Gas sensor and sensor element

Abstract

Gas sensor (100), including: an element body comprising an oxygen ion-conducting solid electrolyte layer (1, 2, 3, 4, 5, 6) and having a measuring gas flow section within the element body for introducing and allowing a measuring gas to flow; a main pump cell (21) designed to pump oxygen out of a first inner cavity (20) of the measuring gas flow section to adjust an oxygen concentration in the first inner cavity (20); an auxiliary pump cell (50) designed to pump oxygen out of a second inner cavity (40) downstream of the first inner cavity (20) of the measuring gas flow section to adjust an oxygen concentration in the second inner cavity (40); a measuring electrode (44) which is arranged on an inner circumferential surface of a measuring chamber (61) downstream of the second inner cavity (40) of the measuring gas flow section; a reference electrode (42) which is arranged inside the element body and to which a reference gas is to be introduced, wherein the reference gas serves as a reference for detecting a concentration of a specific gas in the measuring gas; a voltage measurement unit (82) designed to measure a voltage (V2) between the reference electrode (42) and the measuring electrode (44); and a unit (92) for detecting the concentration of a specific gas, which is designed to detect, on the basis of the measuring voltage (V2), a detection value dependent on oxygen originating from the specific gas in the measuring chamber (61), and to detect, on the basis of the detection value, the concentration of the specific gas in the measuring gas, wherein the main pump cell (21) comprises an inner main pump electrode (22) which is arranged in the first inner cavity (20), the auxiliary pump cell (50) comprises an inner auxiliary pump electrode (51) which is arranged in the second inner cavity (40), the inner main pump electrode (22), the inner auxiliary pump electrode (51) and the measuring electrode (44) each contain a catalytically active noble metal, the inner main pump electrode (22) does not contain a noble metal with the ability to inhibit the catalytic activity of the catalytically active noble metal on the specific gas, and the inner auxiliary pump electrode (51) contains the precious metal with the ability to inhibit catalytic activity.

Description

TECHNICAL AREA

[0001] The present invention relates to gas sensors and sensor elements. STATE OF THE ART

[0002] Gas sensors for detecting the concentration of a specific gas, such as NOx, in a sample gas, such as vehicle exhaust, are known in the prior art. For example, PTL 1 discloses a gas sensor comprising a stack of oxygen-ion-conducting solid electrolyte layers and electrodes arranged on the solid electrolyte layers. This gas sensor detects the NOx concentration as follows: First, oxygen is pumped from a sample gas flow section within a sensor element to the outside of the sensor element, or from the outside of the sensor element to the sample gas flow section, to adjust the oxygen concentration in the sample gas flow section. After the oxygen concentration is adjusted, the NOx in the sample gas is reduced.The NOx concentration in the sample gas is determined based on the current flowing through an electrode (measuring electrode) within the sensor element, depending on the oxygen concentration after reduction. PTL 2 discloses a gas sensor for detecting the ammonia concentration in a sample gas. This gas sensor detects the ammonia concentration by oxidizing ammonia with oxygen in the sample gas to convert it to NOx and detecting the concentration of the NOx derived from ammonia in the same way as in PTL 1.

[0003] PTL 1 also discloses that an inner pump electrode, arranged in the measuring gas flow section of a pump cell for adjusting the oxygen concentration, is a cermet electrode composed of Pt and ZrO2 and containing 1% Au. If the inner pump electrode contains Au, it can prevent the internal pump electrode from reducing NOx. On the other hand, PTL 3 discloses that during the use of such a gas sensor, Au evaporates from the electrode of the pump cell and is deposited on an electrode of a sensor cell for detecting the NOx concentration in the measuring gas, and that as a result, the detection accuracy of the NOx concentration decreases. Furthermore, PTL 4, PTL 5, and PTL 7 each disclose a gas sensor. PTL 6 describes electrodes, electrochemical elements, gas sensors, and gas measurement methods. DOCUMENT LISTPATENT DOCUMENTS PTL 1: JP 2014 - 190 940 A PTL 2: JP 2011 - 039 041 A PTL 3: JP 6 447 568 B2 PTL 4: DE 102017 008 086 A1 PTL 5: DE 10 2017 009 119 A1 PTL 6: DE 102 61 299 A1 PTL 7: DE 101 06 171 A1 REVELATION OF THE INVENTION

[0004] If the internal pump electrode reduces NOx, the detection accuracy of the NOx concentration decreases, which is why the internal pump electrode must contain gold. On the other hand, as described above, if the internal pump electrode contains gold, a problem arises in that the detection accuracy of the NOx concentration decreases during use of the gas sensor.

[0005] The present invention was made with regard to the aforementioned problem. A main objective of the invention is to maintain the accuracy of measuring the concentration of a specific gas over a long period of time.

[0006] After conducting intensive research to solve the aforementioned problem, the inventors found that if the atmosphere around the inner main pump electrode is not a low-oxygen atmosphere, even if the inner main pump electrode does not contain Au, only a small amount of NOx is reduced by the inner main pump electrode. Accordingly, the inventors found that the inner main pump electrode need not contain Au, whereas prior art has assumed that this is essential when measuring the concentration of a specific gas in a sample gas that is not a low-oxygen atmosphere, thus completing the present invention.

[0007] A gas sensor according to the present invention comprises: an element body comprising an oxygen ion-conducting solid electrolyte layer and having a measuring gas flow section within the element body for introducing and allowing a measuring gas to flow; a main pump cell designed to pump oxygen out of a first inner cavity of the measuring gas flow section to adjust an oxygen concentration in the first inner cavity; an auxiliary pump cell designed to pump oxygen out of a second inner cavity downstream of the first inner cavity of the measuring gas flow section in order to adjust an oxygen concentration in the second inner cavity; a measuring electrode that is arranged on an inner circumferential surface of a measuring chamber downstream of the second inner cavity of the measuring gas flow section; a reference electrode arranged within the element body for introducing a reference gas, which serves as a reference for measuring the concentration of a specific gas in the measuring gas; a voltage measurement unit designed to detect a voltage between the reference electrode and the measuring electrode; and a unit for detecting the concentration of a specific gas, designed to detect, based on the measuring voltage, a detection value dependent on oxygen originating from the specific gas in the measuring chamber, and to detect, based on the detection value, the concentration of the specific gas in the measuring gas, wherein the main pump cell comprises an inner main pump electrode which is arranged in the first inner cavity, the auxiliary pump cell includes an inner auxiliary pump electrode, which is arranged in the second inner cavity, The inner main pump electrode, the inner auxiliary pump electrode and the measuring electrode each contain a catalytically active noble metal, the inner main pump electrode does not contain a noble metal with the ability to inhibit the catalytic activity of the catalytically active noble metal on the specific gas, and The inner auxiliary pump electrode contains the precious metal with the ability to inhibit catalytic activity.

