SENSOR ELEMENT

A sensor element includes: a sensor electrode disposed on one main surface of a solid electrolyte layer to face a measurement gas introduction space, and having an oxygen decomposition ability and a NOx decomposition ability; a monitor electrode disposed on the one main surface of the solid electrolyte layer to face the measurement gas introduction space, and having the oxygen decomposition ability; and a reference electrode disposed on the other main surface of the solid electrolyte layer to face a reference gas introduction space, wherein a heater element of a heater part overlaps 50% or more of an area of each of the sensor electrode and the monitor electrode in plan view.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority from Japanese application JP 2019-155840, filed on Aug. 28, 2019, the contents of which is hereby incorporated by reference into this application.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a gas sensor for determining concentration of nitrogen oxides (NOx), and, in particular, to arrangement of electrodes in a sensor element thereof.

Description of the Background Art

As a gas sensor (NOx sensor) including a sensor element containing an oxygen-ion conductive solid electrolyte as a main component, a gas sensor that adjusts an oxygen concentration of a measurement gas using a pump cell, and then measures concentration of NOx in the measurement gas based on a value of a difference between a current flowing through a monitor cell for pumping only oxygen and a current flowing through a sensor cell for pumping oxygen and NOx has already been known (see Japanese Patent Application Laid-Open No. 2016-20894, for example).

In a case where a gas sensor as described above is installed on an exhaust path of a vehicle for use, it is desirable that the gas sensor can be used stably for a long time. On the other hand, the gas sensor is used in a state of a sensor element being heated to a high temperature for activation of a solid electrolyte, so that electrodes of the sensor element are deteriorated upon long-term exposure to the high temperature to affect a pumping ability of each cell. Thus, in a case of a gas sensor for determining the NOx concentration based on the value of the difference between a current value of the monitor cell and a current value of the sensor cell as disclosed in Japanese Patent Application Laid-Open No. 2016-20894, it is desirable that deterioration behaviors of electrodes of both cells be the same in terms of maintaining NOx sensing accuracy.

SUMMARY

The present invention relates to a gas sensor for determining concentration of nitrogen oxides (NOx), and is, in particular, directed to arrangement of electrodes of a sensor element thereof.

According to the present invention, a sensor element for a gas sensor measuring concentration of NOx in a measurement gas includes: an oxygen-ion conductive solid electrolyte layer; a measurement gas introduction space into which the measurement gas is introduced; a reference gas introduction space into which a reference gas is introduced; a heater part to heat the sensor element; a sensor electrode disposed on one main surface of the solid electrolyte layer to face the measurement gas introduction space, and having an oxygen decomposition ability and a NOx decomposition ability; a monitor electrode disposed on the one main surface of the solid electrolyte layer to face the measurement gas introduction space, and having the oxygen decomposition ability; and a reference electrode disposed on the other main surface of the solid electrolyte layer to face the reference gas introduction space, wherein the sensor electrode, the reference electrode, and the solid electrolyte layer constitute a sensor cell as an electrochemical pump cell, the monitor electrode, the reference electrode, and the solid electrolyte layer constitute a monitor cell as an electrochemical pump cell, and, in plan view from a side of the one main surface, a heater element of the heater part overlaps 50% or more of an area of each of the sensor electrode and the monitor electrode.

Deterioration of the sensor element is thus limited to a degree allowable in actual use even in a case where the gas sensor is in continuous use.

It is thus an object of the present invention to provide a sensor element for a gas sensor enabling suppression of deterioration and securement of NOx measurement accuracy even when being in continuous use for a long time.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

<General Configuration of Gas Sensor>

A general configuration of a gas sensor100including a sensor element101according to the present embodiment will be described first. In the present embodiment, the gas sensor100is a NOx sensor for sensing NOx and measuring concentration thereof using the sensor element101.FIG. 1is a vertical sectional view along a longitudinal direction in the vicinity of a leading end surface E1of the sensor element101.FIG. 2is a sectional view perpendicular to the longitudinal direction in the vicinity of the leading end surface E1of the sensor element101. A surface opposite the leading end surface E1along the longitudinal direction of the sensor element101(hereinafter, an element longitudinal direction) is referred to as a proximal end surface E2.

