JPS6039809Y2 - oxygen sensor - Google Patents

Description

[Detailed Description of the Invention] The present invention mainly relates to improving the thermal shock resistance of an oxygen sensor that measures the partial pressure of oxygen in exhaust gas discharged from an internal combustion engine or the like.

As an oxygen sensor for measuring the partial pressure of oxygen in exhaust gas used in a purification system for exhaust gas emitted from an internal combustion engine, etc., a zirconia solid electrolyte 1 having a tubular shape with one end closed as shown in Fig. 1 is used. Electrodes 2 and 3 are provided on both sides, and the detection part 4 is inserted into the exhaust gas 5 so that the outer electrode 3 is brought into contact with the exhaust gas, and the inner electrode 2 is brought into contact with a reference gas, such as the atmosphere, according to the principle of an oxygen concentration battery. Oxygen sensors that measure oxygen partial pressure in exhaust gas 5 are known.

In such an oxygen sensor, the detection part 4 inserted into the exhaust gas 5 is subjected to thermal shock due to heating and cooling by the exhaust gas 5, and if the thermal shock is large, the zirconia solid electrolyte 1 will be damaged. It had serious shortcomings.

Therefore, in order to alleviate the thermal shock of the detection part 4, a large number of openings 6 in the form of circular holes or slits are provided on the outer periphery of the detection part 4.
Generally, a protective cover 7 made of metal is installed.

However, even with the protective cover 7, under the most severe exhaust gas conditions, the detection unit 4 receives a thermal shock of up to 5 per second.
It was not possible to completely prevent damage to the zirconia solid electrolyte due to thermal shock even at 0°C.

The oxygen sensor of the present invention improves the shortcomings of conventional oxygen sensors and has excellent thermal shock resistance, with the solid electrolyte hardly being damaged by thermal shock even when exposed to the most severe exhaust gas conditions. Electrodes are provided on both the inner and outer surfaces of a zirconia solid electrolyte that has a tubular shape with one end closed, and the inner electrode part is brought into contact with the reference gas while the outer electrode part is brought into contact with the exhaust gas. In an oxygen sensor that measures the partial pressure of oxygen in the exhaust gas, the ratio of the outer surface area of the detection part of the zirconia solid electrolyte inserted into the exhaust gas to the volume of the part is 10a+!
/cI1 or more, and the outer diameter of the detection part is 6wn or less in any part.

In other words, the present invention examines in detail the thermal shock destruction phenomenon of solid electrolytes, and examines the shape, heat-receiving area, heat capacity of the detection part, and the damage to the detection part due to heating, cooling rate, temperature distribution, etc. when the detection part receives thermal shock. As a result of a detailed study of the relationship between It is based on

In other words, the conventional idea was that the higher the temperature rise or cooling rate of the detection part, the greater the thermal shock, and the more likely it would be damaged. However, for the detection part as a whole, it is less likely to be damaged if the temperature rise or cooling rate is high and it can easily follow the temperature change on the surface, and for this purpose, the ratio of the volume of the detection part to the heat receiving area of the detection part is important. It was determined that the above ratio must be above a certain value in order to prevent damage even under the most severe exhaust gas conditions.

Furthermore, it has been found that in oxygen sensors with such a shape, the shape has a large effect on thermal shock resistance.

Next, the reason for the numerical limitation of the present invention will be explained based on a specific example.

Among commercially available zirconia solid electrolytes, the bending strength is approximately 1700.
ka/art's relatively high strength zirconia solid electrolyte A
Using two types of zirconia solid electrolytes, zirconia solid electrolyte B and zirconia solid electrolyte B, which have a relatively low bending strength of about 1200 kg/ctl, we prototyped a 5-gas oxygen sensor with the detection part shape shown in Table 1. It is attached to the exhaust pipe and subjected to 10 cycles of approximately 5,000 thermal shocks per second by exhaust gas.
The failure rate of solid electrolytes was comparatively measured.

The failure rate of the solid electrolyte in the detection part was as shown in Table 1.

As is clear from Table 1, the relationship between the surface area and volume ratio of the detection part and the failure rate is shown in Figure 4 for solid electrolyte A and Figure 5 for solid electrolyte B. It has been found that there is a correlation between the ratio of surface area to volume of the detection part, the outer diameter of the detection part, and the failure rate.

In other words, if we explain the reason for the numerical limitation of the present invention, the fourth
As is clear from Fig. 5 and Fig. 5, when the ratio of the surface area to the volume of the detection part is less than 10 cJ/-, the damage rate due to thermal shock increases regardless of the type of zirconia solid electrolyte or the outer diameter of the detection part. cannot be reduced to zero, so the ratio of the surface area to the volume of the detection part is 1.
0c+I! /cJ or more is required.

Further, even when the ratio is 10 cy#/cr1 or more, the outer diameter of the detection part must be 6 wIt or less in order to make the failure rate substantially zero.

The reason why the surface area to volume ratio of the detection part and the outer diameter of the detection part have a clear correlation with thermal shock resistance is that the larger the surface area of the detection part, that is, the heat receiving area, the higher the temperature of the detection part. The larger the rate of change is, and the smaller the volume, or heat capacity, of the zirconia solid electrolyte in the detection part, the faster the temperature change in the detection part becomes.If the rate of temperature change in the entire detection part is large, the detection It is thought that the strain stress due to the temperature distribution of the part is reduced, making it difficult to cause damage due to thermal shock, and the outer diameter dimension as a shape factor contributes to this, determining the limit value at which thermal shock damage occurs. It will be done.

In the present invention, the part called the detection part is the part inserted into the exhaust gas of the zirconia solid electrolyte 4 provided with electrodes as shown in FIGS. and the part beyond the opening position closest to the exhaust pipe wall of the louver-shaped opening 6, that is, the second part.
The tip portion is indicated by a broken line in the figures and FIG. 3.

It should be noted that in the portion near the exhaust pipe wall inside the broken line, there is no opening 6 in the protective cover, so the flow velocity of the exhaust gas is slow, so there is little thermal shock and there is almost no damage.

Therefore, we will exclude this from consideration. As detailed above, the oxygen sensor of the present invention in which the ratio of the outer surface area of the detection part of the zirconia solid electrolyte to the volume of the part is 10 Cri/d or more and the outer diameter of the detection part is 6 Cri/d or less is suitable for use in internal combustion engines. The material is hardly damaged even by thermal shock caused by exhaust gas under the most severe conditions, and can be used stably for a long period of time, making it extremely useful industrially.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram showing a cross-sectional structure of a specific example of an oxygen sensor, FIGS. The figure is an explanatory diagram showing the relationship between the surface area/volume ratio and outer diameter of the detection part of the zirconia solid electrolyte and the damage rate due to thermal shock. 1...zirconia solid electrolyte, 2,3...
...Electrode, 4...Detection section, 5...Exhaust gas, 6...Opening, 7...Protective cover

Claims (1)

[Scope of utility model registration request] Electrodes are provided on both the inner and outer surfaces of a zirconia solid electrolyte that has a tubular shape with one end closed, and the inner electrode part is brought into contact with the reference gas while the outer electrode part is brought into contact with the exhaust gas. In an oxygen sensor that measures the partial pressure of oxygen in the exhaust gas, the ratio of the outer surface area of the detection part of the zirconia solid electrolyte inserted into the exhaust gas to the volume of the part is lOc! 1! /C71 or more, and the outer diameter of the detection portion is 6wn or less in any part.