ruthenium, iridium, rhodium, a predetermined platinum group metal such as palladium,
Alternatively, it is a film made of a conductive material mainly composed of such a platinum group metal. In particular, it is preferable to use a conductor containing platinum as a main component for the measurement electrode. More specifically, a paste formed by mixing powder of a metal material used for such an electrode, ceramics, resin, and a solvent is applied to the surface of the molded body by using a method such as screen printing, and the molded body is coated. To form an oxygen sensor element. In order to improve the durability of the electrode, a porous ceramic sheet is further laminated on the electrode, or a ceramic coating layer is formed by a usual plasma spraying method or a frame spraying method.

[0011] The oxygen sensor element obtained as described above can be used, for example, in the presence of platinum such as chloroplatinic acid, amino nitrite, tetrachloroammonium salt and the like, including some conductive salts such as sulfamic acid, sulfuric acid and ammonium phosphate. Perform plating bath by immersing in salt. Thereafter, using a cermet electrode as a negative electrode and plate or wire-shaped platinum or graphite as an anode, a DC power source is connected to perform electrolytic plating to form a platinum layer on the electrode surface. The temperature of the plating bath varies depending on the type of the platinum salt used for the plating bath solution.
A range of ° C is appropriate. The current density of the current flowing from the DC power supply is preferably about 0.01 to 10 A / dm ² .

Before performing the electrolytic plating, as a pretreatment, the cermet electrode portion of the oxygen sensor element is immersed in an HF solution or an HBF ₄ solution, or is subjected to electrolytic etching to obtain a fresh platinum electrode. If the surface is exposed and a platinum layer is formed on the platinum surface by electrolytic plating, the low-temperature operability can be further improved.

Further, before performing the electroplating, a suspension of the oxygen ion conductive solid electrolyte powder or a solution of a metal chloride compound capable of forming an oxygen ion conductive solid electrolyte by performing a heat treatment is applied to the gap of the cermet electrode layer. Even if heat treatment is performed after the impregnation, the effect of improving low-temperature operability can be enhanced. Note that the temperature of the heat treatment in this case is lower than the firing temperature at the time of firing the substrate.

[0014]

FIG. 1 is a sectional view showing an oxygen concentration detector main body according to the present invention. For example, 96 mol% of zirconia and
As a sintering aid, 1.2 wt% of Al ₂ O ₃ ,
After adding 0.5 wt% of SiO ₂ and performing wet pulverization for 30 hours, a binder and a solution are mixed with the dried powder,
The zirconia material 1 in the form of a slurry was tape-shaped using a doctor blade or the like. The zirconia material in the form of a tape is cut to produce a sheet 1.

As the electrode material, a mixture of a heat-resistant metal such as platinum, rhodium, iridium and palladium and ceramic particles was used. These metal powders are mixed with a resin and a solvent to form a cermet paste, and the cermet paste is applied to predetermined portions of the front and back surfaces of the plate-like sheet 1 by means of screening printing or the like. 5. The heater layer 4 is stacked, and the oxygen sensor element is completed by simultaneous firing.

FIG. 2 shows a method of manufacturing an oxygen sensor element according to the present invention.
FIG. 1 is a diagram showing an example of an electrolytic plating apparatus for performing the above. plating
The tank 10 is filled with a Pt plating solution 11 and the heater 12
Heat solution 11 to about 70 ° C. Set in plating solution 11
The anode 14 connected to the positive electrode of the current device 13 and the
Immerse oxygen sensor element 15 (cathode) connected to eggplant electrode
Then, a direct current was passed to perform plating. Table 1 below
As shown in FIG.
The current density was 0.01 A / dm^Two,
0.5A / dm^Two, 0.8A / dm^Two, 10A / d
m^Two, 50A / dm ^TwoAnd change the plating time to 2
Oxygen sensor that has been plated for
A device was prepared. Plating was performed as a comparative example.
Not available unprocessed products. Pt and Z are used as electrode materials.
rO_TwoUsed at a ratio of 65:35 vol%
Was. In addition, the samples of Experimental Examples 3 to 7 and untreated products (ratio
For Comparative Example 1), pretreatment was performed in a 0.5% HF solution.
Is going. For samples 1 to 7, plating
After the treatment, washing with ion-exchanged water, 120 ° C
Dry for 2 hours at the temperature of
The sensor was completed and its performance was evaluated. About unprocessed products
And assemble it into a metal case without any processing
The oxygen sensor was completed, and the performance was evaluated similarly. sex
Evaluation of these oxygen sensors_ThreeH₈ 560cc / min, d
A: For combustion gas of 14.25 l / min, λ = 1.065, CO 45cc / mi
Non-equilibrium at 250 ° C gas temperature where λ = 1.059 by adding n
Keep the oxygen sensor element temperature at 350 ° C in gas
The output was measured and characteristics were evaluated.