As illustrated inFIGS. 1 and 2, the sensor element101generally has a configuration in which an insulating spacer layer31and an insulating layer40are stacked on a side of one main surface11of a solid electrolyte layer10, and an insulating spacer layer32and a heater part60are stacked on a side of the other main surface12of the solid electrolyte layer10. The spacer layers31and32and the insulating layer40are made of alumina, for example.

The solid electrolyte layer10is a layer made of oxygen-ion conductive ceramic, such as zirconia (yttria stabilized zirconia).

In the vicinity of the leading end surface E1of the sensor element101, a space in which the spacer layer31is not disposed is present between the one main surface11of the solid electrolyte layer10and the insulating layer40. The space is referred to as a measurement gas introduction space51. A gas inlet511is provided in the spacer layer31to face the leading end surface E1, and a diffusion control part512formed of a porous body is buried between the gas inlet511and the measurement gas introduction space51. The diffusion control part512provides predetermined diffusion resistance to a measurement gas introduced through the gas inlet511into the measurement gas introduction space51.

On the one main surface11of the solid electrolyte layer10, an oxygen pump electrode21, a sensor electrode22, and a monitor electrode23are disposed to face the measurement gas introduction space51. As illustrated inFIG. 1, the oxygen pump electrode21is disposed at a location closer to the leading end surface E1than the sensor electrode22and the monitor electrode23are in the measurement gas introduction space51. The measurement gas introduced into the measurement gas introduction space51thereby reaches the sensor electrode22and the monitor electrode23after reaching the oxygen pump electrode21.

On the other hand, the sensor electrode22and the monitor electrode23are arranged parallel with respect to the element longitudinal direction in a case illustrated inFIGS. 1 and 2, but this is just an example. The sensor electrode22and the monitor electrode23may be arranged in series.

The oxygen pump electrode21is made of a cermet of a Pt—Au alloy and zirconia, and has an oxygen decomposition ability. The Pt—Au alloy preferably has an Au content of 20 wt % or less.

The sensor electrode22is made of a cermet of a Pt—Rh alloy and zirconia, and has the oxygen decomposition ability and a NOx decomposition ability. The Pt—Rh alloy preferably has an Rh content of 80 wt % or less.

Furthermore, the monitor electrode23and a reference electrode24are made of a cermet of Pt and zirconia, and have the oxygen decomposition ability.

On the other hand, in a predetermined range from the proximal end surface E2to the vicinity of the leading end surface E1of the sensor element101, a space in which the spacer layer32is not disposed is present between the other main surface12of the solid electrolyte layer10and the heater part60. The space is referred to as a reference gas introduction space52. Atmospheric air as a reference gas is introduced into the reference gas introduction space52from a side of the proximal end surface E2. The measurement gas introduction space51and the reference gas introduction space52are isolated from each other to prevent the measurement gas introduced into the former from being introduced into the latter.

On the other main surface12of the solid electrolyte layer10, the reference electrode24is disposed to face the reference gas introduction space52. That is to say, the reference electrode24is disposed to always be in contact with the atmospheric air as the reference gas.

Furthermore, in the gas sensor100, an oxygen pump cell71, a sensor cell72, and a monitor cell73are constituted.

The oxygen pump cell71is an electrochemical pump cell constituted by the oxygen pump electrode21, the reference electrode24, and the solid electrolyte layer10. In the oxygen pump cell71, a voltage is applied between the oxygen pump electrode21and the reference electrode24by a variable power supply81disposed external to the sensor element101. Oxygen in the measurement gas is decomposed by application of the voltage to allow an oxygen-ion current (oxygen pump current I0) to flow through the solid electrolyte layer10. In the oxygen pump cell71, the voltage applied by the variable power supply81is feedback controlled so that the oxygen pump current I0has a magnitude having a value responsive to a desired value of an oxygen concentration in the measurement gas introduction space51.

The sensor cell72is an electrochemical pump cell constituted by the sensor electrode22, the reference electrode24, and the solid electrolyte layer10. In the sensor cell72, a constant voltage is applied between the sensor electrode22and the reference electrode24by a power supply82disposed external to the sensor element101. In the sensor cell72, NOx in the measurement gas adjusted by the oxygen pump cell71to have the oxygen concentration of the desired value and a tiny amount of oxygen still remaining in the measurement gas adjusted above are decomposed by application of the voltage to allow an oxygen-ion current I1to flow through the solid electrolyte layer10.

