GAS SENSOR

A gas sensor detects a specific gas concentration in a measurement-object gas and includes an element body and one or more pump cells. The element body includes an oxygen-ion-conductive solid electrolyte layer and is provided with a measurement-object gas flow section therein. The measurement-object gas flow section receives a measurement-object gas and allows the measurement-object gas to flow therethrough. The one or more pump cells each have an inner electrode and an outer pump electrode and pump out oxygen from around the inner electrode to around the outer pump electrode. The inner electrode is disposed in the measurement-object gas flow section and contains a catalytically-active noble metal. At least one pump cell of the one or more pump cells pumps out the oxygen by applying a repeatedly on-off controlled pump current between a measurement electrode and the outer pump electrode.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to gas sensors.

2. Description of the Related Art

A known gas sensor in the related art detects the concentration of a specific gas, such as NOx, in a measurement-object gas, such as exhaust gas of an automobile. For example, Patent Literature 1 describes a gas sensor including a layered body of a plurality of oxygen-ion-conductive solid electrolyte layers, a main pump cell, and a measurement pump cell. The main pump cell includes an inner pump electrode disposed in a measurement-object gas flow section inside the layered body, and also includes an outer pump electrode disposed outside the layered body. The measurement pump cell includes a measurement electrode disposed in the measurement-object gas flow section, and also includes an outer pump electrode. When this gas sensor is to detect the NOx concentration, the main pump cell first pumps out oxygen from the measurement-object gas flow section or pumps oxygen into the measurement-object gas flow section, so that the oxygen concentration in the measurement-object gas flow section is adjusted. Subsequently, after the oxygen concentration is adjusted, NOx in the measurement-object gas is reduced, and oxygen produced as a result of the reduction is pumped out by the measurement pump cell. Based on a pump current flowing through the measurement pump cell, the NOx concentration in the measurement-object gas is detected.

CITATION LIST

Patent Literature

SUMMARY OF THE INVENTION

Each of the electrodes included in the pump cells of the gas sensor is catalytically active (i.e., has catalytic performance) and serves as a catalyst for a reaction where oxygen turns into ions. However, as the gas sensor is used, the catalytic activity of this electrode may change, as in a case where the catalytic activity decreases (i.e., is deactivated). A change in the catalytic activity may sometimes lead to a problem in the gas sensor, such as deterioration in the sensitivity for detecting the specific gas concentration.

The present invention has been made to solve such a problem, and a main object thereof is to suppress a change in the catalytic activity of an electrode due to use of the gas sensor.

The present invention provides the following solutions for achieving the main object mentioned above.

A gas sensor according to the present invention detects a specific gas concentration in a measurement-object gas and includes an element body and one or more pump cells. The element body includes an oxygen-ion-conductive solid electrolyte layer and is provided with a measurement-object gas flow section therein. The measurement-object gas flow section receives a measurement-object gas and allows the measurement-object gas to flow therethrough. The one or more pump cells each have an inner electrode and an outer electrode and pump out oxygen from around the inner electrode to around the outer electrode. The inner electrode is disposed in the measurement-object gas flow section and contains a catalytically-active noble metal. The outer electrode is disposed in an area to be exposed to the measurement-object gas at an outer side of the element body. At least one pump cell of the one or more pump cells pumps out the oxygen by applying a repeatedly on-off controlled pump current between the inner electrode and the outer electrode.

This gas sensor includes the one or more pump cells. At least one of the pump cells pumps out the oxygen from around the inner electrode to around the outer electrode by applying the repeatedly on-off controlled pump current between the inner electrode and the outer electrode. By applying the repeatedly on-off controlled pump current (referred to as “intermittent pump current” hereinafter) in this manner, a change in the catalytic activity of the inner electrode due to use of the gas sensor can be suppressed, as compared with a case where a continuous pump current is applied to a pump cell. A conceivable reason for this will be described below. When oxygen is to be pumped out from around the inner electrode by applying the pump current between the inner electrode and the outer electrode, the surrounding oxygen turns into oxygen ions inside the inner electrode. The oxygen ions serve as electron carriers and travel toward the outer electrode. During this process, if the pump current is a continuous electric current, a reaction where some noble metals in the inner electrode oxidize and a reaction where oxygen ions are released as a result of reduction of the oxidized noble metals both occur. When both reactions reach a state of equilibrium, some of the noble metals in the inner electrode are in a constantly oxidized state. Since oxidized noble metals tend to evaporate more easily than before they are oxidized, the noble metals in the inner electrode tend to decrease with use of the gas sensor, thus causing the catalytic activity of the inner electrode to change. On the other hand, the following description relates to a case where an intermittent pump current is used and is intermittently applied such that the intermittent pump current becomes an average electric current equal to the continuous pump current. In this case, the pump current applied to the inner electrode during an on mode is a value larger than the continuous pump current. Accordingly, the oxygen inside the inner electrode turns into ions and travel toward the outer electrode more during the on mode of the intermittent pump current than when the pump current is applied continuously, so that the oxygen concentration inside the inner electrode decreases. In such a state where the oxygen concentration inside the inner electrode is low, the aforementioned noble metals in the inner electrode are less likely to oxidize, and rather, reduction of oxidized noble metals may occur. Thus, a decrease in the aforementioned noble metals in the inner electrode with use of the gas sensor is suppressed. Furthermore, electric current hardly flows to the inner electrode when the pump current is in an off mode, so that oxidization of the aforementioned noble metals in the inner electrode is less likely to occur. As a result, oxidization of the noble metals in the inner electrode is suppressed in both an on mode and an off mode. It is thus conceivable that a change in the catalytic activity of the inner electrode due to use of the gas sensor is suppressed.

In the gas sensor according to the present invention, the one or more pump cells may include a main pump cell, an auxiliary pump cell, and a measurement pump cell. The main pump cell has an inner main pump electrode serving as the inner electrode and an outer main pump electrode serving as the outer electrode. The inner main pump electrode is disposed in a first internal cavity in the measurement-object gas flow section. The main pump cell pumps out oxygen from the first internal cavity. The auxiliary pump cell has an inner auxiliary pump electrode serving as the inner electrode and an outer auxiliary pump electrode serving as the outer electrode. The inner auxiliary pump electrode is disposed in a second internal cavity provided downstream of the first internal cavity in the measurement-object gas flow section. The auxiliary pump cell pumps out oxygen from the second internal cavity. The measurement pump cell has an inner measurement electrode serving as the inner electrode and an outer measurement electrode serving as the outer electrode. The inner measurement electrode is disposed in a measurement chamber provided downstream of the second internal cavity in the measurement-object gas flow section. The measurement pump cell pumps out oxygen produced in the measurement chamber from the specific gas. At least one of the main pump cell, the auxiliary pump cell, and the measurement pump cell pumps out the oxygen from around the inner electrode by applying the repeatedly on-off controlled pump current between the inner electrode and the outer electrode. Accordingly, with regard to a pump cell that is caused to operate by receiving an intermittent pump current among the main pump cell, the auxiliary pump cell, and the measurement pump cell, a change in the catalytic activity of the inner electrode due to use of the gas sensor can be suppressed.

In this case, the measurement pump cell may pump out the oxygen by applying a measurement pump current serving as the repeatedly on-off controlled pump current between the inner measurement electrode and the outer measurement electrode. In other words, among the main pump cell, the auxiliary pump cell, and the measurement pump cell, at least the measurement pump cell may be caused to operate by using an intermittent pump current. Accordingly, a change in the catalytic activity of the inner measurement electrode due to use of the gas sensor can be suppressed. A change in the catalytic activity of the inner measurement electrode has a greater effect on the sensitivity for detecting the specific gas concentration in the gas sensor than a change in the catalytic activity of the inner main pump electrode and the inner auxiliary pump electrode. Therefore, deterioration in the detection sensitivity may be readily minimized by suppressing a change in the catalytic activity of the inner measurement electrode.

In this case, the gas sensor according to the present invention may further include a reference electrode, a measurement-voltage detection device, a measurement-pump-cell control device, and a specific-gas-concentration detection device. The reference electrode is disposed inside the element body and receives a reference gas serving as a reference for detection of the specific gas concentration. The measurement-voltage detection device detects a measurement voltage between the reference electrode and the inner measurement electrode. The measurement-pump-cell control device controls the measurement pump current based on the measurement voltage during a second period so as to set an oxygen concentration in the measurement chamber to a predetermined low concentration. The second period is one of a first period in which a change has occurred in the measurement voltage due to the measurement pump current being in an on mode and a second period in which the change in the measurement voltage has receded as compared with the first period due to the measurement pump current being in an off mode. The specific-gas-concentration detection device detects the specific gas concentration in the measurement-object gas based on the measurement pump current. The effect that the measurement pump current has on the measurement voltage during the second period is smaller than during the first period. Therefore, the oxygen concentration in the measurement chamber is adjusted by controlling the measurement pump current based on the measurement voltage during the second period, and the specific gas concentration in the measurement-object gas is detected based on the measurement pump current, whereby the specific gas concentration can be accurately detected.