[0008] In this gas sensor, the main pump cell and the auxiliary pump cell each pump out oxygen to adjust the oxygen concentration in the sample gas introduced into the sample gas flow section. Consequently, the sample gas, with its adjusted oxygen concentration, reaches the measuring chamber. Based on the measuring voltage, this gas sensor detects the value dependent on the oxygen originating from the specific gas in the measuring chamber and, based on this detected value, determines the concentration of the specific gas in the sample gas. This process occurs even if the inner main pump electrode contains a noble metal with the ability to inhibit catalytic activity (e.g.,Since the inner main pump electrode does not contain the precious metal with the catalytic inhibitor Au, little of the specific gas or the oxide derived from the specific gas is reduced by the inner main pump electrode if the sample gas introduced into the sample gas flow section is not a low-oxygen atmosphere. Consequently, the gas sensor according to the present invention exhibits sufficient detection accuracy for the concentration of a specific gas. Furthermore, because the inner main pump electrode does not contain the precious metal with the catalytic inhibitory properties, the evaporation and deposition of the precious metal on the measuring electrode during use of the gas sensor are inhibited.Consequently, the gas sensor according to the present invention can maintain its detection accuracy for the concentration of a specific gas for a long period of time when used as a gas sensor to measure the concentration of a specific gas in a sample gas that is not an atmosphere with a low oxygen concentration. That is, the gas sensor according to the present invention is particularly well suited for measuring the concentration of a specific gas in a sample gas that is not an atmosphere with a low oxygen concentration.

[0009] Here, “does not contain the precious metal with the capacity to inhibit catalytic activity” refers to the fact that it is essentially free of the precious metal with the capacity to inhibit catalytic activity; i.e., the precious metal with the capacity to inhibit catalytic activity may be present as an accidental impurity.

[0010] Here, if the specific gas is an oxide, "oxygen derived from the specific gas in the measuring chamber" can refer to oxygen produced when the specific gas itself is reduced within the measuring chamber. If the specific gas is a non-oxide, "oxygen derived from the specific gas in the measuring chamber" can refer to oxygen produced when a gas obtained by converting the specific gas to an oxide is reduced within the measuring chamber. Furthermore, the unit for sensing the concentration of a specific gas can, as a sensing value, detect a measuring pump current that flows when oxygen derived from the specific gas in the measuring chamber is pumped out of the measuring chamber based on the measuring voltage, such that the oxygen concentration in the measuring chamber is at a predetermined low concentration.The element body can be a stack comprising a plurality of oxygen ion-conducting solid electrolyte layers stacked on top of each other.

[0011] The internal auxiliary pump electrode of the gas sensor according to the present invention can contain Au as a precious metal with the ability to inhibit catalytic activity.

[0012] A sensor element according to the present invention comprises: an element body comprising an oxygen ion-conducting solid electrolyte layer and having a measuring gas flow section within the element body for introducing and allowing a measuring gas to flow; a main pump cell designed to pump oxygen out of a first inner cavity of the measuring gas flow section to adjust an oxygen concentration in the first inner cavity; an auxiliary pump cell designed to pump oxygen out of a second inner cavity downstream of the first inner cavity of the measuring gas flow section in order to adjust an oxygen concentration in the second inner cavity; a measuring electrode arranged on an inner circumferential surface of a measuring chamber downstream of the second inner cavity of the measuring gas flow section; and a reference electrode arranged within the element body for introducing a reference gas, which serves as a reference for measuring the concentration of a specific gas in the measuring gas; wherein the main pump cell comprises an inner main pump electrode which is arranged in the first inner cavity, the auxiliary pump cell includes an inner auxiliary pump electrode, which is arranged in the second inner cavity, The inner main pump electrode, the inner auxiliary pump electrode and the measuring electrode each contain a catalytically active noble metal, the inner main pump electrode does not contain a noble metal with the ability to inhibit the catalytic activity of the catalytically active noble metal on the specific gas, and The inner auxiliary pump electrode contains the precious metal with the ability to inhibit catalytic activity.

[0013] As with the gas sensor described above according to the present invention, this sensor element can be used to detect the concentration of the specific gas in the sample gas. Furthermore, as with the gas sensor described above according to the present invention, the inner main pump electrode of this sensor element does not contain the noble metal with the catalytic inhibiting properties, whereas the inner auxiliary pump electrode does contain the noble metal with the catalytic inhibiting properties. Consequently, the sensor element according to the present invention can maintain its detection accuracy for the concentration of a specific gas for a long period of time when used to detect the concentration of a specific gas in a sample gas that is not an atmosphere with a low oxygen concentration.The sensor element according to the present invention is particularly suitable for measuring the concentration of a specific gas in a measuring gas that is not an atmosphere with a low oxygen concentration. BRIEF DESCRIPTION OF THE DRAWINGS Fig. Figure 1 is a schematic sectional view of a gas sensor 100. Fig. Figure 2 is a block diagram showing electrical connections between a control unit 90 and individual cells. Fig. Figure 3 is a graph showing the relationship between the NO concentration and the pump current Ip2 for gas sensors of experimental examples 1 to 4. Fig. Figure 4 is a schematic sectional view of a sensor element 201. DETAILED DESCRIPTION OF THE INVENTION

[0014] Embodiments of the present invention are described below with reference to the drawings. Fig.Figure 1 is a schematic sectional view showing, as an overview, an example setup of a gas sensor 100 according to an embodiment of the present invention. Fig.Figure 2 is a block diagram showing the electrical connections between a control unit 90 and individual cells. This gas sensor 100 is attached, for example, to a pipe, such as the exhaust pipe of an internal combustion engine, such as a gasoline or diesel engine. The gas sensor 100 detects the concentration of a specific gas, such as NOx or ammonia, in the exhaust gas from an internal combustion engine, which serves as the measuring gas. In this embodiment, the gas sensor 100 is configured to measure the NOx concentration as the concentration of the specific gas. The gas sensor 100 comprises a sensor element 101 with an elongated, rectangular parallelepiped shape, individual cells 21, 41, 50, and 80 to 83, each comprising a portion of the sensor element 101, and a control unit 90 configured to control the entire gas sensor 100.

[0015] The sensor element 101 is a layered body comprising six layers: 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, arranged in that order from the bottom side as seen in the drawing. Each of the six layers consists of an oxygen-ion-conducting solid electrolyte layer containing, for example, zirconium oxide (ZrO2). The solid electrolyte forming these six layers is high-density and gas-tight. This sensor element 101 is manufactured, for example, by stacking ceramic green layers corresponding to the individual layers on top of each other, for example, after a predefined processing and circuit structure printing process, and then firing the stacked ceramic green layers to fuse them together.

[0016] A gas inlet 10, a first diffusion rate restriction section 11, a buffer chamber 12, a second diffusion rate restriction section 13, a first inner cavity 20, a third diffusion rate restriction section 30, a second inner cavity 40, a fourth diffusion rate restriction section 60, and a third inner cavity 61 are configured to be adjacent to one another such that they are located in the aforementioned order between the lower surface of the second solid electrolyte layer 6 and the upper surface of the first solid electrolyte layer 4 at the front end (on the left end in the Fig. 1) of the sensor element 101 are connected to each other.