On the other hand, the monitor cell73is an electrochemical pump cell constituted by the monitor electrode23, the reference electrode24, and the solid electrolyte layer10. In the monitor cell73, a constant voltage is applied between the monitor electrode23and the reference electrode24by a power supply83disposed external to the sensor element101. In the monitor cell73, a tiny amount of oxygen still remaining in the measurement gas adjusted by the oxygen pump cell71to have the oxygen concentration of the desired value is decomposed by application of the voltage to allow an oxygen-ion current I2to flow through the solid electrolyte layer10.

In the gas sensor100, a NOx concentration is determined based on the correlation between the concentration of NOx in the measurement gas and a value of a difference between the oxygen-ion current I1flowing through the sensor cell72and the oxygen-ion current I2flowing through the monitor cell73. The value of the difference is hereinafter also referred to as a NOx corresponding current value.

Arrangement of the oxygen pump electrode21, the sensor electrode22, the monitor electrode23, and the reference electrode24illustrated inFIGS. 1 and 2is just an example. Arrangement is not limited to this arrangement, and various arrangements can be used. In particular, as for the sensor electrode22, the monitor electrode23, and the reference electrode24, various arrangements can be used as long as requirements described below are met.

The heater part60has a configuration in which a heater element62and a pair of heater leads63are sandwiched between a pair of insulating ceramic layers61(611,612) stacked on a side of the other main surface12of the solid electrolyte layer10. The heater element62and the pair of heater leads63are disposed symmetrically along an element width direction (a left-right direction inFIG. 2).

The heater element62is a resistance heating body disposed in a predetermined range in the vicinity of the leading end surface E1of the sensor element101. Opposite ends of the heater element62are connected to the pair of heater leads63as a current-carrying path disposed along the element longitudinal direction. The heater element62generates heat by being powered by a heater power supply, which is not illustrated, disposed external to the sensor element101through the heater leads63.

The sensor element101is heated to a predetermined temperature (an element driving temperature) of 600° C. to 950° C. through heat generation of the heater element62for activation of the solid electrolyte layer10, when being in use. The sensor element101is not necessarily required to be uniformly heated, and may be heated to have a temperature varied with location.

The heater element62is disposed symmetrically with respect to the element longitudinal direction and meanderingly to go and return along the element longitudinal direction between a portion connected to one of the heater leads63and a portion connected to the other one of the heater leads63. In other words, the heater element62is disposed to have at least two turns on a side of the leading end surface E1and at least one turn on the side of the proximal end surface E2. More particularly, in a case where there are n turns on the side of the proximal end surface E2, there are n+1 turns on the side of the leading end surface E1.

Each of the turns of the heater element62on the side of the leading end surface E1and the side of the proximal end surface E2may be arcuate or rectangular.

<Arrangement of Electrodes and Heater>

In the gas sensor100according to the present embodiment, the measurement gas having a temperature of approximately several hundred degrees Celsius is introduced into the measurement gas introduction space51provided in the sensor element101, which is heated to the predetermined element driving temperature. And then, as described above, the concentration of NOx in the measurement gas is determined based on the NOx corresponding current value, which is the value of the difference between the oxygen-ion current I1flowing through the sensor cell72and the oxygen-ion current I2flowing through the monitor cell73. In terms of securing measurement accuracy (NOx sensitivity) even in a case where the gas sensor100is in continuous use for a long time, it is preferable that the sensor electrode22of the sensor cell72and the monitor electrode23of the monitor cell73be heated on similar temperature conditions when the gas sensor100is in use to reduce a variation in thermoelectromotive force between them and an inter-cell IR drop.

Heating the sensor electrode22and the monitor electrode23in the same manner means that deterioration behaviors of the sensor electrode22and the monitor electrode23when the gas sensor100is in continuous use become approximately equal to each other. If the deterioration behaviors of both electrodes are equal to each other, measurement accuracy is likely to be kept over a relatively long time.

