JPS637342B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is applicable to exhaust gases in which reducing gases such as CO and hydrocarbons (HC) coexist with oxidizing gases such as NO _x and O ₂ , such as lean combustion This invention relates to an exhaust gas sensor that detects oxidizing gas in engine exhaust gas (burn type).

Traditionally, stabilized ZrO ₂ has been used as this type of sensor.
When Pt is attached as electrodes on the inside and outside of a closed sintered tube, and the outside is exposed to exhaust gas and the inside is exposed to air, a large electromotive force is generated in exhaust gas containing excess reducing gas, and Then, the electromotive force becomes small and suddenly changes around λ=1 (λ is the ratio of the actual air-fuel ratio to the stoichiometric air-fuel ratio). Also, TiO ₂ or MgCo ₂ O ₄ has λ=
The resistance changes rapidly near 1. Whereas TiO ₂ has low resistance in the state of excess reducing gas,
MgCo ₂ O ₄ differs in that it becomes higher, but in any case, the rapid change around λ=1 was utilized.

Recently, the lean burn system has been attracting attention because the amount of harmful gases such as CO, HC, and NOx in the exhaust gas from the engine is small, and it can also reduce fuel consumption. There is no sensor whose output changes suddenly, and only a small amount of ZrO ₂
In an electromotive force type sensor that uses oxygen as an electrolyte, it is possible to obtain an electromotive force change of about 29.5 mV for a single-digit change in oxygen partial pressure. The electromotive force also changes depending on the ambient temperature, and to prevent this, it is recommended to add Fe ₂ O ₃ to the electrolyte. However, it has the disadvantage of poor sensitivity and reliability, and has not been put into practical use.

The present invention generates thermoelectromotive force when a temperature difference is applied to an oxide semiconductor that undergoes n/p conversion under a certain oxygen partial pressure, and the polarity of the thermoelectromotive force forms a boundary between the oxygen partial pressure that causes n/p conversion. This method detects oxidizing gases in exhaust gas by utilizing the fact that the

According to the sensor of the present invention, whether the partial pressure of excess oxidizing gas in the exhaust gas of a lean-burn engine is detected is higher or lower than a certain value based on the polarity of the thermoelectromotive force of the sensor, so that the engine can be controlled. is easy.

Oxide semiconductors used in the sensor substrate of the present invention include perovskite oxides as described below, as well as spinel oxides such as magnesium chromate (MgCr ₂ O ₄ ). All of these exhibit mixed conductivity of electrons and oxygen ions.

Examples of the present invention will be described below.

Figures 1 and 2 show the structure of an exhaust gas sensor according to an embodiment of the present invention. In the figures, 1 is a sensor base made of a fired body of perovskite oxide, which is the base of the sensor of the present invention, and 2 is a Pt sensor base. -Ir
3 is a heating resistance wire made of Kanthal; 4 is a base sintered body mainly composed of Al ₂ O ₃ ;
5 is a lead for thermoelectromotive force measurement made of a heat-resistant alloy such as nichrome, stainless steel, or Kovar; 6
is a heater lead made of the same material as the lead 5.

Next, an overview of the operation of the exhaust gas sensor of the present invention with the above configuration will be described.

First, the base sintered body 4 is attached to the exhaust pipe extending from the engine exhaust port to the muffler, just before the muffler, so that the sensor base 1 enters the exhaust gas. When a current pulse is passed through the heating resistance wire 3, a temperature difference occurs between the two electrodes and a thermoelectromotive force is generated. When the partial pressure of the oxidizing gas exceeds the n-p conversion point of the sensor substrate 1, the potential of the electrode on the high temperature side changes from positive to negative, thereby making it possible to detect the oxidizing gas.

Next, the effects of the exhaust gas sensor of the present invention will be explained using a specific example.

Among perovskite compounds, M ₁ TiO ₃ ,
M ₁ ZrO ₃ (M ₁ :Ca, Sr, Ba, Pb, Cd),
Those such as M ₂ NbO ₃ and M ₂ TaO ₃ (M ₂ :Ag, K, Na) that tend to form oxides with excessive combination of B-site elements exhibit n-type behavior, and the resistance depends on the gas composition. The change in is as shown by the solid line in Figure 3.

Furthermore, when using an element that forms an oxide with excess oxygen at the B site, such as M ₃ CoO ₃ or M ₃ MnO ₃ (M ₃ : the first rare element), it exhibits p-type behavior and the gas The resistance changes depending on the composition as shown by the dotted line in FIG. Note that the characteristics shown in FIG. 3 are obtained when the temperature is kept constant at 500°C.