In the example where the measurement pump cell pumps out the oxygen by applying the repeatedly on-off controlled measurement pump current, the gas sensor according to the present invention may further include a reference electrode, a measurement-voltage detection device, a measurement-pump-cell control device, and a specific-gas-concentration detection device. The reference electrode is disposed inside the element body and receives a reference gas serving as a reference for detection of the specific gas concentration. The measurement-voltage detection device detects a measurement voltage between the reference electrode and the inner measurement electrode. The measurement-pump-cell control device controls the measurement pump current so as to set an average value of the measurement pump current to a predetermined target value. The specific-gas-concentration detection device detects the specific gas concentration in the measurement-object gas based on the measurement voltage during a second period. The second period is one of a first period in which a change has occurred in the measurement voltage due to the measurement pump current being in an on mode and a second period in which the change in the measurement voltage has receded as compared with the first period due to the measurement pump current being in an off mode. Accordingly, the specific gas concentration in the measurement-object gas can be detected. Moreover, the specific gas concentration in the measurement-object gas is detected based on the measurement voltage during the second period, whereby the specific gas concentration can be accurately detected.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will now be described with reference to the drawings.FIG. 1is a schematic cross-sectional view schematically illustrating an example of the configuration of a gas sensor100according to an embodiment of the present invention.FIG. 2is a block diagram illustrating an electrical connection relationship between a controller90and individual cells. This gas sensor100is attached to a pipe, such as an exhaust gas pipe of an internal combustion engine. The gas sensor100detects the concentration of a specific gas, such as NOx or ammonia, in a measurement-object gas, such as exhaust gas of the internal combustion engine. In this embodiment, the gas sensor100measures the NOxconcentration as the specific gas concentration. The gas sensor100includes a sensor element101having an elongated rectangular parallelepiped shape, individual cells21,41,50, and80to83each including a part of the sensor element101, and the controller90that controls the entire gas sensor100.

The sensor element101is an element including a layered body in which six layers, namely a first substrate layer1, a second substrate layer2, a third substrate layer3, a first solid electrolyte layer4, a spacer layer5, and a second solid electrolyte layer6, are layered in this order from the bottom side, as viewed in the drawing. Each of the six layers is formed of an oxygen-ion-conductive solid electrolyte layer containing, for example, zirconia (ZrO2). The solid electrolyte forming these six layers is dense and gastight. This sensor element101is manufactured, for example, by stacking ceramic green sheets corresponding to the individual layers on top of each other, for example, after predetermined processing and circuit pattern printing, and then firing the stacked ceramic green sheets so that they are combined together.

A gas inlet10, a first diffusion control section11, a buffer space12, a second diffusion control section13, a first internal cavity20, a third diffusion control section30, a second internal cavity40, a fourth diffusion control section60, and a third internal cavity61are formed adjacent to each other so as to communicate in the above order between the lower surface of the second solid electrolyte layer6and the upper surface of the first solid electrolyte layer4on the front end side (on the left end side inFIG. 1) of the sensor element101.

The gas inlet10, the buffer space12, the first internal space20, the second internal space40, and the third internal space61constitute a space within the sensor element101. The space is provided in such a manner that a portion of the spacer layer5is hollowed out. The top of the space is defined by the lower surface of the second solid electrolyte layer6, the bottom of the space is defined by the upper surface of the first solid electrolyte layer4, and sides of the space are defined by side surfaces of the spacer layer5.

The first diffusion control section11, the second diffusion control section13, and the third diffusion control section30are each provided as two laterally elongated slits (i.e., the longitudinal direction of the openings is perpendicular to the figure). The fourth diffusion control section60is provided as a single laterally elongated slit (i.e., the longitudinal direction of the opening is perpendicular to the figure) formed as a clearance under the lower surface of the second solid electrolyte layer6. The section extending from the gas inlet10to the third internal cavity61is also referred to as “measurement-object gas flow section”.

A reference gas introduction space43is disposed between the upper surface of the third substrate layer3and the lower surface of the spacer layer5at a position farther away from the front end side than the measurement-object gas flow section. The reference gas introduction space43is defined at both sides by the side surfaces of the first solid electrolyte layer4. As an example of a reference gas for NOx concentration measurement, air is introduced into the reference gas introduction space43.

An air introduction layer48is a porous ceramic layer. The reference gas is introduced into the air introduction layer48through the reference gas introduction space43. The air introduction layer48is formed so as to cover a reference electrode42.

The reference electrode42is formed between the upper surface of the third substrate layer3and the first solid electrolyte layer4. As described above, the air introduction layer48leading to the reference gas introduction space43is disposed around the reference electrode42. As described later, the reference electrode42can be used to measure the oxygen concentrations (oxygen partial pressures) in the first internal cavity20, the second internal cavity40, and the third internal cavity61. The reference electrode42is formed as a porous cermet electrode (e.g., a cermet electrode composed of Pt and ZrO2).

The gas inlet10of the measurement-object gas flow section is exposed to the external space. The measurement-object gas is taken into the sensor element101through the gas inlet10from the external space. The first diffusion control section11applies a predetermined diffusion resistance to the measurement-object gas taken in through the gas inlet10. The buffer space12is provided for guiding the measurement-object gas introduced by the first diffusion control section11to the second diffusion control section13. The second diffusion control section13applies a predetermined diffusion resistance to the measurement-object gas introduced to the first internal cavity20from the buffer space12. When the measurement-object gas is to be introduced to the first internal cavity20from outside the sensor element101, the measurement-object gas quickly taken into the sensor element101through the gas inlet10due to pressure fluctuations of the measurement-object gas in the external space (i.e., pulsations of exhaust pressure if the measurement-object gas is exhaust gas of an automobile) is not directly introduced to the first internal cavity20but is introduced to the first internal cavity20after the pressure fluctuations of the measurement-object gas are negated through the first diffusion control section11, the buffer space12, and the second diffusion control section13. Accordingly, the pressure fluctuations of the measurement-object gas to be introduced to the first internal cavity20can be made substantially negligible. The first internal cavity20is provided as a space for adjusting the oxygen partial pressure in the measurement-object gas introduced via the second diffusion control section13. The oxygen partial pressure is adjusted by actuating a main pump cell21.

The main pump cell21is an electrochemical pump cell composed of an inner pump electrode22having a ceiling electrode portion22adisposed over substantially an entire portion of the lower surface of the second solid electrolyte layer6that faces the first internal cavity20, an outer pump electrode23disposed on a region of the upper surface of the second solid electrolyte layer6that corresponds to the ceiling electrode portion22aso as to be exposed to the external space, and a portion of the second solid electrolyte layer6that is located between the inner pump electrode22and the outer pump electrode23.

The inner pump electrode22is formed on portions of the upper and lower solid electrolyte layers (the second solid electrolyte layer6and the first solid electrolyte layer4) that define the first internal cavity20and portions of the spacer layer5that give the sidewalls of the first internal cavity20. Specifically, the ceiling electrode portion22ais formed on a portion of the lower surface of the second solid electrolyte layer6that gives the ceiling surface of the first internal cavity20. A bottom electrode portion22bis formed on a portion of the upper surface of the first solid electrolyte layer4that gives the bottom surface of the first internal cavity20. Side electrode portions (not shown) are formed on portions of the sidewall surfaces (inner surfaces) of the spacer layer5that form both sidewalls of the first internal cavity20so as to join together the ceiling electrode portion22aand the bottom electrode portion22b. Thus, the inner pump electrode22is provided as a tunnel-like structure in the area where the side electrode portions are disposed.

The inner pump electrode22contains a catalytically-active noble metal (e.g., at least one of Pt, Rh, Pd, Ru, and Ir). The inner pump electrode22also contains a noble metal (e.g., Au) having a catalytic-activity inhibition ability for inhibiting the catalytic activity of the catalytically-active noble metal against the specific gas. Accordingly, the inner pump electrode22that comes into contact with the measurement-object gas is composed of a material with a weakened reduction ability against the specific gas (i.e., NOx) component of the measurement-object gas. The inner pump electrode22is preferably formed of a cermet containing a noble metal and an oxygen-ion-conductive oxide (i.e., ZrO2). Moreover, the inner pump electrode22is preferably porous. In this embodiment, the inner pump electrode22is a porous cermet electrode composed of Pt and ZrO2and containing 1% Au.

Similar to the inner pump electrode22, the outer pump electrode23contains a catalytically-active noble metal. Similar to the inner pump electrode22, the outer pump electrode23may contain a noble metal having a catalytic-activity inhibition ability, and may be formed of a cermet. The outer pump electrode23is preferably porous. In this embodiment, the outer pump electrode23is a porous cermet electrode composed of Pt and ZrO2.

In the main pump cell21, the desired pump voltage Vp0is applied between the inner pump electrode22and the outer pump electrode23so that a pump current Ip0flows between the inner pump electrode22and the outer pump electrode23in either a positive or negative direction. Thus, oxygen can be pumped from the first internal cavity20to the external space or from the external space to the first internal cavity20.

To detect the oxygen concentration (oxygen partial pressure) in the atmosphere in the first internal cavity20, the inner pump electrode22, the second solid electrolyte layer6, the spacer layer5, the first solid electrolyte layer4, the third substrate layer3, and the reference electrode42form an electrochemical sensor cell, namely, an oxygen-partial-pressure detection sensor cell80for main pump control.

The oxygen concentration (oxygen partial pressure) in the first internal cavity20can be determined by measuring an electromotive force (voltage V0) in the main-pump-control oxygen-partial-pressure detection sensor cell80. Furthermore, feedback control is performed on the pump voltage Vp0of a variable power source24such that the voltage V0becomes a target value, whereby the pump current Ip0is controlled. Accordingly, the oxygen concentration in the first internal cavity20can be maintained at a predetermined fixed value.

The third diffusion control section30creates a predetermined diffusion resistance to the measurement-object gas whose oxygen concentration (oxygen partial pressure) has been controlled in the first internal cavity20by the operation of the main pump cell21and guides the measurement-object gas into the second internal cavity40.

The second internal cavity40is provided as a space for further adjusting, using an auxiliary pump cell50, the oxygen concentration (oxygen partial pressure) of the measurement-object gas introduced through the third diffusion control section30after the oxygen partial pressure is adjusted in advance in the first internal cavity20. Thus, the oxygen concentration in the second internal cavity40can be maintained at a constant value with high accuracy so that the gas sensor100can measure the NOx concentration with high accuracy.