[0017] The gas inlet opening 10, the buffer chamber 12, the first inner chamber 20, the second inner chamber 40, and the third inner chamber 61 form a space within the sensor element 101. The space is provided such that a section of the spacer layer 5 is hollowed out. The top of the space is defined by the lower surface of the second solid electrolyte layer 6, the bottom of the space is defined by the upper surface of the first solid electrolyte layer 4, and the sides of the space are defined by the side surfaces of the spacer layer 5.

[0018] The first diffusion rate restriction section 11, the second diffusion rate restriction section 13, and the third diffusion rate restriction section 30 are each provided as two laterally elongated slots (i.e., the longitudinal direction of the openings is perpendicular to the figure). The fourth diffusion rate restriction section 60 is provided as a single laterally elongated slot (i.e., the longitudinal direction of the opening is perpendicular to the figure) formed as a space beneath the lower surface of the second solid electrolyte layer 6. The section extending from the gas inlet 10 to the third internal cavity 61 is also referred to as the "measuring gas flow section."

[0019] A reference gas introduction chamber 43 is located between the upper surface of the third substrate layer 3 and the lower surface of the spacer layer 5, at a position further away from the front end than the measuring gas flow section. The reference gas introduction chamber 43 is defined on both sides by the side surfaces of the first solid electrolyte layer 4. As an example of a reference gas for a NOx concentration measurement, air is introduced into the reference gas introduction chamber 43.

[0020] An air introduction layer 48 is a porous ceramic layer. The reference gas is introduced into the air introduction layer 48 through the reference gas introduction chamber 43. The air introduction layer 48 is configured to cover a reference electrode 42.

[0021] The reference electrode 42 is formed between the upper surface of the third substrate layer 3 and the first solid electrolyte layer 4. As described above, the air introduction layer 48, which leads to the reference gas introduction chamber 43, is arranged around the reference electrode 42. As described later, the reference electrode 42 can be used to measure the oxygen concentrations (oxygen partial pressures) in the first inner cavity 20, the second inner cavity 40, and the third inner cavity 61. The reference electrode 42 is configured as a porous cermet electrode (e.g., a cermet electrode composed of Pt and ZrO2).

[0022] The gas inlet 10 of the measuring gas flow section is open to the outside. The measuring gas is drawn from the outside through the gas inlet 10 into the sensor element 101. The first diffusion rate restriction section 11 creates a predetermined diffusion resistance against the measuring gas drawn in through the gas inlet 10. The buffer chamber 12 is provided to guide the measuring gas introduced by the first diffusion rate restriction section 11 into the second diffusion rate restriction section 13. The second diffusion rate restriction section 13 creates a predetermined diffusion resistance against the measuring gas introduced from the buffer chamber 12 into the first inner cavity 20.When the sample gas is introduced from outside the sensor element 101 into the first inner cavity 20, the sample gas, which is rapidly drawn into the sensor element 101 through the gas inlet 10 due to pressure variations in the sample gas in the external space (pulsations of the exhaust pressure if the sample gas is vehicle exhaust), is not introduced directly into the first inner cavity 20. Rather, the sample gas is introduced into the first inner cavity 20 after concentration variations in the sample gas have been eliminated by the first diffusion rate restriction section 11, the buffer chamber 12, and the second diffusion rate restriction section 13. Consequently, the concentration variations in the sample gas introduced into the first inner cavity 20 are almost negligible.The first internal cavity 20 is provided as a space for adjusting the oxygen partial pressure in the measuring gas introduced through the second diffusion rate restriction section 13. This oxygen partial pressure is adjusted by the operation of a main pump cell 21.

[0023] The main pump cell 21 is an electrochemical pump cell composed of an inner pump electrode 22 with an upper electrode section 22a arranged over substantially the entire section of the lower surface of the second solid electrolyte layer 6 facing the first inner cavity 20, an outer pump electrode 23 arranged on a region of the upper surface of the second solid electrolyte layer 6 corresponding to the upper electrode section 22a, so that it is exposed to the outer space, and a section of the second solid electrolyte layer 6 located between the inner pump electrode 22 and the outer pump electrode 23.

[0024] The inner pump electrode 22 is formed on sections of the upper and lower solid electrolyte layers (the second solid electrolyte layer 6 and the first solid electrolyte layer 4), which define the first inner cavity 20, and on sections of the spacer layer 5, which form the side walls of the first inner cavity 20. In particular, the upper electrode section 22a is formed on a section of the lower surface of the second solid electrolyte layer 6, which forms the upper surface of the first inner cavity 20. A lower electrode section 22b is formed on a section of the upper surface of the first solid electrolyte layer 4, which forms the lower surface of the first inner cavity 20.Side electrode sections (not shown) are formed on sections of the side wall surfaces (inner surfaces) of the spacer layer 5, which form both side walls of the first inner cavity 20, thus connecting the upper electrode section 22a and the lower electrode section 22b. Consequently, the inner pump electrode 22 is provided as a tunnel-like structure in the area where the side electrode sections are arranged.

[0025] The inner pump electrode 22 and the outer pump electrode 23 are designed as porous cermet electrodes (e.g. cermet electrodes composed of Pt and ZrO2).

[0026] In the main pump cell 21, the desired pump voltage Vp0 is applied between the inner pump electrode 22 and the outer pump electrode 23 such that a pump current Ip0 flows between the inner pump electrode 22 and the outer pump electrode 23 in either a positive or negative direction. Consequently, oxygen can be pumped from the first inner cavity 20 to the outer cavity or from the outer cavity to the first inner cavity 20.

[0027] To detect the oxygen concentration (oxygen partial pressure) in the atmosphere in the first inner cavity 20, 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 form an electrochemical sensor cell, namely an oxygen partial pressure detection sensor cell 80 for main pump control.

[0028] The oxygen concentration (oxygen partial pressure) in the first inner cavity 20 can be determined from the electromotive force V0, which is measured in the oxygen partial pressure sensing sensor cell 80 for main pump control. Furthermore, the pump current Ip0 is controlled by regulating the pump voltage Vp0 of a variable power supply 24 such that the electromotive force V0 remains constant. Consequently, the oxygen concentration in the first inner cavity 20 can be maintained at a predetermined constant value.

[0029] The third diffusion rate restriction section 30 creates a predetermined diffusion resistance against the measuring gas, whose oxygen concentration (oxygen partial pressure) in the first inner cavity 20 has been set by the operation of the main pump cell 21, and directs the measuring gas into the second inner cavity 40.

