Gas sensor

Abstract

This gas sensor 100 comprises a sensor element 101 and a control device 95. The sensor element 101 includes an element body 102, a main pump cell 21, a first measurement pump cell 50, a second measurement pump cell 41, and a reference electrode 42. The first measurement pump cell 50 includes a first measurement electrode 51 (second inner electrode). The second measurement pump cell 41 includes a second measurement electrode 44. The control device 95 performs first measurement electrode deterioration determination processing for determining deterioration of the first measurement electrode 51 on the basis of whether the absolute value of a voltage V2 between the second measurement electrode 44 and the reference electrode 42 is included in a prescribed high voltage region during the execution of main pump control processing and first measurement pump control processing and during the stop of second measurement pump control processing.

Description

Gas Sensor

[0001] The present invention relates to a gas sensor.

[0002] Conventionally, gas sensors for measuring the concentrations of water and carbon dioxide in a measurement gas, such as automobile exhaust gas, have been known. For example, Patent Document 1 (JP-A-2005-102626) describes a gas sensor for measuring the concentrations of water vapor and carbon dioxide components in a measurement gas, comprising a sensor element having an oxygen-ion conductive solid electrolyte layer and an internal gas flow section. The gas flow section includes a gas inlet, a first diffusion-controlling section, a first internal cavity, a second diffusion-controlling section, and a second internal cavity, which are connected in this order. The main pump cell includes a main inner pump electrode disposed in the first internal cavity and an outer pump electrode disposed on the outer surface of the sensor element. The first measurement pump cell includes a first measurement inner pump electrode and an outer pump electrode disposed in the second internal cavity. The second measurement pump cell includes a second measurement inner pump electrode and an outer pump electrode disposed on the opposite side of the first measurement inner pump electrode from the second diffusion-controlling section. In this gas sensor, the oxygen partial pressure in the first internal space is adjusted by the main pump cell so that substantially all of the water vapor and carbon dioxide components in the measurement gas are decomposed in the first internal space. The first measurement pump cell supplies oxygen to the second internal space so that hydrogen produced by decomposition of the water vapor component is selectively combusted (oxidized) in the second internal space, and the concentration of the water vapor component in the measurement gas is measured based on the magnitude of the current flowing during this process. The second measurement pump cell supplies oxygen near the surface of the second measurement inner pump electrode so that carbon monoxide produced by decomposition of the carbon dioxide component is selectively combusted (oxidized) near the surface of the second measurement inner pump electrode, and the concentration of the carbon dioxide component in the measurement gas is measured based on the magnitude of the current flowing during this process.

[0003] Furthermore, Patent Document 2 describes a gas sensor for measuring the concentration of a water vapor component in a measurement gas, in which the metal component of the measurement inner pump electrode for measuring the concentration of the water vapor component is made of an alloy of gold and a noble metal other than gold (e.g., platinum), which makes the measurement inner pump electrode inactive to carbon monoxide and enables the measurement inner pump electrode to selectively burn (oxidize) hydrogen, thereby enabling the concentration of the water vapor component to be determined with high accuracy even when the measurement gas contains carbon dioxide.

[0004] Patent No. 5918177 Patent No. 6469462

[0005] In such gas sensors, the second inner electrode (the first measuring inner pump electrode in Patent Document 1) for oxidizing hydrogen may deteriorate with use of the gas sensor, resulting in a decrease in the accuracy of measuring the water concentration (concentration of water vapor components).

[0006] The present invention has been made to solve such problems, and a main object of the present invention is to determine the deterioration of the second inner electrode.

[0007] In order to achieve the above-mentioned main object, the present invention employs the following means.

[0008] [1] A gas sensor of the present invention is a gas sensor comprising a sensor element and a control device, for measuring a water concentration and / or a carbon dioxide concentration in a measurement gas, wherein the sensor element comprises: an element body having an oxygen ion conductive solid electrolyte layer and having a measurement gas flow section therein through which the measurement gas is introduced and circulated; a first pump cell comprising: a first inner electrode disposed in a first chamber of the measurement gas flow section and a first outer electrode disposed on an outer surface of the element body; a second pump cell comprising: a second inner electrode disposed in a second chamber located downstream of the first chamber in the measurement gas flow section and a second outer electrode disposed on the outer surface of the element body; a third pump cell comprising: a third inner electrode disposed in a third chamber located downstream of the second chamber in the measurement gas flow section and a third outer electrode disposed on the outer surface of the element body; and a reference electrode disposed inside the element body so as to come into contact with a reference gas; and the control device comprises: a first pump cell control process for controlling the first pump cell to pump oxygen from around the first inner electrode to around the first outer electrode, thereby reducing water and carbon dioxide in the measurement gas in the first chamber; a second pump cell control process for controlling the second pump cell to pump oxygen from around the second outer electrode to around the second inner electrode, thereby oxidizing, in the second chamber, hydrogen produced by the reduction of water in the first chamber; a third pump cell control process for controlling the third pump cell to pump oxygen from around the third outer electrode to around the third inner electrode, thereby oxidizing, in the third chamber, carbon monoxide produced by the reduction of carbon dioxide in the first chamber; and a water concentration measurement process for measuring the water concentration in the measurement gas based on a second pump current flowing through the second pump cell by the second pump cell control process, and / or a carbon dioxide concentration measurement process for measuring the carbon dioxide concentration in the measurement gas based on a third pump current flowing through the third pump cell by the third pump cell control process,and performing a second inner electrode deterioration determination process to determine deterioration of the second inner electrode based on whether an absolute value of a third voltage during execution of the second pump cell control process and during stoppage of the third pump cell control process is within a predetermined high voltage range.

[0009] In this gas sensor, the control device performs a second inner electrode deterioration determination process to determine deterioration of the second inner electrode based on whether the absolute value of the third voltage during execution of the first and second pump cell control processes and while the third pump cell control process is stopped is within a predetermined high-voltage range. When the second inner electrode deteriorates, the hydrogen oxidation ability of the second inner electrode decreases, causing some of the hydrogen that reaches the second chamber to reach the third chamber without being oxidized. In this case, the absolute value of the third voltage becomes larger than when almost no hydrogen reaches the third chamber. Therefore, deterioration of the second inner electrode can be determined based on whether the absolute value of the third voltage is within the predetermined high-voltage range. The inventors confirmed this through experiments, analyses, and the like.

[0010] [2] In the above-described gas sensor (the gas sensor described in [1]), the measurement gas may be exhaust gas from an internal combustion engine, and the control device may perform the second inner electrode deterioration determination process while the internal combustion engine is in a fuel-cut state or is stopped. The exhaust gas from the internal combustion engine during a fuel-cut state or while the engine is stopped has a lower carbon dioxide concentration than the exhaust gas from the internal combustion engine during operation other than when the engine is in a fuel-cut state. Therefore, the influence of carbon monoxide reaching the third chamber is smaller and the influence of hydrogen becomes dominant with respect to the magnitude of the absolute value of the third voltage. Therefore, the timing for performing the second inner electrode deterioration determination process is suitable when the internal combustion engine is in a fuel-cut state or is stopped.