The auxiliary pump cell50is an auxiliary electrochemical pump cell composed of an auxiliary pump electrode51having a ceiling electrode portion51adisposed over substantially an entire portion of the lower surface of the second solid electrolyte layer6that faces the second internal cavity40, the outer pump electrode23(the outer electrode is not limited to the outer pump electrode23, but may be any suitable electrode outside the sensor element101), and the second solid electrolyte layer6.

The auxiliary pump electrode51is disposed within the second internal cavity40in a tunnel-like structure similar to the aforementioned inner pump electrode22provided in the first internal cavity20. Specifically, the tunnel-like structure is formed such that the second solid electrolyte layer6that provides a ceiling surface for the second internal cavity40is provided with the ceiling electrode51a, the first solid electrolyte layer4that provides a bottom surface for the second internal cavity40is provided with a bottom electrode51b, and side electrodes (not shown) that connect the ceiling electrode51aand the bottom electrode51bare formed on opposite wall surfaces of the spacer layer5that provide sidewalls for the second internal cavity40. The auxiliary pump electrode51is similar to the inner pump electrode22in being formed by using a material with a weakened reduction ability against the NOxcomponent in the measurement-object gas.

In detail, the auxiliary pump electrode51contains a catalytically-active noble metal (e.g., at least one of Pt, Rh, Pd, Ru, and Ir). The auxiliary pump electrode51also contains a noble metal (e.g., Au) having the aforementioned catalytic-activity inhibition ability. The auxiliary pump electrode51is preferably formed of a cermet containing a noble metal and an oxygen-ion-conductive oxide (i.e., ZrO2). Moreover, the auxiliary pump electrode51is preferably porous. In this embodiment, the auxiliary pump electrode51is a porous cermet electrode composed of Pt and ZrO2and containing 1% Au.

In the auxiliary pump cell50, the desired voltage Vp1is applied between the auxiliary pump electrode51and the outer pump electrode23. Thus, oxygen can be pumped from the atmosphere in the second internal cavity40to the external space or from the external space to the second internal cavity40.

To control the oxygen partial pressure in the atmosphere in the second internal cavity40, the auxiliary pump electrode51, the reference electrode42, the second solid electrolyte layer6, the spacer layer5, the first solid electrolyte layer4, and the third substrate layer3form an electrochemical sensor cell, namely, an oxygen-partial-pressure detection sensor cell81for auxiliary pump control.

The auxiliary pump cell50performs pumping in accordance with a variable power source52that is voltage-controlled based on an electromotive force (voltage V1) detected by the auxiliary-pump-control oxygen-partial-pressure detection sensor cell81. Accordingly, the oxygen partial pressure in the atmosphere within the second internal cavity40is controlled to a low partial pressure that substantially has no effect on NOxmeasurement.

In addition, a pump current Ip1is used for controlling the electromotive force of the main-pump-control oxygen-partial-pressure detection sensor cell80. In detail, the pump current Ip1is input as a control signal to the main-pump-control oxygen-partial-pressure detection sensor cell80, and the voltage V0thereof is controlled, whereby the gradient of the oxygen partial pressure in the measurement-object gas introduced to the second internal cavity40from the third diffusion control section30is controlled to be constantly fixed. In the case of application as a NOxsensor, the oxygen concentration within the second internal cavity40is maintained at a fixed value of about 0.001 ppm in accordance with the functions of the main pump cell21and the auxiliary pump cell50.

The fourth diffusion control section60creates predetermined diffusion resistance to the measurement-object gas whose oxygen concentration (oxygen partial pressure) has been controlled in the second internal cavity40by the operation of the auxiliary pump cell50and guides the measurement-object gas into the third internal cavity61. The fourth diffusion control section60functions to limit the amount of NOx flowing into the third internal cavity61.

The third internal cavity61is provided as a space for processing associated with the measurement of the nitrogen oxide (NOx) concentration in the measurement-object gas introduced through the fourth diffusion control section60after the oxygen concentration (oxygen partial pressure) is adjusted in advance in the second internal cavity40. NOx concentration measurement is mainly performed in the third internal cavity61by the operation of a measurement pump cell41.

The measurement pump cell41measures the NOxconcentration in the measurement-object gas within the third internal cavity61. The measurement pump cell41is an electrochemical pump cell constituted of a measurement electrode44provided on the upper surface of the first solid electrolyte layer4facing the third internal cavity61, the outer pump electrode23, the second solid electrolyte layer6, the spacer layer5, and the first solid electrolyte layer4. The measurement electrode44is a porous cermet electrode composed of a material with an enhanced reduction ability against the NOxcomponent in the measurement-object gas than the inner pump electrode22. The measurement electrode44also functions as a NOxreduction catalyst that reduces the NOxexisting in the atmosphere within the third internal cavity61.

In detail, the measurement electrode44contains a catalytically-active noble metal (e.g., at least one of Pt, Rh, Pd, Ru, and Ir). The content of a noble metal having the aforementioned catalytic-activity inhibition ability in the measurement electrode44is smaller than the content thereof in the main pump cell21and the auxiliary pump electrode51. The measurement electrode44preferably does not contain a noble metal having the catalytic-activity inhibition ability. The measurement electrode44is preferably formed of a cermet containing a noble metal and an oxygen-ion-conductive oxide (i.e., ZrO2). Moreover, the measurement electrode44is preferably porous. In this embodiment, the measurement electrode44is a porous cermet electrode composed of Pt, Rh, and ZrO2.

The measurement pump cell41pumps out oxygen produced by the decomposition of nitrogen oxide in the atmosphere around the measurement electrode44. The amount of oxygen produced can be detected as a pump current Ip2.

Furthermore, in order to detect the oxygen partial pressure around the measurement electrode44, the first solid electrolyte layer4, the third substrate layer3, the measurement electrode44, and the reference electrode42constitute an electrochemical sensor cell, that is, a measurement-pump-control oxygen-partial-pressure detection sensor cell82. A pulse power source46is controlled based on an electromotive force (voltage V2) detected by the measurement-pump-control oxygen-partial-pressure detection sensor cell82. The pulse power source46applies the pump current Ip2, which is repeatedly on-off controlled, between the measurement electrode44and the outer pump electrode23. The pulse power source46serves as an electric current source. The measurement pump cell41operates in accordance with this pump current Ip2.

The measurement-object gas introduced to the second internal cavity40reaches the measurement electrode44in the third internal cavity61via the fourth diffusion control section60under a condition where the oxygen partial pressure is controlled. The nitrogen oxide in the measurement-object gas surrounding the measurement electrode44is reduced (2NO→N2+O2), so that oxygen is produced. Then, the produced oxygen is to undergo pumping by the measurement pump cell41. During the pumping of the oxygen, the pump current Ip2applied by the pulse power source46is controlled such that the voltage V2detected by the measurement-pump-control oxygen-partial-pressure detection sensor cell82reaches a target state. Because the amount of oxygen produced around the measurement electrode44is proportional to the concentration of the nitrogen oxide in the measurement-object gas, the nitrogen oxide concentration in the measurement-object gas is calculated by using the pump current Ip2in the measurement pump cell41.

An oxygen-partial-pressure detection device serving as an electrochemical sensor cell may be constituted by combining the measurement electrode44, the first solid electrolyte layer4, the third substrate layer3, and the reference electrode42. Thus, an electromotive force according to a difference between the amount of oxygen produced as a result of reduction of the NOx component in the atmosphere surrounding the measurement electrode44and the amount of oxygen contained in reference air can be detected, so that the concentration of the NOx component in the measurement-object gas can be determined accordingly.

Furthermore, the second solid electrolyte layer6, the spacer layer5, the first solid electrolyte layer4, the third substrate layer3, the outer pump electrode23, and the reference electrode42constitute an electrochemical sensor cell83. The oxygen partial pressure in the measurement-object gas outside the sensor can be detected in accordance with an electromotive force (voltage Vref) obtained by the sensor cell83.

In the gas sensor100having the above configuration, the measurement pump cell41receives the measurement-object gas whose oxygen partial pressure is constantly maintained at a fixed low value (i.e., a value that substantially has no effect on NOxmeasurement) as a result of actuation of the main pump cell21and the auxiliary pump cell50. Thus, the NOxconcentration in the measurement-object gas can be ascertained based on the pump current Ip2, used by the measurement pump cell41for pumping out oxygen produced by NOxreduction, substantially in proportion to the NOxconcentration in the measurement-object gas.

To increase the oxygen ion conductivity of the solid electrolyte, the sensor element101further includes a heater section70that functions as a temperature regulator to heat and maintain the temperature of the sensor element101. The heater section70includes a heater connector electrode71, a heater72, a through-hole73, a heater insulating layer74, and a pressure relief vent75.

The heater connector electrode71is formed in contact with the lower surface of the first substrate layer1. The heater connector electrode71is connected to an external power supply so that the heater section70can be externally powered.

The heater72is an electrical resistor formed between the second substrate layer2and the third substrate layer3. The heater72is connected to the heater connector electrode71through the through-hole73. The heater72is externally powered through the heater connector electrode71to generate heat, thereby heating and maintaining the temperature of the solid electrolyte forming the sensor element101.

The heater72is embedded over the entire region from the first internal cavity20to the third internal cavity61so that the temperature of the entire sensor element101can be adjusted to a temperature that activates the solid electrolyte.

The heater insulating layer74is an insulating layer covering the upper and lower surfaces of the heater72and formed of an insulator such as alumina. The heater insulating layer74is formed in order to ensure electrical insulation between the second substrate layer2and the heater72and electrical insulation between the third substrate layer3and the heater72.

The pressure relief vent75extends through the third substrate layer3and the air introduction layer48so as to communicate with the reference gas introduction space43. The pressure relief vent75is formed in order to mitigate an increase in internal pressure due to a temperature increase in the heater insulating layer74.