[0030] The second inner cavity 40 is designed as a space for further adjusting the oxygen concentration (oxygen partial pressure) of the measuring gas, which is introduced through the third diffusion rate restriction section 30, after the oxygen partial pressure has been preset in the first inner cavity 20 using an auxiliary pump cell 50. Consequently, the oxygen concentration in the second inner cavity 40 can be maintained at a constant value with high accuracy, so that the gas sensor 100 can measure the NOx concentration with high accuracy.

[0031] The auxiliary pump cell 50 is an electrochemical auxiliary pump cell composed of an auxiliary pump electrode 51 with an upper electrode section 51a, which is substantially located on an entire section of the lower surface of the second solid electrolyte layer 6, which is directed towards the second inner cavity 40, the outer pump electrode 23 (the outer electrode is not limited to the outer pump electrode 23, but can be any suitable electrode outside the sensor element 101), and the second solid electrolyte layer 6.

[0032] This auxiliary pump electrode 51 is provided in the second inner cavity 40 as a tunnel-like structure, similar to the inner pump electrode 22 located in the first inner cavity 20. Specifically, the upper electrode section 51a is arranged on a section of the second solid electrolyte layer 6, which forms the upper surface of the second inner cavity 40. A lower electrode section 51b is formed on a section of the first solid electrolyte layer 4, which forms the lower surface of the second inner cavity 40. Side electrode sections (not shown) are formed on sections of both side wall surfaces of the spacer layer 5, which form the side walls of the second inner cavity 40, such that the upper electrode section 51a and the lower electrode section 51b are connected to each other. Consequently, the auxiliary pump electrode 51 is provided as a tunnel-like structure.

[0033] In the auxiliary pump cell 50, the desired voltage Vp1 is applied between the auxiliary pump electrode 51 and the outer pump electrode 23. Consequently, oxygen can be pumped from the atmosphere in the second inner cavity 40 to the outer cavity or from the outer cavity to the second inner cavity 40.

[0034] To control the oxygen partial pressure in the atmosphere in the second inner cavity 40, the auxiliary pump 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 form an electrochemical sensor cell, namely an oxygen partial pressure detection sensor cell 81 for auxiliary pump control.

[0035] The auxiliary pump cell 50 performs pumping using a variable power supply 52, the voltage of which is controlled based on the electromotive force V1, which is detected in the oxygen partial pressure sensing sensor cell 81 for auxiliary pump control. Consequently, the oxygen partial pressure in the atmosphere in the second inner cavity 40 can be adjusted to a partial pressure that has essentially no effect on the NOx measurement.

[0036] Furthermore, a pump current Ip1 is used to control the electromotive force of the oxygen partial pressure sensing sensor cell 80 for main pump control. Specifically, the pump current Ip1 is fed as a control signal into the oxygen partial pressure sensing sensor cell 80 for main pump control to control the electromotive force V0, ensuring that the gradient of the oxygen partial pressure in the measuring gas introduced from the third diffusion rate limiting section 30 into the second inner cavity 40 remains constant. When the gas sensor 100 is used as a NOx sensor, the oxygen concentration in the second inner cavity 40 is maintained at a constant value of approximately 0.001 ppm by the operation of the main pump cell 21 and the auxiliary pump cell 50.

[0037] The fourth diffusion rate restriction section 60 generates a predetermined diffusion resistance against the sample gas, whose oxygen concentration (oxygen partial pressure) in the second inner cavity 40 has been set by the operation of the auxiliary pump cell 50, and directs the sample gas into the third inner cavity 61. The fourth diffusion rate restriction section 60 acts to limit the amount of NOx flowing into the third inner cavity 61.

[0038] The third inner cavity 61 is designed as a processing chamber for the measurement of the nitrogen oxide (NOx) concentration in the sample gas, which has been introduced through the fourth diffusion rate restriction section 60 after the oxygen concentration (oxygen partial pressure) has been preset in the second inner cavity 40. The NOx concentration is primarily measured in the third inner cavity 61 by operating a measuring pump cell 41.

[0039] The measuring pump cell 41 measures the NOx concentration in the sample gas in the third inner cavity 61. The measuring pump cell 41 is an electrochemical pump cell composed of a measuring electrode 44, which is formed on a section of the upper surface of the first solid electrolyte layer 4 facing the third inner cavity 61, the outer pump electrode 23, the second solid electrolyte layer 6, the spacer layer 5, and the first solid electrolyte layer 4. The measuring electrode 44 also acts as an NOx reduction catalyst for reducing NOx present in the atmosphere within the third inner cavity 61.

[0040] The measuring pump cell 41 pumps out oxygen that has been generated by the decomposition of nitrogen oxide in the atmosphere around the measuring electrode 44. The amount of oxygen generated can be measured as the pump current Ip2.

[0041] To detect the oxygen partial pressure around the measuring electrode 44, the first solid electrolyte layer 4, the third substrate layer 3, the measuring electrode 44, and the reference electrode 42 form an electrochemical sensor cell, namely an oxygen partial pressure detection sensor cell 82 for measuring pump control. A variable power supply 46 is controlled based on the electromotive force V2, which is detected in the oxygen partial pressure detection sensor cell 82 for measuring pump control.

[0042] The sample gas, which is directed into the second inner cavity 40, flows through the fourth diffusion rate restriction section 60 at a set oxygen partial pressure and reaches the measuring electrode 44 in the third inner cavity 61. The nitrogen oxide in the sample gas around the measuring electrode 44 is reduced, generating oxygen (2NO → N2 + O2). The resulting oxygen is pumped through the measuring pump cell 41. During this process, the voltage Vp2 of the variable power supply 46 is controlled such that the electromotive force V2, which is detected in the oxygen partial pressure sensing sensor cell 82 for measuring pump control, remains constant. Since the amount of oxygen generated around the measuring electrode 44 is proportional to the nitrogen oxide concentration in the sample gas, the nitrogen oxide concentration in the sample gas is calculated from the pump current Ip2 through the measuring pump cell 41.

[0043] 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 form an electrochemical sensor cell 83. The oxygen partial pressure in the measuring gas outside the gas sensor 100 can be detected from the electromotive force Vref generated by the sensor cell 83.

[0044] In the gas sensor 100 with the aforementioned setup, the main pump cell 21 and the auxiliary pump cell 50 are operated to supply a sample gas, whose oxygen partial pressure has been maintained at a constant low value (a value that has essentially no effect on the NOx measurement), to the measuring pump cell 41. Consequently, the NOx concentration in the sample gas can be determined based on the pump current Ip2, which flows when oxygen, generated by the reduction of NOx, is pumped out of the measuring pump cell 41 in a manner essentially proportional to the NOx concentration in the sample gas.

[0045] To increase the oxygen ion conductivity of the solid electrolyte, the sensor element 101 further comprises a heating element section 70, which serves as a temperature control device for heating and maintaining the temperature of the sensor element 101. The heating element section 70 comprises a heating element connection electrode 71, a heating element 72, a through-hole 73, a heating element insulating layer 74, and a pressure relief device 75.

