JPH03216546A - Activation treatment of oxygen sensor element - Google Patents

Abstract

PURPOSE:To accurately control the theoretical air/fuel ratio in low temp. gas by heat-treating an oxygen sensor element in reductive gas containing unsaturated hydrocarbon and/or aromatic hydrocarbon. CONSTITUTION:N2, H2 and CO2 gases are sent into a catalytic conversion furnace 30 to prepare base gas which is, in turn, sent into an activation treatment furnace 31 while C3H6 is added to the base gas as unsaturated hydrocarbon. A prepared oxygen sensor element is sent into the activation treatment furnace 31 to be heat-treated under a predetermined condition. By this method, the theoretical air/fuel ratio in low temp. gas is accurately controlled by the oxygen sensor element and the shift quantity of an electromotive force curve after heat treatment in the atmosphere can be reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for activating an oxygen sensor element, and more specifically, it is a method for activating an oxygen sensor element, and more specifically, it is capable of accurately determining the air-fuel ratio even in a region where the exhaust gas temperature of an internal combustion engine such as an automobile is low. The present invention relates to a method for activating an oxygen sensor element that can be controlled and has excellent low-temperature operability.

[Prior Art] It has been known that sintered silconium oxide (zirconia porcelain) etc. stabilized with CaO or Y20's'''g can serve as solid electrolytes with oxygen ion conductivity. Using this oxygen ion conductive solid electrolyte such as silconia porcelain, the oxygen concentration of the exhaust gas, which is the gas to be measured emitted from internal combustion engines such as automobiles, is detected based on the principle of an oxygen concentration battery. It is known to control the fuel ratio.

Conventionally, this type of oxygen sensor element, which is an oxygen concentration detector, has a fixed electrode, such as a porous platinum electrode, on the inner and outer surfaces of a bottomed cylindrical or flat plate-shaped silconia solid electrolyte body with a passage inside. The inner electrode is brought into contact with the atmosphere and serves as a reference electrode exposed to a reference gas having a reference oxygen concentration, while the outer electrode is exposed to exhaust gas, which is the gas to be measured, and serves as a measurement electrode. The oxygen intensity in the exhaust gas is measured by detecting an electromotive force based on the difference in oxygen concentration between the reference electrode and the measurement electrode.

[Problems to be Solved by the Invention] In recent years, stricter regulations on exhaust gas emitted from internal combustion engines have required accurate air-fuel ratio control in the low exhaust gas temperature region immediately after engine startup and during idling. Furthermore, as engines become more sophisticated, there is a demand for oxygen sensor elements that can accurately control the air-fuel ratio even if the sensor element is installed in a low-temperature area at the rear of the exhaust pipe due to engine room space limitations.

However, in conventional oxygen sensor elements, the catalytic ability of the electrode decreases at low temperatures, so the electromotive force characteristic curve shifts to the excess air lean side, and when the exhaust gas temperature is below 400°C, the stoichiometric air-fuel ratio could not be detected accurately.

In order to compensate for the decrease in the catalytic ability of the oxygen sensor element in the low temperature range as described above, a sensor element in which the electrode portion is heated with a heater has been devised. However, even if a heater is installed, the side of the sensor element exposed to the gas to be measured (exhaust gas) is
When the temperature of the exhaust gas is low, the gas cools the exhaust gas and reduces the catalytic activity of the electrode, making it easier for the electromotive force curve to deviate.

As electrodes for oxygen sensor elements, sensor elements such as stabilized or partially stabilized zirconia are made by mixing catalyst metal and ceramic powder to increase durability under severe usage conditions such as long periods or extremely high temperatures. It is known that the cermet electrode is baked into the base or is baked integrally with the base. However, in order to increase the durability of cermet electrodes, they are generally baked or integrally fired at a high temperature of 1000°C, preferably I:100'C or higher, resulting in a significant decrease in catalytic activity, which limits the range of their use. The drawback is that it is limited.

In general, the electrodes of oxygen sensor elements are not limited to cermet electrodes, but also include those made by plating methods, vapor deposition methods, etc., to increase the adhesion strength with the underlying solid electrolyte body, improve gas permeability within the electrode, or make it porous. For the purpose of 800°C ~ 1
Heat treatment at a temperature of 400''C is appropriately performed.However, most of these heat treatments are performed in the atmosphere, which oxidizes the electrode surface and reduces the catalytic activity of the electrode.

Furthermore, even when heat treatment is performed in a hydrogen or carbon monoxide atmosphere, the element temperature may rise to around 500 to 700°C during the assembly and inspection process of the oxygen sensor element, and most of this temperature is in the atmosphere. Therefore, the electrode surface of the oxygen sensor element at the time of shipment has high equilibration, with oxides of the metals forming the electrode formed thereon. This oxide has the problem of significantly shifting the electromotive force curve of the oxygen sensor element toward the lean side in low-temperature exhaust gas.

The present invention has been made in view of the above circumstances, and its purpose is to improve low-temperature operability and enhance the effect by heat-treating an oxygen sensor element under specific conditions. The object of the present invention is to provide an oxygen sensor element that does not easily disappear even after heating in an oxidizing atmosphere.

[A Thousand Steps to Solve the Problem] That is, according to the present invention, an oxygen ion conductive solid electrolyte body comprising at least an oxygen ion conductive solid electrolyte, a measuring electrode, a reference electrode, and a porous protective layer that protects the measuring electrode. A method for activating an oxygen sensor element is provided, which comprises heat-treating the sensor element in a reducing gas containing an unsaturated hydrocarbon and/or an aromatic hydrocarbon.

The present invention is characterized in that an oxygen sensor element is heat-treated under predetermined conditions in a reducing gas containing one of unsaturated hydrocarbons and aromatic hydrocarbons, or a mixture thereof. It was discovered that the theoretical air combustion control of the oxygen sensor element in low-temperature gas becomes more accurate, and that the shift amount of the electromotive force curve after heat treatment in the atmosphere is reduced.

The present invention can be applied to various known solid electrolytes, and in particular, zirconia can be used with certain stabilizers, such as yttria (Y2).
0:l), calcia (CaO). Macnesia (Mg
O). Using stabilized or partially stabilized zirconia material containing Ytterbia (Yb203) etc.,
Suitable for elements in the shape of a cylinder with a bottom, a cylinder, a plate, etc. obtained from a pressure molding method such as a rubber press method, a lamination method such as a thick film method, or a known method such as winding a thick film into a cylindrical shape. It will be used.

Methods for forming electrodes on these elements include conventionally known plating methods, sputter link methods, thermal decomposition of catalytic metal salts, and baking a cermet paste of electrode metal and ceramics onto the surface of the molded body. A variety of methods may be used as appropriate, such as a paste baking method, or a cermet paste co-firing method in which the cermet paste is printed or applied onto the element wood body and then fired together with the element body.

The electrodes formed by these methods use known catalytic metals such as Pt and Pd. It is made of a platinum group metal such as Rh or a conductive material mainly composed of these platinum group metals. In this case, the platinum group metal may be one type or two or more types.

The porous ceramic coating layer formed on the measurement electrode exposed to the gas to be measured can be formed according to various known methods, such as spinel (Mg
O-A Lee-03). Using ceramic materials such as zirconia and alumina, they can be thermally sprayed using plasma spraying or flame spraying to form a coating layer of a predetermined thickness, or printed on electrodes as ceramic powder slurry, or tipped into the slurry. It is formed by baking after pinking, or by applying it to cermet 1 and baking it together, and the structure of the porous coating layer is not only a single layer but also a multilayer structure of two or more layers. There is no difference.

The unsaturated hydrocarbon used in the present invention includes ethylene (C2H4), which is in a gaseous state at room temperature. Propylene (C:l
Hs), phthylene (C4H8), bentene (CsI
Hydrocarbons with double bonds such as {+o) and acetylene (
Typical examples include hydrocarbons having a triple bond such as C2H2).

In addition, aromatic hydrocarbons have a vapor pressure of 4m at room temperature.
Typical examples include benzene, toluene, xylene, etc., which have a molecular weight higher than mtlg.

As the reducing gas, H2, CO, or a mixture thereof diluted with N2/C02 or a mixture thereof, or a gas obtained by incomplete combustion of natural gas, LPG, etc. may be used, and in particular, H2 or In~40vo
l%, N2 or 10-40vol%C O2 or 0.1
It is preferable to use a gas having a composition in the range of 5 to 5 vol% and 5 to 40 vol% of CO.