[0011] [3] In the gas sensor described in [1] or [2], the second inner electrode may include a catalytically active first-class noble metal and a second-class noble metal that suppresses the catalytic activity of the first-class noble metal toward carbon monoxide. When the second inner electrode includes both the first and second-class noble metals, carbon monoxide generated from carbon dioxide in the first chamber is prevented from being oxidized around the second inner electrode before reaching the third inner electrode. This prevents a decrease in the accuracy of measuring the carbon dioxide concentration based on the third pump current flowing through the third inner electrode. However, when the second inner electrode includes the second-class noble metal, evaporation of the second-class noble metal from the second inner electrode occurs as the gas sensor is used, resulting in a decrease in the accuracy of measuring the water concentration and / or the carbon dioxide concentration. Therefore, when the second inner electrode includes the second-class noble metal, it is highly important to determine the deterioration of the third inner electrode.

[0012] [4] In the above-described gas sensor (the gas sensor described in any one of [1] to [3]), two or more of the first outer electrode, the second outer electrode, and the third outer electrode may be common electrodes.

[0013] 1 is a schematic cross-sectional view showing an example of the configuration of a gas sensor 100. FIG. 2 is a block diagram showing the electrical connection relationship between a control device 95 and each cell, etc. FIG. 3 is a flowchart showing an example of a processing routine. FIG. 4 is a graph showing the change in voltage V2 depending on whether or not the first measurement electrode 51 has deteriorated. FIG. 5 is a graph showing the relationship between the carbon monoxide concentration and the hydrogen concentration and the voltage V2. FIG. 6 is a schematic cross-sectional view of a sensor element 201 of a modified example.

[0014] Next, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a cross-sectional view illustrating an example of the configuration of a gas sensor 100 according to an embodiment of the present invention. FIG. 2 is a block diagram illustrating the electrical connections between a control device 95, each cell, and a heater 72. The gas sensor 100 is attached to a pipe, such as an exhaust gas pipe of an internal combustion engine. The gas sensor 100 measures the concentration of a specific gas in the measured gas, which is exhaust gas from the internal combustion engine. In this embodiment, the gas sensor 100 measures the water concentration and carbon dioxide concentration as the specific gas concentration. The gas sensor 100 includes a sensor element 101 having an element body 102 shaped like an elongated rectangular parallelepiped; cells 21, 41, 50, 80-83 included in the sensor element 101; a heater unit 70 provided inside the sensor element 101; and a control device 95 having variable power supplies 24, 46, 52, and a heater power supply 76, and controlling the entire gas sensor 100. The longitudinal direction of the sensor element 101 (left-right direction in Figure 1) is defined as the front-to-back direction, the thickness direction of the sensor element 101 (up-to-down direction in Figure 1) is defined as the up-to-down direction, and the width direction of the sensor element 101 (direction perpendicular to the front-to-back direction and up-to-down direction) is defined as the left-to-right direction.

[0015] The element body 102 is a laminate formed by stacking six layers, in this order from bottom to top, from the drawing: a first substrate layer 1, a second substrate layer 2, a third substrate layer 3, a first solid electrolyte layer 4, a spacer layer 5, and a second solid electrolyte layer 6, each of which is made of an oxygen-ion conductive solid electrolyte layer such as zirconia (ZrO). The solid electrolyte forming these six layers is dense and airtight. The element body 102 is manufactured, for example, by laminating ceramic green sheets corresponding to each layer after performing predetermined processing and printing a circuit pattern thereon, and then firing the resulting sheets to integrate them.

[0016] At the front end side of the sensor element 101 (element body 102), between the lower surface of the second solid electrolyte layer 6 and the upper surface of the first solid electrolyte layer 4, a gas inlet 10, a first diffusion-controlling section 11, a buffer space 12, a second diffusion-controlling section 13, a first internal space 20, a third diffusion-controlling section 30, a second internal space 40, a fourth diffusion-controlling section 60, and a third internal space 61 are formed adjacent to each other and communicate with each other in this order.

[0017] The gas inlet 10, buffer space 12, first internal space 20, second internal space 40, and third internal space 61 are spaces inside the sensor element 101, which are defined by hollowing out the spacer layer 5, with an upper portion defined by the underside of the second solid electrolyte layer 6, a lower portion defined by the upper surface of the first solid electrolyte layer 4, and sides defined by the side surfaces of the spacer layer 5.

[0018] The first diffusion-controlling portion 11, the second diffusion-controlling portion 13, and the third diffusion-controlling portion 30 are each provided as two horizontally elongated slits (with the opening extending longitudinally in the direction perpendicular to the drawing). The fourth diffusion-controlling portion 60 is provided as a single horizontally elongated slit (with the opening extending longitudinally in the direction perpendicular to the drawing) formed as a gap with the underside of the second solid electrolyte layer 6. The portion from the gas inlet 10 to the third internal space 61 is also referred to as a measurement gas flow portion.

[0019] The sensor element 101 (element body 102) includes a reference gas inlet 49 that passes a reference gas from the outside of the sensor element 101 to the reference electrode 42 when measuring the concentration of a specific gas. The reference gas inlet 49 includes a reference gas inlet space 43 and a reference gas inlet layer 48. The reference gas inlet space 43 is a space extending inward from the rear end surface of the sensor element 101. The reference gas inlet space 43 is located between the upper surface of the third substrate layer 3 and the lower surface of the spacer layer 5, and is laterally bounded by the side surface of the first solid electrolyte layer 4. The reference gas inlet space 43 opens at the rear end surface of the sensor element 101, and this opening functions as an inlet 49a of the reference gas inlet 49. The reference gas is introduced into the reference gas inlet space 43 from the inlet 49a. The reference gas inlet 49 introduces the reference gas introduced from the inlet 49a into the reference electrode 42 while providing a predetermined diffusion resistance to the reference gas. In this embodiment, the reference gas is the atmosphere.

[0020] The reference gas introduction layer 48 is provided between the upper surface of the third substrate layer 3 and the lower surface of the first solid electrolyte layer 4. The reference gas introduction layer 48 is a porous body made of ceramics such as alumina. A portion of the upper surface of the reference gas introduction layer 48 is exposed within the reference gas introduction space 43. The reference gas introduction layer 48 is formed to cover the reference electrode 42. The reference gas introduction layer 48 allows the reference gas to flow from the reference gas introduction space 43 to the reference electrode 42.

[0021] The reference electrode 42 is an electrode formed in a manner sandwiched between the upper surface of the third substrate layer 3 and the first solid electrolyte layer 4, and as described above, is surrounded by a reference gas introduction layer 48 that connects to the reference gas introduction space 43. As will be described later, the reference electrode 42 can be used to measure the oxygen concentration (oxygen partial pressure) in the first internal space 20, the second internal space 40, and the third internal space 61. The reference electrode 42 is formed as a porous cermet electrode (for example, a cermet electrode of Pt and ZrO).