[0046] The heating device connection electrode 71 is in contact with the lower surface of the first substrate layer 1. The heating device connection electrode 71 is connected to an external power supply, so that the heating device section 70 can be externally powered.

[0047] The heating element 72 is an electrical resistor formed between the second substrate layer 2 and the third substrate layer 3. The heating element 72 is connected to the heating element connection electrode 71 via the through-hole 73. The heating element 72 is externally powered via the heating element connection electrode 71, generating heat which warms the solid electrolyte forming the sensor element 101 and maintains its temperature.

[0048] The heating device 72 is embedded in the entire area from the first inner cavity 20 to the third inner cavity 61 in such a way that the temperature of the entire sensor element 101 can be set to a temperature that activates the solid electrolyte.

[0049] The heating element insulating layer 74 is an insulating layer that covers the upper and lower surfaces of the heating element 72 and is formed from an insulator, such as aluminum oxide. The heating element insulating layer 74 is designed to ensure electrical insulation between the second substrate layer 2 and the heating element 72, and electrical insulation between the third substrate layer 3 and the heating element 72.

[0050] The pressure relief device 75 extends through the third substrate layer 3 and the air inlet layer 48, thus connecting to the reference gas inlet chamber 43. The pressure relief device 75 is designed to reduce any increase in internal pressure due to a temperature rise in the heating element insulation layer 74.

[0051] The inner pump electrode 22, the auxiliary pump electrode 51, and the measuring electrode 44 each contain a catalytically active noble metal. The catalytically active noble metal can be, for example, at least one of Pt, Rh, Ir, Ru, and Pd. The outer pump electrode 23 and the reference electrode 42 also contain the catalytically active noble metal. The auxiliary pump electrode 51 further contains a noble metal with the ability to inhibit the catalytic activity of the catalytically active noble metal on the specific gas. Consequently, the auxiliary pump electrode 51 has a reduced ability to reduce the NOx component in the measuring gas. The noble metal with the ability to inhibit the catalytic activity can be, for example, Au. In contrast, the inner pump electrode 22 does not contain the noble metal with the ability to inhibit the catalytic activity. The measuring electrode 44 preferably does not contain the noble metal with the ability to inhibit the catalytic activity.The outer pump electrode 23 and the reference electrode 42 preferably do not contain the noble metal that inhibits the catalytic activity. Electrodes 22, 23, 42, 44, and 51 are each preferably formed from a cermet containing a noble metal and an oxygen-ion-conducting oxide (e.g., ZrO2). Electrodes 22, 23, 42, 44, and 51 are each preferably porous. In this embodiment, electrodes 22, 23, 42, and 44 are porous cermet electrodes composed of Pt and ZrO2, and the auxiliary pump electrode 51 is a porous cermet electrode composed of Pt and ZrO2 containing 1% Au.

[0052] The control unit 90 is a microprocessor comprising, for example, a CPU 92 and a memory 94. The control unit 90 receives the electromotive force V0, which was detected in the oxygen partial pressure sensing sensor cell 80 for main pump control, the electromotive force V1, which was detected in the oxygen partial pressure sensing sensor cell 81 for auxiliary pump control, the electromotive force V2, which was detected in the oxygen partial pressure sensing sensor cell 82 for measuring pump control, the electromotive force Vref, which was detected in the sensor cell 83, the pump current Ip0, which was detected in the main pump cell 21, the pump current Ip1, which was detected in the auxiliary pump cell 50, and the pump current Ip2, which was detected in the measuring pump cell 41.The control unit 90 transmits control signals to the variable power supply 24 of the main pump cell 21, the variable power supply 52 of the auxiliary pump cell 50 and the variable power supply 46 of the measuring pump cell 41.

[0053] The control unit 90 regulates the pump voltage Vp0 of the variable power supply 24 such that the electromotive force V0 is at the target value (referred to as target value V0*) (i.e., so that the oxygen concentration in the first inner cavity 20 is at a constant target concentration). Consequently, the pump current Ip0 varies depending on the oxygen concentration in the measuring gas.

[0054] The control unit 90 also regulates the voltage Vp1 of the variable power supply 52 such that the electromotive force V1 remains at a constant value (referred to as target value V1*) (i.e., so that the oxygen concentration in the second inner cavity 40 is at a predetermined low oxygen concentration, which has essentially no effect on the NOx measurement). In addition, the control unit 90 sets the target value V0* of the electromotive force V0 based on the pump current Ip1 flowing at voltage Vp1 such that the pump current Ip1 remains at a constant value (referred to as target value Ip1*) (regulation). Consequently, the gradient of the oxygen partial pressure in the sample gas introduced into the second inner cavity 40 by the third diffusion rate limiting section 30 remains constant.Furthermore, the oxygen partial pressure in the atmosphere is adjusted to a low partial pressure in the second inner cavity 40, which has essentially no effect on the NOx measurement.

[0055] The control unit 90 also regulates the voltage Vp2 of the variable power supply 46 such that the electromotive force V2 remains at a constant value (referred to as the target value V2*) (i.e., so that the oxygen concentration in the third inner cavity 61 is at a predetermined low concentration). Consequently, oxygen is pumped out of the third inner cavity 61 such that the concentration of oxygen produced by the reduction of NOx in the sample gas is essentially zero in the third inner cavity 61. The control unit 90 detects the pump current Ip2 as the detection value dependent on oxygen originating from the specific gas (here NOx) in the third inner cavity 61 and calculates the NOx concentration in the sample gas based on the pump current Ip2.

[0056] Memory 94 stores a relationship formula between the pump current Ip2 and the NOx concentration, for example in the form of a linear function. This relationship formula can be determined experimentally beforehand.

[0057] An example of the use of the gas sensor 100 constructed in this way is described below. It is assumed that the CPU 92 of the control unit 90 is operated to control the pump cells 21, 41, and 50 described above and to detect the voltages V0, V1, V2, and Vref of the sensor cells 80 to 83 described above. In this state, when the sample gas is introduced from the gas inlet 10, the sample gas first passes through the first diffusion rate restriction section 11, the buffer chamber 12, and the second diffusion rate restriction section 13 in the order mentioned above and reaches the first inner cavity 20. The oxygen concentration in the sample gas is then adjusted in the first inner cavity 20 by the main pump cell 21 and in the second inner cavity 40 by the auxiliary pump cell 50. After adjustment, the sample gas reaches the third inner cavity 61.The CPU 92 detects the NOx concentration in the measuring gas based on the detected pump current Ip2 and the relationship formula stored in the memory 94.

