JPS6161343B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an oxygen sensor having a transition metal oxide layer that detects the oxygen partial pressure of engine exhaust gas, such as automobile exhaust gas, in which fuel and oxygen are reacted by a change in electrical resistance.

In the three-way catalytic gas purification system, which is adopted as a measure against automobile exhaust gas pollution, the air-fuel ratio must be controlled within a very narrow range of the equivalence point, and an oxygen sensor is used to detect the residual amount after combustion. Oxygen is sensed and fed back to the fuel supply system. It is known that it is appropriate to use a transition metal oxide such as titanium dioxide as a material for an oxygen sensor that detects changes in oxygen partial pressure in exhaust gas based on changes in electrical resistance. However, even if such an oxygen sensor is used, if the fuel is not completely combusted in the internal combustion engine and unburned fuel remains, the corresponding amount of oxygen will be included in the exhaust gas.
Even if the oxygen sensor measurement results are fed back, the air-fuel ratio cannot be accurately controlled. For this purpose, a platinum group metal as an oxidation catalyst is supported, for example, inside calcined titanium dioxide, and its catalytic action promotes the combustion of unburned materials. However, at this time, if the contact between the catalyst and the exhaust gas is not sufficient, the oxidation reaction will not take place completely, and unburned matter will still remain.

SUMMARY OF THE INVENTION An object of the present invention is to provide an oxygen sensor that detects the oxygen content in the exhaust gas of an engine in which fuel and oxygen are reacted after unburned fuel is reacted with oxygen as much as possible.

This object, according to the present invention, comprises a transition metal oxide layer on a ceramic substrate, a metal electrode in contact with the transition metal oxide layer, and a thermally sprayed ceramic protective layer covering at least the oxide layer. This is achieved by supporting fine particles of a platinum group oxidation catalyst in the pores.

Embodiments of the present invention will be described below with reference to the drawings. Japanese Patent Application Publication No. 55-18922 filed by the applicant
An oxygen sensor having a structure as shown in FIG. 1 is known from a publication. Electrode platinum thick films 2 and 3 are baked onto a substrate 1 made of an alumina round rod using a printing technique. Next, the titanium dioxide layer 4 is similarly baked as a thick film so as to fill the gap between the electrode thick films 2 and 3. Furthermore, a ceramic protective layer 5 is provided thereon by plasma spraying magnesia spinel. This protective layer 5 is designed to prevent erosion or chemical action caused by exhaust gas on the platinum thick films 2 and 3 and the titanium dioxide thick film 4, and is a heat-insulating protective layer based on its thermal conductivity, which is 1/2 that of alumina in the spinel film. This is useful for alleviating thermal strain caused by rapid heating and cooling of the titanium dioxide film 4 due to exhaust gas. This sprayed protective layer 5 is a porous material having pores with an inner diameter of 1000 Å or less, and the exhaust gas contacts the oxide film 4 through these pores and changes its resistance. This change is detected through the electrode platinum films 2 and 3. After thermal spraying of this protective layer, according to the invention, e.g.
It is impregnated with an aqueous or organic solution of a platinum compound such as H ₂ PtCl ₆ and calcined at a temperature of 600°C and then 800°C. As a result, metallic platinum fine particles adhere to the walls of the pores inside the protective layer 5. The platinum fine particles act as an oxidation catalyst and promote the combustion of unburned components in the exhaust gas passing through the pores of the protective layer 5.

FIG. 2 shows another embodiment, in which parts corresponding to those in FIG. 1 are given the same reference numerals. In this case, a rectangular alumina substrate 1 is used, and the substrate 1 has through holes 6 on diagonal lines. On top of this, strip-shaped electrode platinum thick films 2 and 3 are printed and baked. Furthermore, a titanium dioxide thick film 4 and an electrode platinum thick film second layer 31 are provided thereon.
form. Next, insert the iron-nickel alloy wire 7 into the through hole 6.
After the holes in the platinum films 2 and 3 communicating therewith are made to pass through freely, they are bent and brought into contact with the surfaces of the platinum films 2 and 3, respectively, and fixed with an adhesive. Subsequently, one end of the thin gold wire 8 is welded onto the electrode films 2 and 3, respectively, and the other end is welded to the iron-nickel alloy wire 7. A ceramic protective layer 5 is formed thereon by spraying magnesia spinel in the same manner as in FIG. This protective layer 5 is made of alloy wire 7
is clamped to the electrode films 2 and 3 to ensure electrical connection between the alloy wire 7 and the electrode films 2 and 3 even if the thin wire 8 should break. Next, the protective layer 5 is impregnated with a platinum solution in the same manner as in the case of FIG. 1 and fired, so that the protective layer 5 supports the platinum fine particles. During the process in which the exhaust gas that has reached the top of the protective layer 5 reaches the oxide film 4 through the pores in the protective layer 5, it comes into contact with the platinum supported on the walls of the pores, and its catalytic action removes any unused substances in the exhaust gas. Fuel is burned. The resistance value of the oxide film 4 corresponding to the remaining oxygen content can be detected from the outside via the alloy wire 7 and fed back to the fuel supply system.

In actual manufacturing, the oxygen sensor shown in Figure 2 is formed in multiple pieces on a large rectangular substrate, and then finally divided into parts using the so-called tangential break method, which involves folding them at grid-like grooves pre-prepared on the substrate. Mass produce.

As described above, the oxygen sensor according to the present invention has a platinum group catalyst supported within the porous thermally sprayed protective layer covering the transition metal oxide layer, and the process in which the gas whose oxygen content is to be detected reaches the oxide layer. Because the pores of less than 1000 Å make sufficient contact with the catalyst attached to the wall surface, almost complete combustion of unreacted fuel occurs. Therefore, data regarding the air-fuel ratio is much more reliable than when the catalyst is supported on an oxide fired body having pores with an inner diameter of about 0.5 μm. The oxygen sensor according to the present invention can be used not only for detecting oxygen content in automobile exhaust gas, but also for increasing reliability in determining residual oxygen content in combustion exhaust gas from other engines.

Furthermore, this protective layer is directly exposed to combustion exhaust gases, and its excellent erosion resistance, thermal shock resistance, and thermal cycling properties can significantly improve the reliability and lifespan of the sensor. It is.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of one embodiment of the present invention, and FIG. 2 is a sectional view of another embodiment. 1... Substrate (substrate), 2, 3, 31... Platinum electrode thick film, 4... Titanium dioxide thick film, 5... Thermal sprayed protective layer.

Claims (1)

[Claims] 1. A structure comprising a transition metal oxide layer on a ceramic substrate, a metal electrode in contact with the transition metal oxide layer, and a sprayed ceramic protective layer covering at least the oxide layer, in which platinum group oxidation catalyst fine particles are supported in the pores of the protective layer. An oxygen sensor characterized by: