JP2002181772A - Method of adjusting characteristic of oxygen sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of adjusting the characteristics of an oxygen sensor by which the characteristic variation of the sensor resulting from the use of the sensor can be suppressed. SOLUTION: The element 11 of the oxygen sensor is provided with a detecting plate 15 composed of a solid electrolyte and an exhaust chamber 26 formed in the element 11. The detecting plate 15 is provided with an outer electrode 20 which comes into contact with an exhaust gas and in inner electrode 21 positioned in the chamber 26. The inner electrode 21 is reduced by applying a first DC voltage across the outer and inner electrodes 20 and 21 by respectively using the electrodes 20 and 21 as an anode and a cathode. Thereafter, the impedance of the element 11 (detecting plate 15) is increased by applying a second DC voltage which is opposite in direction to the first DC voltage and is lower than the first voltage across the electrodes 20 and 21. The second DC voltage is set to such a value that does not cause any oxygen defect in the vicinity of the outer electrode 20 of the detecting plate 15.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for adjusting the characteristics of an oxygen sensor used for air-fuel ratio feedback control of an internal combustion engine.

[0002]

2. Description of the Related Art As this type of oxygen sensor, there are known a so-called concentration cell type, a limiting current type, and an oxide semiconductor type.

FIG. 5 schematically shows the structure of a limiting current type oxygen sensor. FIG. 5
As shown in FIG. 1, the oxygen sensor includes an element 101 made of a solid electrolyte such as zirconia.
A space 102 is formed inside 1. Element 101
Is provided with an outer electrode 104 in contact with the exhaust and an inner electrode 103 located in the inner space 102 so as to sandwich a part of the same element 101. The element 10
A heater 105 for heating the element 101 and activating the element 101 is embedded in the inside of the element 1.

In such an oxygen sensor, after the element 101 is heated to its activation temperature by the heater 105,
A predetermined voltage is applied between the electrodes 103 and 104. Then, the oxygen concentration of the exhaust gas is detected based on the magnitude of the current value (limit current value) flowing between the electrodes 103 and 104 at that time.

[0005] By the way, such an oxygen sensor is an element 1
01 on the green sheet which is to
Is formed by applying a platinum paste or the like, and then firing the green sheet.
03, 104 is oxidized and its function as a catalyst electrode is reduced. Therefore, conventionally, the oxygen sensor is placed in a reducing gas atmosphere such as hydrogen gas or propane gas, and is heated again to a predetermined temperature to reduce the oxidized electrodes 103 and 104. Like that.

However, even if such a reduction treatment is performed, the inner electrode 103 located in the internal space 102 is hardly brought into contact with the reducing gas, so that the activation may not be sufficiently performed. Therefore, the following process for further activating the inner electrode 103 is performed. That is, in this process, the inner electrode 103 is set as a cathode, the outer electrode 104 is set as an anode, and
DC voltage is applied between 03 and 104. As a result, oxygen ions are separated from the inner electrode 103, and the oxygen ions flow to the outer electrode 104, so that the inner electrode 103
Is reduced electrically. Then, both of the inner electrode 103 and the outer electrode 104 are activated through these processes.

[0007]

Since the limit current value tends to change not only with the oxygen concentration of the exhaust gas but also with the temperature of the element 101, the heater 105 is required to accurately detect the oxygen concentration. , It is necessary to maintain the temperature of the element 101 at a predetermined activation temperature. For this reason, in such an oxygen sensor, the fact that the impedance of the element 101 has temperature dependency is used, and the heater current is controlled based on this impedance to maintain the temperature of the element 101 at a predetermined activation temperature. I am trying to do it. That is, the impedance (target impedance) of the element 101 when the element 101 is placed under the activation temperature is obtained in advance by an experiment or the like. Then, each of the electrodes 103, 1
The impedance (actual impedance) of the element 101 is detected based on the relationship between the voltage applied between the electrodes 04 and the current flowing between the electrodes 103 and 104, and the heater current is adjusted so that the actual impedance matches the target impedance. To control.