[0022] In the measurement gas flow section, the gas inlet 10 is a section that opens to the external space, and the measurement gas is introduced into the sensor element 101 from the external space through the gas inlet 10. The first diffusion-controlling section 11 is a section that imparts a predetermined diffusion resistance to the measurement gas introduced through the gas inlet 10. The buffer space 12 is a space provided for guiding the measurement gas introduced through the first diffusion-controlling section 11 to the second diffusion-controlling section 13. The second diffusion-controlling section 13 is a section that imparts a predetermined diffusion resistance to the measurement gas introduced from the buffer space 12 into the first internal space 20. When the measurement gas is introduced from the outside of the sensor element 101 into the first internal space 20, the measurement gas is suddenly taken into the sensor element 101 through the gas inlet 10 due to pressure fluctuations of the measurement gas in the external space (exhaust pressure pulsations in the case where the measurement gas is automobile exhaust gas). The measurement gas is not introduced directly into the first internal space 20, but is introduced into the first internal space 20 after the pressure fluctuations of the measurement gas are canceled out through the first diffusion-controlling part 11, buffer space 12, and second diffusion-controlling part 13. This makes the pressure fluctuations of the measurement gas introduced into the first internal space 20 almost negligible. The first internal space 20 is provided as a space for adjusting the oxygen partial pressure in the measurement gas introduced through the second diffusion-controlling part 13. The oxygen partial pressure is adjusted by operating the main pump cell 21.

[0023] The main pump cell 21 is an electrochemical pump cell including an inner pump electrode 22 having a ceiling electrode portion 22 a provided on almost the entire lower surface of the second solid electrolyte layer 6 facing the first internal space 20, an outer pump electrode 23 provided on the upper surface of the second solid electrolyte layer 6 in a region corresponding to the ceiling electrode portion 22 a so as to be exposed to the outside of the sensor element 101, and the second solid electrolyte layer 6, the spacer layer 5, and the first solid electrolyte layer 4 which form a current path between these electrodes.

[0024] The inner pump electrode 22 is formed across the upper and lower solid electrolyte layers (the second solid electrolyte layer 6 and the first solid electrolyte layer 4) that define the first internal space 20 and the spacer layer 5 that provides the side walls. Specifically, a ceiling electrode portion 22a is formed on the lower surface of the second solid electrolyte layer 6 that provides the ceiling surface of the first internal space 20, and a bottom electrode portion 22b is formed on the upper surface of the first solid electrolyte layer 4 that provides the bottom surface. Side electrode portions (not shown) are formed on the side wall surfaces (inner surfaces) of the spacer layer 5 that constitute both side wall portions of the first internal space 20 so as to connect the ceiling electrode portion 22a and the bottom electrode portion 22b, and are arranged in a tunnel-like structure at the locations where the side electrode portions are provided.

[0025] In the main pump cell 21, by applying a desired voltage Vp0 between the inner pump electrode 22 and the outer pump electrode 23 and flowing a pump current Ip0 in a positive or negative direction between the inner pump electrode 22 and the outer pump electrode 23, it is possible to pump oxygen from the first internal space 20 out to the external space, or to pump oxygen from the external space into the first internal space 20.

[0026] In addition, in order to detect the oxygen concentration (oxygen partial pressure) in the atmosphere in the first internal space 20, an electrochemical sensor cell, i.e., an oxygen partial pressure detection sensor cell 80 for controlling the main pump, is configured by the inner pump electrode 22, the second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, the third substrate layer 3, and the reference electrode 42.

[0027] The oxygen concentration (oxygen partial pressure) in the first internal space 20 can be determined by measuring the electromotive force (voltage V0) in the oxygen partial pressure detection sensor cell 80 for controlling the main pump. Furthermore, the pump current Ip0 is controlled by feedback controlling the voltage Vp0 of the variable power supply 24 so that the voltage V0 becomes a target value. This adjusts the oxygen concentration in the first internal space 20.

[0028] The third diffusion-controlling section 30 is a section that imparts a predetermined diffusion resistance to the measurement gas whose oxygen concentration (oxygen partial pressure) has been controlled by the operation of the main pump cell 21 in the first internal space 20, and guides the measurement gas into the second internal space 40.

[0029] The second internal space 40 is provided as a space for adjusting the oxygen partial pressure of the measurement gas introduced through the third diffusion-controlling section 30 using the first measurement pump cell 50, and for performing processing related to measuring the water concentration in the measurement gas.

[0030] The first measurement pump cell 50 is an electrochemical pump cell including a first measurement electrode 51 having a ceiling electrode portion 51 a provided on substantially the entire lower surface of the second solid electrolyte layer 6 facing the second internal space 40, an outer pump electrode 23 (not limited to the outer pump electrode 23, but any appropriate electrode disposed on the outer surface of the sensor element 101 will suffice), the second solid electrolyte layer 6, the spacer layer 5, and the first solid electrolyte layer 4.

[0031] The first measurement electrode 51 is disposed in the second internal space 40 in a tunnel-shaped structure similar to the inner pump electrode 22 provided in the first internal space 20. That is, a ceiling electrode portion 51a is formed on the second solid electrolyte layer 6 that provides the ceiling surface of the second internal space 40, and a bottom electrode portion 51b is formed on the first solid electrolyte layer 4 that provides the bottom surface of the second internal space 40. Side electrodes (not shown) connecting the ceiling electrode portion 51a and the bottom electrode portion 51b are formed on both wall surfaces of the spacer layer 5 that provide the side walls of the second internal space 40, forming a tunnel-shaped structure.

[0032] In the first measurement pump cell 50, by applying a desired voltage Vp1 between the first measurement electrode 51 and the outer pump electrode 23, it is possible to pump oxygen in the atmosphere within the second internal space 40 out to the external space or pump oxygen from the external space into the second internal space 40.

[0033] In addition, in order to control the oxygen partial pressure in the atmosphere within the second internal space 40, the first measurement electrode 51, the reference electrode 42, the second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, and the third substrate layer 3 constitute an electrochemical sensor cell, i.e., an oxygen partial pressure detection sensor cell 81 for controlling the first measurement pump.

[0034] The first measurement pump cell 50 performs pumping using a variable power supply 52 whose voltage is controlled based on the electromotive force (voltage V1) detected by the first measurement pump control oxygen partial pressure detection sensor cell 81. As a result, the oxygen partial pressure in the atmosphere within the second internal space 40 is adjusted by the pump current Ip1 flowing through the first measurement pump cell 50.

[0035] The fourth diffusion-controlling section 60 is a section that imparts a predetermined diffusion resistance to the measurement gas whose oxygen concentration (oxygen partial pressure) has been controlled by the operation of the first measurement pump cell 50 in the second internal space 40, and guides the measurement gas into the third internal space 61.

[0036] The third internal space 61 is provided as a space for adjusting the oxygen partial pressure of the measurement gas introduced through the fourth diffusion-controlling section 60 using the second measurement pump cell 41, and for performing processing related to measuring the carbon dioxide concentration in the measurement gas.

[0037] The second measurement pump cell 41 is an electrochemical pump cell including a second measurement electrode 44 provided on the upper surface of the first solid electrolyte layer 4 facing the third internal space 61, an outer pump electrode 23 (not limited to the outer pump electrode 23, but any suitable electrode disposed on the outer surface of the sensor element 101 will suffice), the second solid electrolyte layer 6, the spacer layer 5, and the first solid electrolyte layer 4.

[0038] In the second measurement pump cell 41, by applying a desired voltage Vp2 between the second measurement electrode 44 and the outer pump electrode 23, it is possible to pump oxygen in the atmosphere within the third internal space 61 into the external space, or pump oxygen from the external space into the second internal space 40.

[0039] In addition, in order to detect the oxygen partial pressure around the second measurement electrode 44, the first solid electrolyte layer 4, the third substrate layer 3, the second measurement electrode 44, and the reference electrode 42 constitute an electrochemical sensor cell, i.e., an oxygen partial pressure detection sensor cell 82 for controlling the second measurement pump.

