Gas sensor and sensor element

Abstract

Gas sensor (100), including: an element body (1, 2, 3, 4, 5, 6) containing an oxygen ion-conducting solid electrolyte layer (4, 6), wherein a test object gas flow section is provided within the element body (1, 2, 3, 4, 5, 6) to allow a test object gas to be introduced into and flow through the test object gas flow section; a main pump cell (21) that pumps oxygen from a first interior space (20) to adjust the oxygen concentration of the first interior space (20), wherein the first interior space (20) is provided in the gas flow section of the object being measured; an auxiliary pump cell (50) that pumps oxygen from a second interior space (40) to adjust the oxygen concentration of the second interior space (40), wherein the second interior space (40) is provided downstream of the first interior space (20) in the gas flow section of the object being measured; a pre-pump cell (15) that pumps oxygen into a pre-chamber (12) to prevent the object gas from reaching the first interior space (20) in a state where the object gas is an oxygen-depleted atmosphere, the pre-chamber (12) being provided upstream of the first interior space (20) in the object gas flow section; a measuring electrode (44) which is arranged on an inner circumferential surface of a measuring chamber (61), wherein the measuring chamber (61) is provided downstream of the second interior space (40) in the object-gas flow section; a reference electrode (42) which is arranged within the element body (1, 2, 3, 4, 5, 6) and into which a reference gas is to be introduced, wherein the reference gas serves as a reference for detecting a specific gas concentration in the gas of the object being measured; a voltage detection device (82) that detects a measurement voltage (V2) present between the reference electrode (42) and the measuring electrode (44); and a specific gas concentration detection device (92) which, based on the measuring voltage (V2), obtains a detection value (Ip2) according to the oxygen generated in the measuring chamber (61) and, based on the detection value (Ip2), detects the specific gas concentration in the gas of the object being measured, wherein the oxygen is oxygen derived from the specific gas, wherein the pre-pump cell (15) includes an internal pre-pump electrode (16) arranged in the pre-chamber (12), the main pump cell (21) includes an inner main pump electrode (22) arranged in the first interior space (20), the auxiliary pump cell (50) includes an inner auxiliary pump electrode (51) arranged in the second interior space (40), the inner pre-pump electrode (16), the inner main pump electrode (22), the inner auxiliary pump electrode (51) and the measuring electrode (44) each contain a noble metal with catalytic activity, wherein at least one of the inner pre-pump electrode (16) and the inner main pump electrode (22) does not contain a noble metal with a catalytic suppression capability, wherein the catalytic suppression capability is a capability to suppress the catalytic activity of the noble metal exhibiting the catalytic activity shown for the specific gas, and the internal auxiliary pump electrode (51) contains a precious metal with the ability to suppress catalytic activity.

Description

TECHNICAL FIELD

[0001] The present invention relates to a gas sensor and a sensor element. TECHNICAL BACKGROUND

[0002] In the prior art, a gas sensor is known for detecting the specific gas concentration, such as NOx, in a measurement target gas, such as automobile exhaust. For example, Patent Literature 1 describes a gas sensor having a laminated body including a plurality of oxygen ion-conductive solid electrolyte layers and electrodes on the solid electrolyte layers. When the NOx concentration is to be detected with this gas sensor, oxygen is first pumped inward or outward through a measurement target gas flow portion located inside the sensor element and a portion outside the sensor element, thereby adjusting the oxygen concentration of the measurement target gas flow portion.Subsequently, NOx in the target gas, which is a gas after adjusting the oxygen concentration, is reduced, and the NOx concentration in the target gas is detected based on a current flowing across an electrode (measuring electrode) within the sensor element according to the oxygen concentration after reduction. Furthermore, Patent Literature 2 describes a gas sensor for detecting the ammonia concentration in a target gas. The gas sensor detects the ammonia concentration as follows: Ammonia is converted into NOx by oxidation with the oxygen present in the target gas, and the concentration of NOx derived from the ammonia is detected using a method similar to Patent Literature 1.

[0003] Furthermore, Patent Literature 1 describes the following: An inner pumping electrode of a pumping cell for adjusting the oxygen concentration is located in the measurement target gas flow section, and the inner pumping electrode is a cermet electrode containing Pt and ZrO2 and containing 1% Au. Since the inner pumping electrode contains Au, the inner pumping electrode is prevented from reducing NOx. On the other hand, Patent Literature 3 describes the following: In conjunction with the use of a gas sensor, Au evaporates from an electrode of a pumping cell and adheres to an electrode of a sensor cell to detect the NOx concentration in a measurement target gas. As a result, the detection accuracy of the NOx concentration decreases. Patent Literature 4 discloses a sensor element of a gas sensor. Furthermore, Patent Literature 5 describes a gas sensor. CITATION LISTPATENT LITERATURE

[0004] PTL 1: JP 2014 - 190 940 A; PTL 2: JP 2011 - 39 041 A; PTL 3: JP 6 447 568 B2; PTL 4: DE 103 08 395 A1; PTL 5: DE 11 2016 004 257 T5 DISCLOSURE OF THE INVENTION

[0005] As described above, it was necessary to include Au in the inner pumping electrode because when the inner pumping electrode reduces NOx, NOx concentration detection accuracy decreases. On the other hand, as described above, there was a problem that the presence of Au in the inner pumping electrode decreases NOx concentration detection accuracy associated with the use of the gas sensor.

[0006] The present invention has been developed to solve such problems, and a main object of the present invention is to maintain a specific gas concentration detection accuracy over a long period of time.

[0007] To solve the problems described above, the inventors conducted careful studies and subsequently found that when an environment of the inner pumping electrode is not an oxygen-deficient atmosphere, substantially no reduction of NOx by the inner pumping electrode occurs, even if the inner pumping electrode does not contain Au. Furthermore, it was found that the need to include Au in the inner pumping electrode, which is considered essential in the prior art, can be eliminated by ensuring that oxygen is pumped into the measurement target gas flow section. Accordingly, the present invention was completed.

[0008] A gas sensor of the present invention includes an element body, a main pump cell, an auxiliary pump cell, a pre-pump cell, a measuring electrode, a reference electrode, a measuring voltage detection device, and a specific gas concentration detection device. The element body includes an oxygen ion-conductive solid electrolyte layer, and a measurement target gas flow section is provided within the element body so that a measurement target gas can be introduced into the measurement target gas flow section and flows through the measurement target gas flow section. The main pump cell pumps oxygen from a first internal space to adjust the oxygen concentration of the first internal space, the first internal space being provided in the measurement target gas flow section.The auxiliary pump cell pumps oxygen from a second internal space to adjust an oxygen concentration of the second internal space, the second internal space being provided downstream of the first internal space in the measurement object gas flow section. The pre-pump cell pumps oxygen into a pre-chamber to prevent the measurement object gas from reaching the first internal space in a state where the measurement object gas is an oxygen-deficient atmosphere, the pre-chamber being provided upstream of the first internal space in the measurement object gas flow section. The measuring electrode is arranged on an inner peripheral surface of a measuring chamber, the measuring chamber being provided downstream of the second internal space in the measurement object gas flow section. The reference electrode is arranged inside the element body. A reference gas is to be introduced into the reference electrode. The reference gas serves as a reference for detecting a specific gas concentration in the measurement object gas.The measurement voltage detection device detects a measurement voltage applied between the reference electrode and the measurement electrode. The specific gas concentration detection device obtains a detection value corresponding to the oxygen generated in the measurement chamber based on the detection voltage and detects the specific gas concentration in the measurement object gas based on the detection value, where the oxygen is oxygen derived from the specific gas. The fore-pump cell includes an inner fore-pump electrode arranged in the pre-chamber. The main pump cell includes an inner main pump electrode arranged in the first internal space. The auxiliary pump cell includes an inner auxiliary pump electrode arranged in the second internal space. The inner fore-pump electrode, the inner main pump electrode, the inner auxiliary pump electrode, and the measurement electrode each contain a noble metal with catalytic activity.At least one of the inner pre-pump electrode and the inner main pump electrode does not contain a noble metal capable of suppressing catalytic activity. The ability to suppress catalytic activity is the ability of the noble metal to suppress the catalytic activity exhibited for the specific gas. The inner auxiliary pump electrode contains a noble metal capable of suppressing catalytic activity.

[0009] In the gas sensor, the pre-pump cell pumps oxygen into the pre-chamber provided in front of the first internal space. This is to prevent the measured object gas from reaching the first internal space in a state where the measured object gas is an oxygen-deficient atmosphere. Then, the main pump cell and the auxiliary pump cell individually pump oxygen from the measured object gas containing oxygen as described above into the pre-chamber. Thus, the oxygen concentration of the measured object gas is adjusted. As a result, the measured object gas enters the measuring chamber after the oxygen concentration is adjusted. Furthermore, based on the measuring voltage, the gas sensor obtains a detection value according to the oxygen generated in the measuring chamber, where the oxygen is derived from the specific gas, and based on the obtained detection value, the gas sensor detects the specific gas concentration in the measured object gas.It should be noted that, in the gas sensor of the present invention, as described above, the pre-pumping cell supplies oxygen to the measurement target gas before adjusting the oxygen concentration. Thus, even in a case where the measurement target gas is an oxygen-poor atmosphere before being introduced into the measurement target gas flow section, the environments of the inner pre-pumping electrode and the inner main pumping electrode are prevented from becoming an oxygen-poor atmosphere. Therefore, even in a case where at least one of the inner pre-pumping electrode and the inner main pumping electrode does not contain a noble metal (e.g., Au) capable of suppressing catalytic activity, neither reduction of a specific gas by the electrode nor reduction of an oxide derived from the specific gas by the electrode is likely to occur. This achieves sufficient specific gas detection accuracy.Since at least one of the inner pre-pumping electrode and the inner main pumping electrode does not contain a noble metal capable of suppressing catalytic activity, a case in which noble metal evaporates and adheres to the measuring electrode, which may otherwise occur in connection with the use of the gas sensor, is prevented. Accordingly, the gas sensor of the present invention has a specific gas concentration detection accuracy that is maintained over a long period of time.

[0010] It should be noted that the term "measurement target gas that is an oxygen-poor atmosphere" includes cases where the measurement target gas is a rich atmosphere gas with unburned fuel. Furthermore, the phrase "does not contain a precious metal capable of suppressing catalytic activity" essentially means that a precious metal capable of suppressing catalytic activity is not included, and therefore, it is possible that one or more precious metals capable of suppressing catalytic activity are included as incidental impurities. The first internal space may be referred to as the "oxygen concentration adjustment chamber." The main pump cell may be referred to as the "adjustment pump cell."

[0011] It should be noted that in the case of an oxide, the term "oxygen generated in the measuring chamber, where the oxygen originates from the specific gas" may refer to the oxygen generated when the specific gas itself is reduced in the measuring chamber. If the specific gas is a non-oxide gas, the term "oxygen generated in the measuring chamber, where the oxygen originates from the specific gas" may refer to the oxygen generated when the specific gas is converted into an oxide and the resulting gas is reduced in the measuring chamber. In addition, the specific gas concentration detection device can obtain the detection value as follows.Based on the measurement voltage, oxygen generated in the measurement chamber can be pumped out of the measurement chamber. The oxygen is oxygen extracted from the specific gas, allowing the oxygen concentration in the measurement chamber to reach a predetermined low concentration. When pumping is performed, a measurement pump current flows. The measurement pump current can be the detection value. The element body can be a laminated body with a plurality of stacked oxygen-ion-conductive solid electrolyte layers.

[0012] The gas sensor of the present invention may further include a pre-pump control device. The pre-pump control device can control the pre-pump cell so that a constant pre-pump current flows through the pre-pump cell. With this configuration, oxygen can be supplied to a measurement target gas, which is an oxygen-poor atmosphere in the pre-chamber, through relatively simple control.