[0008] Such control of the heater current is performed on the premise that the impedance of the element 101 at a predetermined temperature is constant. However, as described above, in order to activate the inner electrode 103, both electrodes 103, 1
When a DC voltage is applied between the electrodes 104, oxygen ions in the element 101 move from the inner electrode 103 side to the outer electrode 104 side, and oxygen vacancies of the element 101 near the inner electrode 103 increase. , The impedance with respect to the temperature decreases. When the oxygen sensor is used in this state, the oxygen vacancies are gradually filled with oxygen ions as the oxygen sensor is used, and accordingly, the impedance of the element 101 at a predetermined temperature also gradually increases. .

As a result, even if the heater current is controlled on the basis of the impedance, the temperature of the element 101 cannot be maintained at the activation temperature, causing an unnecessary rise in the temperature. For this reason, the relationship between the oxygen concentration of the exhaust gas and the limit current value cannot be kept constant, the reliability of the oxygen concentration detected by the oxygen sensor decreases, and the element temperature increases due to an excessive rise in the element temperature. Damages.

As a method for adjusting the characteristics of such an oxygen sensor, for example, as disclosed in Japanese Patent Application Laid-Open No. Hei 6-265522, an alternating voltage is applied between both electrodes to activate these electrodes. Although there are some, there is no consideration of the change over time of the element impedance as described above, and there is still room for improvement in this respect.

The problem caused by such a change over time in the element impedance is not limited to the limiting current type oxygen sensor exemplified above, but is also common to a concentration cell type or an oxide semiconductor type oxygen sensor. It has become.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method for adjusting the characteristics of an oxygen sensor that can suppress a change over time in element impedance due to use. .

[0013]

The means for achieving the above object and the effects thereof will be described below. First, according to the first aspect of the present invention, an element for detecting oxygen concentration in which at least a part is made of a solid electrolyte and a space is formed therein, and the element is located on the side of the internal space in the solid electrolyte portion of the element. A method for adjusting characteristics of an oxygen sensor, comprising: an inner electrode provided on a surface; and an outer electrode provided on a surface of the solid electrolyte portion opposite to the inner electrode with the solid electrolyte portion interposed therebetween. A first step of applying a first DC voltage between the two electrodes, and a second step of increasing the impedance of the solid electrolyte portion after applying the first DC voltage. This is the gist.

In the first step, when a first DC voltage is applied between both electrodes, the electrode on the cathode side is reduced, while
Oxygen vacancies near the cathode of the solid electrolyte portion increase, and the impedance of the solid electrolyte portion decreases.
However, in the next second step, a process of previously increasing the impedance of the solid electrolyte portion which has been reduced as described above is performed. Therefore, even if the oxygen sensor is subsequently used, it is possible to suppress a temporal change in element impedance due to the use.

According to a second aspect of the present invention, in the method of adjusting a characteristic of the oxygen sensor according to the first aspect, the first DC voltage in the first step is such that the inner electrode is a cathode, and the outer electrode is a cathode. Is applied between the two electrodes as an anode.

According to the characteristic adjusting method, at least the inner electrode of the two electrodes can be activated. According to a third aspect of the present invention, there is provided the oxygen sensor characteristic adjusting method according to the first or second aspect, wherein the second DC voltage has a magnitude opposite to the first DC voltage and smaller than the first DC voltage. The gist is that the DC voltage is applied between the two electrodes in the second step.

According to the above-described characteristic adjusting method, between the two electrodes,
A second DC voltage opposite to the first DC voltage is applied. By applying a DC voltage in the opposite direction to the first DC voltage in this manner, oxygen ions are supplied to oxygen vacancies in the solid electrolyte portion near the electrodes that have increased when the first DC voltage is applied, The oxygen vacancies can be reduced. In addition, since the magnitude of the second DC voltage is set smaller than the first DC voltage, the fixed electrolyte in the vicinity of the electrode set as the cathode when the second DC voltage is applied is set. An increase in oxygen vacancies in the portion can be suppressed as much as possible. Therefore, the impedance of the solid electrolyte part can be suitably increased.