[0040] The variable power supply 46 is controlled based on the electromotive force (voltage V2) detected by the oxygen partial pressure detection sensor cell 82 for controlling the second measurement pump, and the voltage Vp2 of the variable power supply 46 is applied to the second measurement pump cell 41. As a result, the oxygen partial pressure in the atmosphere in the third internal space 61 is adjusted by the pump current Ip2 flowing through the second measurement pump cell 41.

[0041] Furthermore, the second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, the third substrate layer 3, the outer pump electrode 23, and the reference electrode 42 constitute an electrochemical sensor cell 83, and the electromotive force (voltage Vref) obtained by this sensor cell 83 makes it possible to detect the oxygen partial pressure in the measurement gas outside the sensor.

[0042] Here, the electrodes 22, 23, 42, 44, and 51 will be described. The inner pump electrode 22, the first measurement electrode 51, and the second measurement electrode 44 each contain a catalytically active first-class precious metal. Examples of the first-class precious metal include at least one of Pt, Rh, Ir, Ru, and Pd. The outer pump electrode 23 and the reference electrode 42 also contain a first-class precious metal. The first measurement electrode 51 preferably contains a second-class precious metal that suppresses the catalytic activity of the first-class precious metal toward carbon monoxide. The first measurement electrode 51 contains the second-class precious metal, thereby weakening its ability to oxidize carbon monoxide. Examples of the second-class precious metal include Au. The inner pump electrode 22 and the second measurement electrode 44 do not contain the second-class precious metal. The outer pump electrode 23 and the reference electrode 42 also preferably do not contain the second-class precious metal. Each of the electrodes 22, 23, 42, 44, and 51 is preferably a cermet containing a noble metal and a solid electrolyte (e.g., ZrO) having oxygen ion conductivity. Each of the electrodes 22, 23, 42, 44, and 51 is preferably a porous body. In this embodiment, the first measurement electrode 51 is a porous cermet electrode of Pt containing 1% Au and ZrO. The inner pump electrode 22, the outer pump electrode 23, the reference electrode 42, and the second measurement electrode 44 are all porous cermet electrodes of Pt and ZrO.

[0043] In order to increase the oxygen ion conductivity of the solid electrolyte, the sensor element 101 includes a heater unit 70 that adjusts the temperature by heating and maintaining the temperature of the sensor element 101. The heater unit 70 includes a heater connector electrode 71, a heater 72, a through-hole 73, a heater insulating layer 74, and a pressure release hole 75.

[0044] The heater connector electrode 71 is an electrode formed in a manner to contact the lower surface of the first substrate layer 1. By connecting the heater connector electrode 71 to a heater power supply 76 (see FIG. 2), it is possible to supply power from the heater power supply 76 to the heater section 70.

[0045] The heater 72 is an electrical resistor sandwiched between the second substrate layer 2 and the third substrate layer 3. The heater 72 is connected to a heater connector electrode 71 via a through hole 73, and generates heat when power is supplied from a heater power supply 76 through the heater connector electrode 71, thereby heating and keeping warm the solid electrolyte that forms the sensor element 101.

[0046] In addition, the heater 72 is embedded throughout the entire area from the first internal space 20 to the third internal space 61, making it possible to adjust the temperature of the entire sensor element 101 to a temperature at which the solid electrolyte described above is activated.

[0047] The heater insulating layer 74 is an insulating layer made of an insulator such as alumina and formed on the upper and lower surfaces of the heater 72. The heater insulating layer 74 is formed for the purpose of obtaining electrical insulation between the second substrate layer 2 and the heater 72, and between the third substrate layer 3 and the heater 72.

[0048] The pressure release hole 75 is a portion that penetrates the third substrate layer 3 and the reference gas introduction layer 48 and is provided so as to communicate with the reference gas introduction space 43, and is formed for the purpose of alleviating the increase in internal pressure that accompanies a rise in temperature within the heater insulation layer 74.

[0049] 2, the control device 95 includes the variable power supplies 24, 46, and 52, the heater power supply 76, and a control unit 96. The control unit 96 is a microprocessor including a CPU 97 and a storage unit 98. The storage unit 98 is a rewritable nonvolatile memory capable of storing, for example, various programs and data. The control unit 96 receives the voltage V0 of the oxygen partial pressure detection sensor cell 80 for controlling the main pump, the voltage V1 of the oxygen partial pressure detection sensor cell 81 for controlling the first measurement pump, the voltage V2 of the oxygen partial pressure detection sensor cell 82 for controlling the second measurement pump, the voltage Vref of the sensor cell 83, the pump current Ip0 flowing through the main pump cell 21, the pump current Ip1 flowing through the first measurement pump cell 50, and the pump current Ip2 flowing through the second measurement pump cell 41. The control unit 96 also outputs control signals to the variable power supplies 24, 52, 46 to control the voltages Vp0, Vp1, Vp2 output by the variable power supplies 24, 52, 46, thereby controlling the main pump cell 21, the first measurement pump cell 50, and the second measurement pump cell 41. The control unit 96 also outputs control signals to the heater power supply 76 to control the power supplied from the heater power supply 76 to the heater 72. The memory unit 98 also stores target values ​​V0*, V1*, V2*, etc., which will be described later. The CPU 97 of the control unit 96 controls the cells 21, 50, 41 by referring to these target values ​​V0*, V1*, V2*.

[0050] The control unit 96 performs a main pump control process (an example of a first pump cell control process) to control the main pump cell 21 to pump oxygen from around the inner pump electrode 22 to around the outer pump electrode 23. Specifically, the control unit 96 controls the main pump cell 21 by feedback-controlling the voltage Vp0 of the variable power supply 24 so that the voltage V0 becomes a target value V0*. The target value V0* is set as a value that causes the oxygen concentration in the first internal space 20 to be a predetermined low concentration that is low enough to reduce substantially all of the water and carbon dioxide in the measurement gas. By performing this main pump control process, water in the measurement gas is reduced to generate hydrogen and oxygen, and carbon dioxide in the measurement gas is reduced to generate carbon monoxide and oxygen in the first internal space 20. The generated oxygen is pumped from around the inner pump electrode 22 to around the outer pump electrode 23 by the pump current Ip0 flowing through the main pump cell 21.

[0051] The control unit 96 performs a first measurement pump control process (an example of a second pump cell control process) to control the first measurement pump cell 50 so as to pump oxygen from the periphery of the outer pump electrode 23 to the periphery of the first measurement electrode 51. Specifically, the control unit 96 controls the first measurement pump cell 50 by feedback-controlling the voltage Vp1 of the variable power supply 52 so that the voltage V1 becomes a target value V1*. The target value V1* is set as a value that causes the oxygen concentration in the second internal space 40 to become a predetermined concentration that substantially oxidizes all of the hydrogen in the second internal space 40. By performing this first measurement pump control process, hydrogen generated by the reduction of water in the first internal space 20 is oxidized in the second internal space 40, thereby generating water again. The pump current Ip1 flowing through the first measuring pump cell 50 correlates with the amount of oxygen pumped into the second internal space 40 to oxidize the hydrogen therein, and thus correlates with the amount of water in the measurement gas in the first internal space 20, which generated the hydrogen in the second internal space 40. Therefore, the pump current Ip1 correlates with the water concentration in the measurement gas, and the water concentration in the measurement gas can be measured based on the pump current Ip1. The control unit 96 derives the water concentration in the measurement gas based on the pump current Ip1, for example, using a correspondence relationship between the pump current Ip1 and the water concentration stored in the memory unit 98. The correspondence relationship between the pump current Ip1 and the water concentration can be experimentally determined in advance as a relational expression (e.g., a linear or quadratic function) or a map. Hereinafter, the process of measuring the water concentration in the measurement gas based on the pump current Ip1 will be referred to as a water concentration measurement process.