[0013] The gas sensor of the present invention may further include a storage device. The storage device may store information related to a relationship formula representing a relationship between the detection value and the specific gas concentration. Regardless of whether a measurement target gas outside the element body is an oxygen-poor atmosphere, the specific gas concentration detection device can detect the specific gas concentration using the same relationship formula stored in the storage device. In this way, the gas sensor of the present invention can accurately detect the specific gas concentration without different relationship formulas for the case where the measurement target gas is an oxygen-poor atmosphere and the case where the measurement target gas is not an oxygen-poor atmosphere.This allows the gas sensor to detect the specific gas concentration easily and precisely.

[0014] In the gas sensor of the present invention, the specific gas concentration detecting device detects the specific gas concentration, and the specific gas concentration may be a concentration corrected based on an oxygen concentration of the measurement target gas located outside the element body. It should be noted that even if the actual concentration (real concentration) of a specific gas in a measurement target gas remains constant, the detection value may change with the oxygen concentration of a measurement target gas located outside the element body, and in this case, the measured specific gas concentration based on the detection value also changes. Accordingly, by detecting the specific gas concentration with the oxygen concentration-based correction incorporating, the specific gas concentration measurement accuracy is improved.The term "detecting the specific gas concentration and the specific gas concentration is a concentration corrected based on an oxygen concentration of the measurement object gas" includes the following: cases where the specific gas concentration is detected based on a detection value obtained after the oxygen concentration-based correction; and cases where, when detecting the specific gas concentration based on the detection value, the oxygen concentration-based correction is performed and the corrected specific gas concentration is detected.

[0015] In this case, the gas sensor of the present invention may further include a pre-pumping control device and an oxygen concentration detection device. The pre-pumping control device may control the pre-pumping cell so that a constant pre-pumping current flows through the pre-pumping cell. The oxygen concentration detection device may detect the oxygen concentration of the measurement object gas located outside the element body. The oxygen concentration may be detected based on the constant pre-pumping current, a pumping current that flows when the main pumping cell pumps oxygen from the first internal space so that the oxygen concentration of the first internal space reaches a target concentration, and the target concentration. The specific gas concentration detection device may correct the specific gas concentration using the oxygen concentration detected by the oxygen concentration detection device.It should be noted that the constant pre-pump flow flowing through the pre-pump cell corresponds to the oxygen flow rate pumped by the pre-pump cell into the measurement target gas flow section. Furthermore, the pump flow of the main pump cell corresponds to the oxygen flow rate pumped from the first internal space. Thus, the oxygen concentration of a measurement target gas outside the element body can be detected based on the currents and the target concentration. That is, the oxygen concentration required for correction can be detected by the gas sensor of the present invention.

[0016] The gas sensor of the present invention may further include a gas-side sensing target electrode disposed at a portion to be exposed to the sensing target gas located outside the element body. The pre-pumping cell can pump oxygen from the vicinity of the gas-side sensing target electrode into the pre-chamber. With this configuration, the following is possible: Compared with, for example, a case where oxygen is pumped into the pre-chamber from the vicinity of the reference electrode, a decrease in measurement accuracy that may occur when the potential of the reference electrode changes due to a voltage drop caused by the pumping current is suppressed.

[0017] In the gas sensor of the present invention, the measurement target gas can be exhaust gas from an internal combustion engine, the reference gas can be air, and the pre-pumping cell can pump oxygen from the vicinity of the reference electrode into the pre-chamber. With this configuration, the following is possible: Compared, for example, to a case where oxygen is pumped inward from exhaust gases located outside the element body, oxygen can be pumped into the pre-chamber with a low applied voltage, because air has a higher oxygen concentration than exhaust gases.

[0018] In the gas sensor of the present invention, neither the inner pre-pumping electrode nor the inner main-pumping electrode can contain a noble metal capable of suppressing catalytic activity. With this configuration, the effect of maintaining specific gas concentration detection accuracy over a longer period of time is improved.

[0019] In the gas sensor of the present invention, the inner auxiliary pump electrode contains the noble metal having the ability to suppress the catalytic activity, and the noble metal may include Au.

[0020] A sensor element of the present invention includes an element body, a main pump cell, an auxiliary pump cell, a pre-pump cell, a measuring electrode, a reference electrode, a measuring voltage detection device, and a specific gas concentration detection device. The element body includes an oxygen ion-conductive solid electrolyte layer, and a measurement target gas flow section is provided within the element body so that a measurement target gas can be introduced into the measurement target gas flow section and flows through the measurement target gas flow section. The main pump cell pumps oxygen from a first internal space to adjust an oxygen concentration of the first internal space, the first internal space being provided in the measurement target gas flow section.The auxiliary pump cell pumps oxygen from a second internal space to adjust the oxygen concentration of the second internal space, the second internal space being provided downstream of the first internal space in the measurement object gas flow section. The pre-pumping cell pumps oxygen into a pre-chamber being provided upstream of the first internal space in the measurement object gas flow section. The measuring electrode is arranged on an inner peripheral surface of a measuring chamber, the measuring chamber being provided downstream of the second internal space in the measurement object gas flow section. The reference electrode is arranged within the element body. A reference gas is to be introduced into the reference electrode. The reference gas serves as a reference for detecting a specific gas concentration in the measurement object gas. The pre-pumping cell includes an internal pre-pumping electrode arranged in the pre-chamber.The main pump cell includes an inner main pump electrode arranged in the first internal space. The auxiliary pump cell includes an inner auxiliary pump electrode arranged in the second internal space. The inner pre-pumping electrode, the inner main pump electrode, the inner auxiliary pump electrode, and the measuring electrode each contain a noble metal with catalytic activity. At least one of the inner pre-pumping electrode and the inner main pump electrode does not contain a noble metal with the ability to suppress catalytic activity. The ability to suppress catalytic activity is the ability of the noble metal that has the catalytic activity exhibited for the specific gas to suppress the catalytic activity. The inner auxiliary pump electrode contains a noble metal with the ability to suppress catalytic activity.

[0021] Using the sensor element, the specific gas concentration in a measurement target gas can be detected, as in the above-described gas sensor of the present invention. Furthermore, in the sensor element, as in the above-described gas sensor of the present invention, at least one of the inner pre-pump electrode and the inner main pump electrode does not contain a noble metal capable of suppressing catalytic activity, and the inner auxiliary pump electrode contains a noble metal capable of suppressing catalytic activity. Therefore, when the sensor element is used to detect the specific gas concentration in a measurement target gas, specific gas concentration detection accuracy can be maintained over a long period of time. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic cross-sectional view of a gas sensor 100. Fig.2 is a block diagram illustrating an electrical connection relationship between a controller 90 and individual cells. Fig. 3 is a diagram illustrating a relationship between the oxygen concentration of a measurement object gas and a pump current Ip0. Fig. 4 is a diagram illustrating a relationship between the oxygen concentration of a measurement object gas and a pump current Ip2. Fig. Figure 5 is a diagram showing, in enlarged view, an area of Fig. 4, which corresponds to oxygen concentrations of 10 vol% or less. Fig. Figure 6 is a graph illustrating time changes of the pump current in a case where a setpoint Ip0s* is 0 mA. Fig. Figure 7 is a graph illustrating time changes of the pump current in a case where the setpoint Ip0s* is 1 mA. Fig. 8 is a graph illustrating a relationship between a NO concentration and the pump current Ip2 with respect to the gas sensors of Experimental Examples 1 to 8. Fig. 9 is a schematic cross-sectional view of a sensor element 201. BEST MODE FOR CARRYING OUT THE INVENTION

[0022] An embodiment of the present invention will now be described with reference to the drawings. Fig. 1 is a schematic cross-sectional view schematically illustrating an example of a configuration of a gas sensor 100 which is an embodiment of the present invention. Fig.2 is a block diagram illustrating an electrical connection relationship between a controller 90 and individual cells. The gas sensor 100 is attached to a pipe, such as an exhaust pipe of an internal combustion engine, examples of which include gasoline and diesel engines. The gas sensor 100 detects the specific gas concentration in a measurement object gas, which is the exhaust gas of an internal combustion engine. Examples of the specific gas are NOx and ammonia. In the present embodiment, the specific gas concentration measured by the gas sensor 100 is the concentration of NOx. The gas sensor 100 includes a sensor element 101, cells 15, 21, 41, 50, and 80 to 83, and a controller 90. The sensor element 101 has an elongated parallelepiped shape. Each of the cells 15, 21, 41, 50 and 80 to 83 includes a portion of the sensor element 101. The controller 90 controls the gas sensor 100 as a whole.

[0023] The sensor element 101 is a laminated body element in which six layers, namely 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, are laminated in this order from the bottom side, as shown in the drawing. Each of the six layers is composed of an oxygen ion-conductive solid electrolyte layer containing, for example, zirconium oxide (ZrO2). Furthermore, the solid electrolyte constituting each of the six layers is dense and gas-tight. The sensor element 101 is manufactured, for example, as follows. Ceramic green sheets corresponding to the respective layers are subjected to, for example, a predetermined process and circuit pattern printing. The resulting sheets are then laminated and subjected to firing to unify them.

[0024] On the front (left side of the Fig. 1) of the sensor element 101, a gas inlet port 10, a first diffusion rate limiting portion 11, a buffer space 12, a second diffusion rate limiting portion 13, a first inner space 20, a third diffusion rate limiting portion 30, a second inner space 40, a fourth diffusion rate limiting portion 60, and a third inner space 61 are formed adjacent to each other so as to communicate 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.

[0025] The gas inlet port 10, the buffer space 12, the first internal space 20, the second internal space 40, and the third internal space 61 form a space within the sensor element 101. The space is provided such that a portion of the spacer layer 5 is hollowed out. The top of the space is defined by the bottom surface of the second solid electrolyte layer 6, the bottom of the space is defined by the top surface of the first solid electrolyte layer 4, and the sides of the space are defined by side surfaces of the spacer layer 5.

[0026] The first diffusion rate limiting portion 11, the second diffusion rate limiting portion 13, and the third diffusion rate limiting portion 30 are each provided as two horizontally extending slits (whose openings have a longitudinal direction in a direction perpendicular to the drawing). Furthermore, the fourth diffusion rate limiting portion 60 is provided as a horizontally extending slit (whose opening has a longitudinal direction in a direction 20 perpendicular to the drawing) formed as a gap with respect to the bottom surface of the second solid electrolyte layer 6. Note that the area extending from the gas inlet port 10 to the third internal space 61 is also referred to as the "measured object gas flow portion."

[0027] Furthermore, a reference gas introduction space 43 is provided at a position farther from the front side as the measurement object gas flow section. This position is located between the upper surface of the third substrate layer 3 and the lower surface of the spacer layer 5, and one side of the reference gas introduction space 43 is defined by a side surface of the first solid electrolyte layer 4. A reference gas for measuring the NOx concentration is introduced into the reference gas introduction space 43. Examples of the reference gas include air.

[0028] An air introduction layer 48 is a layer formed from a porous ceramic material. A reference gas can be introduced into the air introduction layer 48 via the reference gas introduction space 43. Furthermore, the air introduction layer 48 is configured to cover the reference electrode 42.

[0029] The reference electrode 42 is an electrode formed so as to be sandwiched 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 is coupled to the reference gas introduction space 43, is provided in the vicinity of the reference electrode 42. Furthermore, as described later, the reference electrode 42 can be used to measure the oxygen concentrations (oxygen partial pressures) of 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 (e.g., a cermet electrode containing Pt and ZrO2).