According to a fourth aspect of the present invention, in the method for adjusting characteristics of an oxygen sensor according to the third aspect, the second DC voltage in the second step is set to a cathode side of the two electrodes. The gist is that the size is set so as not to cause oxygen deficiency in the solid electrolyte portion located near the electrode.

According to the characteristic adjusting method, when the second DC voltage is applied between the two electrodes, the impedance of the solid electrolyte portion is increased without causing oxygen defects in the fixed electrolyte portion near the cathode. Will be able to do that.
Therefore, the impedance of the solid electrolyte part can be more suitably increased.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the method for adjusting the characteristics of an oxygen sensor according to the present invention will be described below.

First, the configuration of an oxygen sensor to which the characteristic adjusting method according to the present embodiment is applied will be described.
The oxygen sensor is mounted on the exhaust pipe such that its elements are located inside the exhaust pipe (both not shown) of the internal combustion engine.

As shown in FIG. 1, the element 11 includes a plate-shaped detection unit 12 and a plate-shaped heater 13 and has a structure in which these are stacked. Detector 12
Has an interposed plate 14 stacked on the heater 13 and a detection plate 15 stacked on the interposed plate 14. On the detection plate 15, a diffusion-controlling layer 16 and the diffusion-controlling layer 16
Dense layers 18 covering the upper surfaces of the layers are sequentially laminated. Furthermore,
A trap layer 19 is formed so as to cover the entire outer peripheral surfaces of the detection unit 12, the heater 13, the diffusion-controlling layer 16, and the dense layer 18. The detection plate 15 is formed of a solid electrolyte material, for example, oxygen ion conductive zirconia. The interposition plate 14 is formed of, for example, alumina (Al2O3).

On the other hand, the diffusion-controlling layer 16 is formed of a porous metal oxide (for example, alumina (Al 2 O 3)) having excellent corrosion resistance and clogging resistance to exhaust gas.
It has a plurality of pores (not shown). The exhaust gas in contact with the diffusion-controlling layer 16 can pass through the pores inside the diffusion-controlling layer 16 in the width direction (the left-right direction in FIG. 1).

The trap layer 19 has a function of trapping soot and the like contained in the exhaust gas and thereby preventing the soot and the like from adhering to the electrodes, the detection plate 15 and the like. For example, it is formed of alumina (Al2O3).

The interposition plate 14 is formed with a concave portion 24 extending from one surface to near the other surface in the longitudinal direction, and an inner portion surrounded by an inner peripheral surface of the concave portion 24 and a lower surface of the detection plate 15. The discharge chamber 26 is defined by the space. The discharge chamber 26 is communicated with the outside.

As shown in FIG. 2, an inner electrode 21 is provided on the lower surface of the detection plate 15 so as to be located in the discharge chamber 26, and an inner electrode 21 is provided on the upper surface of the detection plate 15.
The outer electrode 20 is provided so as to sandwich the same detection plate 15 between them. Each of these electrodes 20, 21 is formed of platinum (Pt), and is electrically connected to an output terminal (not shown) via a lead portion also made of platinum. A predetermined voltage (for example, 0.5 V) is applied between the electrodes 20 and 21 via these output terminals, leads and the like.

The heater 13 is formed of an insulator (for example, alumina (Al 2 O 3)).
A heating section 30 made of platinum is formed on the upper surface of the. Next, the operation of the oxygen sensor will be described.