[0052] The control unit 96 performs a second measurement pump control process (an example of a third pump cell control process) to control the second measurement pump cell 41 so as to pump oxygen from the periphery of the outer pump electrode 23 to the periphery of the second measurement electrode 44. Specifically, the control unit 96 controls the second measurement pump cell 41 by feedback-controlling the voltage Vp2 of the variable power supply 46 so that the voltage V2 becomes a target value V2*. The target value V2* is set as a value such that the oxygen concentration in the third internal space 61 becomes a predetermined concentration that substantially all of the carbon monoxide in the third internal space 61 is oxidized. By performing this second measurement pump control process, carbon monoxide generated by the reduction of carbon dioxide in the first internal space 20 is oxidized in the third internal space 61, thereby generating carbon dioxide again. The pump current Ip2 flowing through the second measuring pump cell 41 correlates with the amount of oxygen pumped into the third internal space 61 to oxidize the carbon monoxide in the third internal space 61, and thus correlates with the amount of carbon dioxide in the measurement gas in the first internal space 20, which generated the carbon monoxide in the third internal space 61. Therefore, the pump current Ip2 correlates with the carbon dioxide concentration in the measurement gas, and the carbon dioxide concentration in the measurement gas can be measured based on the pump current Ip2. The control unit 96 derives the carbon dioxide concentration in the measurement gas based on the pump current Ip2, for example, using a correspondence relationship between the pump current Ip2 and the carbon dioxide concentration stored in the memory unit 98. The correspondence relationship between the pump current Ip2 and the carbon dioxide concentration can be experimentally determined in advance as a relational expression (e.g., a linear or quadratic function) or a map. Hereinafter, the process of measuring the carbon dioxide concentration in the measurement gas based on the pump current Ip2 will be referred to as a carbon dioxide concentration measurement process.

[0053] Both hydrogen and carbon monoxide generated in the first internal space 20 reach the second internal space 40. However, hydrogen has a faster gas diffusion rate and is more likely to combine with oxygen. Therefore, in the second internal space 40, hydrogen can be selectively oxidized between hydrogen and carbon monoxide by the first measurement pump control process. Since almost no hydrogen reaches the third internal space 61 downstream of the second internal space 40, carbon monoxide can be oxidized by the second measurement pump control process. In this embodiment, as described above, the first measurement electrode 51 contains a second-class noble metal, which weakens its ability to oxidize carbon monoxide. Therefore, in the vicinity of the first measurement electrode 51, i.e., in the second internal space 40, hydrogen can be more selectively oxidized between hydrogen and carbon monoxide by the first measurement pump control process.

[0054] The control unit 96 performs a heater control process in which it outputs a control signal to the heater power supply 76 to control the temperature of the heater 72 to a target temperature (e.g., 800°C). The target temperature of the heater 72 is determined as the temperature at which the solid electrolyte is activated, plus a margin. The temperature of the heater 72 can be expressed as a linear function of the resistance of the heater 72. Therefore, in the heater control process, the control unit 96 calculates the resistance of the heater 72 as a value that can be regarded as the temperature of the heater 72 (a value that can be converted into temperature), and then feedback-controls the heater power supply 76 so that the calculated resistance value becomes the target resistance value (the resistance value corresponding to the target temperature). The control unit 96 can, for example, acquire the voltage of the heater 72 and the current flowing through the heater 72 and calculate the resistance of the heater 72 based on the acquired voltage and current. The control unit 96 may calculate the resistance of the heater 72 using, for example, a three-terminal method or a four-terminal method. When energizing the heater 72 , the heater power supply 76 adjusts the power supplied to the heater 72 by changing the value of the voltage applied to the heater 72 based on a control signal from the control unit 96 , for example.

[0055] The control device 95, including the variable power supplies 24, 46, 52 and the heater power supply 76 shown in FIG. 2, is actually connected to each electrode inside the sensor element 101 via lead wires (not shown) formed within the sensor element 101 and connector electrodes (not shown) formed on the rear end side of the sensor element 101 (only the heater connector electrode 71 is shown in FIG. 1).

[0056] When the gas sensor 100 configured as described above is used, the CPU 97 of the control unit 96 first performs the heater control process described above to control the temperature of the heater 72 to the target temperature. Once the temperature of the heater 72 reaches the target temperature (or near the target temperature), the CPU 97 starts controlling the pump cells 21, 41, and 50 (main pump control process, first measurement pump control process, and second measurement pump control process) and acquiring the voltages V0, V1, V2, and Vref from the sensor cells 80 to 83. While continuously performing these processes, the control unit 96 also repeatedly performs a water concentration measurement process and a carbon dioxide concentration measurement process, and executes a deterioration determination process for the first measurement electrode 51, which will be described later. In this embodiment, the period from the start to the end of the heater control process is considered to be one use of the gas sensor 100. The control unit 96 starts the heater control process upon receiving a command from the engine ECU (not shown), for example, when the internal combustion engine starts operating, and ends the heater control process upon receiving a command from the engine ECU when the internal combustion engine stops operating.

[0057] Next, a description will be given of an example of a process for determining deterioration of the first measurement electrode 51. Fig. 3 is a flowchart showing an example of a processing routine including the process for determining deterioration of the first measurement electrode 51. This routine is stored in, for example, the storage unit 98 of the control unit 96, and is repeatedly executed by the CPU 97.

[0058] 3 is executed, the CPU 97 first determines whether the internal combustion engine is in a fuel-cut state or is stopped (step S100). For example, the CPU 97 obtains information from the engine ECU that can identify whether the internal combustion engine is in a fuel-cut state or is stopped, and makes the determination in step S100 based on the obtained information. Alternatively, the CPU 97 may detect the oxygen concentration in the measurement gas surrounding the sensor element 101 based on the voltage Vref of the sensor cell 83 described above, and make the determination in step S100 based on whether the detected oxygen concentration is within a high-concentration range that indicates a fuel-cut state or a stopped state. If the CPU 97 determines in step S100 that the internal combustion engine is neither in a fuel-cut state nor stopped, it terminates this routine.