The controller90is a microprocessor including, for example, a CPU92and a memory94. The controller90receives the voltage V0detected by the main-pump-control oxygen-partial-pressure detection sensor cell80, the voltage V1detected by the auxiliary-pump-control oxygen-partial-pressure detection sensor cell81, the voltage V2detected by the measurement-pump-control oxygen-partial-pressure detection sensor cell82, the voltage Vref detected by the sensor cell83, the pump current Ip0detected by the main pump cell21, and the pump current Ip1detected by the auxiliary pump cell50. Furthermore, the controller90outputs control signals to the variable power source24of the main pump cell21, the variable power source52of the auxiliary pump cell50, and the pulse power source46of the measurement pump cell41, so as to control the cells21,50, and41.

The controller90performs feedback control on the pump voltage Vp0of the variable power source24based on the voltage V0so as to set the voltage V0to a target value (referred to as “target value V0*”) (i.e., to set the oxygen concentration in the first internal cavity20to a target concentration). Thus, the pump current Ip0varies depending on the concentration of oxygen contained in the measurement-object gas.

The controller90also performs feedback control on the voltage Vp1of the variable power source52based on the voltage V1so as to set the voltage V1to a target value (referred to as “target value V1*”) (i.e., to set the oxygen concentration in the second internal cavity40to a predetermined low oxygen concentration that substantially has no effect on NOxmeasurement). In addition, the controller90sets the target value V0* of the voltage V0(i.e., performs feedback control) based on the pump current Ip1so as to set the pump current Ip1flowing in accordance with the voltage Vp1to a target value (referred to as “target value Ip1*”). Accordingly, the gradient of the oxygen partial pressure in the measurement-object gas introduced to the second internal cavity40from the third diffusion control section30is controlled to be constantly fixed. Moreover, the oxygen partial pressure in the atmosphere within the second internal cavity40is controlled to a low partial pressure that substantially has no effect on NOxmeasurement.

Moreover, the controller90performs feedback control on the pump current Ip2of the pulse power source46based on the voltage V2so as to set the voltage V2to a target value (referred to as “target value V2*”) (i.e., to set the oxygen concentration in the third internal cavity61to a predetermined low concentration). Thus, oxygen produced as a result of reduction of the NOx in the measurement-object gas in the third internal cavity61is pumped out from the third internal cavity61such that the oxygen becomes substantially zero. Then, the controller90determines the NOxconcentration in the measurement-object gas based on the pump current Ip2serving as a value according to the oxygen produced in the third internal cavity61from the specific gas (i.e., NOx).

The memory94stores therein, for example, a relational expression (e.g., a linear function) or a map as a correspondence relationship between the pump current Ip2and the NOxconcentration. Such a relational expression or a map can be preliminarily obtained from tests.

An example of how the gas sensor100having the above-described configuration is used will be described below. It is assumed that the CPU92of the controller90is controlling the pump cells21,41, and50described above and is acquiring the voltages V0, V1, V2, and Vref from the sensor cells80to83described above. In this state, when the measurement-object gas is introduced through the gas inlet10, the measurement-object gas first passes through the first diffusion control section11, the buffer space12, and the second diffusion control section13and reaches the first internal cavity20. Then, the oxygen concentration in the measurement-object gas is adjusted by the main pump cell21and the auxiliary pump cell50in the first internal cavity20and the second internal cavity40. After the adjustment, the measurement-object gas reaches the third internal cavity61. The CPU92then detects the NOx concentration in the measurement-object gas based on the pump current Ip2and the correspondence relationship stored in the memory94.

The operation of the measurement pump cell41and the control of the measurement pump cell41by the controller90will be described here in detail.FIG. 3illustrates temporal changes in the pump current Ip2and the voltage V2. The upper section ofFIG. 3indicates the temporal change in the pump current Ip2, and the lower section indicates the temporal change in the voltage V2. With regard to the pump current Ip2, the direction in which oxygen can be pumped out from around the measurement electrode44to around the outer pump electrode23is defined as “positive”. InFIG. 3, the upward direction of the ordinate axis is defined as “positive direction”. The positive direction of the pump current Ip2is the direction of an arrow of the pump current Ip2inFIG. 1, and is the direction in which electric current flows from the measurement electrode44toward the outer pump electrode23at the outer side of the sensor element101. With regard to the voltage V2, a state where the electric potential of the reference electrode42is higher than that of the measurement electrode44is defined as “positive”. InFIG. 3, the upward direction of the ordinate axis is defined as “positive direction”.

The pulse power source46of the measurement pump cell41applies the repeatedly on-off controlled pump current Ip2, that is, the intermittent pump current Ip2, between the measurement electrode44and the outer pump electrode23, so that oxygen is pumped out from around the measurement electrode44. In this embodiment, as shown inFIG. 3, the pump current Ip2is a pulse-wave electric current that is repeatedly on-off controlled in cycles T. For example, when the pump current Ip2rises from 0 A at a time point t1and transitions to an on mode, the pump current Ip2becomes a maximum current Ip2max and maintains this state until an ON time Ton elapses and a time point t4is reached. When the pump current Ip2falls at the time point t4and transitions to an off mode, the pump current Ip2becomes 0 A until an OFF time Toff elapses and a time point t7is reached. Although the actual pump current Ip2requires a short period of time for rising from the time point t1and for falling from the time point t4, this is omitted inFIG. 3. Moreover, there may be a case where electric current slightly flows due to the effect of, for example, noise even while the pump current Ip2output from the pulse power source46is in the off mode. However, such a case is omitted inFIG. 3.

As mentioned above, the CPU92of the controller90performs feedback control on the pump current Ip2of the pulse power source46based on the voltage V2so as to set the oxygen concentration in the third internal cavity61to the predetermined low concentration. An amount of oxygen that the measurement pump cell41pumps out from the third internal cavity61during one cycle (i.e., cycle T) in accordance with the pump current Ip2is proportional to an average value Ip2ave (see a single-dot chain line in the upper section ofFIG. 3) of the pump current Ip2during one cycle. Therefore, for example, the CPU92varies the average value Ip2ave by outputting a control value to the pulse power source46to change at least one of parameters including the proportion (i.e., duty ratio) of the ON time Ton occupying the cycle T, the cycle T, and the maximum current Ip2max. Alternatively, the CPU92may output the average value Ip2ave as a control value to the pulse power source46, and the pulse power source46may vary at least one of the aforementioned parameters based on the control value. In this embodiment, the CPU92outputs the duty ratio (or a variation of the duty ratio) as a control value to the pulse power source46, and the pulse power source46varies the duty ratio of the pump current Ip2based on the control value. For example, if the voltage V2is smaller than the target value V2*, the CPU92controls the pulse power source46to increase the duty ratio of the pump current Ip2(i.e., to extend the ON time Ton without changing the cycle T), thereby reducing the oxygen concentration in the third internal cavity61.FIG. 3shows an example where the second ON time Ton (between time points t7and t8) is twice as long as the first ON time Ton (between the time points t1and t4), such that the average value Ip2ave in the second cycle T is twice as large as that in the first cycle T.FIG. 3also shows that, with the average value Ip2ave being doubled, the oxygen concentration in the third internal cavity61is reduced, such that the voltage V2is higher with a value V2b′ at a time point t9than a value V2bat the time point t7.

The pump current Ip2controlled by the CPU92in this manner ultimately becomes an electric current according to the oxygen produced in the third internal cavity61from the specific gas (i.e., NOx). Therefore, the CPU92can determine the NOx concentration in the measurement-object gas based on this pump current Ip2. For example, a value of a parameter (i.e., duty ratio) of the pump current Ip2used as a control value by the CPU92is proportional to the NOx concentration in the measurement-object gas. Therefore, by storing the correspondence relationship between the two in the memory94, the CPU92can determine the NOx concentration based on the control value output to the pulse power source46by the CPU92and the correspondence relationship stored in the memory94. Likewise, the CPU92can also determine the NOx concentration by using a value (e.g., average value Ip2ave) determined based on the control value.

The following description relates to a comparative example where the pump current Ip2is a continuous electric current.FIG. 4is a schematic cross-sectional view of a gas sensor900according to a comparative example.FIG. 5illustrates a temporal change in the pump current Ip2of the gas sensor900. Components of the gas sensor900that are identical to those of the gas sensor100are given the same reference signs as those inFIG. 1, and detailed descriptions thereof will be omitted. The definition of the direction of the pump current Ip2is the same as that of the gas sensor100, and the upward direction of the ordinate axis inFIG. 5is defined as “positive direction”. Furthermore, inFIG. 5, the waveform of the pump current Ip2shown inFIG. 3is indicated by a single-dot chain line as a comparison. The gas sensor900includes a measurement pump cell941in place of the measurement pump cell41. The measurement pump cell941is similar to the measurement pump cell41except that the measurement pump cell941includes a variable power source946in place of the pulse power source46. The variable power source946applies a pump voltage Vp2between the measurement electrode44and the outer pump electrode23. Accordingly, the measurement pump cell941receives a continuous pump current Ip2, as shown inFIG. 5. A controller of the gas sensor900is not shown, but is similar to the controller90in that the controller receives the voltages V0, V1, V2, and Vref and the pump currents Ip0and Ip1and outputs control signals to the variable power sources24and52. Furthermore, the controller of the gas sensor900performs feedback control on the pump voltage Vp2of the variable power source946based on the input voltage V2so as to set the voltage V2to a target value. Thus, oxygen produced as a result of reduction of the NOx in the measurement-object gas in the third internal cavity61is pumped out from the third internal cavity61by the measurement pump cell941such that the oxygen becomes substantially zero. For example, when the controller is to perform feedback control on the pump voltage Vp2at every cycle T identical to that inFIG. 3, the pump current Ip2flowing to the measurement pump cell941changes at every cycle T, as shown inFIG. 5, while flowing continuously. Although the actual pump current Ip2requires a short period of time to completely change at every cycle T, this is omitted inFIG. 5. The controller of the gas sensor900then acquires the pump current Ip2from the measurement pump cell941and calculates the NOx concentration in the measurement-object gas based on this pump current Ip2.