In the oxygen sensor according to the present embodiment, as described above, the heater current is controlled such that the actual impedance matches the previously determined target impedance, and the element 11 is activated at an activation temperature (for example, 750). ℃)
The oxygen concentration of the exhaust is detected. This actual impedance is detected as follows. That is, first, the predetermined voltage applied between the electrodes 20 and 21 is boosted for a very short time (for example, 100 ms) by the voltage ΔV (for example, 0.1 V), and the change amount ΔI of the detection current accompanying the boosting is raised. Is detected. Then, the actual impedance is calculated based on the relationship between the boosted voltage ΔV and the amount of change ΔI of the detected current.

The oxygen concentration of the exhaust gas is detected by the oxygen sensor activated through the control of the heater current as described below. The exhaust gas that has passed through the trap layer 19 is
Further, it passes through the pores of the diffusion-controlling layer 16 and reaches the outer electrode 20 in a diffusion-controlled state. And oxygen contained in the exhaust is ionized by the catalytic ability of platinum,
The oxygen ions move from the outer electrode 20 through the inside of the detection plate 15 to the inner electrode 21 so that each electrode 2
A current (limit current) flows between 0 and 21. The magnitude of this limit current (the amount of movement of oxygen ions per unit time) changes according to the oxygen concentration of the exhaust gas, since the applied voltage between the electrodes 20 and 21 is constant at the predetermined voltage. Therefore, the oxygen concentration of the exhaust gas can be detected based on the magnitude of the limit current value. The oxygen ions that have passed through the inside of the detection plate 15 are introduced into the discharge chamber 26,
It is discharged from the same room 26 to the outside.

Next, the procedure for manufacturing the element 11 will be described. First, a platinum paste is applied to both surfaces of a plate-like material (green sheet) serving as the detection plate 15 to form a pattern serving as each of the electrodes 20 and 21. Thereafter, a plurality of plate-like materials constituting the detection plate 15, the interposition plate 14, and the heater 13 are sequentially laminated and pressed, and these materials are fired to be integrally formed. Thereafter, a diffusion-controlling layer 16 and a dense layer 18 are sequentially formed on the upper surface of the detection plate 15.
Thereafter, a trap layer 19 is formed so as to cover the entire outer peripheral surface of each of these materials.

The characteristics of the element 11 thus manufactured are then adjusted through the following steps [1] to [3] (see FIG. 3). [1] First, the element 11 is replaced with hydrogen gas, propane gas, or the like.
Place in a reducing gas atmosphere and heat to very high temperatures. In this step, the reducing gas and the oxygen in both electrodes 20 and 21 are combined and burned, so that the oxidizing two electrodes 2
0, 21 will be reduced.

[2] Next, at a high temperature (for example, 750 ° C.), a first DC voltage (for example,
1.8V). As a result, the oxygen bonded to the inner electrode 21 is ionized, and the oxygen ions move from the inner electrode 21 so that the inner electrode 21 is electrically reduced. At this time, oxygen in the vicinity of the inner electrode 21 on the detection plate 15 also becomes oxygen ions and moves toward the outer electrode 22.
Oxygen vacancies are generated in the vicinity. The generation of oxygen vacancies in this manner causes the element 11 (detection plate 1
The impedance of 5) temporarily decreases.

[3] Next, at a high temperature (for example, 750 ° C.), the magnitude of the first DC voltage is opposite to that of the first DC voltage in the above step [2] between both electrodes 20 and 21. A second DC voltage smaller than the voltage is applied. Note that the magnitude of the second DC voltage is a magnitude that does not cause oxygen defects near the cathode side electrode (outer electrode 20) (for example,
0.5V). In this step, oxygen in the outside air is ionized by the catalytic ability of the outer electrode 20, and the oxygen ions move toward the inner electrode 21. Therefore, oxygen ions are supplied to the increased oxygen vacancies near the inner electrode 21 in the element 11, and the oxygen vacancies are reduced. As a result, the impedance of the element 11 increases as the oxygen vacancy decreases.