[0059] If the CPU 97 determines in step S100 that the internal combustion engine is in a fuel-cut state or is stopped, it stops the second measurement pump control process (step S110) and measures the voltage V2 in that state (step S120). That is, the CPU 97 measures the voltage V2 while the main pump control process and the first measurement pump control process are being executed and while the second measurement pump control process is stopped. The CPU 97 then performs a deterioration determination process for the first measurement electrode 51 based on the measured voltage V2 (step S130). The CPU 97 determines whether the first measurement electrode 51 has deteriorated based on whether the absolute value of the measured voltage V2 is within a predetermined high-voltage range. For example, the CPU 97 determines that the first measurement electrode 51 has deteriorated if the absolute value of the measured voltage V2 is greater than a predetermined threshold value V2ref1, and determines that the first measurement electrode 51 has not deteriorated if the absolute value of the measured voltage V2 is equal to or less than the threshold value V2ref1. If the absolute value of the voltage V2 is greater than the threshold value V2ref1 in step S130, the CPU 97 turns on the first measurement electrode deterioration flag (step S140), starts (resumes) the second measurement pump control process (step S150), and ends this routine. If the absolute value of the voltage V2 is equal to or less than the threshold value V2ref1 in step S130, the CPU 97 performs step S150 without turning on the first measurement electrode deterioration flag, and ends this routine. Note that if the CPU 97 determines in step S130 that the first measurement electrode 51 is deteriorated, it preferably notifies other devices such as the engine ECU and a user such as the driver of an abnormality in the gas sensor 100. Furthermore, if the first measurement electrode deterioration flag is on, the CPU 97 may not perform at least one of the water concentration measurement process and the carbon dioxide concentration measurement process. The CPU 97 turns off the first measurement electrode deterioration flag based on operation by the operator when the deterioration of the first measurement electrode 51 is resolved, for example, after replacing the sensor element 101 in which the first measurement electrode 51 has deteriorated.

[0060] FIG. 4 is a graph showing the change in voltage V2 depending on whether or not the first measurement electrode 51 has deteriorated. FIG. 5 is a graph showing the relationship between the carbon monoxide concentration and the hydrogen concentration and the voltage V2. The inventors conducted the following experiment on the gas sensor 100 to obtain the graphs shown in FIGS. 4 and 5. First, a gas sensor 100 including an unused (initial state) sensor element 101 and a gas sensor 100 including a sensor element 101 after deterioration of the first measurement electrode 51 were prepared. For the deteriorated sensor element 101, the heater control process and the first measurement pump control process were executed for 1,000 hours while the tip side of the sensor element 101 was exposed to exhaust gas from an internal combustion engine, thereby deteriorating the first measurement electrode 51.

[0061] Next, the relationship between the water concentration in the measurement gas and the voltage V2 was investigated for both the initial sensor element 101 and the deteriorated sensor element 101, and the graph shown in FIG. 4 was obtained. Specifically, the gas sensor 100 including the initial sensor element 101 was attached to a pipe so that the tip of the sensor element 101 protruded into the pipe. Next, a model gas was flowed through the pipe as the measurement gas, and the control unit 96 was caused to execute the heater control process, the main pump control process, the first measurement pump control process, and the second measurement pump control process. The model gas used was a gas containing nitrogen as the base gas, a carbon dioxide concentration of 10%, and gradually varying water concentrations. Steps S110 and S120 of FIG. 3 were then executed at regular intervals, i.e., while the main pump control process and the first measurement pump control process were being executed and the second measurement pump control process was stopped, and multiple data sets corresponding to the measured voltage V2 and the water concentration of the model gas at that time were obtained. Using a similar method, multiple data sets corresponding to the voltage V2 and water concentration of the deteriorated sensor element 101 were also obtained. These data are plotted in the graph shown in FIG.

[0062] Furthermore, the relationship between the carbon monoxide concentration and hydrogen concentration and the voltage V2 was investigated for the sensor element 101 in the initial state, and the graph shown in FIG. 5 was obtained. Specifically, as in the above-described experiment, the gas sensor 100 was attached to a pipe, and the voltage V2 was measured while a model gas was flowing as the measurement gas. However, the control unit 96 was caused to execute the heater control process, but the main pump control process, the first measurement pump control process, and the second measurement pump control process were not executed. A gas containing carbon monoxide and nitrogen as the base gas was used as the model gas. The voltage V2 was measured while gradually changing the carbon monoxide concentration of the model gas, and multiple pieces of data correlating the voltage V2 with the carbon monoxide concentration were obtained. Similarly, a gas containing hydrogen and nitrogen as the base gas was used as the model gas, and the voltage V2 was measured while gradually changing the hydrogen concentration of the model gas, and multiple pieces of data correlating the voltage V2 with the hydrogen concentration were obtained. These data were plotted on the graph shown in FIG. 5.

[0063] As can be seen from Figure 4, in the initial state of the sensor element 101, the voltage V2 was a substantially constant value (approximately 870 mV) regardless of the water concentration. This value was substantially equal to the value of the voltage V2 when the carbon monoxide concentration was 10% in Figure 5. On the other hand, in the deteriorated sensor element 101, when the water concentration was 5% or less, the voltage V2 was the same as the voltage V2 of the initial state sensor element 101, but when the water concentration was 10% or more, the value of the voltage V2 was higher than that of the initial state sensor element 101. Furthermore, the value of the voltage V2 of the deteriorated sensor element 101 at this time was substantially equal to the value of the voltage V2 (approximately 970 mV) when the hydrogen concentration was 10% or more in Figure 5. This is thought to be because the hydrogen oxidation ability of the first measuring electrode 51 decreases as the first measuring electrode 51 deteriorates, and at least a portion of the hydrogen that reaches the second internal space 40 reaches the second measuring electrode 44 in the third internal space 61 without being oxidized, even when the first measurement pump control process is being executed. Using this, in step S130 of FIG. 3 described above, whether the first measurement electrode 51 has deteriorated is determined by determining whether the voltage V2 is within a predetermined high-voltage region. The predetermined high-voltage region can be determined in advance through experiments or the like as a region that the absolute value of the voltage V2 does not reach when carbon monoxide is present around the second measurement electrode 44 but is almost completely absent, but reaches when hydrogen is present around the second measurement electrode 44. In this embodiment, the threshold V2ref1 described above is set to 920 mV as a value that distinguishes between the voltage V2 at the highest carbon monoxide concentration of 15% in FIG. 5 and the voltage V2 at the lowest hydrogen concentration of 0.1% in FIG. 5, and the region exceeding this threshold is defined as the high-voltage region. Note that the voltage V2 varies depending on the structure of the sensor element 101 (e.g., the positions of the second measurement electrode 44 and the reference electrode 42 relative to the heater 72), so the values ​​of the voltage V2 and the threshold V2ref1 shown in FIGS. 4 and 5 are merely examples.

[0064] During the second measurement pump control process, the voltage V2 is controlled to the target value V2*. Therefore, the voltage V2 does not correspond to the hydrogen concentration or carbon monoxide concentration around the second measurement electrode 44. Therefore, it is not possible to determine the deterioration of the first measurement electrode 51 based on the voltage V2. This is the reason why the voltage V2 is measured in step S120 with the second measurement pump control process stopped in step S110. Furthermore, if the main pump control process is not performed, hydrogen is not produced from the water in the measurement gas. Furthermore, even if the main pump control process is being performed, if the first measurement pump control process is not performed, hydrogen will reach the second measurement electrode 44 even if the first measurement electrode 51 is not deteriorated. Therefore, the voltage V2 is measured in step S120 during the main pump control process and the first measurement pump control process.

[0065] 4, even with the deteriorated sensor element 101, when the water concentration is 5% or less, the voltage V2 is the same as the voltage V2 of the initial sensor element 101. This is thought to be because even with the deteriorated first measurement electrode 51, if only a small amount of hydrogen reaches the second internal space 40, all of the hydrogen can be oxidized, and no hydrogen reaches the second measurement electrode 44. Therefore, if the first measurement electrode 51 deteriorates further, the voltage V2 will exceed the threshold value V2ref1 even when the water concentration is 5% or less, and deterioration can be detected based on the voltage V2.