[0058] As described above, the inner pump electrode 22 does not contain the noble metal with the ability to inhibit catalytic activity, whereas the auxiliary pump electrode 51 does contain the noble metal with the ability to inhibit catalytic activity. The reason for this is explained below. The inventors provided gas sensors of experimental examples 1 to 4, which had the same structure as gas sensor 100, but differed in the presence or absence of Au in the inner pump electrode 22 and the auxiliary pump electrode 51, as shown in Table 1. For all experimental examples 1 to 4, the inner pump electrode 22 and the auxiliary pump electrode 51 were porous cermet electrodes composed of a noble metal and ZrO₂. In Table 1, "0.8" means that the electrodes contained Pt and Au as noble metals and that the mass percentage of Au relative to Pt in the electrodes is 0.8 wt.%.In Table 1, “-” means that the electrodes contained only Pt as a precious metal and no Au. [Table 1] Mass percentage of Au relative to Pt in the electrodes [wt%] Internal pump electrode Auxiliary pump electrode Experimental Example 1 0,8 0,8 Experimental Example 2 - - Experimental Example 3 - 0,8 Experimental Example 4 0,8 -

[0059] The gas sensors of experimental examples 1 to 4 were each investigated with respect to the relationship between the concentration of a specific gas in a sample gas and the pump current Ip2 in the case where the sample gas was not an atmosphere with a low oxygen concentration. Three model gases containing 0 ppm, 250 ppm, and 500 ppm NO as the specific gas component were prepared and used as the sample gas. Nitrogen was used as the base gas for all three model gases, the humidity concentration was set to 3 vol%, and the oxygen concentration was set to 10 vol%. The temperature of the model gases was 250 °C. The model gases were passed through a 20 mm diameter pipe at a flow rate of 50 L / min. The relationship between the NO concentration and the pump current Ip2 for the gas sensors of experimental examples 1 to 4 is shown in Table 2 and the Fig. 3 shown. [Table 2] NO concentration [ppm] Pump current Ip2 [µA] Experimental Example 1 Experimental Example 2 Experimental Example 3 Experimental Example 4 0 0,09 0,09 0,09 0,09 250 0,67 0,09 0,67 0,09 500 1,24 0,09 1,24 0,09

[0060] As can be seen from the data in Table 2 and the Fig.As can be seen from the results shown in Figure 3, experimental examples 2 and 4, in which the auxiliary pump electrode 51 did not contain Au, showed a small change in Ip2 when the NO concentration was changed, and the pump current Ip2 was nearly 0 µA. This is possibly due to the fact that NO was reduced by the catalytic activity of the auxiliary pump electrode 51 before it reached the measuring electrode 44. In contrast, experimental examples 1 and 3, in which the auxiliary pump electrode 51 contained Au, showed a proportional relationship between the NO concentration and Ip2. Furthermore, the values ​​of Ip2, which corresponded to those of the NO concentration for experimental examples 1 and 3, were nearly identical. That is, whether the inner pump electrode 22 contained Au or not did not affect the pump current Ip2.These results show that NO is not reduced by the inner pump electrode 22 even when, while the auxiliary pump electrode 51 contains Au, the inner pump electrode 22 does not. Based on these results, the inventors found that the inner pump electrode 22 need not contain Au if the measuring gas is not an atmosphere with a low oxygen concentration. Based on these results, the inner pump electrode 22 of the gas sensor 100 according to this embodiment contains no Au, whereas the auxiliary pump electrode 51 contains Au. That is, experimental example 3 corresponds to the gas sensor 100 according to this embodiment and is therefore an example of the gas sensor according to the present invention. Experimental examples 1, 2, and 4 correspond to comparative examples.

[0061] The reason for the results discussed above is assumed to be as follows. During the use of the gas sensor 100, the main pump cell 21 and the auxiliary pump cell 50 are controlled by the CPU 92 in the manner described above such that they pump out oxygen when the sample gas is not an atmosphere with a low oxygen concentration. Consequently, the relationship between the oxygen concentrations around the gas inlet 10 and the electrodes in the sample gas flow section is assumed to be as follows: (around the gas inlet 10) > (around the inner pump electrode 22) > (around the auxiliary pump electrode 51) > (around the measuring electrode 44). That is, the oxygen concentration is higher around the inner pump electrode 22 than around the auxiliary pump electrode 51.NOx is less likely to be reduced at higher oxygen concentrations; therefore, it is less likely that NOx will be reduced by the inner pump electrode 22, even if the inner pump electrode 22 does not contain the noble metal with the ability to inhibit catalytic activity (here, Au). On the other hand, it is more likely that NOx will be reduced by the auxiliary pump electrode 51, since the measuring gas reaches the area around the auxiliary pump electrode 51 after oxygen has been pumped out through the main pump cell 21. However, the reduction of NOx can be inhibited because the auxiliary pump electrode 51 contains Au. Consequently, the gas sensor 100 according to this embodiment has a sufficiently low tendency to reduce NOx before the measuring gas reaches the measuring electrode 44 and therefore has sufficient detection accuracy for the concentration of a specific gas.

[0062] If the inner pump electrode 22 contains Au, Au can evaporate from the inner pump electrode 22 and be deposited on the measuring electrode 44 during use of the gas sensor 100. The deposition of Au on the measuring electrode 44 inhibits the catalytic activity of the measuring electrode 44 and consequently leads to insufficient reduction of NOx around the measuring electrode 44. As a result, the actual pump current Ip2 decreases compared to the correct pump current Ip2, which corresponds to the NOx concentration, thereby reducing the detection accuracy for the concentration of a specific gas. In contrast, since the inner pump electrode 22 of the gas sensor 100, according to this embodiment, does not contain the noble metal with the ability to inhibit catalytic activity, the evaporation of the noble metal during use of the gas sensor 100 can be inhibited, and therefore the reduction in detection accuracy during use can be reduced.

[0063] Consequently, according to this embodiment, the gas sensor 100 can maintain its detection accuracy for the concentration of a specific gas for a long period of time. In contrast, for example, as in experimental examples 2 and 4, if the auxiliary pump electrode 51 does not contain Au, the detection accuracy for the concentration of a specific gas is already reduced when the gas sensor is first used. For example, as in experimental example 1, if the inner pump electrode 22 contains Au, the detection accuracy for the concentration of a specific gas tends to decrease during use of the gas sensor. That is, the long-term stability of the gas sensor decreases.

[0064] Although the auxiliary pump electrode 51 contains Au, the Au in the auxiliary pump electrode 51 exhibits a relatively low tendency to evaporate. This will be explained. The evaporation of Au from an electrode, as described above, is more likely to occur at a higher oxygen concentration. For example, in the case of an electrode containing Pt and Au, Pt is more likely to be oxidized to form PtO2 at a higher oxygen concentration. PtO2 has a stronger tendency to evaporate than Pt because PtO2 has a higher saturation vapor pressure than Pt. When Pt evaporates in the form of PtO2, the remaining Au also tends to evaporate. This is because Au alone has a higher saturation vapor pressure than a Pt-Au alloy.In contrast, Au in the auxiliary pump electrode 51 exhibits a relatively low tendency to evaporate because, as described above, the oxygen concentration around the auxiliary pump electrode 51 is lower. Consequently, the reduction in detection accuracy during the use of the gas sensor 100, as described above, is less likely to occur, even if the auxiliary pump electrode 51 contains Au.