The solid line in FIG. 4 shows the change in the element impedance of the oxygen sensor subjected to the above-described characteristic adjustment processing due to its use. The dashed line in FIG. 4 shows, as a comparative example, a change in the element impedance of the oxygen sensor when step [3] in the characteristic adjustment processing is omitted.

As shown in FIG. 4, in the oxygen sensor which has been subjected to the characteristic adjustment processing, the element impedance is increased in advance through the step [3] prior to its use. (Rt-R1) can be suppressed to be smaller than the variation (Rt-R2) of the comparative example. And, by suppressing the temporal change of the element impedance in this way, unnecessary rise of the element temperature during the heater current control is suppressed, and the detection accuracy is lowered due to the excessive rise of the element temperature, and The occurrence of damage to the element 11 is suppressed.

As described above, according to the present embodiment, the following effects can be obtained. (1) The temporal change of the element impedance due to the use of the oxygen sensor can be suppressed, and the accompanying reduction in detection accuracy and the occurrence of damage to the element 11 can be suppressed.

(2) Since the magnitude of the second DC voltage is set smaller than the magnitude of the first DC voltage, the element near the outer electrode 20 which is set to the cathode when the second DC voltage is applied 11 can be suppressed as much as possible. Therefore, the impedance of the element 11 can be suitably increased.

(3) The magnitude of the second DC voltage is set so as not to cause oxygen deficiency in the element plate 15 near the outer electrode 20 set on the cathode side. The impedance of the element 11 can be increased without causing oxygen vacancies.

The above embodiment may be modified and implemented as follows. In the above embodiment, the reduction process of the inner electrode 21 is performed through the above steps [2] and [3], and the reduced device impedance is increased. However, the reduction of the outer electrode 20 is similarly performed. Processing can be performed. In this case, in step [2], the inner electrode 21 is set as an anode and the outer electrode 20 is set as a cathode. In step [3], conversely, the outer electrode 20 is set as an anode and the inner electrode 21 is set as a cathode. Make settings for each cathode.

In the above embodiment, the magnitude of the first DC voltage applied to reduce the electrodes is set to 1.8 V, but the invention is not limited to this. However, when the applied voltage is lower than 1.5 V, the reduction of the electrode tends to be insufficient, and when the applied voltage is higher than 2.0 V, the detection plate 15
May change its characteristics as a solid electrolyte. Therefore, the magnitude of the first DC voltage is 1.5 V to 2.0 V
It is desirable to set in the range.

In the above embodiment, the magnitude of the second DC voltage applied to increase the impedance of the detection plate 15 is set to 0.5 V. However, the present invention is not limited to this. By setting the second DC voltage to a voltage lower than the first DC voltage, it is possible to suppress an increase in oxygen defects at least in the vicinity of the cathode of the detection plate 15. However, when the magnitude of the second DC voltage is set to be smaller than 0.1 V, the flow of oxygen ions in the detection plate 15 hardly occurs. Therefore, the magnitude of the second DC voltage is 0.1
It is desirable to set in the range of V to 0.5V.

In the above embodiment, in order to adjust the characteristics of the element 11 manufactured through the baking process, the element 11 is heated in an atmosphere of a reducing gas (step [1]), A DC voltage was applied between the electrodes 20 and 21 (steps [2] and [3]). In addition to these steps, an inert gas may be supplied to the surfaces of the electrodes 20 and 21 when the element 11 is fired. According to such a characteristic adjusting method, the oxidation of both electrodes 20 and 21 can be suppressed as much as possible, and as a result, the above-mentioned steps [2] and [3]
Processing time can be shortened.

In the above embodiment, as the process for increasing the impedance (step [3]), the two electrodes 20 and 2 are used.
Although a DC voltage is applied between the two, the element 11 may be immersed in a hydroxide aqueous solution containing oxygen ions instead. By such a characteristic adjustment method,
The oxygen deficiency increased in the step [2] is reduced by oxygen ions in the aqueous hydroxide solution,
The impedance of the element 11 can be suitably increased.