[0066] A specific example of deterioration of the first measurement electrode 51 is a decrease in the catalytic activity due to the sintering of the first-class precious metal contained in the electrode, resulting in a decrease in the oxidation ability of the electrode. Furthermore, if the first measurement electrode 51 contains a second-class precious metal, the evaporation of the second-class precious metal may reduce the three-phase interface between the precious metal, solid electrolyte, and the measured gas in the first measurement electrode 51, increasing the resistance of the first measurement electrode 51 and making it difficult for the pump current Ip1 to flow (i.e., reducing the hydrogen oxidation ability of the first measurement electrode 51).

[0067] Here, the correspondence between the components of this embodiment and the components of the present invention will be clarified. The sensor element 101 of this embodiment corresponds to the sensor element of the present invention, and the control device 95 corresponds to the control device. The element body 102 corresponds to the element body, the first internal space 20 corresponds to the first chamber, the inner pump electrode 22 corresponds to the first inner electrode, the main pump cell 21 corresponds to the first pump cell, the second internal space 40 corresponds to the second chamber, the first measurement electrode 51 corresponds to the second inner electrode, the first measurement pump cell 50 corresponds to the second pump cell, the third internal space 61 corresponds to the third chamber, the second measurement electrode 44 corresponds to the third inner electrode, the second measurement pump cell 41 corresponds to the third pump cell, the outer pump electrode 23 corresponds to the first outer electrode, the second outer electrode, and the third outer electrode, and the reference electrode 42 corresponds to the reference electrode. The main pump control process corresponds to the first pump cell control process, the first measurement pump control process corresponds to the second pump cell control process, and the second measurement pump control process corresponds to the third pump cell control process. The pump current Ip1 corresponds to the second pump current, the pump current Ip2 corresponds to the third pump current, the voltage V2 corresponds to the third voltage, and the corrected pump current Ip2ad corresponds to the corrected third pump current. The deterioration determination process for the first measurement electrode 51 corresponds to the second inner electrode deterioration determination process.

[0068] According to the gas sensor 100 of this embodiment described above, the control device 95 performs a deterioration determination process for the first measurement electrode 51 based on whether the absolute value of the voltage V2 during execution of the main pump control process and the first measurement pump control process and while the second measurement pump control process is stopped is within a predetermined high-voltage range. As described above, when the first measurement electrode 51 deteriorates, hydrogen reaches the second measurement electrode 44, increasing the absolute value of the voltage V2. Therefore, deterioration of the first measurement electrode 51 can be determined based on whether the absolute value of the voltage V2 is within the predetermined high-voltage range.

[0069] The gas to be measured is exhaust gas from an internal combustion engine, and the control device 95 performs the deterioration determination process for the first measuring electrode 51 while the internal combustion engine is being cut off or stopped. Here, the exhaust gas (gas in the exhaust gas pipe) while the internal combustion engine is being cut off or stopped has a lower carbon dioxide concentration than the exhaust gas while the internal combustion engine is operating other than when the fuel is cut off. Therefore, the influence of carbon monoxide reaching the third internal space 61 on the absolute value of the voltage V2 is smaller, and the influence of hydrogen becomes dominant. Therefore, the timing for performing the deterioration determination process for the first measuring electrode 51 is suitable while the internal combustion engine is being cut off or stopped.

[0070] Furthermore, the first measurement electrode 51 contains a first-class precious metal having catalytic activity and a second-class precious metal that suppresses the catalytic activity of the first-class precious metal against carbon monoxide. As described above, when the first measurement electrode 51 contains both the first-class precious metal and the second-class precious metal, carbon monoxide generated from carbon dioxide in the first internal space 20 is prevented from being oxidized around the first measurement electrode 51 before reaching the second measurement electrode 44. This prevents a decrease in the accuracy of measuring the carbon dioxide concentration based on the pump current Ip2 flowing through the second measurement electrode 44. However, when the first measurement electrode 51 contains a second-class precious metal, one mode of deterioration of the first measurement electrode 51 is evaporation of the second-class precious metal from the first measurement electrode 51 as the gas sensor 100 is used, resulting in a decrease in the accuracy of measuring the water concentration and carbon dioxide concentration. Therefore, when the first measurement electrode 51 contains a second-class precious metal, it is highly important to determine deterioration of the first measurement electrode 51.

[0071] It goes without saying that the present invention is not limited to the above-described embodiment, and can be embodied in various forms as long as they fall within the technical scope of the present invention.

[0072] For example, in the above-described embodiment, the CPU 97 may perform step S120 after a predetermined waiting time Tw1 has elapsed since executing step S110. The waiting time Tw1 may be determined in advance based on the time from when the second measurement pump control process is stopped (the pump current Ip2 is stopped) until the voltage V2 reaches a value corresponding to the hydrogen concentration in the third internal space 61. The waiting time Tw1 may be, for example, several milliseconds to a dozen milliseconds.

[0073] In the above-described embodiment, the CPU 97 performs the deterioration determination process for the first measurement electrode 51 when the internal combustion engine is in a fuel-cut state or is stopped. However, this is not limited to this. As shown in FIG. 5 , the voltage V2 due to carbon monoxide around the second measurement electrode 44 is different from the voltage V2 due to hydrogen. Therefore, even if carbon monoxide is present around the second measurement electrode 44, it is possible to determine whether hydrogen is present around the second measurement electrode 44 based on, for example, the measured value of the voltage V2 and the threshold value V2ref1. Therefore, the CPU 97 can perform the deterioration determination process for the first measurement electrode 51 not only during a fuel-cut state (when the carbon dioxide concentration in the measurement gas is low). For example, instead of step S100, the CPU 97 may determine whether the deterioration determination process for the first measurement electrode 51 has not been performed or whether a predetermined time T1 has elapsed since the previous execution of the process for the first measurement electrode 51. If the deterioration determination process has not been performed or if the predetermined time T1 has elapsed since the previous execution of the process for the first measurement electrode 51, the CPU 97 may perform step S110 and subsequent steps. The predetermined time T1 may be set, for example, to the time from the initial state until the first measurement electrode 51 is likely to deteriorate, or a shorter time, and may be, for example, a few seconds to a few minutes, or 1,000 hours to a few thousand hours.

[0074] In the above-described embodiment, the CPU 97 measures the water concentration and the carbon dioxide concentration in the measurement gas by executing the water concentration measurement process and the carbon dioxide concentration measurement process. However, the control device 95 may measure only one of the water concentration and the carbon dioxide concentration in the measurement gas by executing only one of the water concentration measurement process and the carbon dioxide concentration measurement process.

[0075] In the above-described embodiment, the outer pump electrode 23 serves as a first outer electrode paired with the inner pump electrode 22 in the main pump cell 21, a second outer electrode paired with the first measurement electrode 51 in the first measurement pump cell 50, and a third outer electrode paired with the second measurement electrode 44 in the second measurement pump cell 41. That is, the first to third outer electrodes are configured as a common outer pump electrode 23. However, this is not limiting. For example, two of the first to third outer electrodes may be configured as a common outer pump electrode 23, and the remaining one may be an electrode independent of the outer pump electrode 23 and provided on the outer surface of the element body 102 so as to be in contact with the measurement gas. Alternatively, the first to third outer electrodes may be independent electrodes and provided on the outer surface of the element body 102 so as to be in contact with the measurement gas.