[0065] The correspondences between the elements of this embodiment and the elements of the present invention are shown below. The stack of six layers of this embodiment, namely the first substrate layer 1, the second substrate layer 2, the third substrate layer 3, the first solid electrolyte layer 4, the spacer layer 5, and the second solid electrolyte layer 6, stacked in the aforementioned order, corresponds to an element body of the present invention. The first inner cavity 20 corresponds to a first inner cavity. The main pump cell 21 corresponds to a main pump cell. The second inner cavity 40 corresponds to a second inner cavity. The auxiliary pump cell 50 corresponds to an auxiliary pump cell. The third inner cavity 61 corresponds to a measuring chamber. The measuring electrode 44 corresponds to a measuring electrode. The reference electrode 42 corresponds to a reference electrode.The oxygen partial pressure sensing sensor cell 82 for measuring pump control corresponds to a measuring voltage sensing unit. The pump current Ip2 corresponds to a sensing value. The CPU 92 of the control unit 90 corresponds to a unit for sensing the concentration of a specific gas. The inner pump electrode 22 corresponds to an inner main pump electrode. The auxiliary pump electrode 51 corresponds to an inner auxiliary pump electrode.

[0066] The inner pump electrode 22 of the gas sensor 100 described above, according to this embodiment, does not contain the noble metal with the ability to inhibit catalytic activity (e.g., Au); however, a small amount of the specific gas is reduced by the inner pump electrode 22 if the sample gas introduced into the sample gas flow section is not an atmosphere with a low oxygen concentration. Furthermore, since the inner pump electrode 22 does not contain the noble metal with the ability to inhibit catalytic activity, the evaporation and deposition of the noble metal on the measuring electrode 44 during use of the gas sensor 100 can be inhibited.Consequently, the Gas Sensor 100 can maintain its detection accuracy for the concentration of a specific gas over a long period when used in applications where the concentration of a specific gas is measured in a sample gas that is not a low-oxygen atmosphere. That is, the Gas Sensor 100 is particularly suitable for measuring the concentration of a specific gas in a sample gas that is not a low-oxygen atmosphere.

[0067] It should be noted that the present invention is not in any way limited to the embodiment described above, but can be implemented in various embodiments which are within the technical scope of the invention.

[0068] Although the second diffusion rate restriction section 13 is located between the buffer space 12 and the first inner cavity 20 in the embodiment described above, the present invention is not limited to this. For example, the second diffusion rate restriction section 13 can be omitted, and the buffer space 12 and the first inner cavity 20 can form a single space.

[0069] Although the gas sensor 100, according to the embodiment described above, is configured to detect the NOx concentration as the concentration of the specific gas, the present invention is not limited thereto. Rather, the gas sensor 100 can be configured to detect the concentration of an oxide other than the concentration of the specific gas. If the specific gas is an oxide, oxygen is produced when the specific gas itself is reduced in the third inner cavity 61, as is the case in the embodiment described above; therefore, the CPU 92 can detect the value depending on the oxygen and thus detect the concentration of the specific gas. Alternatively, the specific gas can be a non-oxide, such as ammonia. If the specific gas is a non-oxide, the specific gas is converted into an oxide (e.g., ammonia is converted into NO).Since oxygen is produced when the converted gas is reduced in the third inner cavity 61, the CPU 92 can detect the oxygen-dependent value and determine the concentration of the specific gas. For example, because the inner pump electrode 22 contains the catalytically active noble metal described above, the specific gas in the first inner cavity 20 can be converted into an oxide. Since ammonia is converted to NO as an oxide, the measurement of the ammonia concentration is essentially carried out according to the same principle as the measurement of the NOx concentration.

[0070] Although the sensor element 101 of the gas sensor 100, according to the embodiment described above, comprises the first inner cavity 20, the second inner cavity 40, and the third inner cavity 61, the present invention is not limited thereto. For example, as in the case of a sensor element 201 in the Fig.4, the third inner cavity 61 is omitted. In the sensor element 201 according to the in the Fig.In the modification shown in Figure 4, the gas inlet 10, the first diffusion rate restriction section 11, the buffer chamber 12, the second diffusion rate restriction section 13, the first inner cavity 20, the third diffusion rate restriction section 30, and the second inner cavity 40 are arranged adjacent to one another such that they are connected to each other in the aforementioned order between the lower surface of the second solid electrolyte layer 6 and the upper surface of the first solid electrolyte layer 4. The measuring electrode 44 is arranged on the upper surface of the first solid electrolyte layer 4 in the second inner cavity 40. The measuring electrode 44 is covered by a fourth diffusion rate restriction section 45. The fourth diffusion rate restriction section 45 is a porous film made of a ceramic, such as aluminum oxide (Al₂O₃).As with the fourth diffusion rate restriction section 60 of the embodiment described above, the fourth diffusion rate restriction section 45 serves to limit the amount of NOx flowing into the measuring electrode 44. The fourth diffusion rate restriction section 45 also acts as a protective film for the measuring electrode 44. The upper electrode section 51a of the auxiliary pump electrode 51 is designed to extend over the measuring electrode 44. As in the embodiment described above, the sensor element 201 thus configured can detect the NOx concentration, for example, based on the pump current Ip2. In this case, the area around the measuring electrode 44 acts as a measuring chamber.

[0071] Although in the embodiment described above the outer pump electrode 23 acts as the outer main pump electrode of the main pump cell 21, as the outer auxiliary pump electrode of the auxiliary pump cell 50, and as the outer measuring electrode of the measuring pump cell 41, the present invention is not limited thereto. In addition to the outer pump electrode 23, one or more outer main pump electrodes, outer auxiliary pump electrodes, and outer measuring electrodes can be arranged outside the element body such that they contact the measuring gas.

[0072] Although the sensor element body 101, according to the embodiment described above, is a stack comprising a plurality of solid electrolyte layers (layers 1 to 6), the present invention is not limited thereto. The sensor element body 101 can comprise at least one oxygen-ion-conducting solid electrolyte layer and can have a measuring gas flow section within the element body. For example, layers 1 to 5, which are separated from the second solid electrolyte layer 6 in the Fig. The first layer is different, and the layers are made of materials other than solid electrolytes (e.g., aluminum oxide layers). In this case, the electrodes of the sensor element 101 can be arranged on the second solid electrolyte layer 6. For example, the measuring electrode 44 can be located in the Fig.The reference electrode 42 can be arranged on the lower surface of the second solid electrolyte layer 6. Furthermore, the reference gas introduction chamber 43 can be arranged in the spacer layer 5 instead of in the first solid electrolyte layer 4. The air introduction layer 48 can be arranged 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. The reference electrode 42 can be arranged on the lower surface of the second solid electrolyte layer 6 on the rear side of the third inner cavity 61.