[0030] In the measurement object gas flow section, the gas inlet port 10 is a portion open to an outside space. A measurement object gas can be sucked into the sensor element 101 from an outside space through the gas inlet port 10. The first diffusion rate limiting section 11 is a portion that imparts a predetermined diffusion resistance to the measurement object gas sucked through the gas inlet port 10. The buffer space 12 is a space provided for guiding the measurement object gas introduced from the first diffusion rate limiting section 11 to the second diffusion rate limiting section 13. The buffer space 12 also serves as a space (pre-chamber) for pumping oxygen into the measurement object gas introduced through the first diffusion rate limiting section 11. The pumping of oxygen into the buffer space 12 is achieved by the operation of a pre-pumping cell 15.The second diffusion rate limiting section 13 is a section that imparts a predetermined diffusion resistance to the measurement target gas introduced from the buffer space 12 into the first internal space 20. The measurement target gas is introduced into the first internal space 20 from outside the sensor element 101 as follows. When pressure fluctuations of a measurement target gas in an external space (exhaust pressure pulsations when the measurement target gas is automobile exhaust gas) occur, the measurement target gas is quickly drawn into the sensor element 101 through the gas inlet port 10. Subsequently, the measurement target gas is not directly introduced into the first internal space 20, but is introduced into the first internal space 20 after concentration fluctuations of the measurement target gas have been eliminated by the first diffusion rate limiting section 11, the buffer space 12, and the second diffusion rate limiting section 13.As a result, the concentration fluctuations of the target gas upon introduction into the first internal space 20 are essentially negligible. The first internal space 20 is provided as a space for adjusting the oxygen partial pressure in the target gas introduced through the second diffusion rate limiting section 13. The oxygen partial pressure is adjusted by the operation of a main pump cell 21.

[0031] The pre-pump cell 15 is an electrochemical pump cell including a pre-pump electrode 16, an outer pump electrode 23, and the second solid electrolyte layer 6 disposed between the electrodes. The pre-pump electrode 16 is provided on substantially the entire surface of a portion of the lower surface of the second solid electrolyte layer 6, the portion facing the buffer space 12. The outer pump electrode 23 is disposed on a portion to be exposed to a measurement target gas located outside the sensor element 101. Of a plurality of electrodes in the measurement target gas flow portion, the pre-pump electrode 16 is an electrode located most upstream.A pump voltage Vp0s can be applied by a variable power supply 17 provided between the pre-pump electrode 16 and the outer pump electrode 23, thereby passing a pump current Ip0s between the pre-pump electrode 16 and the outer pump electrode 23. This allows the pre-pump cell 15 to pump oxygen from an external space into the buffer space 12.

[0032] The main pump cell 21 is an electrochemical pump cell having an inner pump electrode 22, an outer pump electrode 23, and the second solid electrolyte layer 6 sandwiched between the electrodes. The inner pump electrode 22 includes a ceiling electrode portion 22a provided on substantially the entire surface of a portion of the lower surface of the second solid electrolyte layer 6, the portion facing the first internal space 20. The outer pump electrode 23 is provided on a region of the upper surface of the second solid electrolyte layer 6, the region corresponding to the ceiling electrode portion 22a. The outer pump electrode 23 is arranged to be exposed to an external space.

[0033] The inner pumping electrode 22 is formed to extend along portions of the upper and lower solid electrolyte layers (second solid electrolyte layer 6 and first solid electrolyte layer 4) defining the first inner space 20 and along portions of the spacer layer 5 serving as side walls.Specifically, the ceiling electrode portion 22a is formed on a portion of the lower surface of the second solid electrolyte layer 6, the portion serving as the ceiling surface of the first internal space 20; a lower electrode portion 22b is formed on a portion of the upper surface of the first solid electrolyte layer 4, the portion serving as the lower surface of the first internal space 20; side electrode portions (not shown) are formed on portions of the sidewall surfaces (inner surfaces) of the spacer layer 5, the portions constituting corresponding sidewall portions of the first internal space 20, the side electrode portions connecting the ceiling electrode portion 22a to the lower electrode portion 22b; and thus, in the region where the side electrode portions are arranged, the structure has a tunnel shape.

[0034] The inner pumping electrode 22 and the outer pumping electrode 23 are each formed as a porous cermet electrode (e.g. cermet electrode with Pt and ZrO2).

[0035] The main pump cell 21 can pump oxygen from the first interior space 20 to an exterior space and oxygen from an exterior space to the first interior space 20. This can be done by applying a desired pump voltage Vp0 between the inner pump electrode 22 and the outer pump electrode 23, thereby conducting a pump current Ip0 in a positive or negative direction between the inner pump electrode 22 and the outer pump electrode 23.

[0036] Furthermore, an electrochemical sensor cell, namely an oxygen partial pressure detection sensor cell 80 for controlling the main pump, is configured to detect the oxygen concentration (oxygen partial pressure) of the atmosphere in the first internal space 20. The electrochemical sensor cell includes 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.

[0037] The oxygen concentration (oxygen partial pressure) in the first internal space 20 can be determined by measuring an electromotive force V0 of the oxygen partial pressure detection sensor cell 80 to control the main pump. Furthermore, the pump voltage Vp0 of a variable power supply 24 is fed back so that the electromotive force V0 becomes a constant electromotive force, thereby controlling the pump current Ip0. This allows the oxygen concentration in the first internal space 20 to be maintained at a predetermined constant value.

[0038] The third diffusion rate limiting section 30 is a section that imparts a predetermined diffusion resistance to the measurement object gas having an oxygen concentration (oxygen partial pressure) controlled in the first internal space 20 by the operation of the main pump cell 21 and guides the measurement object gas into the second internal space 40.

[0039] The second internal space 40 is provided as a space for further adjusting the oxygen partial pressure of the measurement target gas using an auxiliary pump cell 50, which is introduced into the second internal space 40 via the third diffusion rate limiting section 30 after the oxygen concentration (oxygen partial pressure) in the first internal space 20 has been adjusted in advance. With this configuration, the oxygen concentration in the second internal space 40 is precisely maintained at a constant concentration, so that the NOx concentration can be accurately measured by the gas sensor 100.

[0040] The auxiliary pump cell 50 is an electrochemical auxiliary pump cell including an auxiliary pump electrode 51, an outer pump electrode 23, and the second solid electrolyte layer 6 (here, the outer pump electrode 23 is not limited to the above-described outer pump electrode 23, and it is sufficient that the electrode be a suitable electrode outside the sensor element 101). The auxiliary pump electrode 51 includes a ceiling electrode portion 51a provided on substantially the entire surface of a portion of the lower surface of the second solid electrolyte layer 6, the portion facing the second internal space 40.

[0041] The auxiliary pump electrode 51 is arranged in the second internal space 40, and the structure of the auxiliary pump electrode 51 has a tunnel shape similar to that of the inner pump electrode 22 arranged in the first internal space 20 as described above. That is, the ceiling electrode portion 51a is formed on a portion of the second solid electrolyte layer 6, which portion serves as the ceiling surface of the second internal space 40; a bottom electrode portion 51b is formed on a portion of the first solid electrolyte layer 4, which portion serves as the bottom surface of the second internal space 40; side electrode portions (not shown) are formed on portions of the respective wall surfaces of the spacer layer 5, which portions serve as side walls of the second internal space 40, the side electrode portions connecting the ceiling electrode portion 51a to the bottom electrode portion 51b; and thus has a tunnel-shaped structure.

[0042] The auxiliary pump cell 50 can pump oxygen present in the atmosphere of the second internal space 40 into an external space and can pump oxygen from an external space into the second internal space 40. This can be done by applying a desired voltage Vp1 between the auxiliary pump electrode 51 and the external pump electrode 23.

[0043] Furthermore, an electrochemical sensor cell, namely an oxygen partial pressure detection sensor cell 81 for controlling the auxiliary pump, is configured to control the oxygen partial pressure of the atmosphere in the second internal space 40. The electrochemical sensor cell includes 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.

[0044] Note that the auxiliary pump cell 50 performs pumping with a variable power supply 52, and the voltage of the power supply 52 is controlled based on an electromotive force V1 detected by the oxygen partial pressure detection sensor cell 81 for controlling the auxiliary pump. With this configuration, the oxygen partial pressure present in the atmosphere of the second internal space 40 can be controlled to a low partial pressure that has substantially no influence on the measurement of NOx.

[0045] In addition, a pump current Ip1 is additionally used to control the electromotive force of the oxygen partial pressure detection sensor cell 80 for controlling the main pump. Specifically, the pump current Ip1, which is a control signal, is input to the oxygen partial pressure detection sensor cell 80 for controlling the main pump, thereby controlling the electromotive force V0. Accordingly, the gradient of the oxygen partial pressure present in the measurement target gas, which is introduced into the second internal space 40 via the third diffusion rate limiting section 30, is controlled to a constant gradient. When the gas sensor is used as a NOx sensor, the oxygen concentration in the second internal space 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.

[0046] The fourth diffusion rate limiting section 60 is a section that imparts a predetermined diffusion resistance to the measurement target gas having an oxygen concentration (oxygen partial pressure) controlled in the second internal space 40 by the operation of the auxiliary pump cell 50, and guides the measurement target gas into the third internal space 61. The fourth diffusion rate limiting section 60 functions to limit the amount of NOx flowing into the third internal space 61.

[0047] The third interior space 61 is provided as a space for performing a method for measuring the nitrogen oxide (NOx) concentration in the measurement target gas. The measurement target gas is introduced into the third interior space 61 through the fourth diffusion rate limiting section 60 after the oxygen concentration (oxygen partial pressure) in the second interior space 40 has been preset. The NOx concentration is measured primarily by operating a measuring pump cell 41 in the third interior space 61.

[0048] The measuring pump cell 41 measures the NOx concentration in the measurement target gas in the third internal space 61. The measuring pump cell 41 is an electrochemical pump cell including a measuring electrode 44, 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 is provided on a portion of the upper surface of the first solid electrolyte layer 4, the portion facing the third internal space 61. The measuring electrode 44 also functions as a NOx reduction catalyst that reduces the NOx present in the atmosphere of the third internal space 61.

[0049] The measuring pump cell 41 can detect a pump current Ip2 by pumping out the oxygen generated by the decomposition of nitrogen oxide in the atmosphere around the measuring electrode 44 and determine the pump current Ip2 as the amount of oxygen generated.

[0050] Furthermore, an electrochemical sensor cell, namely an oxygen partial pressure detection sensor cell 82 for controlling the measuring pump, is configured to detect the oxygen partial pressure in the vicinity of the measuring electrode 44. The electrochemical sensor cell includes the first solid electrolyte layer 4, the third substrate layer 3, the measuring electrode 44, and the reference electrode 42. A variable power supply 46 is controlled based on an electromotive force V2 detected by the oxygen partial pressure detection sensor cell 82 for controlling the measuring pump.

[0051] After being guided into the second internal space 40, the measured object gas flows through the fourth diffusion rate limiting section 60 under a situation where the oxygen partial pressure is controlled and reaches the measuring electrode 44 located in the third internal space 61. Nitrogen oxide in the measured object gas in the vicinity of the measuring electrode 44 is reduced (2NO -> N2+O2) to generate oxygen. The generated oxygen is then pumped by the measuring pump cell 41. At this time, a voltage Vp2 of the variable power supply 46 is controlled so that the electromotive force V2 detected by the oxygen partial pressure detection sensor cell 82 for controlling the measuring pump becomes a constant electromotive force. The amount of oxygen generated in the vicinity of the measuring electrode 44 is proportional to the nitrogen oxide concentration in the measured object gas.Accordingly, the nitrogen oxide concentration present in the measurement object gas is calculated using the pump current Ip2 of the measuring pump cell 41.

[0052] Furthermore, an electrochemical sensor cell 83 is configured, which includes the second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, the third substrate layer 3, the external pumping electrode 23, and the reference electrode 42. The partial pressure of oxygen present in a measurement target gas outside the sensor can be detected based on an electromotive force Vref obtained by the sensor cell 83.

[0053] In the gas sensor 100 configured as described above, the measured object gas has an oxygen partial pressure maintained at a constant low value (a value that has substantially no influence on the measurement of NOx) due to the operation of the main pump cell 21 and the auxiliary pump cell 50, and in this state, the measured object gas is supplied to the measuring pump cell 41. Accordingly, the NOx concentration in the measured object gas can be determined based on the pump current Ip2 pumped out from the measuring pump cell 41 when oxygen is generated by reducing NOx, which is substantially proportional to the NOx concentration in the measured object gas.