The content of the undistributed hydrocarbon and/or aromatic hydrocarbon in the reducing gas is preferably in the range of 100 ppm to 10 vol%, particularly preferably 500 ppm to 5 vol%. Furthermore, if the number of carbons in the main chain of unsaturated hydrocarbons or aromatic hydrocarbons exceeds 6, the amount of carbon deposited during treatment will increase significantly, so the content in the reducing gas should be 500 ppm to 1. Preferably vol%. Containing unsaturated hydrocarbons and/or aromatic hydrocarbons
If it is less than ppl1, the above-mentioned activation effect of the oxygen sensor element will be weak, and the variations in the obtained products will be large. Moreover, if it exceeds 10 vol%, carbon may precipitate during heat treatment, which may deteriorate the electrode.

= On the other hand, the heat treatment temperature is preferably in the range of 300 to 1100°C (7), more preferably 500 to 850°C. The temperature during processing does not need to be constant, but can be between 300 and 1100°.
A heat cycle between C and C may be applied. Heat treatment temperature: 30
0℃? If the temperature is also lowered, the above-mentioned activation effect of the oxygen sensor element may be weakened, and the variations in the obtained products may increase. Furthermore, if the heat treatment temperature exceeds 1100° C., sintering or wear of the metal used for the electrodes may progress rapidly, resulting in poor conduction of the element.

Further, the heat treatment time is preferably 1 to 48 hours, and preferably 5 to 16 hours.
Time is more preferable. If the heat treatment time is less than 1 hour,
The activation effect is weak, resulting in large variations in the products obtained, and if the heat treatment time exceeds 48 hours, the metal used for the electrode will rapidly sinter and wear out, which may result in poor conduction of the element.

Even if the process of the present invention is applied only to the part of the oxygen sensor assembly (finished product) that is exposed to the gas to be measured, the same effect can be obtained as when the process is applied to the entire element at the stage of a single element. .

The treatment of the present invention may be performed intermittently, and the atmosphere in the treatment furnace is changed to a reducing gas using unsaturated hydrocarbons and N2, Co.
It is also possible to alternately replace gases such as (2) or atmosphere, and apply a cycle of rich φ lean atmosphere.

In addition, in the present invention, by using a heater for heating the electrode portion, the performance of the electrode is improved and the electrode can be used in a wider range of temperatures, thereby increasing the effects of the present invention.

[Examples] Hereinafter, the present invention will be explained in more detail based on illustrated examples, but the present invention is not limited to these examples.

(Example 1) FIG. 1 is a cross-sectional explanatory diagram showing an example of an oxygen sensor element.

In Figure 1, 10 is 4 mol% Y203-96 mol%
For ZrO2, l. 2wt%AM20
．． It is a flat silconia porcelain which is a solid electrolyte and contains 0.9 wt% SiO2, and Pt on both the inside and outside surfaces.
A cermet electrode material consisting of 85 wt% and 15 wt% of the solid electrolyte having the above composition was placed 9 cm from the element closing tip.
What about the outside measurement after partially applying it? An electrode l6 and an inner reference electrode l4 are provided, respectively. And measurement electrode l6
A porous protective layer 18 is formed further outside.

On the other hand, an insulating layer 20 with a heater 22 sandwiched therebetween is formed inside the zirconia ceramic lO, and these are integrally fired to produce an oxygen sensor element. Note that 12 is a reference gas introduction hole, and the reference electrode 14 comes into contact with the air introduced through this hole.

FIG. 2 is a flow diagram showing an example of the activation processing apparatus for an oxygen sensor element of the present invention. N2. H2 and C
The O gas is sent to the catalytic conversion reactor 30, where the C O/C
A base gas is created by converting the ratio of O / H2 /N2 = 10/2/50/38, and then propylene (C: I H 6) as an unsaturated hydrocarbon is converted to Q for 100 of the base gas. ．． It was added at a rate of 5 vol % and sent to a 3 liter activation treatment furnace.

On the other hand, the oxygen sensor element manufactured as described above was sent into an activation treatment furnace 3l, and an activation treatment was performed under the following conditions.

(Processing time: 5 cm IX1) The oxygen sensor element subjected to the activation process as described above was evaluated as follows.