In the gas sensor100according to the present embodiment, in view of these points, planar arrangement of the sensor electrode22, the monitor electrode23, and the heater element62of the sensor element101at least meets a requirement (a) below, and further planar arrangement of them and the reference electrode24preferably meets at least one of requirements (b) to (e) below, so that deterioration thereof is limited to a degree allowable in use even in a case where the gas sensor100is in continuous use:

(a) The heater element62overlaps 50% or more of the area of each of the sensor electrode22and the monitor electrode23;

(b) The heater element62overlaps 80% or more of the area of each of the sensor electrode22and the monitor electrode23;

(c) Each of the sensor electrode22and the monitor electrode23has a region to overlap both of the heater element62and the reference electrode24, and the reference electrode24overlaps 50% or more of the area of each of the sensor electrode22and the monitor electrode23;

(d) The sensor electrode22and the monitor electrode23are disposed at locations closer to the proximal end surface E2in a range of disposition of the heater element62along the element longitudinal direction; and

(e) The sensor electrode22and the monitor electrode23are disposed parallel with respect to the element longitudinal direction.

There can be various cases and variations of specific arrangement of the sensor electrode22, the monitor electrode23, and the heater element62, and further the reference electrode24meeting the requirement (a) and further the requirements (b) to (e).

EXAMPLES

Eight types of gas sensors100having different specific arrangements of the sensor electrode22, the monitor electrode23, the reference electrode24, and the heater element62were manufactured.

More particularly, as examples, six types of gas sensors100(Examples 1 to 6) at least meeting the requirement (a) were manufactured. On the other hand, as comparative examples, two types of gas sensors100(Comparative Examples 1 and 2) failing to meet any of the requirements (a) to (e) were manufactured.

FIGS. 3 to 10respectively illustrate planar arrangements of main components in the vicinity of the leading end surfaces E1of sensor elements101of the gas sensors100of Examples 1 to 6 and Comparative Examples 1 and 2. More particularly,FIGS. 3 to 10each illustrate arrangement in the vicinity of the leading end surface E1of the sensor element101in plan view from the side of the one main surface11of the solid electrolyte layer10.

Features of arrangements of the main components of the gas sensors100of Examples 1 to 6 and Comparative Examples 1 and 2 are shown in Table 1 as a list. Percentages (of the area) of overlaps of the heater element62and the reference electrode24with the sensor electrode22and the monitor electrode23, results of determination (Determination 1) based on a NOx sensitivity change rate, and results of determination (Determination 2) based on a difference in thermoelectromotive force between the sensor cell72and the monitor cell73described below are shown in Table 2 as a list.

In the sensor element101of Example 1, as illustrated inFIG. 3, the heater element62has two first turns62t1on the side of the leading end surface E1, one second turn62t2on the side of the proximal end surface E2, two first linear portions62s1extending along the element longitudinal direction between the first turns62t1and respective tapered ends of the pair of heater leads63, and two second linear portions62s2extending along the element longitudinal direction between the respective first turns62t1and the second turn62t2.

The first turns62t1and the second turn62t2are each arcuate. The first linear portions62s1are disposed outward along the element width direction, and the second linear portions62s2are disposed inward along the element width direction. All the linear portions are arranged at regular intervals along the element width direction. The measurement gas introduction space51is provided, along the element longitudinal direction, in a range from a location closer to the leading end surface E1than the first turns62t1are to the second turn62t2, and, along the element width direction, in a range sandwiched between the two first linear portions62s1.

In the measurement gas introduction space51, in plan view from the side of the one main surface11of the solid electrolyte layer10, the sensor electrode22and the monitor electrode23are disposed parallel in shapes that a longitudinal direction of each of the electrodes matches the element longitudinal direction, at locations which are intermediate in a presence range of the heater element62along the element longitudinal direction and where those electrodes overlap the respective second linear portions62s2in plan view. That is to say, the sensor element101of Example 1 meets the requirement (e). The area of the overlap is 50% of the area of each of the sensor electrode22and the monitor electrode23. That is to say, the sensor element101of Example 1 meets the requirement (a).

The oxygen pump electrode21is disposed closer to the leading end surface E1than the sensor electrode22and the monitor electrode23in the measurement gas introduction space51.

On the other hand, the reference electrode24is disposed to be rectangular in plan view at a location closer to the proximal end surface E2than the second turn62t2is. That is to say, the reference electrode24does not overlap the sensor electrode22and the monitor electrode23.

As described above, the sensor element101of Example 1 meets the requirements (a) and (e).

As illustrated inFIG. 4, the sensor element101of Example 2 has a similar configuration to that of Example 1 except that the sensor electrode22and the monitor electrode23are disposed closer to the proximal end surface E2than those of Example 1 are to thereby be separated from the oxygen pump electrode21along the element longitudinal direction compared with those of Example 1. That is to say, the sensor element101of Example 2 meets the requirements (a), (d), and (e).