[0054] To improve the oxygen ion conductivity of the solid electrolyte, the sensor element 101 additionally includes a heating unit 70, which serves to adjust the temperature for heating the sensor element 101 and maintain the temperature. The heating unit 70 includes a heater connection electrode 71, a heater 72, a through-hole 73, a heater insulation layer 74, and a pressure relief hole 75.

[0055] The heater connection electrode 71 is an electrode configured to contact the lower surface of the first substrate layer 1. The heater unit 70 can be externally powered by connecting the heater connection electrode 71 to an external power source.

[0056] The heater 72 is an electrical resistor formed by sandwiching the top and bottom of the second substrate layer 2 and the third substrate layer 3. The heater 72 is connected to the heater connection electrode 71 via the through-hole 73 and generates heat upon external current input via the heater connection electrode 71, thereby heating and maintaining the temperature of the solid electrolyte forming the sensor element 101.

[0057] In addition, the heater 72 is embedded over an entire area extending from the first interior space 20 to the third interior space 61 and can thus adjust the temperature of the sensor element 101 as a whole to a temperature at which the solid electrolyte becomes active.

[0058] The heater insulating layer 74 is an insulating layer disposed adjacent to the upper and lower surfaces of the heater 72 and formed of an insulating material such as aluminum oxide. The heater insulating layer 74 is configured to provide electrical insulation between the second substrate layer 2 and the heater 72 and electrical insulation between the third substrate layer 3 and the heater 72.

[0059] The pressure relief hole 75 is a portion extending through the third substrate layer 3 and the air introduction layer 48 and communicating with the reference gas introduction space 43. The pressure relief hole 75 is formed to mitigate an increase in internal pressure due to a temperature increase within the heater insulation layer 74.

[0060] The pre-pumping electrode 16, the inner pumping electrode 22, the auxiliary pumping electrode 51, and the measuring electrode 44 each contain a noble metal having catalytic activity. The noble metal having catalytic activity may be, for example, at least one of Pt, Rh, Ir, Ru, and Pd. The outer pumping electrode 23 and the reference electrode 42 also each contain a noble metal having catalytic activity. The auxiliary pumping electrode 51 also contains a noble metal having a catalytic activity suppressing ability, that is, a capability of suppressing the catalytic activity of the aforementioned noble metal exhibited for the specific gas. With this configuration, the capability of the auxiliary pumping electrode 51 to reduce the NOx component present in the measuring target gas is reduced. Examples of the noble metal having a catalytic activity suppressing ability include Au.On the other hand, at least one of the pre-pump electrode 16 and the inner pump electrode 22 does not contain a noble metal capable of suppressing catalytic activity. It is preferable that the measuring electrode 44 does not contain a noble metal capable of suppressing catalytic activity. It is also preferable that neither the outer pump electrode 23 nor the reference electrode 42 contains a noble metal capable of suppressing catalytic activity. It is preferable that each of the electrodes 16, 22, 23, 42, 44, and 51 is a cermet electrode containing a noble metal and an oxide with oxygen ion conductivity (e.g., ZrO2). It is preferable that each of the electrodes 16, 22, 23, 42, 44, and 51 is a porous member. In the present embodiment, each of the electrodes 16, 22, 23, 42, 44 and 51 is a porous cermet electrode containing Pt and ZrO2.In addition, the auxiliary pump electrode 51 is a porous cermet electrode containing Pt and ZrO2 and containing 1% Au.

[0061] The controller 90 is a microprocessor including a CPU 92, a memory 94, and the like. The controller 90 inputs the electromotive force V0, the electromotive force V1, the electromotive force V2, the electromotive force Vref, the pump current Ip0s, the pump current Ip0, the pump current Ip1, and the pump current Ip2. The electromotive force V0 is detected by the oxygen partial pressure detection sensor cell 80 to control the main pump. The electromotive force V1 is detected by the oxygen partial pressure detection sensor cell 81 to control the auxiliary pump. The electromotive force V2 is detected by the oxygen partial pressure detection sensor cell 82 to control the metering pump. The electromotive force Vref is detected by the sensor cell 83. The pump current Ip0s is detected by the pre-pump cell 15. The pump current Ip0 is detected by the main pump cell 21. The pump current Ip1 is detected by the auxiliary pump cell 50.The pump current Ip2 is detected by the measuring pump cell 41. In addition, the controller 90 outputs control signals to the variable power supply 17 of the backing pump cell 15, 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.

[0062] The controller 90 regulates the voltage Vp0s of the variable power supply 17 so that the pump current Ip0s of the pre-pump cell 15 reaches a setpoint Ip0s*. The controller 90 controls the voltage Vp0s to pump oxygen into the buffer space 12 and does not control the voltage Vp0s to pump oxygen out of the buffer space 12. Furthermore, in the present embodiment, the setpoint Ip0s* for the controller 90 is set to a constant value. The set value Ip0s* is set to a value such that even in a case where a measurement object gas outside the sensor element 101 is an oxygen-poor atmosphere (e.g., atmosphere with an oxygen concentration less than or equal to 0.1 vol%, less than 0.2 vol%, less than 1 vol%, or the like), the measurement object gas after oxygen is pumped inward by the pre-pumping cell 15 (ie, the measurement object gas to be introduced into the first internal space 20) is not an oxygen-poor atmosphere.It should be noted that in a case where the air-fuel ratio of the measurement object gas is lower than the stoichiometric air-fuel ratio, that is, the measurement object gas is a rich atmosphere, unburned fuel is included in the measurement object gas, and thus the oxygen concentration can be determined by the amount of oxygen required for sufficient combustion of the fuel. In this case, the oxygen concentration is expressed as a negative value. Accordingly, for example, the target value Ip0s* is set as follows. First, a minimum oxygen concentration of the exhaust gases of an internal combustion engine using the gas sensor 100 is investigated in advance. The minimum oxygen concentration is a minimum among the oxygen concentrations under various operating conditions (in some cases, the oxygen concentration may decrease to a negative value).Subsequently, the setpoint Ip0s* is set based on the amount of oxygen required to increase the minimum oxygen concentration of the measurement target gas to an oxygen concentration higher than the oxygen concentration of an oxygen-poor atmosphere (e.g., oxygen concentration greater than 0.1 vol%, greater than or equal to 0.2 vol%, greater than or equal to 1 vol%, or the like). Since the setpoint Ip0s* is set to a constant value, the controller 90 controls the pre-pumping cell 15 so that a constant oxygen flow rate is pumped into the buffer space 12. The value of the setpoint Ip0s* can be appropriately set based on an experiment as described above. For example, the setpoint Ip0s* can be 0.5 mA or more and 3 mA or less.

[0063] The controller 90 regulates the pump voltage Vp0 of the variable power supply 24 so that the electromotive force V0 reaches a setpoint (called "setpoint V0*") (i.e., so that the oxygen concentration in the first internal space 20 becomes a constant setpoint concentration). Accordingly, the pump current Ip0 changes depending on the concentration of oxygen present in the measurement object gas and the oxygen flow rate pumped inward by the pre-pumping cell 15.

[0064] Furthermore, the controller 90 controls the voltage Vp1 of the variable power supply 52 so that the electromotive force V1 reaches a constant value (called the "set value V1*") (that is, so that the oxygen concentration in the second internal space 40 becomes a predetermined low-oxygen concentration, which has substantially no influence on the measurement of NOx). In addition, the controller 90 sets (regulates) the set value V0* of the electromotive force V0 based on the pump current Ip1 so that the pump current Ip1 flowing due to the voltage Vp1 reaches a constant value (called the "set value Ip1*"). As a result, the gradient of the oxygen partial pressure in the measurement target gas to be introduced into the second internal space 40 through the third diffusion rate limiting section 30 is consistently a constant gradient.In addition, the partial pressure of the oxygen present in the atmosphere of the second interior space 40 is controlled to a low partial pressure which has essentially no influence on the measurement of NOx.

[0065] Further, the controller 90 controls the voltage Vp2 of the variable power supply 46 so that the electromotive force V2 reaches a constant value (referred to as "set value V2*") (i.e., so that the oxygen concentration in the third internal space 61 becomes a predetermined low concentration). Accordingly, oxygen is pumped from the third internal space 61 so that the amount of oxygen generated when NOx in the measurement target gas is reduced in the third internal space 61 becomes substantially zero. Subsequently, the controller 90 obtains the pump current Ip2 and calculates the concentration of NOx present in the measurement target gas based on the pump current Ip2. The pump current Ip2 is a detection value corresponding to the oxygen generated in the third internal space 61, where the oxygen is oxygen derived from a specific gas (in this case, NOx).

[0066] A relationship formula representing a relationship between the pump current Ip2 and the NOx concentration is stored in memory 94. The relationship formula can be, for example, a linear function formula. The relationship formula can be determined through prior experimentation.

[0067] An application example of the gas sensor 100 configured as described above will be described below. Assume that the CPU 92 of the controller 90 is in a state where the CPU 92 controls the above-described pump cells 15, 21, 41, and 50 and obtains the voltages V0, V1, V2, and Vref from the above-described sensor cells 80 to 83. In this state, when the measurement object gas is introduced through the gas inlet port 10, the measurement object gas first passes through the first diffusion rate limiting section 11 and is then introduced into the buffer space 12. In the buffer space 12, oxygen is pumped into the measurement object gas by the pre-pumping cell 15. Thereafter, the measurement object gas containing the pumped oxygen reaches the first internal space 20.Subsequently, in the first internal space 20 and the second internal space 40, the oxygen concentration of the measurement object gas is adjusted by the main pump cell 21 and the auxiliary pump cell 50, and the adjusted measurement object gas reaches the third internal space 61. Subsequently, the CPU 92 detects the concentration of NOx in the measurement object gas based on the obtained pump current Ip2 and the relationship formula stored in the memory 94.

[0068] Thus, oxygen is pumped into the buffer space 12 from the pre-pumping cell 15 to prevent the measurement target gas from being introduced into the first internal space 20 in a state where the measurement target gas is an oxygen-deficient atmosphere, as described above. The reasons for this will be described. The inventors investigated the pumping currents Ip0 and Ip2 obtained by varying the oxygen concentration value of the measurement target gas and the set value Ip0s*. The measurement target gas was a gas before it was introduced into the gas inlet port 10. A customized model gas was used as the measurement target gas. In the model gas, the base gas was nitrogen, the specific gas component was 500 ppm NO, and the fuel gas was 1000 ppm carbon monoxide gas and 1000 ppm ethylene gas. The model gas was adjusted so that the water concentration was 5 vol% and the oxygen concentration was 0.005 to 20 vol%.The temperature of the model gas was 250°C, and the model gas was passed through a 20 mm diameter tube at a flow rate of 50 l / min. Note that in the gas sensor used in this study (hereinafter referred to as the "Gas Sensor of Reference Example"), the pre-pump electrode 16 and the inner pump assembly 22 contained Au, as did the auxiliary pump electrode 51. Except for this feature, the gas sensor of Reference Example is the same as the gas sensor 100 of the present embodiment.

[0069] Fig. Figure 3 is a graph showing a relationship between the oxygen concentration of a measurement object gas and the pump current Ip0. Fig. 3 illustrates cases where the setpoint Ip0s* was 0 mA, the setpoint Ip0s* was 1 mA, and the setpoint Ip0s* was 2 mA. The left graph of Fig. Figure 3 is an enlarged view of the right-hand graphic portion of Fig.3, which is surrounded by a dashed line (in the left graph, the horizontal axis is on a log scale). Fig. Figure 4 is a graph showing a relationship between the oxygen concentration of a measurement object gas and the pump current Ip2. Fig. 4 shows the same cases as in Fig. 3. Fig. 5 is a graphic showing an enlarged view of an area of Fig. 4, which corresponds to oxygen concentrations of 10 vol% or less. In Fig. 5, the horizontal axis is on a log scale. The oxygen concentration on the horizontal axis is the oxygen concentration of the fitted model gas, ie, the oxygen concentration of a measurement object gas that is outside the sensor element 101. In addition, the A / F of the model gas is also shown in parentheses on the horizontal axis of Fig.5. The A / F is a value measured with a MEXA-730λ manufactured by HO-RIBA, Ltd.