If the average value Ip2ave inFIG. 3and the pump current Ip2indicated by a solid line inFIG. 5are the same, the amount of oxygen pumped out from the third internal cavity61at every cycle T is the same. Therefore, even in a case where the pump current Ip2is applied intermittently as in this embodiment instead of the pump current Ip2being applied continuously as in the comparative example, the CPU92can still adjust the oxygen concentration in the third internal cavity61and measure the NOx concentration. Moreover, because the gas sensor100uses the pulse power source46serving as an electric current source, the control value output to the pulse power source46by the CPU92is a value according to the pump current Ip2. Therefore, without having to receive the pump current Ip2from the measurement pump cell41, the CPU92can ascertain the pump current Ip2based on the control value set by the CPU92and determine the NOx concentration.

In the gas sensor100, the pump current Ip2is intermittently applied to the measurement pump cell41so that a change in the catalytic activity of the measurement electrode44due to use of the gas sensor100can be suppressed, as compared with a case where the pump current Ip2is continuously applied to the measurement pump cell941, as in the gas sensor900. A conceivable reason for this will be described below.

When oxygen is to be pumped out from around the measurement electrode44by applying the pump current Ip2between the measurement electrode44and the outer pump electrode23, the surrounding oxygen (i.e., oxygen mainly produced as a result of NOx reduction) turns into oxygen ions (O2+4e−→2O2−) inside the measurement electrode44. The oxygen ions serve as electron carriers and travel through the solid electrolyte layers (i.e., the layers4to6) toward the outer pump electrode23. During this process, if the pump current Ip2is a continuous electric current, a reaction where some noble metals (i.e., Pt and Rh) in the measurement electrode44oxidize and a reaction where oxygen ions are released as a result of reduction of the oxidized noble metals (i.e., PtO, PtO2, and Rh2O3) both occur. When both reactions reach a state of equilibrium, some of the noble metals in the measurement electrode44are in a constantly oxidized state. Since oxidized noble metals tend to evaporate more easily than before they are oxidized, the noble metals in the measurement electrode44tend to decrease with use of the gas sensor900, thus causing the catalytic activity of the measurement electrode44to decrease. Moreover, the oxidization of the noble metals in the measurement electrode44may conceivably cause the microstructure of the measurement electrode44to change. This may conceivably cause the three-phase interface among the measurement electrode44, the pores in the measurement electrode44, and the first solid electrolyte layer4or the two-phase interface between the measurement electrode44and the first solid electrolyte layer4to decrease in surface area, thus causing the catalytic activity of the measurement electrode44to decrease.

On the other hand, the following description relates to a case where an intermittent pump current Ip2is used in the gas sensor100and is intermittently applied to the measurement pump cell41such that the intermittent pump current Ip2becomes an average electric current (i.e., average value Ip2ave) equal to the continuous pump current Ip2in the gas sensor900. In this case, the pump current Ip2(i.e., maximum current Ip2max) applied to the measurement electrode44during an on mode (e.g., between the time points t1and t4inFIG. 3) in the gas sensor100is a value larger than the continuous pump current Ip2in the gas sensor900(seeFIGS. 3 and 5). Accordingly, the oxygen inside the measurement electrode44turns into ions and travel toward the outer pump electrode23more during the on mode of the intermittent pump current Ip2than when the pump current Ip2is applied continuously, so that the oxygen concentration inside the measurement electrode44decreases. In such a state where the oxygen concentration inside the measurement electrode44is low, the aforementioned noble metals in the measurement electrode44are less likely to oxidize, and rather, reduction of oxidized noble metals may occur. Thus, in the gas sensor100, a decrease in the aforementioned noble metals in the measurement electrode44with use thereof is suppressed, as compared with the gas sensor900. Furthermore, in the gas sensor100, electric current hardly flows to the measurement electrode44when the pump current Ip2is in an off mode (e.g., between the time points t4and t7inFIG. 3), so that oxidization of the aforementioned noble metals in the measurement electrode44is less likely to occur. As a result, in the gas sensor100, oxidization of the noble metals in the measurement electrode44is suppressed in both an on mode and an off mode of the pump current Ip2. It is thus conceivable that a decrease in the catalytic activity of the measurement electrode44due to use of the gas sensor100is suppressed therein, as compared with the gas sensor900. Since a lower catalytic activity of the measurement electrode44leads to suppressed NOx reduction in the third internal cavity61, the pump current Ip2(more specifically, the average value Ip2ave) decreases, thus resulting deteriorated sensitivity for detecting the NOx concentration. In the gas sensor100according to this embodiment, such deteriorated sensitivity for detecting the NOx concentration with use thereof can be suppressed. Therefore, the gas sensor100according to this embodiment can maintain the measurement accuracy over a long period of use, and can thus extend the lifespan thereof.

The voltage V2is a voltage between the measurement electrode44and the reference electrode42and is basically a value according to the oxygen concentration in the third internal cavity61. However, because the electric potential of the measurement electrode44changes intermittently when the pulse power source46intermittently applies the pump current Ip2thereto, the voltage V2also varies periodically in a pulsating manner in response to this effect. In detail, the waveform of the voltage V2has a first period in which a change is occurring as a result of the pump current Ip2being controlled to an on mode, and a second period in which the change has receded as a result of the pump current Ip2being controlled to an off mode. For example, inFIG. 3, with regard to the voltage V2, a change begins (i.e., the voltage V2begins to rise) from the time point t1, a value V2a(i.e., maximum value) corresponding to a maximum change is reached at the time point t4, the change begins to recede (i.e., the voltage V2begins to fall) from the time point t4, and a value V2b(i.e., minimum value) where the change has receded the most is reached at the time point t7. In this case, the range from the value V2bto the value V2aof the voltage V2during the duration of a single cycle T of the pump current Ip2is defined as 0% to 100%, and the first period and the second period of the voltage V2are set using this range as a reference. In detail, a period in which the voltage V2is 90% or higher (i.e. between the time points t3and t5) is defined as the first period, and the length thereof is defined as a first duration T1. A period from when the pump current Ip2is controlled to an off mode and the voltage V2becomes 10% or lower to when the voltage V2begins to rise as a result of the pump current Ip2being controlled to an on mode in the subsequent cycle (i.e. between the time points t6and t7) is defined as a second period, and the length thereof is defined as a second duration T2. InFIG. 3, the voltage V2reaches the value V2acorresponding to the first maximum change at the time point t4at which the pump current Ip2is controlled to an off mode. However, if the ON time Ton is long, there may be a case where the voltage V2may reach the value V2aprior to the time point t4and remain in this state until the time point t4.

Accordingly, in this embodiment, the voltage V2varies periodically in response to the effect of the pump current Ip2. During the second period, the effect that the pump current Ip2has on the voltage V2is smaller, as compared with the first period. Therefore, the voltage V2during the second period is a value that indicates the oxygen concentration in the third internal cavity61more accurately than the voltage V2during the first period. Accordingly, the CPU92preferably performs the above-described control of the pump current Ip2based on the voltage V2during the second period. Thus, the CPU92can accurately set the oxygen concentration in the third internal cavity61close to the predetermined low concentration, and can also accurately determine the NOx concentration in the measurement-object gas. In this embodiment, the CPU92acquires the voltage V2during the second period, compares the acquired voltage V2with the target value V2*, and sets the control value to be output to the pulse power source46. Furthermore, as shown inFIG. 3, the voltage V2during the second period also changes with time, and the effect of the pump current Ip2tends to decrease toward the end of the second period (i.e., the time point t7inFIG. 3). This is conceivably due to the effect of a capacity component in the measurement pump cell41, such as a capacity component of the measurement electrode44. Thus, the CPU92preferably controls the pump current Ip2based on the voltage V2at the latest possible timing in the second period. For example, the CPU92may control the pump current Ip2based on the voltage V2at any timing in the latter half of the second period.

The CPU92may control the pump current Ip2based on the voltage V2during the second period in each of a plurality of cycles T (e.g., based on an average value of a plurality of voltages V2acquired in different cycles). For example, if the pump current Ip2is controlled based only on the voltage V2during the second period in one cycle, the control of the pump current Ip2may sometimes become unstable due to, for example, a fluctuation or a deviation of the actual average value Ip2ave relative to the proper average value Ip2ave corresponding to the NOx concentration. In contrast, the CPU92controls the pump current Ip2based on, for example, the average value of the voltages V2during the second periods in the plurality of cycles T, so that the control is less likely to become unstable. In this embodiment, the CPU92stores the voltage V2in the memory94at a predetermined timing during the second period for every cycle T, and controls the pump current Ip2based on an average value of multiple times' (e.g., three times') worth of most-recently-stored voltages V2.

The CPU92may control the main pump cell21based on the voltage V0during the second period of the voltage V2. Likewise, the CPU92may control the auxiliary pump cell50based on the voltage V1during the second period of the voltage V2. The CPU92may set the target value V0* based on the pump current Ip1during the second period of the voltage V2.

The correspondence relationship between the components in this embodiment and the components in the present invention will now be clarified. The first substrate layer1, the second substrate layer2, the third substrate layer3, the first solid electrolyte layer4, the spacer layer5, and the second solid electrolyte layer6according to this embodiment correspond to an element body according to the present invention. The main pump cell21, the auxiliary pump cell50, and the measurement pump cell41each correspond to a pump cell. The inner pump electrode22, the auxiliary pump electrode51, and the measurement electrode44each correspond to an inner electrode. The outer pump electrode23corresponds to an outer electrode. Furthermore, the inner pump electrode22corresponds to an inner main pump electrode, the auxiliary pump electrode51corresponds to an inner auxiliary pump electrode, the measurement electrode44corresponds to an inner measurement electrode, the outer pump electrode23corresponds to an outer main pump electrode, an outer auxiliary pump electrode, and an outer measurement electrode, and the third internal cavity61corresponds to a measurement chamber. Moreover, the pump current Ip2corresponds to a measurement pump current, the voltage V2corresponds to a measurement voltage, the measurement-pump-control oxygen-partial-pressure detection sensor cell82corresponds to a measurement-voltage detection device, and the controller90corresponds to a measurement-pump-cell control device and a specific-gas-concentration detection device.