[0073] Although, in the embodiment described above, the control unit 90 sets the target value V0* of the electromotive force V0 based on the pump current Ip1 such that the pump current Ip1 is at the target value Ip1* (regulation) and regulates the pump voltage Vp0 such that the electromotive force V0 is at the target value V0*, another control method is also possible. For example, the control unit 90 can regulate the pump voltage Vp0 based on the pump current Ip1 such that the pump current Ip1 is at the target value Ip1*. That is, the detection of the electromotive force V0 from the oxygen partial pressure sensing sensor cell 80 to the main pump control and the setting of the target value V0* can be omitted, and the control unit 90 can directly control the pump voltage Vp0 based on the pump current Ip1 (and thereby control the pump current Ip0).

[0074] Although not described in the embodiment above, the gas sensor 100 is preferably used to measure the concentration of a specific gas in a sample gas with an oxygen concentration greater than 0.1% by volume. That is, a "sample gas that is not an atmosphere with a low oxygen concentration" can be a sample gas with an oxygen concentration greater than 0.1% by volume. In experimental examples 1 to 4 described above, the oxygen concentration in the sample gas reaching the second inner cavity 40 (= oxygen concentration at the outlet of the third diffusion rate restriction section 30) was measured as 0.1% by volume. This suggests that the auxiliary pump electrode 51 must have contained Au, since the oxygen concentration around the auxiliary pump electrode 51 was no more than 0.1% by volume.On the other hand, as can be seen from the relationship described above between the oxygen concentrations around the electrodes in the sample gas flow section, the oxygen concentration around the inner pump electrode 22 was more than 0.1 volume%. This explains why NO was not reduced even though the inner pump electrode 22 did not contain Au. Consequently, if the sample gas has an oxygen concentration of more than 0.1 volume%, the reduction of the specific gas or the oxide derived from the specific gas by the inner pump electrode 22 can be more reliably inhibited even if the inner pump electrode 22 does not contain the noble metal with the ability to inhibit catalytic activity. That is, the requirement for the inner pump electrode 22 to contain the noble metal with the ability to inhibit catalytic activity can be more reliably ruled out.The gas sensor 100 is preferably used to measure the concentration of a specific gas in a sample gas with an oxygen concentration of 1 volume% or more. In this case, the requirement for the inner pump electrode 22 to contain the precious metal with the ability to inhibit catalytic activity can be eliminated even more reliably.

[0075] In the embodiment described above, the main pump cell 21 can pump oxygen out of the first inner cavity 20 so that the oxygen concentration in the sample gas reaching the second inner cavity 40 is not less than 0.1% by volume. In this case, the probability of the oxygen concentration around the inner pump electrode 22 becoming low can be reduced. The reduction of the specific gas or the oxide derived from the specific gas by the inner pump electrode 22 can therefore be more reliably inhibited if the inner pump electrode 22 does not contain the noble metal with the ability to inhibit catalytic activity. The CPU 92 preferably controls the main pump cell 21 such that this oxygen pumping is carried out.For example, the permissible range of the target value V0* described above can be experimentally determined in advance such that the oxygen concentration in the sample gas reaching the second inner cavity 40 is not less than 0.1 volume%. If the target value V0* is set based on the pump current Ip1, the CPU 92 can set the target value V0* within this permissible range. COMMERCIAL APPLICABILITY

[0076] The present invention is applicable to the industrial manufacture of gas sensors for detecting the concentration of a specific gas, such as NOx, in a measuring gas, such as motor vehicle exhaust.

Claims

[1] Gas sensor (100), comprising: an element body comprising an oxygen ion-conducting solid electrolyte layer (1, 2, 3, 4, 5, 6) and having a measuring gas flow section within the element body for introducing and allowing a measuring gas to flow; a main pump cell (21) designed to pump oxygen out of a first inner cavity (20) of the measuring gas flow section to adjust an oxygen concentration in the first inner cavity (20); an auxiliary pump cell (50) designed to pump oxygen out of a second inner cavity (40) downstream of the first inner cavity (20) of the measuring gas flow section to adjust an oxygen concentration in the second inner cavity (40); a measuring electrode (44) which is arranged on an inner circumferential surface of a measuring chamber (61) downstream of the second inner cavity (40) of the measuring gas flow section; a reference electrode (42) which is arranged inside the element body and to which a reference gas is to be introduced, wherein the reference gas serves as a reference for detecting a concentration of a specific gas in the measuring gas; a voltage measurement unit (82) designed to measure a voltage (V2) between the reference electrode (42) and the measuring electrode (44); and a unit (92) for detecting the concentration of a specific gas, which is designed to detect, on the basis of the measuring voltage (V2), a detection value dependent on oxygen originating from the specific gas in the measuring chamber (61), and to detect, on the basis of the detection value, the concentration of the specific gas in the measuring gas, wherein the main pump cell (21) comprises an inner main pump electrode (22) which is arranged in the first inner cavity (20), the auxiliary pump cell (50) comprises an inner auxiliary pump electrode (51) which is arranged in the second inner cavity (40), the inner main pump electrode (22), the inner auxiliary pump electrode (51) and the measuring electrode (44) each contain a catalytically active noble metal, the inner main pump electrode (22) does not contain a noble metal with the ability to inhibit the catalytic activity of the catalytically active noble metal on the specific gas, and the inner auxiliary pump electrode (51) contains the precious metal with the ability to inhibit catalytic activity. [2] Gas sensor (100) according to claim 1, wherein the inner auxiliary pump electrode (51) contains Au as a precious metal with the ability to inhibit catalytic activity. [3] Sensor element (101), comprising: an element body comprising an oxygen ion-conducting solid electrolyte layer (1, 2, 3, 4, 5, 6) and having a measuring gas flow section within the element body for introducing and allowing a measuring gas to flow; a main pump cell (21) designed to pump oxygen out of a first inner cavity (20) of the measuring gas flow section to adjust an oxygen concentration in the first inner cavity (20); an auxiliary pump cell (50) designed to pump oxygen out of a second inner cavity (40) downstream of the first inner cavity (20) of the measuring gas flow section to adjust an oxygen concentration in the second inner cavity (40); a measuring electrode (44) arranged on an inner circumferential surface of a measuring chamber (61) downstream of the second inner cavity (40) of the measuring gas flow section; and a reference electrode (42) which is arranged inside the element body and to which a reference gas is to be introduced, wherein the reference gas serves as a reference for detecting a concentration of a specific gas in the measuring gas; wherein the main pump cell (21) comprises an inner main pump electrode (22) which is arranged in the first inner cavity (20), the auxiliary pump cell (50) comprises an inner auxiliary pump electrode (51) which is arranged in the second inner cavity (40), the inner main pump electrode (22), the inner auxiliary pump electrode (51) and the measuring electrode (44) each contain a catalytically active noble metal, the inner main pump electrode (22) does not contain a noble metal with the ability to inhibit the catalytic activity of the catalytically active noble metal on the specific gas, and the inner auxiliary pump electrode (51) contains the precious metal with the ability to inhibit catalytic activity.

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