[0076] In the above-described embodiment, the sensor element 101 of the gas sensor 100 includes the first internal cavity 20, the second internal cavity 40, and the third internal cavity 61. However, this is not limiting. For example, as shown in a modified sensor element 201 of FIG. 6 , the third internal cavity 61 may not be included. In the modified sensor element 201 of FIG. 6 , a gas inlet 10, a first diffusion-controlling portion 11, a buffer space 12, a second diffusion-controlling portion 13, the first internal cavity 20, the third diffusion-controlling portion 30, and the second internal cavity 40 are adjacently formed and communicated in this order between the lower surface of the second solid electrolyte layer 6 and the upper surface of the first solid electrolyte layer 4. The second measurement electrode 44 is disposed on the upper surface of the first solid electrolyte layer 4 within the second internal cavity 40. The second measurement electrode 44 is covered with a fourth diffusion-controlling portion 45. The fourth diffusion rate-controlling portion 45 is a film made of a porous ceramic material such as alumina (Al2O3). Similar to the fourth diffusion rate-controlling portion 60 of the above-described embodiment, the fourth diffusion rate-controlling portion 45 provides a predetermined diffusion resistance to the measurement gas in the second internal space 40 and guides the gas to the second measurement electrode 44. The fourth diffusion rate-controlling portion 45 also functions as a protective film for the second measurement electrode 44. The ceiling electrode portion 51a of the first measurement electrode 51 extends to directly above the second measurement electrode 44. Even with this configuration, the sensor element 201 can measure the carbon dioxide concentration based on the pump current Ip2 flowing through the second measurement pump cell 41, as in the above-described embodiment. In the sensor element 201 of FIG. 6, the periphery of the second measurement electrode 44 functions as a third chamber. That is, the periphery of the second measurement electrode 44 plays a role similar to that of the third internal space 61.

[0077] In the above-described embodiment, the element body 102 of the sensor element 101 is a laminate having multiple solid electrolyte layers (layers 1 to 6), but this is not limiting. The element body of the sensor element 101 only needs to have at least one oxygen-ion conductive solid electrolyte layer and have a measurement gas flow section provided therein. For example, in FIG. 1 , layers 1 to 5 other than the second solid electrolyte layer 6 may be structural layers made of a material other than a solid electrolyte (e.g., a layer made of alumina). In this case, each electrode of the sensor element 101 may be disposed on the second solid electrolyte layer 6. For example, the second measurement electrode 44 in FIG. 1 may be disposed on the underside of the second solid electrolyte layer 6. Furthermore, the reference gas introduction space 43 may be provided in the spacer layer 5 instead of the first solid electrolyte layer 4, the reference gas introduction layer 48 may be provided between the second solid electrolyte layer 6 and the spacer layer 5 instead of between the first solid electrolyte layer 4 and the third substrate layer 3, and the reference electrode 42 may be provided behind the third internal space 61 and on the underside of the second solid electrolyte layer 6.

[0078] This application claims priority from Japanese Patent Application No. 2023-109463, filed on July 3, 2023, the entire contents of which are incorporated herein by reference.

[0079] The present invention can be used in a gas sensor for measuring the water concentration and / or carbon dioxide concentration in a measurement gas such as an automobile exhaust gas.

[0080] 1 First substrate layer, 2 Second substrate layer, 3 Third substrate layer, 4 First solid electrolyte layer, 5 Spacer layer, 6 Second solid electrolyte layer, 10 Gas inlet, 11 First diffusion rate-controlling section, 12 Buffer space, 13 Second diffusion rate-controlling section, 20 First internal space, 21 Main pump cell, 22 Inner pump electrode, 22a Ceiling electrode section, 22b Bottom electrode section, 23 Outer pump electrode, 24 Variable power supply, 30 Third diffusion rate-controlling section, 40 Second internal space, 41 Second measurement pump cell, 42 Reference electrode, 43 Reference gas introduction space, 44 Second measurement electrode, 45 Fourth diffusion rate-controlling section, 46 Variable power supply, 48 Reference gas introduction layer, 49 Reference gas introduction section, 49a Inlet section, 50 First measurement pump cell, 51 First measurement electrode, 51a Ceiling electrode section, 51b Bottom electrode section, 52 REFERENCE SIGNS LIST, 60: fourth diffusion rate-controlling portion, 61: third internal cavity, 70: heater portion, 71: heater connector electrode, 72: heater, 73: through hole, 74: heater insulating layer, 75: pressure release hole, 76: heater power supply, 80: oxygen partial pressure detection sensor cell for controlling main pump, 81: oxygen partial pressure detection sensor cell for controlling first measurement pump, 82: oxygen partial pressure detection sensor cell for controlling second measurement pump, 83: sensor cell, 95: control device, 96: control portion, 97: CPU, 98: memory portion, 100: gas sensor, 101, 201: sensor element, 102: element body.

Claims

1. A gas sensor comprising a sensor element and a control device for measuring a water concentration and / or a carbon dioxide concentration in a measured gas, the sensor element comprising: an element body having an oxygen ion conductive solid electrolyte layer and having a measured gas flow section therein for introducing and flowing the measured gas; a first pump cell comprising: a first inner electrode disposed in a first chamber of the measured gas flow section and a first outer electrode disposed on an outer surface of the element body; a second pump cell comprising: a second inner electrode disposed in a second chamber located downstream of the first chamber of the measured gas flow section and a second outer electrode disposed on the outer surface of the element body; a third pump cell comprising: a third inner electrode disposed in a third chamber located downstream of the second chamber of the measured gas flow section and a third outer electrode disposed on the outer surface of the element body; and a reference electrode disposed inside the element body so as to come into contact with a reference gas; and the control device comprising: a first pump cell control process for controlling the first pump cell to pump oxygen from around the first inner electrode to around the first outer electrode, thereby reducing water and carbon dioxide in the measured gas in the first chamber; a second pump cell control process for controlling the second pump cell to pump oxygen from around the second outer electrode to around the second inner electrode, thereby oxidizing hydrogen produced by the reduction of water in the first chamber, in the second chamber; a third pump cell control process for controlling the third pump cell to pump oxygen from around the third outer electrode to around the third inner electrode, thereby oxidizing carbon monoxide produced by the reduction of carbon dioxide in the first chamber, in the third chamber; a water concentration measurement process for measuring a water concentration in the measured gas based on a second pump current flowing in the second pump cell by the second pump cell control process, and / or a carbon dioxide concentration measurement process for measuring a carbon dioxide concentration in the measured gas based on a third pump current flowing in the third pump cell by the third pump cell control process,a second inner electrode deterioration determination process is performed to determine deterioration of the second inner electrode based on whether an absolute value of a third voltage during execution of the second pump cell control process and during stoppage of the third pump cell control process is within a predetermined high voltage region.

2. The gas sensor according to claim 1, wherein the measurement gas is exhaust gas from an internal combustion engine, and the control device performs the second inner electrode deterioration determination process while the internal combustion engine is in a fuel cut or stopped state.

3. The gas sensor according to claim 1 or 2, wherein the second inner electrode includes a first type precious metal having catalytic activity and a second type precious metal that suppresses the catalytic activity of the first type precious metal for carbon monoxide.

4. The gas sensor according to claim 1 or 2, wherein at least two of the first outer electrode, the second outer electrode and the third outer electrode are common electrodes.