[0070] As can be seen from the Fig. 4 and Fig. As can be seen in Figure 5, the values ​​of the pump currents Ip2 corresponding to the same oxygen concentration were the same in all cases where the set point Ip0s* was 0 mA, the set point Ip0s* was 1 mA, and the set point Ip0s* was 2 mA, in the case of an oxygen concentration of the model gas greater than or equal to 1 vol%. In contrast, a pump current Ip2 generated in a case where the set point Ip0s* was 0 mA, that is, the fore-pump cell 15 was not pumping oxygen in at all, had a value smaller than a pump current Ip2 generated in a case where the fore-pump cell 15 was pumping oxygen in. That is, the sensitivity of the pump current Ip2 to the NOx concentration was reduced.

[0071] Fig.Figure 3 confirms that even with the same oxygen concentrations of the model gases, the larger the setpoint Ip0s*, the larger the pump current Ip0. Regarding the increase in the pump current Ip0, which was determined compared to the pump current Ip0 obtained when the setpoint Ip0s* was 0 mA, the pump current Ip0 in the case where the setpoint Ip0s* was 2 mA was not twice the pump current Ip0 obtained when the setpoint Ip0s* was 1 mA. That is, the increase in the pump current Ip0 was not directly proportional to the setpoint Ip0s*. The reason for this is that even with a large setpoint Ip0s*, part of the oxygen pumped into the buffer chamber 12 escapes to the outside through the gas inlet opening 10 and thus not all of the pumped oxygen reaches the first interior chamber 20. In addition, Fig.3, that the pump current Ip0 only had a negative value when Ip0s* was 0 mA and the concentration of the model gas was less than or equal to 0.1 vol% (cases on the left side of Fig. 3, in which the oxygen concentration was 0.005 vol%, 0.01 vol% or 0.1 vol%). Thus, the Fig. 3 to 5, the sensitivity of the pump current Ip2 was low when the pump current Ip0 had a negative value. Negative values ​​of the pump current Ip0 mean that the main pump cell 21 pumps oxygen into the first internal space 20 (pumps oxygen so that the oxygen partial pressure in the first internal space 20 reaches the set value V0*), not pumps oxygen out of the first internal space 20. That is, negative values ​​of the pump current Ip0 mean that the oxygen concentration of the measurement target gas to be introduced into the first internal space 20 is lower than the oxygen concentration represented by the set value V0*.

[0072] The results described above show that a specific gas measurement accuracy decreases when the oxygen concentration of the measurement object gas to be introduced into the first internal space 20 is low. In contrast, when the pre-pump cell 15 is operated, the measurement object gas is introduced into the first internal space 20 after the pre-pump cell 15 has supplied oxygen as described above, and thus, as shown in Fig. 3, the value of Ip0 can be increased (ie, the oxygen concentration of the measurement object gas to be introduced into the first internal space 20 can be increased). Therefore, the measurement object gas is unlikely to reach the first internal space 20 in a state where the measurement object gas is an oxygen-poor atmosphere, and thus a decrease in measurement accuracy that may occur when the measurement object gas is an oxygen-poor atmosphere is suppressed. From the results in Fig.3 to 5, it is assumed that if the pre-pumping cell 15 pumps oxygen into the buffer space 12 such that a measurement object gas with an oxygen concentration less than or equal to 0.1 vol% does not reach the first internal space 20, i.e., the oxygen concentration of the measurement object gas reaching the first internal space 20 is greater than 0.1 vol%, a decrease in measurement accuracy can be suppressed. Furthermore, it is assumed that the pre-pumping cell 15 is to be operated such that the oxygen concentration of the measurement object gas 47 reaching the first internal space 20 is preferably greater than or equal to 0.2 vol%, and more preferably greater than or equal to 1 vol%.

[0073] If the pre-pumping cell 15 does not pump oxygen inside, the measurement accuracy decreases when the measurement object gas is an oxygen-poor atmosphere. The reason for this is unknown, but may be, for example, as follows. One possible reason for this is that when a measurement object gas that is an oxygen-poor atmosphere is introduced into the first inner space 20, the inner pumping electrode 22 acts as a catalyst to reduce NOx in the first inner space 20 before the measurement object gas reaches the third inner space 61. Another possible reason is as follows. In the case where the measurement object gas is a rich atmosphere, unburned components such as hydrocarbon (HC) and carbon monoxide exist in the measurement object gas. NOx may react with the components, and therefore NOx tends to be reduced in the first inner space 20.For example, in a gasoline engine, the air-fuel ratio of the measurement target gas often remains at or near the stoichiometric air-fuel ratio, and therefore the measurement target gas may be an oxygen-poor atmosphere throughout. Even in this case, the specific gas concentration can be accurately determined as long as oxygen is pumped inward through the pre-pumping cells 15. Furthermore, the controller 90 regulates the setpoint V0* so that the pump current Ip1 reaches a constant value. This may also be related to a decrease in measurement accuracy that may occur when the measurement target gas is an oxygen-poor atmosphere. For example, in a case where the oxygen concentration of the measurement target gas to be introduced into the first internal space 20 is temporarily reduced, there will be a time delay before the second internal space 40 is affected by the influence.Accordingly, there is a time delay until the set value V0* is changed to an appropriate value based on the pump current Ip1, and thus a phenomenon may occur in which oxygen in the first internal space 20 is temporarily pumped out excessively. Thus, it is possible that in the event that the oxygen concentration in the first internal space 20 decreases sharply due to the phenomenon, a reduction of NOx in the first internal space 20 may occur. In contrast, when the pre-pumping cell 15 is operated, it is assumed that a decrease in measurement accuracy is suppressed for the following reason: since oxygen is supplied from the pre-pumping cell 15, the oxygen concentration in the first internal space 20 does not decrease enough to reduce NOx in the first internal space 20 even if a temporary decrease in the oxygen concentration of the measurement target gas occurs as described above.

[0074] Furthermore, in the gas sensor 100 of the present embodiment, peak noise in the pumping current Ip1 and the pumping current Ip2, which occurs when the atmosphere of the measurement target gas suddenly changes from a rich atmosphere to a lean atmosphere or vice versa, can be suppressed. Using the above-described gas sensor of the reference example, the inventors investigated the behavior of the pumping currents Ip0, Ip1, and Ip2 generated when a measurement target gas to be introduced into the gas inlet port 10 was suddenly changed from a rich atmosphere to a lean atmosphere. The measurement target gases used were customized model gases. The following model gases were prepared: a rich atmosphere gas with an oxygen concentration of 0.05 vol% and a lean atmosphere gas with an oxygen concentration of 0.65 vol%.After 30 seconds of the rich-atmosphere gas flowing through a tube, the rich-atmosphere gas was replaced with the lean-atmosphere gas. Except for the oxygen concentration, the conditions for the model gases were the same as those for the model gas used in the measurement according to the . Fig. 3 to 5. It should be noted that the model gas contains combustible gases (1000 ppm carbon monoxide gas and 1000 ppm ethylene gas) as described above, and therefore a model gas with an oxygen concentration of 0.05 vol% is a rich atmosphere. Fig. Figure 6 is a graph illustrating the time variations of the pump currents Ip0, Ip1 and Ip2 in a case where the setpoint Ip0s* is 0 mA. Fig. Figure 7 is a graph illustrating the time variations of the pump currents Ip0, Ip1 and Ip2 in a case where the setpoint Ip0s* is 1 mA.

[0075] As can be seen from the Fig. 6 and Fig. 7, at the setpoint Ip0s* of 1 mA ( Fig. 7) in contrast to Fig. 6, the pump current Ip0 did not drop to a negative value and had a constant positive value even during the period in which the measured gas was a rich atmosphere (elapsed time 0 to 30 seconds). In addition, in Fig. 7 the peak noise in the pump currents Ip1 and Ip2, which occurred when changing from the rich atmosphere to the lean atmosphere, compared to Fig.6 is reduced. This is presumably because peak noise tends to occur in the pump currents Ip1 and Ip2 when the pump current Ip0 changes from positive to negative or vice versa. For example, in a gasoline engine, the air-fuel ratio of the measurement target gas often remains at or near the stoichiometric air-fuel ratio, and when the pre-pumping cell 15 is not pumping oxygen into the buffer space 12, the pump current Ip0 may frequently change between positive and negative, and thus peak noise may frequently occur. In the gas sensor 100 of the present embodiment, such changes in the pump current Ip0 between positive and negative can be suppressed.

[0076] It should be noted that in the above-described gas sensor of Reference Example, the pre-pumping electrode 16, the inner pumping electrode 22, and the auxiliary pumping electrode 51 each contain Au. The inventors fabricated gas sensors of Experimental Examples 1 to 8, in which the presence or absence of Au in each of the electrodes was varied, as shown in Table 1. For all Experimental Examples 1 to 8, the pre-pumping electrode 16, the inner pumping electrode 22, and the auxiliary pumping electrode 51 were each a porous cermet electrode containing one or more noble metals and ZrO2. In Table 1, the value "0.8" means that the electrode contains Pt and Au as noble metals, and that the mass percentage of Au relative to Pt in the electrode is 0.8 wt%. In Table 1, the symbol "-" means that the electrode contains Pt exclusively as the noble metal without Au. [Table 1] Mass percentage of Au relative to Pt in the electrode [wt.%] BACKING ELECTRODE INNER PUMP ELECTRODE AUXILIARY PUMP ELECTRODE EXPERIMENTAL EXAMPLE 1 0,8 0,8 0,8 EXPERIMENTAL EXAMPLE 2 - 0,8 0,8 EXPERIMENTAL EXAMPLE 3 0,8 - - EXPERIMENTAL EXAMPLE 4 - - - EXPERIMENTAL EXAMPLE 5 0,8 - 0,8 EXPERIMENTAL EXAMPLE 6 - - 0,8 EXPERIMENTAL EXAMPLE 7 0,8 0,8 - EXPERIMENTAL EXAMPLE 8 - 0,8 -

[0077] For each of the gas sensors in Experimental Examples 1 to 8, the relationship between the specific gas concentration in the target gas and the pump current Ip2 was investigated with a setpoint Ip0s* of 1 mA. The target gases used were adapted to three types of model gases. In the model gases, the specific gas component was NO, and the NO concentration was as follows: 0 ppm, 250 ppm, and 500 ppm. In each of the three types of model gases, the base gas was nitrogen, the water concentration was set to 3 vol%, and the oxygen concentration was set to 1 vol%. The temperature of the model gases was 250°C, and the model gases were passed through a 20 mm diameter pipe at a flow rate of 50 l / min. Table 2 and Fig. 8 show a relationship between the NO concentration and the pumping current Ip2 with respect to the gas sensors of Experimental Examples 1 to 8.