Further, an element other than oxygen ions may be supplied to oxygen vacancies by any means to fill the oxygen vacancies and substantially reduce the oxygen vacancies. In the above embodiment, the characteristic adjustment method according to the present invention is applied to the limiting current type oxygen sensor, but may be applied to the concentration cell type oxygen sensor.

Also, the method for adjusting characteristics according to the present invention comprises:
The present invention may be applied to an oxide semiconductor type oxygen sensor. In an oxide semiconductor type oxygen sensor, the element impedance itself changes according to the oxygen concentration of the exhaust gas, and the oxygen concentration of the exhaust gas is detected based on the change in the element impedance. Therefore, by applying the characteristic adjustment method according to the present invention to such an oxide semiconductor type oxygen sensor,
It is possible to suppress a change with time of the element impedance due to its use, and it is possible to suppress a decrease in the correlation between the element impedance and the oxygen concentration due to the change with time.

Although the embodiments of the present invention have been described above, it should be added that the embodiments of the present invention include the following various embodiments. An element for oxygen concentration detection in which at least a part is formed of a solid electrolyte and a space is formed therein, and an inner electrode disposed on a surface located on the inner space side in the solid electrolyte portion of the element; An outer electrode provided on a surface of the solid electrolyte portion opposite to the inner electrode with the solid electrolyte portion interposed therebetween, wherein the solid electrolyte portion, the inner electrode, and the outer electrode are integrally formed by firing. A method for adjusting the characteristics of an oxygen sensor, the method comprising adjusting at least a step of supplying an inert gas to at least a surface of the inner electrode among the inner electrode and the outer electrode during the baking treatment. Method.

[Brief description of the drawings]

FIG. 1 is a perspective view showing an element structure of an oxygen sensor to which a characteristic adjustment method according to the present invention is applied.

FIG. 2 is a sectional view showing a sectional structure of the oxygen sensor.

FIG. 3 is a process diagram showing one embodiment of a method for adjusting the characteristics of an oxygen sensor according to the present invention.

FIG. 4 is a graph showing a relationship between element impedance and duration of use of an oxygen sensor.

FIG. 5 is a sectional view showing a sectional structure of the oxygen sensor.

[Explanation of symbols]

11 element, 12 detection unit, 13 heater, 14 interposition plate, 15 detection plate, 16 diffusion-controlling layer, 18 dense layer,
19: trap layer, 20: outer electrode, 21: inner electrode,
24: recess, 26: discharge chamber, 30: heating section.

Claims (4)

[Claims] 1. An element for detecting oxygen concentration in which at least a part is made of a solid electrolyte and a space is formed therein, and is disposed on a surface of the element which is located on the side of the internal space in the solid electrolyte portion. A method for adjusting characteristics of an oxygen sensor, comprising: an inner electrode; and an outer electrode disposed on a surface of the solid electrolyte portion opposite to the inner electrode with the solid electrolyte portion interposed therebetween. A first step of applying a first direct-current voltage to the first electrode; and a second step of increasing the impedance of the solid electrolyte portion after applying the first direct-current voltage. Characteristics adjustment method. 2. The method according to claim 1, wherein, in the first step, the first DC voltage is applied between the two electrodes using the inner electrode as a cathode and the outer electrode as an anode. . 3. In the second step, a second DC voltage having a direction opposite to the first DC voltage and smaller in magnitude than the first DC voltage is applied between the two electrodes. Item 3. The method for adjusting characteristics of an oxygen sensor according to Item 1 or 2. 4. In the second step, the second DC voltage is set to a value that does not cause oxygen deficiency in the solid electrolyte portion located in the vicinity of the electrode set on the cathode side of the two electrodes. The method for adjusting characteristics of an oxygen sensor according to claim 3, wherein