A perovskite compound consisting of an element that easily forms an n-type and an element that easily forms a p-type is mixed.
When it becomes a solid solution, an n/p conversion point is created at any point in the air-fuel ratio.

As an example of this, the situation will be explained by changing the mixing ratio of SrTiO ₃ and LaCoO ₃ .
Mixture of molecular ratio of LaCoO ₃ 0.2 to SrTiO ₃ 0.8,
Solid solution (sample 1), SrTiO ₃ 0.6 mixed with 0.4 LaCoO ₃ and solid solution (sample 2),
A mixture of SrTiO ₃ 0.4 and LaCoO ₃ 0.6 as a solid solution (sample 3) and a mixture of SrTiO ₃ 0.2 and SrTiO ₃ 0.8 as a solid solution (sample 4) were prepared using acetate as an aqueous solution. After ratio mixing (only Ti was dispersed in the solution in the form of TiO ₂ powder), water was evaporated off in a rotary distiller, and then exposed to air.
The acetate was thermally decomposed by heating to 400°C, and then calcined in air at 1300°C, 1150°C, 1000°C, and 850°C for 1 hour to produce raw material powders.

This raw material powder contains 64% CaO, 26% B ₂ O ₃ , and 10% SiO ₂
% high melting point glass respectively 0%, 10%, 20%,
Add 30% carboxymethyl cellulose powder and 1% (outer portion) and mix, then add water to this powder at a weight ratio of 1:1 and mix well.
After heating to ~100°C to evaporate water, the sensor base was press-molded at a pressure of 300 kg/cm ² with the Pt--Ir lead inserted. Firing was carried out for 2 hours at a temperature 100°C higher than the calcination temperature. During calcination the carboxymethyl cellulose was oxidized away.
The reason for adding glass is to improve adhesion to the Pt-Ir wire, and also to prevent the sensor from disintegrating if exposed to reducing gas for a long time if it contains a large amount of LaCoO ₃ . It's for a reason.

When these samples were exposed to gas under the same conditions as shown in Fig. 3, the resistance changes were as shown in Fig. 4. That is, O ₂ content was 5% in sample 1, 1% in sample 2,
It was observed that sample 3 had an n/p conversion point at 0.08% CO content, and sample 4 at 0.9%.

In addition, when the sensors of Samples 1 to 3 above were mounted on an engine and the engine was operated with the air-fuel ratio varied from 13.5 to 20, the thermoelectromotive force response when a constant power pulse was passed through the Kanthal wire was as fast as 20 msec, and the response was constant. The values reached were as shown in Figure 5. That is, the change in thermoelectromotive force is significant because the polarity is reversed at the n/p conversion point, and it also changes slightly at the point where λ=1, but this seems to be less of a problem than in the former case. In addition,
Although only the cases of SrTiO ₃ and LaCoO ₃ are shown in the above samples, a change in thermoelectromotive force at the n/p conversion point occurred in both cases. When manganate is used as a p-type compound, λ = 1 as compared to cobaltate.
This is preferable because there is no change in electromotive force at this point. Also,
If a p-type compound other than manganate or cobaltate is used, the catalytic action will be reduced due to the mixed gas of reducing gas and oxidizing gas. That is, although spinel oxides include oxides that perform n/p conversion and have a catalytic effect on air-fuel mixtures, perovskite oxides, among which manganese and cobalt salts, have the best catalytic effect.

As described above, according to the present invention, it is possible to provide an exhaust gas sensor that can accurately detect oxidizing gas in the exhaust gas of a lean-burn engine.

[Brief explanation of the drawing]

1 and 2 are a top view and a sectional view showing an exhaust gas sensor according to an embodiment of the present invention, and FIG.
Figure 4 shows the change in resistance of n or p single perovskite depending on the gas atmosphere, Figure 4 shows the change in resistance of n/p composite perovskite depending on the gas atmosphere, and Figure 5 shows the exhaust gas sensor of the present invention in practice. FIG. 3 is a diagram showing changes in thermoelectromotive force depending on the air-fuel ratio when installed in an engine. 1... Sensor base, 2... Electrode embedded wire, 3...
heating resistance wire.

Claims (1)

[Claims] 1. A sensor base made of an oxide semiconductor that performs n/p conversion under a specific oxygen partial pressure, a pair of electrodes provided on the sensor base, a heater that heats one electrode, and detects the polarity of the thermoelectromotive force between the two electrodes. An exhaust gas sensor consisting of a device that