As is clear from Table 1, No. Except for the sample No. 7, it can be seen that the low-temperature activity of the oxygen sensor of the present invention, which has been subjected to the plating treatment, is improved as compared with the untreated product.

[0018]

[Table 1]

Further, 100 oxygen sensor elements manufactured under the conditions of Experimental Example 4 were prepared, and these elements were cut at a distance of about 12 mm from the tip, placed in a CO reactor and heated to obtain CO gas. Was poured. Gas components flowing out of the reactor were analyzed using gas chromatography. The composition of the gas flowing into the reactor was: CO: 1%, O ₂ : 1
%, He: 98%. The gas flow rate is 100m
1 / min.

As is clear from Table 1, the temperature at which the CO purification rate reaches 100% in the sample subjected to electrolytic plating (Experimental Example 4) is shifted to the lower temperature side by about 40 ° C. as compared with the untreated product. It can be seen that the low-temperature activity was enhanced.

As a second embodiment, before performing an electrolytic plating process on an electrode, a pretreatment is performed in a 10% HCl aqueous solution.
A sample (experimental example 8) subjected to electrolytic etching by passing a current of 0.01 A / dm ^{2 at} a current density, and etching in a 0.5% HF aqueous solution as a pretreatment, and ZrOCl ₂ as a post treatment After impregnating the plated element in an aqueous solution, a sample (Experimental Example 9) was prepared by baking at 1000 ° C. for 10 minutes.
The temperature at which the purification rate reached 100% was examined. As a comparative example, an oxygen sensor element prepared under the same conditions as in Experimental Example 4 of Example 1 (see Table 1) was prepared (Comparative Example 2), and the temperature at which the CO purification rate reached 100% was similarly examined. Was. Table 2 shows the results. The composition of the electrode is the same as in the first embodiment.

[0022]

[Table 2]

As can be seen from Table 2, 0.
Comparative example in which etching was performed only in a 5% HF aqueous solution
Experiment in which electrolytic etching was performed as a pretreatment compared to 2
The oxygen sensor element of Example 8 and 0.5% as pretreatment
Etching in HF aqueous solution and post-processing
Oxygen sensor element of Experimental Example 9 in which
However, the temperature at which the CO purification rate reaches 100% is about 10 to 15
° C lower. From this, as a pretreatment, electrolytic
Etching or ZrOCl as post-processing _TwoWater soluble
Improve the activity of the catalyst by immersing it in liquid
You can see that you can do it.

[0024]

As described above, in the oxygen sensor element of the present invention, since the platinum layer is formed on the surface of the cermet electrode by performing the electrolytic plating, the oxygen sensor element with respect to the oxygen ion conductive solid electrolyte of the cermet electrode is formed. The catalytic activity of the electrode can be favorably maintained while maintaining good adhesion. Therefore, it is possible to provide an oxygen sensor element having good low-temperature activity even from the initial use to after use for a long time. When such an oxygen sensor element is used for controlling the air-fuel ratio of an engine, it is possible to suppress the variation of the control points to a small extent, and also to minimize the temporal change as a sensor.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view for explaining a configuration of an oxygen sensor element of the present invention.

FIG. 2 is a diagram showing a configuration of an electrolytic plating apparatus of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Sensor main body 2, 3 electrode 4 Heater 5 Protective layer 10 Plating tank 11 Plating solution 12 Heater 13 Constant current device 14 Anode 15 Oxygen sensor element

Claims (1)

(57) [Claims] 1. An oxygen concentration detector comprising a substrate made of an oxygen ion conductive solid electrolyte and a pair of cermet electrodes provided on the substrate, wherein at least one surface of the cermet electrode is formed by electrolytic plating. An oxygen concentration detector provided with a platinum layer.