However, the area of the overlap of the sensor electrode22and the monitor electrode23with the heater element62is 80% of the area of each of the sensor electrode22and the monitor electrode23. The sensor element101of Example 2 further meets the requirement (b).

As described above, the sensor element101of Example 2 meets the requirements (a), (b), (d), and (e).

As illustrated inFIG. 5, as for the heater part60, the sensor element101of Example 3 is the same as that of Example 1 in number and arrangement of the first turns62t1, the second turn62t2, the first linear portions62s1, and the second linear portions62s2of the heater element62, but is different from that of Example 1 in that the ends of the heater leads63connected to the respective first linear portions62s1are rectangular, the first turns62t1and the second turn62t2are rectangular, and the distance between the second linear portions62s2is shorter than the distance between the first linear portions62s1and the second linear portions62s2.

The sensor electrode22and the monitor electrode23are disposed parallel as with those of Example 1 at locations closer to the proximal end surface E2along the element longitudinal direction as with those of Example 2. That is to say, the sensor element101of Example 3 meets the requirements (d) and (e). However, the area of the overlap of the sensor electrode22and the monitor electrode23with the heater element62is only 50% of the area of each of the sensor electrode22and the monitor electrode23. That is to say, the sensor element101of Example 3 meets the requirement (a). The oxygen pump electrode21extends to the side of the proximal end surface E2compared with that of Example 2, and thus, a gap from the oxygen pump electrode21to the sensor electrode22and the monitor electrode23is approximately the same as that of Example 1. The reference electrode24is disposed, along the element longitudinal direction, in a range from the first turns62t1to the second turn62t2, and, along the element width direction, in a range in which ends of the reference electrode24just overlap the pair of the second linear portions62s2as a whole. An end of the reference electrode24on the side of the proximal end surface E2is arcuate.

The reference electrode24is thereby disposed to overlap the sensor electrode22and the monitor electrode23in plan view. The area of the overlap of the sensor electrode22and the monitor electrode23with the reference electrode24is 50% of the area of each of the sensor electrode22and the monitor electrode23. That is to say, the sensor element101of Example 3 meets the requirement (c).

As described above, the sensor element101of Example 3 meets the requirements (a), (c), (d), and (e).

As illustrated inFIG. 6, the sensor element101of Example 4 has a similar configuration to that of the sensor element101of Example 3 except that the size of each of the sensor electrode22and the monitor electrode23along the element width direction is reduced. More specifically, the sensor electrode22and the monitor electrode23are disposed so that the area of the overlap with the heater element62and the area of the overlap with the reference electrode24are each 80% of the area of each of the sensor electrode22and the monitor electrode23.

Accordingly, the sensor element101of Example 4 thus meets the requirements (a) to (e).

In the sensor element101of Example 5, as illustrated inFIG. 7, the heater element62has three first turns62t1on the side of the leading end surface E1, two second turns62t2on the side of the proximal end surface E2, two first linear portions62saextending along the element longitudinal direction between first turns62t1disposed outward along the element width direction and respective tapered ends of the pair of heater leads63, two second linear portions62sbextending along the element longitudinal direction between the first turns62t1disposed outward along the element width direction and the second turns62t2, and two third linear portions62scextending along the element longitudinal direction between a first turn62t1disposed inward along the element width direction and the second turns62t2. All the linear portions are arranged at regular intervals along the element width direction.

The first turns62t1and the second turns62t2are each arcuate.

The measurement gas introduction space51is provided, along the element longitudinal direction, in a range from a location closer to the leading end surface E1than the first turns62t1to a location closer to the proximal end surface E2than the second turns62t2are, and, along the element width direction, in a range sandwiched between the two first linear portions62sa.

In the measurement gas introduction space51, the sensor electrode22and the monitor electrode23are disposed parallel in shapes that the element width direction matches the longitudinal direction of each of the electrodes, at locations where the sensor electrode22and the monitor electrode23overlap the respective second turns62t2in plan view. That is to say, the sensor element101of Example 5 meets the requirements (d) and (e). The area of the overlap is 80% of the area of each of the sensor electrode22and the monitor electrode23. That is to say, the sensor element101of Example 5 meets the requirements (a) and (b).