Evaluation method: Keep the sensor element at 400°C,
The excess air ratio in the gas consisting of N2 and air was varied from 0.950 to 1.150, the sensor electromotive force was measured during that time, and the excess air ratio was calculated when the sensor electromotive force reached 0.45V. This was used as a measure of low temperature operability.

In addition, similar evaluations were performed on samples that were heat-treated in the atmosphere at 700°C for 5 hours after the activation treatment.

FIG. 3 shows the input of each sample evaluated as described above.

As is clear from the results in Figure 3, No. Compared to the untreated product No. 1-9, No. 1-1 to 15 and No. l7
, 18 activated products have a higher stoichiometric air-fuel ratio point (in,=1
．． 000). On the other hand, NO
．． 1-6 were unmeasurable.

Furthermore, No. 1-1 to l-5 and No. 1-7, l-
When each of the activated products of No. 8 was heat-treated at 700°C in the air for 5 hours, no. No. 1-1 treated with propylene at 200°C and No. 1-7 and 1-8 treated products without the addition of propylene deteriorated to the same level as the untreated products, indicating that the effect of the activation treatment was not stable.

On the other hand, No. Each of the samples Nos. 1-2 to 1-5 still showed better absorption than the untreated product and the reduced-treated product without addition of propylene, indicating that the effect of the activation treatment was stable.

(Example 2) An oxygen sensor element having the structure shown in Fig. 1 was produced by the same method as in Example 1, and using this element, the following method was carried out according to the flow diagram of the activation processing apparatus shown in Fig. 4. The oxygen sensor element was activated.

As in Example 1, N2, H2 and CO2 gases are fed into the catalytic conversion reactor 30, where a base gas having the same composition as in Example 1 is prepared, and this is placed in a constant temperature bath 32 at 15°C. Benzene (CGH6) was added as an aromatic hydrocarbon at a ratio of 8 vol % to the base gas 100 through the installed pub link container 33, and the mixture was fed into the activation processing furnace 31.

Meanwhile, the oxygen sensor element is sent to the activation processing furnace 31,
Activation treatment was performed under the following conditions.

(Treatment ratio 1 river/sn!% rI) The obtained activated products and untreated products were evaluated in the same manner as in Example 1. The results are shown in Figure 5.

As a result, No. No. 2-6 untreated product. 2
- The activated products 1 to 2-5 showed an air-fuel ratio closer to the stoichiometric air-fuel ratio.

Furthermore, No. 2-1 to 2-5 each activated product,
When heat-treated at 700°C in the atmosphere for 5 hours, the benzene-added product maintained better properties than the non-additive and untreated products, indicating that the effect of the activation treatment was stable. I understand.

[Effects of the Invention] As explained above, according to the method of the present invention, the oxygen sensor element is heat-treated in a reducing gas containing a perturbed hydrocarbon and/or an aromatic hydrocarbon. In addition to making the theoretical air combustion control in gas more accurate, it also has the effect of reducing the amount of shift in the electromotive force curve after heat treatment in the atmosphere.

[Brief explanation of drawings]

Figure 1 is a cross-sectional explanatory diagram showing an example of an oxygen sensor element;
FIG. 3 is a flowchart showing an example of an activation processing apparatus for an oxygen sensor element of the present invention, FIG. 3 is a club showing the activation processing results of an oxygen sensor element in Example 1, and FIG. FIG. 5 is a flowchart showing another example of the activation processing apparatus, and FIG. 5 is a graph showing the activation processing results of the oxygen sensor element in Example 2. lO...Sirconia porcelain as a solid electrolyte, 12...
Reference gas introduction hole, 14... Reference electrode, 16... Measurement electrode, l8... Porous protective layer, 20... Insulating layer, 2
2...Heater, 30...Catalytic conversion furnace, 31...
Activation treatment furnace, 32... constant temperature bath, 33... Hufflink container.

Claims (1)

[Claims] (1) An oxygen sensor element composed of at least an oxygen ion conductive solid electrolyte body, a measuring electrode, a reference electrode, and a porous protective layer that protects the measuring electrode is made of an unsaturated hydrocarbon and/or aromatic hydrocarbon. 1. A method for activating an oxygen sensor element, which comprises performing heat treatment in a reducing gas containing.