The gas sensor100according to this embodiment described in detail above includes the main pump cell21, the auxiliary pump cell50, and the measurement pump cell41as one or more pump cells. The main pump cell21has the inner pump electrode22disposed in the first internal cavity20and also has the outer pump electrode23, and pumps out oxygen from the first internal cavity20. The auxiliary pump cell50has the auxiliary pump electrode51disposed in the second internal cavity40at the downstream side of the first internal cavity20and also has the outer pump electrode23, and pumps out oxygen from the second internal cavity40. The measurement pump cell41has the measurement electrode44disposed in the third internal cavity61at the downstream side of the second internal cavity40and also has the outer pump electrode23, and pumps out oxygen produced in the third internal cavity61from NOx in the measurement-object gas. Of these pump cells, the measurement pump cell41applies the repeatedly on-off controlled pump current Ip2between the measurement electrode44and the outer pump electrode23, so as to pump out oxygen surrounding the measurement electrode44. Accordingly, the gas sensor100can suppress a change (i.e., decrease) in the catalytic activity of the measurement electrode44with use thereof, as compared with a case where the pump current Ip2is continuously applied between the measurement electrode44and the outer pump electrode23, as in the gas sensor900. The repeatedly on-off controlled pump current Ip2has a higher peak value (i.e., maximum current Ip2max) than the continuous pump current Ip2, and thus achieves a high signal-to-noise ratio (S/N ratio) and is resistant to noise.

Of the pump cells21,50, and41, the measurement pump cell41operates by using an intermittent pump current (i.e., pump current Ip2). A change in the catalytic activity of the measurement electrode44has a larger effect on the sensitivity for detecting the NOx concentration of the gas sensor100than a change in the catalytic activity of the inner pump electrode22and the auxiliary pump electrode51. Therefore, deterioration in the detection sensitivity may be readily minimized by suppressing a change in the catalytic activity of the measurement electrode44.

Furthermore, the CPU92controls the pump current Ip2based on the voltage V2during the second period so as to set the oxygen concentration in the third internal cavity61to the predetermined low concentration. Then, the CPU92detects the NOx concentration in the measurement-object gas based on the pump current Ip2. Accordingly, the CPU92can accurately detect the NOx concentration.

The present invention is not limited whatsoever to the above embodiment, and various embodiments are possible so long as they belong within the technical scope of the present invention.

For example, although the intermittent pump current Ip2is an electric current having one rectangular wave per cycle (i.e., a rectangular single pulse current) in the above-described embodiment, as shown inFIG. 3, the present invention is not limited thereto. For example, the pulse power source46may use a burst pulse current, as shown inFIG. 6, as the intermittent pump current Ip2. In this case, the CPU92may vary the average value Ip2ave of the pump current Ip2during one cycle by outputting a control value to the pulse power source46to change at least one of parameters including the proportion (i.e., duty ratio) of an oscillation period TA occupying a cycle T (i.e., burst cycle), the cycle T, the number of pulses during one cycle (i.e., five pulses inFIG. 6), a time Ta of a single oscillation (i.e., pulse), a pulse cycle (Ta+Tb), and the maximum current Ip2max.

In the case where the pump current Ip2is a burst pulse current, as inFIG. 6, the first period of the voltage V2is included in the oscillation period TA, and the second period is included in a non-oscillation period TB. More specifically, in the case where the pump current Ip2is a burst pulse current, the oscillation period TA is regarded as an on period (i.e., between the time points t1and t4inFIG. 3) of the pump current Ip2, the non-oscillation period TB is regarded as an off period (i.e., between the time points t4and t7inFIG. 3) of the pump current Ip2, and the second period is defined based on a method similar to that in the above-described embodiment.

Although the pulse power source46applies a rectangular-wave pulse current as the pump current Ip2in the above-described embodiment, as shown inFIG. 3, the pulse current to be applied is not limited to a rectangular-wave (square-wave) pulse current and may have a sinusoidal half waveform, a triangular waveform, a saw-tooth waveform, or a waveform at the time of discharge, or may have a waveform obtained by combining one or more of these waveforms.

Although not specifically described in the above-described embodiment, each cycle T of the intermittent pump current Ip2inFIGS. 3 and 6may be, for example, 0.1 s or shorter (i.e., a frequency of 10 Hz or higher), 0.02 s or shorter (i.e., a frequency of 50 Hz or higher), or 0.001 s or shorter (i.e., a frequency of 100 Hz or higher). For example, in a case where the controller90outputs the determined NOx concentration to another device, such as an engine ECU of a vehicle, at every predetermined cycle Tout, the cycle T is preferably shorter than or equal to 1/10 of the cycle Tout.

Although the measurement pump cell41among the pump cells21,50, and41included in the gas sensor100operates by using an intermittent pump current (i.e., pump current Ip2) in the above-described embodiment, the present invention is not limited thereto. At least one of the pump cells included in the gas sensor100may simply operate by using an intermittent pump current. For example, the main pump cell21and the measurement pump cell41may operate by using an intermittent pump current.FIG. 7is a schematic cross-sectional view of a gas sensor200according to a modification. Components of the gas sensor200that are identical to those of the gas sensor100are given the same reference signs as those inFIG. 1, and detailed descriptions thereof will be omitted. The gas sensor200includes a main pump cell221in place of the main pump cell21, and includes an auxiliary pump cell250in place of the auxiliary pump cell50. The main pump cell221is similar to the main pump cell21except that the main pump cell221includes a pulse power source224, serving as an electric current source similar to the pulse power source46, in place of the variable power source24. The auxiliary pump cell250is similar to the auxiliary pump cell50except that the auxiliary pump cell250includes a pulse power source252, serving as an electric current source similar to the pulse power source46, in place of the variable power source52. In the gas sensor200according to this modification, the main pump cell221applies an intermittent pulse current so that a change in the catalytic activity of the inner pump electrode22due to use of the gas sensor200can be suppressed. Moreover, the pulse power source252applies an intermittent pulse current so that a change in the catalytic activity of the auxiliary pump electrode51due to use of the gas sensor200can be suppressed. As mentioned above, the inner pump electrode22contains a catalytically-active noble metal and a noble metal having the catalytic-activity inhibition ability. Therefore, when a noble metal in the inner pump electrode22oxidizes and evaporates, it is conceivable that the oxygen pumping performance of the main pump cell21may deteriorate due to a decrease in the catalytic activity of the inner pump electrode22, or the inner pump electrode22may decompose the NOx in the first internal cavity20due to an increase in the catalytic activity. Since either case is not preferable for the gas sensor200, it is significant to suppress such a change (i.e., an increase or decrease) in the catalytic activity of the inner pump electrode22. The same applies to the auxiliary pump electrode51. In a case where the main pump cell221is to pump oxygen into the first internal cavity20from around the outer pump electrode23by using an intermittent pulse current, it is conceivable that a change in the catalytic activity of the outer pump electrode23can be suppressed. For example, in a case where the oxygen concentration in the measurement-object gas is lower than the aforementioned target oxygen concentration in the first internal cavity20(including a case of a rich atmosphere), the controller90controls the pulse power source224such that the main pump cell221pumps oxygen into the first internal cavity20from around the outer pump electrode23. In this case, the controller90may invert the positive and negative signs of pulses of the pulse power source224, so as to switch between a mode where the main pump cell221pumps oxygen out from the first internal cavity20and a mode where the main pump cell221pumps oxygen into the first internal cavity20. Accordingly, a negative pulse current (i.e., a pulse whose electric current value falls from zero to a negative value) may be applied, instead of a positive pulse current as inFIGS. 3 and 6.

Even in a case where a continuous pump current Ip0is applied to the main pump cell21as in the above-described embodiment, an electric current source may be used in place of the variable power source24, and the pump current Ip0may be varied directly by the electric current source. Likewise, even in a case where a continuous pump current Ip1is applied to the auxiliary pump cell50as in the above-described embodiment, an electric current source may be used in place of the variable power source52, and the pump current Ip1may be varied directly by the electric current source.

Although the sensor element101of the gas sensor100includes the first internal cavity20, the second internal cavity40, and the third internal cavity61in the above-described embodiment, the present invention is not limited thereto. For example, the third internal cavity61may be omitted, as in a sensor element301inFIG. 8. In the sensor element301according to a modification shown inFIG. 8, the gas inlet10, the first diffusion control section11, the buffer space12, the second diffusion control section13, the first internal cavity20, the third diffusion control section30, and the second internal cavity40are provided next to each other in a communicating manner in that order between the lower surface of the second solid electrolyte layer6and the upper surface of the first solid electrolyte layer4. The measurement electrode44is disposed on the upper surface of the first solid electrolyte layer4within the second internal cavity40. The measurement electrode44is covered by a fourth diffusion control section45. The fourth diffusion control section45is a film formed of a ceramic porous body composed of, for example, alumina (Al2O3). Similar to the fourth diffusion control section60according to the above-described embodiment, the fourth diffusion control section45has a function of limiting the amount of NOxflowing to the measurement electrode44. Moreover, the fourth diffusion control section45also functions as a protective film for the measurement electrode44. The ceiling electrode51aof the auxiliary pump electrode51is provided to extend to a position directly above the measurement electrode44. The sensor element301having such a configuration is similar to that in the above-described embodiment in being able to detect the NOx concentration based on the pump current Ip2. In this case, the space surrounding the measurement electrode44(i.e., the interior of the fourth diffusion control section45) functions as a measurement chamber.