On the other hand, the reference electrode24is disposed, along the element longitudinal direction, in a range from the first turns62t1to a location closer to the proximal end surface E2than the second turns62t2are, and, along the element width direction, in a range in which the ends of the reference electrode24just overlap the pair of the second linear portions62sbas a whole. The reference electrode24is thereby disposed to overlap the sensor electrode22and the monitor electrode23in plan view. The area of the overlap of the sensor electrode22and the monitor electrode23with the reference electrode24is 95% of the area of each of the sensor electrode22and the monitor electrode23. That is to say, the sensor element101of Example 5 meets the requirement (c). The end of the reference electrode24on the side of the proximal end surface E2is arcuate.

Accordingly, the sensor element101of Example 5 thus meets all the requirements (a) to (e).

As illustrated inFIG. 8, the sensor element101of Example 6 has a similar configuration to that of the sensor element101of Example 1 except that the sensor electrode22and the monitor electrode23are disposed in series along the element longitudinal direction at intermediate locations in a presence range of the heater element62along the element longitudinal direction above one of the second linear portions62s2. More specifically, the sensor electrode22and the monitor electrode23are disposed so that the area of the overlap with the heater element62is 50% of the area of each of the sensor electrode22and the monitor electrode23.

Accordingly, the sensor element101of Example 6 thus meets the requirement (a).

Comparative Example 1

In the sensor element101of Comparative Example 1, as illustrated inFIG. 9, arrangement of the heater element62and the heater leads63, the measurement gas introduction space51, and the oxygen pump electrode21is similar to that of Example 1, but the sensor electrode22is disposed between one of the first linear portions62s1and one of the second linear portions62s2, and the monitor electrode23is disposed between the other one of the first linear portions62s1and the other one of the second linear portions62s2. Furthermore, the electrodes are disposed at different locations along the element longitudinal direction so that the monitor electrode23is disposed closer to the leading end surface E1than the sensor electrode22is. The reference electrode24is disposed to be rectangular at a location between the two second linear portions62s2.

Accordingly, the sensor element101of Comparative Example 1 thus fails to meet any of the requirements (a) to (e).

Comparative Example 2

As illustrated inFIG. 10, the sensor element101of Comparative Example 2 has a similar configuration to that of the sensor element101of Comparative Example 1 except that the sensor electrode22and the monitor electrode23are disposed to overlap the second linear portions62s2of the heater element62. More specifically, the area of the overlap of the sensor electrode22and the monitor electrode23with the second linear portions62s2is 30% of the area of each of the sensor electrode22and the monitor electrode23.

Accordingly, the sensor element101of Comparative Example 2 thus fails to meet any of the requirements (a) to (e).

An accelerated durability test was conducted on the sensor elements101of Examples 1 to 6 and Comparative Examples 1 and 2 having configurations as described above, and NOx sensitivity change rates before and after the test were evaluated. The accelerated durability test is intended as a test to evaluate the degree of deterioration over time.

The accelerated durability test was conducted under conditions below: Each of the gas sensors100was installed onto an exhaust pipe of an engine, and a 40-minute driving pattern configured to have an engine speed in a range of 1500 rpm to 3500 rpm and a load torque in a range of 0 N·m to 350 N·m was repeated until 1000 hours had elapsed. In this case, the element driving temperature was set to 800° C., temperature of the gas was maintained within a range of 200° C. to 600° C., and the NOx concentration was kept a value within a range of 0 ppm to 1500 ppm.

(Determination of NOx Sensitivity Change)

The NOx corresponding current value was determined through NOx measurement using a model gas having a NOx concentration of 500 ppm and an oxygen concentration of 0%, and containing nitrogen as the balance before the start, at 500 hours after the start, and at the end (at 1000 hours after the start) of the accelerated durability test.

Then, change rates of NOx sensitivity (NOx sensitivity change rate) in the respective timings were calculated from the respective NOx corresponding current values determined by measurement using, as a reference (an initial value), the value before the start of the test, and, based on the calculated value, the degree of a change in NOx sensitivity of each of the gas sensors100was determined (Determination 1).

In Determination 1, in a case where (the absolute value of) the NOx sensitivity change rate is 10% or less, it is determined that the change in NOx sensitivity is suitably suppressed, and a circle is marked in Table 2.

In a case where (the absolute value of) the NOx sensitivity change rate is more than 10% and 20% or less, it is determined that the change in NOx sensitivity is suppressed within a range allowable in actual use of the gas sensor100, and a triangle is marked in Table 2.