[0080] [Table 2] NO CONCENTRATION [ppm] PUMP CURRENT Ip2[µA] EXPERIMENTAL EXAMPLE 1 EXPERIMENTAL EXAMPLE 2 EXPERIMENTAL EXAMPLE 3 EXPERIMENTAL EXAMPLE 4 EXPERIMENTAL EXAMPLE 5 EXPERIMENTAL EXAMPLE 6 EXPERIMENTAL EXAMPLE 7 EXPERIMENTAL EXAMPLE 8 0 0,09 0,09 0,09 0,08 0,08 0,08 0,08 0,09 250 0,76 0,76 0,09 0,08 0,76 0,75 0,08 0,09 500 1,43 1,43 0,09 0,08 1,43 1,43 0,08 0,09

[0078] The data shown in Table 2 and Fig.The results shown in Figure 8 indicate that in Experimental Examples 3, 4, 7, and 8, each of which contained no Au in the auxiliary pump electrode 51, the pump current Ip2 was substantially unchanged even with changes in the NO concentration, and the pump current Ip2 was substantially 0 µA. This is presumably because NO was reduced by the catalytic activity of the auxiliary pump electrode 51 before reaching the measuring electrode 44. In contrast, in Experimental Examples 1, 2, 5, and 6, each of which contained Au in the auxiliary pump electrode 51, there was a linear relationship between the NO concentration and the pump current Ip2. Furthermore, in Experimental Examples 1, 2, 5, and 6, the values ​​of the pump currents Ip2 corresponding to each of the NO concentrations were substantially the same. That is, whether the pre-pump electrode contained 16 Au or not and whether the inner pump electrode contained 22 Au or not had no effect on the pump current Ip2.This result indicates that it is sufficient for the auxiliary pump electrode 51 to contain Au. Au is not required in the pre-pumping electrode 16 and the inner pumping electrode 22. Even without the presence of Au, NO reduction by the pre-pumping electrode 16 or the inner pumping electrode 22 does not occur. From this result, the inventors found that when the pre-pumping cell 15 is configured to pump oxygen inward, at least one of the pre-pumping electrode 16 and the inner pumping electrode 22 need not contain Au. Based on this result, the gas sensor 100 of the present embodiment is configured as follows: at least one of the pre-pumping electrode 16 and the inner pumping electrode 22 does not contain Au, while the auxiliary pump electrode 51 contains Au. That is, each of Experimental Examples 2, 5, and 6 corresponds to the gas sensor 100 of the present embodiment and thus to the examples of the gas sensor of the present invention.Each of Experimental Examples 1, 3, 4, 7 and 8 corresponds to a Comparative Example of the present invention.

[0079] The reason for the above result is presumed as follows. First, under the control of the CPU 92 as described above, during use of the gas sensor 100, the pre-pump cell 15 supplies oxygen to the measurement object gas, which is a measurement object gas before the oxygen concentration is adjusted by the main pump cell 21 and the auxiliary pump cell 50. Thus, the pre-pump cell 15 supplies oxygen inward, while the main pump cell 21 and the auxiliary pump cell 50 pump oxygen outward. Accordingly, the magnitude relationship between the oxygen concentrations in the vicinity of the electrodes in the measurement object gas flow section is presumed to be as follows: (vicinity of the pre-pump electrode 16) > (vicinity of the inner pump electrode 22) > (vicinity of the auxiliary pump electrode 51) > (vicinity of the measuring electrode 44).That is, even in a case where the measured object gas is an oxygen-poor atmosphere before the measured object gas is introduced into the measured object gas flow section, the oxygen concentrations in the surroundings of the pre-pumping electrode 16 and the inner pumping electrode 22 are maintained at high oxygen concentrations compared to a surrounding of the auxiliary pumping electrode 51. The higher the oxygen concentration, the less likely NOx reduction is to occur. Therefore, even if at least one of the pre-pumping electrode 16 and the inner pumping electrode 22 does not contain a noble metal (in this case, Au) capable of suppressing catalytic activity, NOx reduction by the electrode is unlikely to occur. On the other hand, the measured object gas reaches a surrounding of the auxiliary pumping electrode 51 after oxygen is pumped out from the main pumping cell 21.As a result, NOx reduction can easily occur in the vicinity of the pumping electrode 51 by the auxiliary pumping electrode 51. However, the auxiliary pumping electrode 51 contains Au and can therefore suppress the reduction of NOx. As can be seen from the above description, in the gas sensor 100 of the present embodiment, the reduction of NOx before NOx reaches the measuring electrode 44 is sufficiently suppressed, so that the specific gas concentration detection accuracy is sufficient.

[0080] Furthermore, in a case where both the pre-pumping electrode 16 and the inner pumping electrode 22 contain Au, Au may adhere to the measuring electrode 44 through evaporation from the electrodes in conjunction with the use of the gas sensor 100. When Au adheres to the measuring electrode 44, the catalytic activity of the measuring electrode 44 is suppressed, and thus NOx in a vicinity of the measuring electrode 44 is not sufficiently reduced. As a result, the actual pumping current Ip2 is reduced from the correct pumping current Ip2 corresponding to the NOx concentration. As a result, the specific gas concentration detection accuracy decreases.In contrast, in the gas sensor 100 of the present embodiment, at least one of the pre-pumping electrode 16 and the inner pumping electrode 22 does not contain a noble metal having the ability to suppress catalytic activity, so that the noble metal evaporation associated with the use of the gas sensor 100 is suppressed and a decrease in detection accuracy associated with the use is suppressed.

[0081] As can be seen from the above description, the gas sensor 100 of the present embodiment has a specific gas concentration detection accuracy that is maintained over a long period of time. In contrast, for example, in a case where the auxiliary pump electrode 51 also does not contain Au, as in Experimental Examples 3, 4, 7, and 8, a specific gas concentration detection accuracy is already low at the beginning of use of the gas sensor. Furthermore, for example, in a case where both the pre-pump electrode 16 and the inner pump electrode 22 contain Au, as in Experimental Example 1, a specific gas concentration detection accuracy tends to decrease in relation to the use of the gas sensor. That is, the service life of the gas sensor decreases.

[0082] It should be noted that although the auxiliary pump electrode 51 contains Au, Au in the auxiliary pump 51 has a relatively low evaporation tendency. This will be described. The tendency of Au to evaporate from an electrode, as described above, increases with increasing oxygen concentration. For example, in an electrode containing Pt and Au, Pt is more likely to be oxidized to PtO2 with increasing oxygen concentration. PtO2 has a higher saturated vapor pressure than Pt and therefore evaporates more easily than Pt. Furthermore, when Pt becomes PtO2 and evaporates, the remaining Au also tends to evaporate. This is because elemental Au has a higher saturated vapor pressure than a Pt-Au alloy. In contrast, Au in the auxiliary pump electrode 51 has a relatively low evaporation tendency because the oxygen concentration around the auxiliary pump electrode 51 is low as described above.Therefore, even if the auxiliary pump electrode 51 contains Au, it is unlikely that a reduction in detection accuracy will occur in conjunction with the use of the gas sensor 100 described above.

[0083] It is preferable that neither the pre-pumping electrode 16 nor the inner pumping electrode 22 contain a noble metal capable of suppressing catalytic activity. With this configuration, the evaporation of a noble metal capable of suppressing catalytic activity is further suppressed. This enhances the effect of maintaining specific gas concentration detection accuracy over a longer period of time.

[0084] Here, the corresponding relationships between the constituent elements of the present embodiment and the constituent elements of the present invention will be clarified. The laminated body of the present embodiment, in which six layers, 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, are laminated in this order, corresponds to the element body of the present invention. The buffer space 12 corresponds to the pre-chamber. The pre-pump cell 15 corresponds to the pre-pump cell. The first internal space 20 corresponds to the first internal space. The main pump cell 21 corresponds to the main pump cell. The second internal space 40 corresponds to the second internal space. The auxiliary pump cell 50 corresponds to the auxiliary pump cell. The third internal space 61 corresponds to the measuring chamber. The measuring electrode 44 corresponds to the measuring electrode.The reference electrode 42 corresponds to the reference electrode. The oxygen partial pressure detection sensor cell 82 for controlling the measuring pump corresponds to the measurement voltage detection device. The pump current Ip2 corresponds to the detection value. The CPU 92 of the controller 90 corresponds to the specific gas concentration detection device. The pre-pump electrode 16 corresponds to the inner pre-pump electrode. The inner pump electrode 22 corresponds to the inner main pump electrode. The auxiliary pump electrode 51 corresponds to the inner auxiliary pump electrode. In addition, the pump current Ip0s corresponds to the pre-pump current. The CPU 92 corresponds to the pre-pump control device. The memory 94 corresponds to the storage device. The pump current Ip0 corresponds to the pump current of the main pump cell. The CPU 92 corresponds to the oxygen concentration detection device. The outer pump electrode 23 corresponds to the gas-side measurement object electrode.

[0085] In the gas sensor 100 of the above-described embodiment, the pre-pumping cell 15 supplies oxygen to the measurement target gas before the oxygen concentration is adjusted by the main pumping cell 21, and thus, even if the measurement target gas is in an oxygen-poor atmosphere before being introduced into the measurement target gas flow section, the surroundings of the pre-pumping electrode 16 and the surroundings of the inner pumping electrode 22 do not become an oxygen-poor atmosphere. Thus, even if at least one of the pre-pumping electrode 16 and the inner pumping electrode 22 does not contain a noble metal (e.g., Au) capable of suppressing catalytic activity, the electrode is unlikely to reduce a specific gas.Since at least one of the pre-pumping electrode 16 and the inner pumping electrode 22 does not contain a noble metal capable of suppressing catalytic activity, a case where noble metal evaporates and adheres to the measuring electrode 44 in conjunction with the use of the gas sensor 100 is suppressed. Accordingly, the gas sensor 100 has a specific gas concentration detection accuracy that is maintained over a long period of time.

[0086] In addition, the CPU 92 controls the pre-pump cell 15 so that a constant pre-pump current (setpoint Ip0s*) flows and thus oxygen can be supplied to a measuring object gas, which is an oxygen-poor atmosphere, in the buffer chamber 12 with a relatively simple control.

[0087] In addition, the CPU 92 detects the specific gas concentration using a common relationship formula stored in the memory 94, regardless of whether a measurement object gas outside the element body is an oxygen-poor atmosphere. As described with reference to Fig. 4 and Fig. As described in Figure 5, the sensitivity of the pump current Ip2 in the gas sensor 100 of the present embodiment tends to remain constant even when the target gas is an oxygen-poor atmosphere. Thus, the gas sensor 100 can accurately detect the specific gas concentration without requiring different relationship formulas for the case where the target gas is an oxygen-poor atmosphere and the case where the target gas is not an oxygen-poor atmosphere. Thus, the gas sensor 100 can detect the specific gas concentration simply and accurately.

[0088] In addition, the pre-pump cell 15 pumps oxygen from the vicinity of the outer pumping electrode 23 into the buffer space 12. With this configuration, the following is possible: Compared, for example, with a case where oxygen is pumped from the vicinity of the reference electrode 42 into the buffer space 12, a decrease in measurement accuracy that may occur when the potential of the reference electrode 42 changes due to a voltage drop caused by the current occurring during pumping is suppressed.

[0089] It should be noted that the present invention is in no way limited to the embodiment described above and can be implemented in a variety of embodiments that fall within the technical scope of the present invention.