Although the outer pump electrode23serves as an outer main pump electrode of the main pump cell21, an outer auxiliary pump electrode of the auxiliary pump cell50, and an outer measurement electrode of the measurement pump cell41in the above-described embodiment, the present invention is not limited thereto. Any one or two of the outer main pump electrode, the outer auxiliary pump electrode, and the outer measurement electrode may be provided at the outer side of the sensor element101independently of the outer pump electrode23.

Although the element body of the sensor element101according to the embodiment described above is a stack including a plurality of solid electrolyte layers (the layers1to6), the present invention is not limited thereto. The element body of the sensor element101may include at least one oxygen-ion-conductive solid electrolyte layer and have a measurement-object gas flow section inside the element body. For example, the layers1to5other than the second solid electrolyte layer6inFIG. 1may be layers formed of materials other than solid electrolytes (e.g., alumina layers). In this case, the electrodes of the sensor element101may be disposed on the second solid electrolyte layer6. For example, the measurement electrode44inFIG. 1may be disposed on the lower surface of the second solid electrolyte layer6. In addition, the reference gas introduction space43may be disposed in the spacer layer5rather than in the first solid electrolyte layer4. The air introduction layer48may be disposed between the second solid electrolyte layer6and the spacer layer5rather than between the first solid electrolyte layer4and the third substrate layer3. The reference electrode42may be disposed on the lower surface of the second solid electrolyte layer6on the rear side of the third internal cavity61.

Although the gas sensor100detects the NOx concentration as a specific gas concentration in the above-described embodiment, the specific gas concentration may alternatively be the concentration of another oxide. In a case where the specific gas is an oxide, oxygen is produced as a result of reduction of the specific gas itself in the third internal cavity61, similarly to the above-described embodiment. Thus, the CPU92can detect the specific gas concentration based on a detection value (e.g., average value Ip2ave) according to this oxygen. Alternatively, the specific gas may be a non-oxide, such as ammonia. In a case where the specific gas is a non-oxide, the specific gas is converted into an oxide in, for example, the first internal cavity20(i.e., is converted into NO by being oxidized in the case of ammonia), so that oxygen is produced as a result of reduction of the oxide in the third internal cavity61after the conversion. Thus, the CPU92can acquire a detection value according to this oxygen and detect the specific gas concentration. Accordingly, whether the specific gas is an oxide or a non-oxide, the gas sensor100can detect the specific gas concentration based on oxygen produced in the third internal cavity61from the specific gas.

As an alternative to the above-described embodiment in which the controller90sets the target value V0* of the voltage V0(i.e., performs feedback control) based on the pump current Ip1so as to set the pump current Ip1to the target value Ip1*, and performs feedback control on the pump voltage Vp0so as to set the voltage V0to the target value V0*, the controller90may perform another control. For example, the controller90may perform feedback control on the pump voltage Vp0based on the pump current Ip1so as to set the pump current Ip1to the target value Ip1*.

Specifically, the controller90may omit the acquisition of the voltage V0from the main-pump-control oxygen-partial-pressure detection sensor cell80and the setting of the target value V0* and may directly control the pump voltage Vp0(and, by extension, the pump current Ip0) based on the pump current Ip1.

Although the pump voltage Vp2of the pulse power source46is controlled such that the voltage V2detected by the measurement-pump-control oxygen-partial-pressure detection sensor cell82becomes the target value V2*, and the NOx concentration in the measurement-object gas is calculated by using the pump current Ip2at that time in the above-described embodiment, the present invention is not limited thereto. For example, the CPU92may perform feedback control on the measurement pump cell41(e.g., control the pump voltage Vp2) so as to set the average value Ip2ave shown inFIG. 3to a predetermined target value Ip2*, and may calculate the NOx concentration by using the voltage V2at that time. With the measurement pump cell41being controlled such that the average value Ip2ave becomes the target value Ip2*, oxygen is pumped out from the third internal cavity61at a substantially fixed flow rate on average, or in other words, in terms of a longer time period than the cycle T. Therefore, the oxygen concentration in the third internal cavity61changes in accordance with the amount of oxygen produced as a result of reduction of the NOx in the measurement-object gas in the third internal cavity61, thus causing the voltage V2to change accordingly. Accordingly, the voltage V2becomes a value according to the NOx concentration in the measurement-object gas. Thus, the NOx concentration can be calculated based on this voltage V2. For example, the correspondence relationship between the voltage V2and the NOx concentration may be preliminarily stored in the memory94. Moreover, the CPU92uses the value of the voltage V2during the second period as a value of the voltage V2for calculating the NOx concentration. As mentioned above, the effect that the pump current Ip2has on the voltage V2is small during the second period. Therefore, the specific gas concentration in the measurement-object gas can be accurately detected by using the voltage V2during the second period.

InFIGS. 3 and 6in the above-described embodiment, the electric current waveform is rectangular in a case where the pump current Ip1is a pulse current. However, as mentioned above, a pulse current requires a short period of time for rising and falling. Specifically, a pulse current actually has a rise time and a fall time, and the waveform of a pulse current is not a perfect rectangle. Therefore, for example, as shown in an enlarged view of a pulse current in the upper section ofFIG. 9, if the pulse width of the pulse current is too small, the actual peak value of the pulse current (i.e., the peak value of the waveform indicated by a dashed line) may sometimes not reach the peak value of an ideal rectangular waveform (i.e., the peak value of the waveform indicated by a solid line) due to the effect of the rise time. In this case, for example, even if the CPU92performs the aforementioned feedback control based on the voltage V2and the target value V2* and outputs a control value to the pulse power source46so as to set the average value Ip2ave to a certain target value (referred to as “target average current Ip2ave*”), the actual average value Ip2ave becomes smaller than the theoretical average value Ip2ave, thus causing the actual average value Ip2ave to deviate from the target average current Ip2ave*. Thus, the controller90may sometimes be not able to properly control the measurement pump cell41, or the S/N ratio of the pump current Ip2may sometimes decrease. In contrast, for example, as shown in an enlarged view of a pulse current in the lower section ofFIG. 9, if the pulse width of the pulse current is large, the actual peak value of the pulse current (i.e., the peak value of the waveform indicated by a dashed line) reaches the peak value of an ideal rectangular waveform (i.e., the peak value of the waveform indicated by a solid line) even with the existence of the rise time. Therefore, if the pulse width is large, the actual average value Ip2ave is less likely to deviate from the theoretical average value Ip2ave and the target average current Ip2ave*. Thus, the controller90preferably controls the measurement pump cell41such that the pulse width of the pump current Ip2does not become too small to an extent that the actual peak value does not reach the peak value of the ideal rectangular waveform. In other words, the controller90preferably controls the pump current Ip2in a range where the pulse width of the pump current Ip2does not fall below a predetermined lower limit value. For example, if the target average current Ip2ave* is small, the controller90preferably reduces the number of pulses. By reducing the number of pulses, the pulse width is increased accordingly for applying the pump current Ip2according to the target average current Ip2ave*, so that the pulse width can be prevented from decreasing. In the example inFIG. 9, the pulse width of the waveform in the upper section is small such that the average value Ip2ave and the target average current Ip2ave* deviate from each other. However, by using the waveform in the lower section as an alternative, the average value Ip2ave and the target average current Ip2ave* are less likely to deviate from each other.FIG. 9illustrates a case where the pump current Ip2is a burst pulse current, similar toFIG. 6, and the waveform in the upper section has six pulses per cycle, whereas the number of pulses in the waveform in the lower section is reduced to two. Instead, the pulse width (i.e., the time Ta per pulse) of the waveform in the lower section ofFIG. 9is three times that of the waveform in the upper section, so that the ideal average value Ip2ave is not varied between the waveform in the upper section and the waveform in the lower section. In the waveform in the lower section ofFIG. 9, the burst-pulse interval (Tb inFIG. 9) is increased (specifically by a factor of 3). The following description relates to an example of a process performed by the controller90in a case where the number of pulses of the pump current Ip2is reduced when the target average current Ip2ave* is small. For example, the controller90repeatedly executes a process involving setting the target average current Ip2ave* by performing feedback control based on the voltage V2and the target value V2*, and controlling the measurement pump cell41so as to set the average value Ip2ave to the set target average current Ip2ave*. In this case, if the value of the set target average current Ip2ave* is a value included in a predetermined low current range (e.g., a value smaller than a predetermined threshold value), the measurement pump cell41is controlled such that a pump current Ip2with a reduced number of pulses is applied, as compared with a case where the value of the set target average current Ip2ave* is a value not included in the predetermined low current range (e.g., a value larger than or equal to the predetermined threshold value).FIG. 10illustrates an example of comparison results between a case where control for reducing the number of pulses is not performed (i.e., the table at the left side) and a case where such control is performed (i.e., the table at the right side).FIG. 10shows the height, width, and number of pulses of the pump current Ip2set by the controller90such that the average value Ip2ave becomes the target average current Ip2ave* in a case where the target average current Ip2ave* is varied between 0.06 μA and 6 μA. The pump current Ip2is a burst pulse current, similar toFIGS. 6 and 9, the pulse height corresponds to Ip2max inFIGS. 6 and 9, the width corresponds to Ta inFIGS. 6 and 9, and the number of pulses corresponds to the number of pulses per cycle. Moreover, the presence and absence of a deviation between the actual average value Ip2ave and the target average current Ip2ave* in a case where the pump current Ip2with the set pulse height, width, and number is applied is also shown. As shown at the left side ofFIG. 10, in a case where the control for reducing the number of pulses is not performed and the number of pulses is fixed at3, the set pulse width is a small value below 10 μs when the target average current Ip2ave* is lower than 0.15 μA. The actual average value Ip2ave in this case is smaller than the ideal average value Ip2ave and deviates from the target average current Ip2ave*. In contrast, in the example at the right side ofFIG. 10, the controller90has performed the control for reducing the number of pulses to 1 when the target average current Ip2ave* is lower than 0.15 μA. As a result, when the target average current Ip2ave* is lower than 0.15 μA, the set number of pulses is reduced to 1, so that the pulse width is set to a value larger than or equal to 10 μs as a value required for achieving the target average current Ip2ave*. Specifically, the pulse width is set to a larger value than that in the table at the left side ofFIG. 10, and the pulse width does not fall below 10 μs. As a result, even when the target average current Ip2ave* is lower than 0.15 μA, a deviation has not occurred between the actual average value Ip2ave and the target average current Ip2ave*. InFIG. 10, the NO concentration corresponding to the target average current Ip2ave* is also shown. In the example at the left side ofFIG. 10, the actual average value Ip2ave deviates from the target average current Ip2ave* when the target average current Ip2ave* is lower than 0.15 μA, that is, when the NO concentration is a low concentration below 50 ppm, thus making it difficult to properly control the measurement pump cell41, as mentioned above, or causing the S/N ratio of the pump current Ip2to decrease. In contrast, in the example at the right side ofFIG. 10, such a problem is less likely to occur even when the NO concentration is a low concentration below 50 ppm.