On the other hand, as for each of the gas sensors100having a NOx sensitivity change rate of more than 20% and thus not corresponding to any of the above-mentioned cases, a cross is marked in Table 2.

(Determination of Difference in Thermoelectromotive Force)

As for each of the sensor elements101of Examples 1 to 6 and Comparative Examples 1 and 2 after the accelerated durability test, a thermoelectromotive force in each of the sensor cell72and the monitor cell73was measured in an ambient atmosphere. The element driving temperature was set to 800° C. A value of a difference in thermoelectromotive force (thermoelectromotive force difference) between them was determined, and, based on the value, the degree of a difference in deterioration between the sensor electrode22and the monitor electrode23was determined (Determination 2).

In Determination 2, in a case where (the absolute value of) the thermoelectromotive force difference is 5 mV or less, it is determined that a significant difference in degree of deterioration between the sensor electrode22and the monitor electrode23is not caused, and a circle is marked in Table 2.

In a case where (the absolute value of) the thermoelectromotive force difference is more than 5 mV and 10 mV or less, it is determined that the difference in deterioration between the sensor electrode22and the monitor electrode23is within a range allowable in actual use of the gas sensor100, and a triangle is marked in Table 2.

On the other hand, as for each of the gas sensors100having (the absolute value of) the thermoelectromotive force difference of more than 10 mV and thus not corresponding to any of the above-mentioned cases, a cross is marked in Table 2.

(Summary of Results of Determination)

FIG. 11is a plot of the NOx sensitivity change rates of the gas sensors100of Examples 1 to 6 and Comparative Examples 1 and 2 against an elapsed time of the accelerated durability test.

As shown inFIG. 11, as for each of the gas sensors100, (the absolute value of) the NOx sensitivity change rate changed monotonically as the elapsed time of the accelerated durability test increased. On the other hand, it was found that (the absolute value of) the NOx sensitivity change rate of each of the gas sensors100of Examples 1 to 6 remained within 20% whereas (the absolute value of) the NOx sensitivity change rate of each of the gas sensors100of Comparative Examples 1 and 2 exceeded 20% after the elapse of 1000 hours.

More specifically, as for each of Examples 2 to 5, the NOx sensitivity change rate after the elapse of 1000 hours was 10% or less as shown by the circle marked in a column “DETERMINATION 1” in Table 2, and it was determined that the change in NOx sensitivity was suitably suppressed. In addition, the thermoelectromotive force difference between the sensor cell72and the monitor cell73was 5 mV or less as shown by the circle marked in a column “DETERMINATION 2” in Table 2, and it was determined that the significant difference in degree of deterioration between the sensor electrode22and the monitor electrode23was not caused.

As for each of Examples 1 and 6, the NOx sensitivity change rate after the elapse of 1000 hours was more than 10% and 20% or less as shown by the triangle marked in the column “DETERMINATION 1” in Table 2, and it was determined that the change in NOx sensitivity was suppressed within the range allowable in actual use of the gas sensor100. In addition, the thermoelectromotive force difference between the sensor cell72and the monitor cell73was more than 5 mV and 10 mV or less as shown by the triangle marked in the column “DETERMINATION 2” in Table 2, and it was determined that the difference in deterioration between the sensor electrode22and the monitor electrode23was within the range allowable in actual use of the gas sensor100.

On the other hand, as for each of Comparative Examples 1 and 2, the NOx sensitivity change rate after the elapse of 1000 hours was more than 20% as shown by the cross marked in the column “DETERMINATION 1” in Table 2. In addition, the thermoelectromotive force difference between the sensor cell72and the monitor cell73was more than 20 mV as shown by the cross marked in the column “DETERMINATION 2” in Table 2.

The above-mentioned results indicate that, as for the gas sensor100at least meeting the requirement (a), the change in NOx sensitivity and the difference in deterioration between the sensor electrode22and the monitor electrode23are suppressed within the range allowable in actual use of the gas sensor100even when the gas sensor100is used for a long time.

In particular, the results of Examples 2 to 5 indicate that, in the gas sensor100meeting at least one of the requirements (b) and (c) and meeting the requirements (d) and (e) in addition to the requirement (a), the change in NOx sensitivity is suitably suppressed, and the significant difference in degree of deterioration between the sensor electrode22and the monitor electrode23is not caused.