[0090] For example, in the above-described embodiment, the CPU 92 detects the specific gas concentration based on the pump current Ip2 and a relationship formula representing a relationship between the pump current Ip2 and the NOx concentration stored in the memory 94, but this configuration is not limiting. For example, the CPU 92 may detect the specific gas concentration corrected based on the oxygen concentration of a measurement target gas located outside the sensor element 101. For example, with reference to Fig.5 according to the data in which the set value Ip0s* is 1 mA or 2 mA, in the case that the oxygen concentration of the measured object gas is constantly less than or equal to 5%, the value of the pump current Ip2 does not change significantly even with a change in the oxygen concentration, provided that the actual concentration (real concentration) of a specific gas is uniform (see Fig. 5). On the other hand, if the oxygen concentration of the target gas can change over a wider range, the pump current Ip2 can change with the oxygen concentration, as shown in Fig. 4. In a case where the pump current Ip2 changes relatively significantly with the oxygen concentration just described, the CPU 92 can detect the specific gas concentration with a correction based on the oxygen concentration. This improves the specific gas concentration measurement accuracy. For example, with reference to Fig.4, according to the data in which the set value Ip0s* is 1 mA or 2 mA, the pump current Ip2 linearly correlates with the oxygen concentration when the specific gas concentration is uniform, and thus the relationship between the oxygen concentration and the pump current Ip2 can be approximated with a linear function. Accordingly, the CPU 92 can derive a corrected pump current by excluding an influence of the oxygen concentration from the pump current Ip2 obtained from the measuring pump cell 41 using the linear function formula (relationship formula for correction), and based on the corrected pump current and the relationship formula stored in the memory 94 in the above-described embodiment, the CPU 92 can detect the specific gas concentration. In this case, the memory 94 may also store the relationship formula for correction.Alternatively, instead of the relationship formula stored in the memory 94 in the above-described embodiment, a relationship formula in which the relationship formula for correction is taken into account, that is, a relationship formula representing a relationship between the pump current Ip2, the specific gas concentration, and the oxygen concentration of a measurement object gas outside the sensor element 101, may be stored, and the CPU 92 may acquire a corrected specific gas concentration using this relationship formula.As with the relational formula stored in the memory 94 in the above-described embodiment, with respect to the relational formula for correction and the relational formula in which the relational formula for correction is also taken into account, the specific gas concentration can be accurately determined by the formula, which is a common formula, regardless of whether a measurement object gas outside the sensor element 101 is an oxygen-poor atmosphere.

[0091] If the CPU 92 performs correction as described above, the CPU 92 can detect the oxygen concentration of the measurement target gas located outside the sensor element 101. Note that the constant pump current Ip0s (i.e., the set value Ip0s*) corresponds to the oxygen flow rate pumped from the pre-pump cell 15 into the buffer space 12. Furthermore, the pump current Ip0 corresponds to the flow rate of oxygen pumped from the first internal space 20. Thus, based on the pump current Ip0s, the pump current Ip0, and the set oxygen concentration in the first internal space 20, the CPU 92 can detect the oxygen concentration of a measurement target gas, which is a gas before the pre-pump cell 15 pumps oxygen in and the main pump cell 21 pumps oxygen out. That is, the CPU 92 can detect the oxygen concentration of the measurement target gas located outside the sensor element 101.Accordingly, the oxygen concentration required for correction can be detected by the gas sensor 100. Furthermore, the CPU 92 can also detect the oxygen concentration of the measurement target gas outside the sensor element 101, based on, for example, a voltage Vref between the reference electrode 42 and the outer pumping electrode 23. Alternatively, the CPU 92 can obtain the oxygen concentration of the measurement target gas located outside the sensor element 101 from a device other than the gas sensor 100, such as another sensor or the engine ECU, and use the oxygen concentration for correction.

[0092] In the embodiment described above, the pre-pump cell 15 pumps oxygen from a vicinity of the outer pumping electrode 23 into the buffer space 12, but this configuration is not limiting. For example, oxygen can be pumped from a vicinity of the reference electrode 42 into the buffer space 12. With this configuration, the following is possible. Compared, for example, with a case where oxygen is pumped inward from an external measurement object gas, oxygen can be pumped into the buffer space 12 with a low applied voltage because the reference gas (in this case, air) has a higher oxygen concentration than the measurement object gas.In contrast, when introducing oxygen from an environment of the outer pumping electrode 23 into the buffer space 12, the voltage Vp0s of the variable power supply 17 must be relatively high, since, particularly when an environment of the outer pumping electrode 23 is an oxygen-poor atmosphere, the generation of oxygen ions by reducing carbon monoxide, water or the like in the measurement object gas is necessary.

[0093] In the above-described embodiment, the second diffusion rate limiting portion 13 is provided between the buffer space 12 and the first internal space 20, but this configuration is not limiting. For example, the second diffusion rate limiting portion 13 may be omitted, and the buffer space 12 and the first internal space 20 may form a single space.

[0094] In the above-described embodiment, the specific gas concentration detected by the gas sensor 100 is the concentration of NOx, but this configuration is not limiting, and the specific gas concentration may be the concentration of another oxide. In the case where the specific gas is an oxide, oxygen is generated when the specific gas itself is reduced in the third internal space 61 as in the above-described embodiment, and accordingly, the CPU 92 can detect the specific gas concentration by obtaining a detection value corresponding to oxygen. Moreover, the specific gas may be a non-oxide gas such as ammonia. When the specific gas is a non-oxide gas, the specific gas may be converted into an oxide (for example, ammonia into NO).When the converted gas is reduced in the third internal space 61, oxygen is generated, and accordingly, the CPU 92 can detect the specific gas concentration by obtaining a detection value corresponding to the oxygen. For example, if the pre-pumping electrode 16 contains a metal with a catalytic function to promote the oxidation of ammonia, the specific gas can be converted into an oxide in the buffer space 12 via the catalytic function of the pre-pumping electrode 16. A similar configuration is possible for the internal pumping electrode 22. Ammonia is converted into an oxide, which is NO, and therefore, the measurement of the ammonia concentration is performed according to substantially the same principle as that of the NOx concentration measurement.

[0095] In the embodiment described above, the CPU 92 controls the pre-pumping cell 15 so that a constant pre-pumping current (setpoint Ip0s*) flows, but this configuration is not limited. For example, the CPU 92 may feed back the voltage Vp0s so that the oxygen concentration in the buffer space 12, which is detected based on the voltage between the pre-pumping electrode 16 and the reference electrode 42, reaches a setpoint. Alternatively, the CPU 92 may control the voltage Vp0s so that the lower the oxygen concentration of an outer side of the sensor element 101, the larger the amount of oxygen to be pumped into the buffer space 12. In this case, the CPU 92 may detect the oxygen concentration of an outer side of the sensor element 101 using the method described above or obtain the oxygen concentration from a device other than the gas sensor 100.In addition, the CPU 92 can control the voltage Vp0s as a constant voltage.

[0096] In the embodiment described above, the target value Ip0s* is set based on the amount of oxygen required to increase the minimum oxygen concentration of the measurement target gas, which is the minimum among the oxygen concentrations under various operating conditions of an internal combustion engine, to an oxygen concentration higher than the oxygen concentration of an oxygen-deficient atmosphere (e.g., oxygen concentration greater than 0.1 vol%, greater than or equal to 0.2 vol%, greater than or equal to 1 vol%, or the like). However, this configuration is not limiting.For example, the set value Ip0s* can be set to a value such that the pump current Ip0 does not drop to a negative value even when a measurement target gas having a minimum oxygen concentration, which is the minimum among oxygen concentrations under various operating conditions of an internal combustion engine, is introduced into the measurement target gas flow section of the sensor element 101. That is, the pre-pumping cell 15 can "prevent the measurement target gas from reaching the first internal space 20 in a state where the measurement target gas is an oxygen-deficient atmosphere" by ensuring that "the pump current Ip0 does not drop to a negative value."In any case, the amount of oxygen to be pumped from the pre-pumping cell 15 into the buffer space 12 can be adjusted by experimentation so that, according to a fluctuation range of a component of the measurement object gas, a decrease in the measurement accuracy within the fluctuation range is suppressed (e.g., so that, as shown in the . Fig. 4 and Fig. 5, a condition in which the sensitivity of the pump current Ip2 to the concentration of NOx decreases does not easily occur.

[0097] In the above-described embodiment, the sensor element 101 of the gas sensor 100 includes the first internal space 20, the second internal space 40, and the third internal space 61, but this configuration is not limiting. For example, the third internal space 61 may not be included as in a sensor element 201 as shown in Fig. 9. In the sensor element 201, which is a modified example from Fig.9, the gas inlet port 10, the first diffusion rate limiting portion 11, the buffer space 12, the second diffusion rate limiting portion 13, the first inner space 20, the third diffusion rate limiting portion 30, and the second inner space 40 are formed adjacent to each other so as to communicate, 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. Furthermore, the measuring electrode 44 is arranged on the upper surface of the first solid electrolyte layer 4 in the second inner space 40. The measuring electrode 44 is covered with a fourth diffusion rate limiting portion 45. The fourth diffusion rate limiting portion 45 is a film formed of a porous ceramic member containing, for example, alumina (Al2O3).Like the fourth diffusion rate limiting portion 60 of the above-described embodiment, the fourth diffusion rate limiting portion 45 serves to limit the amount of NOx flowing into the measuring electrode 44. Moreover, the fourth diffusion rate limiting portion 45 functions as a protective film for the measuring electrode 44. A ceiling electrode portion 51a of the auxiliary pump electrode 51 is formed to extend to a position immediately above the measuring electrode 44. The sensor element 201 configured as described above can also detect the NOx concentration based on, for example, the pump current Ip2, as in the above-described embodiment. In this case, a vicinity of the measuring electrode 44 functions as a measuring chamber.

[0098] In the above-described embodiment, the outer pumping electrode 23 serves as the following electrodes: the gas-side measurement target electrode (outer pre-pumping electrode) of the pre-pumping cell 15, an outer main pumping electrode of the main pumping cell 21, an outer auxiliary pumping electrode of the auxiliary pumping cell 50, and an outer measuring electrode of the measuring pumping cell 41. However, this configuration is not limiting. One or more of the outer pre-pumping electrode, the outer main pumping electrode, the outer auxiliary pumping electrode, and the outer measuring electrode may be an additional electrode other than the outer pumping electrode 23. The additional electrode may be provided outside the element body and contact with a measurement target gas.

[0099] In the above-described embodiment, the element body of the sensor element 101 is a laminated body having a plurality of solid electrolyte layers (layers 1 to 6), but this configuration is not limiting. It is sufficient that the element body of the sensor element 101 has at least one oxygen ion-conductive solid electrolyte layer and a measurement object gas flow section is provided inside. Referring to Fig. 1, for example, each of the layers 1 to 5 that are not the second solid electrolyte layer 6 may be a layer formed from a material other than a solid electrolyte (e.g., a layer of aluminum oxide). In this case, the electrodes to be accommodated in the sensor element 101 may be arranged on the second solid electrolyte layer 6. For example, the measuring electrode 44 may be made of Fig.1 may be arranged on the lower surface 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 air 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 lower surface of the second solid electrolyte layer 6.

[0100] In the embodiment described above, the controller 90 sets (controls) the target value V0* of the electromotive force V0 based on the pump current Ip1 in such a way that the pump current Ip1 reaches a target value Ip1*, and the controller 90 controls the pump voltage Vp0 so that the electromotive force V0 reaches the target value V0*. However, the controller 90 may perform other control. For example, the controller 90 may control the pump voltage Vp0 based on the pump current Ip1 so that the pump current Ip1 reaches a target value Ip1*. That is, the controller 90 may not obtain the electromotive force V0 from the oxygen partial pressure detection sensor cell 80 to control the main pump or to set the target value V0*; the controller 90 may directly control the pump voltage Vp0 (that is, control the pump current Ip0) based on the pump current Ip1.

[0101] The above-described "minimum oxygen concentration, which is the minimum among the oxygen concentrations under various operating conditions of an internal combustion engine," may be, for example, -11 vol% (a value of 11 related to the air-fuel ratio in gasoline engines). For example, when setting the target value Ip0s* as described in the above embodiment, in a case where a measurement target gas with an oxygen concentration of -11 vol% flows into the buffer space 12, an amount of oxygen is required to increase the oxygen concentration of the measurement target gas to an oxygen concentration higher than that of an oxygen-deficient atmosphere (the higher oxygen concentration may be greater than 0.1 vol%, preferably greater than or equal to 0.2 vol%, and more preferably greater than or equal to 1 vol%), the target value Ip0s* may be set based on the amount of oxygen.