As mentioned above, the controller90preferably controls the pump current Ip2in a range where the pulse width of the pump current Ip2does not fall below the predetermined lower limit value. Likewise, the controller90preferably controls the pump current Ip2in a range where the pulse interval (Tb inFIG. 9) of the pump current Ip2does not fall below a predetermined lower limit value. This is because, in a case where the pulse interval is too small, a subsequent pulse may rise before the pulse current completely falls, thus causing a problem similar to the case where the pulse width is too small. In the examples inFIGS. 9 and 10, the pump current Ip2is described as being a burst pulse current. Even in a case where the pump current Ip2is not a burst pulse current, as inFIG. 3, it is preferable that the pump current Ip2be controlled in a range where the pulse width (Ton inFIG. 3) and the pulse interval (Toff inFIG. 3) do not fall below the predetermined lower limit values.

Although the controller90and the power sources24,46, and52are illustrated separately from each other inFIG. 2in the above-described embodiment, the power sources24,46, and52may be regarded as being a part of the controller90.

EXAMPLES

Specific fabrication examples of gas sensors will be described below as practical examples. The present invention is not to be limited to the following practical examples.

Practical Example 1

Practical Example 1 is achieved by fabricating the gas sensor100shown inFIGS. 1 and 2. The sensor element101is fabricated as follows. First, ceramic green sheets corresponding to the individual layers1to6are each formed by mixing zirconia particles having 4 mol % of yttria added thereto as a stabilizer with an organic binder, a dispersant, a plasticizer, and an organic solvent, and then molding the mixture by tape molding. Then, the ceramic green sheets corresponding to the individual layers undergo hole-drilling, where appropriate, and screen-printing of a pattern of a conductive paste for forming electrodes and a circuit, and are subsequently stacked and pressure-bonded, so that a pressure-bonded body is obtained. A pattern for the measurement electrode44is formed by screen-printing a conductive paste having a mixture of zirconia particles and an organic binder onto Pt and Rh serving as noble metals. Then, an unbaked layered body having the size of the sensor element101is cut out from the pressure-bonded body and is baked, whereby the sensor element101is obtained. The fabricated sensor element101is electrically connected with the power sources and the controller90, whereby the gas sensor100according to Practical Example 1 is fabricated. The pump current Ip2to be applied by the pulse power source46is a rectangular single pulse current shown inFIG. 3, has a frequency of 100 Hz (i.e. a cycle T of 0.1 s), and a maximum current Ip2max of 50 μA.

Practical Example 2

In Practical Example 2, a gas sensor100similar to that in Practical Example 1 is fabricated except that the pump current Ip2to be applied by the pulse power source46is changed. The pump current Ip2to be applied by the pulse power source46is a burst pulse current shown inFIG. 6, has a frequency of 100 Hz (i.e. a cycle T of 0.1 s), has five pulses per cycle, and has 50% as the proportion (i.e., duty ratio) of the oscillation period TA occupying the cycle T (burst cycle).

Practical Example 3

In Practical Example 3, a gas sensor100similar to that in Practical Example 1 is fabricated except that the frequency is set to 200 Hz (i.e., the cycle T is set to 0.05 s).

Practical Example 4

In Practical Example 4, a gas sensor100similar to that in Practical Example 2 is fabricated except that the frequency is set to 200 Hz (i.e., the cycle T is set to 0.05 s).

Comparative Example 1

In Comparative Example 1, the gas sensor900according to the comparative example shown inFIG. 4is fabricated. Specifically, in Comparative Example 1, the pump current Ip2to be applied is a continuous electric current. The gas sensor900according to Comparative Example 1 is similar to that in Practical Example 1 except that the gas sensor900includes the measurement pump cell941shown inFIG. 4in place of the measurement pump cell41.

A durability test using a diesel engine is performed with respect to each of Practical Examples 1 to 4 and Comparative Example 1, and the degree of degradation in the measurement electrode44is evaluated. As indicators for evaluating the degree of degradation, the NO-sensitivity change rate before and after the durability test and the NO-sensitivity linear change rate before and after the durability test are measured. In detail, the test is performed as follows. The gas sensor according to Practical Example 1 is attached to an exhaust gas pipe of an automobile. Then, the temperature is set to 800° C. by applying electricity to the heater72, thereby heating the sensor element101. The controller90controls the aforementioned pump cells21,41, and50and acquires the voltages V0, V1, V2, and Vref from the aforementioned sensor cells80to83. In this state, a first model gas having nitrogen as its base gas and having a NO concentration of 500 ppm is made to flow through the pipe. After waiting for a control value from the controller90to the pulse power source46to become stable, the average value Ip2ave is measured as an initial value Ia500 of the detection sensitivity of the gas sensor relative to the NO at 500 ppm. Subsequently, a 40-minute operation pattern with an engine rotation speed ranging from 1500 to 3500 rpm and a load torque ranging from 0 to 350 N·m is repeated as a durability test for 1000 hours. In this case, the gas temperature is between 200° C. and 600° C., and the NOx concentration is between 0 ppm and 1500 ppm. During this 1000-hour period, the aforementioned individual pump cells are being continuously controlled and the voltages are being continuously acquired by the controller90. After the lapse of 1000 hours, the average value Ip2ave is measured by using a method similar to that for the initial value Ia500, and is set as a post-durability-test value Ib500. Then, the NO-sensitivity change rate [%] of the pump current Ip2before and after the durability test performed on the gas sensor according to Practical Example 1 is determined as the sensitivity change rate at the NO concentration of 500 ppm=[1−(post-durability-test value Ib500/initial value Ia500)]×100%. Moreover, the NO-sensitivity linear rate after the durability test performed on the gas sensor according to Practical Example 1 is determined as follows. First, similar to the post-durability-test value Ib500, the average value Ip2ave after a control value has become stable is measured with respect to a second model gas having nitrogen as its base gas and having an NO concentration of 0 ppm, and is set as a post-durability-test value Ib0. Moreover, the average value Ip2ave after a control value has become stable is also measured with respect to a third model gas having nitrogen as its base gas and having an NO concentration of 1500 ppm, and is set as a post-durability-test value Ib1500. Then, an NO-sensitivity gradient A between two points respectively corresponding to the case where the NO concentration in the gas sensor according to Practical Example 1 after the durability test is 0 ppm and the case where the NO concentration is 500 ppm is determined as a gradient A=(Ib500−Ib0)/(500−0). Moreover, an NO-sensitivity gradient B between two points respectively corresponding to the case where the NO concentration in the gas sensor according to Practical Example 1 after the durability test is 0 ppm and the case where the NO concentration is 1500 ppm is determined as a gradient B=(Ib1500−Ib0)/(1500−0). Then, the NO-sensitivity linear rate [%] after the durability test is determined as a NO-sensitivity linear rate=(gradient B)/(gradient A)×100%. For Practical Examples 2 to 4, the NO-sensitivity change rate [%] of the pump current Ip2before and after the durability test and the NO-sensitivity linear rate [%] after the durability test are similarly determined. For Comparative Example 1, the NO-sensitivity change rate [%] and the NO-sensitivity linear rate [%] are similarly determined except that the value of the pump current Ip2after being stable is measured in place of the average value Ip2ave.

Table 1 shows the frequency, the pulse method, the NO-sensitivity change rate, and the NO-sensitivity linear rate with respect to Practical Examples 1 to 4 and Comparative Example 1. A change in the pump current Ip2relative to the NO between the initial value and the state after the durability test decreases with decreasing absolute value of the NO-sensitivity change rate. This implies that deterioration in the detection accuracy with use of the gas sensor is suppressed. Furthermore, as the NO-sensitivity linear rate after the durability test approaches 100%, the NO-sensitivity linearity between three points respectively corresponding to 0 ppm, 500 ppm, and 1500 ppm as the NO concentration values is closer to an ideal state even after the durability test. This implies that deterioration in the detection accuracy with use of the gas sensor is suppressed. It is clear from Table 1 that, with regard to both the NO-sensitivity change rate and the NO-sensitivity linear rate, the results of Practical Examples 1 to 4 are more favorable than that of Comparative Example 1, and deterioration in the detection accuracy with use of the gas sensor is suppressed. This is conceivably because a decrease in the catalytic activity of the measurement electrode44caused by the pump current Ip2flowing thereto is suppressed in Practical Examples 1 to 4 due to the aforementioned reasons. In each of Practical Examples 1 to 4 and Comparative Example 1, the NO-sensitivity linear rate measured before the durability test is approximately 98%.