[0102] It is preferable that the pre-pumping cell 15 pumps oxygen into the buffer space 12 such that, even when a measurement target gas having an arbitrary oxygen concentration within a range of -11 vol% or more and 0.1 vol% or less flows into the buffer space 12, the measurement target gas reaching the first internal space 20 has an oxygen concentration (=oxygen concentration at the exit of the second diffusion rate limiting section 13) of more than 0.1 vol%. In Experimental Examples 1 to 8 described above, the oxygen concentration of the measurement target gas reaching the second internal space 40 (=oxygen concentration at the exit of the third diffusion rate limiting section 30) was examined, and the oxygen concentration was found to be 0.1 vol%.Accordingly, it is assumed that since the oxygen concentration in a vicinity of the auxiliary pump electrode 51 was less than or equal to 0.1 vol%, the auxiliary pump electrode 51 must have contained Au. On the other hand, as can be seen from the above-described magnitude relationship between the oxygen concentrations in the vicinity of the electrodes in the measurement object gas flow section, the oxygen concentrations in the vicinity of the pre-pump electrode 16 and the inner pump electrode 22 were greater than 0.1 vol%, and accordingly, it is assumed that reduction of NO did not occur at the pre-pump electrode 16 or the inner pump electrode 22, although neither electrode contained Au.Therefore, if the pre-pumping cell 15 pumps oxygen into the buffer space 12 such that, even when a measurement target gas having an oxygen concentration in the range of -11 vol% or more and 0.1 vol% or less flows into the buffer space 12, the measurement target gas reaching the first internal space 20 has an oxygen concentration greater than 0.1 vol%, it is possible to more reliably reduce a specific gas by the electrode or reduce an oxide derived from the specific gas by the electrode even if neither the pre-pumping electrode 16 nor the internal pumping electrode 22 contains a noble metal capable of suppressing catalytic activity. That is, it is possible to more reliably eliminate the need for incorporating a noble metal capable of suppressing catalytic activity into the pre-pumping electrode 16 and the internal pumping electrode 22.Furthermore, it is preferable that the CPU 92 controls the pre-pumping cell 15 to pump oxygen inward as just described. For example, when setting the set value Ip0s* in the manner described in the above embodiment, the set value Ip0s* can be set to such a value that even in a case where a measurement target gas with an oxygen concentration of -11 vol% flows into the buffer space 12, the oxygen concentration of the measurement target gas reaching the first internal space 20 can be increased to a value greater than 0.1 vol%.Similarly, in the case where "the CPU 92 controls the voltage Vp0s as a constant voltage" as described in the above modified example, the set value (constant value) of the voltage Vp0s can be set so that, with a pump current Ip0s flowing in a state where the voltage Vp0s is controlled to a constant value, the oxygen concentration of a measurement object gas flowing into the buffer space 12 can be increased from -11 vol% to an oxygen concentration greater than 0.1 vol%. In the case where "the CPU 92 controls the voltage Vp0s so that the oxygen concentration in the buffer space 12 reaches a set value" as described in the above modified example, the set value of the oxygen concentration in the buffer space 12 (set value of the voltage between the pre-pump electrode 16 and the reference electrode 42) can be set to a value slightly greater than 0.1 vol% (or a value greater than 0.1 vol% with a predetermined margin).In the case where “the CPU 92 controls the voltage Vp0s such that the lower the oxygen concentration of an outside of the sensor element 101, the larger the amount of oxygen to be pumped into the buffer space 12” as described in the above-mentioned modified example, the following is possible: the corresponding relationship between the oxygen concentration of an outside of the sensor element 101 and the set value of the voltage Vp0s can be set in advance so that even in a case where a measurement object gas having an oxygen concentration in the range of -11 vol% or more and 0.1 vol% or less flows into the buffer space 12, the oxygen concentration of the measurement object gas reaching the first internal space 20 can be increased to a value greater than 0.1 vol%, and accordingly, the CPU 92 can control the voltage Vp0s based on the corresponding relationship.Furthermore, it is preferable that the pre-pumping cell 15 pumps oxygen into the buffer space 12 such that the measured object gas reaching the first internal space 20 has an oxygen concentration greater than or equal to 1 vol%. More specifically, it is preferable that oxygen be pumped into the buffer space 12 such that, even in a case where a measured object gas having an oxygen concentration in the range of -11 vol% or more and less than 1 vol% flows into the buffer space 12, the measured object gas reaching the first internal space 20 has an oxygen concentration greater than or equal to 1 vol%. With this configuration, it is possible to more reliably eliminate the need for incorporating a noble metal capable of suppressing catalytic activity into the pre-pumping electrode 16 and the internal pumping electrode 22.Furthermore, it is preferred that the CPU 92 controls the pre-pumping cell 15 so that oxygen is pumped inward as just described.

[0103] In the above-described embodiment, the main pump cell 21 can pump oxygen from the first internal space 20 so that the oxygen concentration of the measurement target gas reaching the second internal space 40 becomes not less than 0.1 vol%. With this configuration, the oxygen concentration in a vicinity of the internal pump electrode 22 is prevented from decreasing. Therefore, in the case where the internal pump electrode 22 does not contain a noble metal capable of suppressing catalytic activity, reduction of a specific gas by the internal pump electrode 22 or reduction of an oxide derived from the specific gas by the internal pump electrode 22 is more likely to occur. Furthermore, it is preferable that the CPU 92 controls the main pump cell 21 to pump oxygen out as just described.For example, an allowable range of the above-described set value V0* can be determined in advance through experimentation. The allowable range can be a range that prevents the measurement target gas reaching the second internal space 40 from having an oxygen concentration of less than 0.1 vol%. Furthermore, when setting the set value V0* based on the pump current Ip1, the CPU 92 can adjust the set value V0* to be within the allowable range. INDUSTRIAL APPLICABILITY

[0104] The present invention can be used in a sensor element for detecting the specific gas concentration, such as NOx, in a measurement object gas, such as automobile exhaust gas, and in a gas sensor including the sensor element.

Claims

[1] Gas sensor (100), comprising: an element body (1, 2, 3, 4, 5, 6) containing an oxygen ion-conductive solid electrolyte layer (4, 6), wherein a measurement object gas flow section is provided within the element body (1, 2, 3, 4, 5, 6) to allow a measurement object gas to be introduced into the measurement object gas flow section and to flow through the measurement object gas flow section; a main pump cell (21) that pumps oxygen from a first internal space (20) to adjust an oxygen concentration of the first internal space (20), the first internal space (20) being provided in the measurement object gas flow section; an auxiliary pump cell (50) that pumps oxygen from a second internal space (40) to adjust an oxygen concentration of the second internal space (40), the second internal space (40) being provided downstream of the first internal space (20) in the measurement object gas flow section; a pre-pump cell (15) that pumps oxygen into a pre-chamber (12) to prevent the measurement object gas from reaching the first internal space (20) in a state where the measurement object gas is an oxygen-poor atmosphere, the pre-chamber (12) being provided upstream of the first internal space (20) in the measurement object gas flow section; a measuring electrode (44) arranged on an inner peripheral surface of a measuring chamber (61), the measuring chamber (61) being provided downstream of the second internal space (40) in the measuring object gas flow section; a reference electrode (42) which is arranged within the element body (1, 2, 3, 4, 5, 6) and into which a reference gas is to be introduced, the reference gas serving as a reference for detecting a specific gas concentration in the measurement object gas; a measuring voltage detection device (82) which detects a measuring voltage (V2) present between the reference electrode (42) and the measuring electrode (44); and a specific gas concentration detecting device (92) which, based on the measurement voltage (V2), obtains a detection value (Ip2) according to the oxygen generated in the measuring chamber (61) and, based on the detection value (Ip2), detects the specific gas concentration in the measurement object gas, wherein the oxygen is oxygen derived from the specific gas, wherein the pre-pump cell (15) contains an inner pre-pump electrode (16) arranged in the pre-chamber (12), the main pump cell (21) includes an inner main pump electrode (22) arranged in the first interior space (20), the auxiliary pump cell (50) includes an inner auxiliary pump electrode (51) arranged in the second interior space (40), the inner pre-pump electrode (16), the inner main pump electrode (22), the inner auxiliary pump electrode (51) and the measuring electrode (44) each contain a noble metal with catalytic activity, wherein at least one of the inner pre-pump electrode (16) and the inner main pump electrode (22) does not contain a noble metal having a capability of suppressing the catalytic activity, wherein the capability of suppressing the catalytic activity is a capability of suppressing the catalytic activity of the noble metal having the catalytic activity exhibited for the specific gas, and the inner auxiliary pump electrode (51) contains a noble metal having the ability to suppress the catalytic activity. [2] The gas sensor (100) of claim 1, further comprising a backing pump control device (92) that controls the backing pump cell (15) so that a constant backing pump current (Ip0s) flows through the backing pump cell (15). [3] The gas sensor (100) according to claim 1 or 2, wherein the specific gas concentration detecting means (92) detects the specific gas concentration, the specific gas concentration being a concentration corrected based on an oxygen concentration of the measurement object gas located outside the element body (1, 2, 3, 4, 5, 6). [4] The gas sensor (100) according to any one of claims 1 to 3, further comprising a gas-side measurement object electrode (23) disposed at a portion to be exposed to the measurement object gas located outside the element body (1, 2, 3, 4, 5, 6), wherein the pre-pumping cell (15) pumps oxygen into the pre-chamber (12) from a vicinity of the gas-side measurement object electrode (23). [5] Gas sensor (100) according to one of claims 1 to 3, wherein the measurement object gas is an exhaust gas of an internal combustion engine, the reference gas is air and the pre-pumping cell (15) pumps oxygen from the surroundings of the reference electrode (42) into the pre-chamber (12). [6] The gas sensor (100) according to any one of claims 1 to 5, wherein neither the inner pre-pump electrode (16) nor the inner main pump electrode (22) contains a noble metal capable of suppressing catalytic activity. [7] The gas sensor (100) according to any one of claims 1 to 6, wherein the inner auxiliary pump electrode (51) contains Au as a noble metal having the ability to suppress the catalytic activity. [8] Sensor element (101) comprising: an element body (1, 2, 3, 4, 5, 6) containing an oxygen ion-conductive solid electrolyte layer (4, 6), wherein a measurement object gas flow section is provided within the element body (1, 2, 3, 4, 5, 6) to allow a measurement object gas to be introduced into the measurement object gas flow section and to flow through the measurement object gas flow section; a main pump cell (21) that pumps oxygen from a first internal space (20) to adjust an oxygen concentration of the first internal space (20), the first internal space (20) being provided in the measurement object gas flow section; an auxiliary pump cell (50) that pumps oxygen from a second internal space (40) to adjust an oxygen concentration of the second internal space (40), the second internal space (40) being provided downstream of the first internal space (20) in the measurement object gas flow section; a pre-pump cell (15) which pumps oxygen into a pre-chamber (12), wherein the pre-chamber (12) is provided upstream of the first inner space (20) in the measurement object gas flow section; a measuring electrode (44) arranged on an inner peripheral surface of a measuring chamber (61), the measuring chamber (61) being provided downstream of the second internal space (40) in the measuring object gas flow section; and a reference electrode (42) which is arranged within the element body (1, 2, 3, 4, 5, 6) and into which a reference gas is to be introduced, wherein the reference gas serves as a reference for detecting a specific gas concentration in the measurement object gas, wherein the pre-pump cell (15) contains an inner pre-pump electrode (16) arranged in the pre-chamber (12), the main pump cell (21) includes an inner main pump electrode (22) arranged in the first interior space (20), the auxiliary pump cell (50) includes an inner auxiliary pump electrode (51) arranged in the second interior space (40), the inner pre-pump electrode (16), the inner main pump electrode (22), the inner auxiliary pump electrode (51) and the measuring electrode (44) each contain a noble metal with catalytic activity, wherein at least one of the inner pre-pump electrode (16) and the inner main pump electrode (22) does not contain a noble metal having a capability of suppressing the catalytic activity, wherein the capability of suppressing the catalytic activity is a capability of suppressing the catalytic activity of the noble metal having the catalytic activity exhibited for the specific gas, and the inner auxiliary pump electrode (51) contains a noble metal having the ability to suppress the catalytic activity.

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