Sensor element and gas sensor

Abstract

To provide a sensor element capable of maintaining high measurement accuracy even when temperature of the sensor element changes.SOLUTION: A gas sensor includes a sensor element which includes: a long-sized tabular base part 102 including oxygen ion conductive solid electrolyte layers 1, 2, 3, 4, 5, 6; a measuring target gas flow void 15 having a first inner void 20 communicating with a gas introduction port 10 through a first dispersion rate controlling passage 11; and an oxygen pump cell 21 including an inside-void oxygen pump electrode 22 arranged inside the first inner void 20, and an outside-void oxygen pump electrode 23 corresponding to the inside-void oxygen pump electrode 22. As to an impedance between the inside-void oxygen pump electrode 22 and the outside-void oxygen pump electrode 23, a ratio of a real part of an impedance at a frequency 1 Hz (an ohmic value R1 at 1 Hz) to a real part of an impedance at a frequency 0.1 Hz (an ohmic value R0 at 0.1 Hz) (R1 / R0) is 93% or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a sensor element and a gas sensor for detecting a target gas in a measurement gas. [Background technology]

[0002] Gas sensors are used to detect and measure the concentration of target gas components (oxygen O2, nitrogen oxides NOx, ammonia NH3, hydrocarbons HC, carbon dioxide CO2, etc.) in measurement gases such as automobile exhaust gas. Known examples of such gas sensors include gas sensors equipped with a sensor element using an oxygen ion conductive solid electrolyte such as zirconia (ZrO2) (for example, Japanese Patent No. 6678134, JP 2020-165966 A, and Japanese Patent No. 6934828 A).

[0003] For example, Japanese Patent No. 6678134 discloses a gas sensor element having a solid electrolyte body, a first electrode film provided on one side of the solid electrolyte body, and a second electrode film provided on the other side of the solid electrolyte body. It discloses that, in at least one of the electrode films of the gas sensor element, the electrostatic capacitance of the entire interior of the electrode film and the ratio of the total length of the two-layer interface between the precious metal particles and the solid electrolyte particles per cross section of the electrode film to the total perimeter of the solid electrolyte particles per cross section of the electrode film can be set within a predetermined range, thereby improving the responsiveness of the gas sensor when switching between gases.

[0004] For example, Japanese Patent Application Laid-Open No. 2020-165966 discloses a gas sensor including a sensor element having a reference electrode and a reference gas inlet that introduces a reference gas and allows it to flow to the reference electrode. The gas sensor discloses that the ratio R1 / R2 of the reaction resistance R1 of the reference electrode to the diffusion resistance R2 of the reference gas inlet is set within a predetermined range to suppress a decrease in the detection accuracy of a specific gas concentration due to changes in the oxygen concentration around the reference electrode. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 6678134 [Patent Document 2] Japanese Patent Publication No. 2020-165966 [Patent Document 3] Patent No. 6934828 Summary of the Invention [Problem to be solved by the invention]

[0006] When a gas sensor measures a target gas, the sensor element is typically kept at a high temperature (e.g., about 800°C) so that the solid electrolyte constituting the sensor element exhibits oxygen ion conductivity. However, due to external environmental factors, such as a high flow rate of the target gas, the sensor element of the gas sensor may be cooled and temporarily reach a temperature lower than the normal operating temperature. Furthermore, when the gas sensor is installed in the exhaust path of a vehicle, the vibrations of the vehicle may increase the resistance of the electrical connections inside the gas sensor (poor contacts), causing fluctuations in the temperature control of the sensor element and resulting in a change in the temperature of the sensor element.

[0007] When such a temperature change occurs in the sensor element, it is undesirable that the measurement accuracy of the gas to be measured decreases.

[0008] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a gas sensor that can maintain high measurement accuracy even if the temperature of the sensor element changes for some reason. [Means for solving the problem]

[0009] The present inventors have conducted extensive research and found that in a gas sensor, high measurement accuracy can be maintained by setting the resistance of the oxygen pump cell that pumps oxygen in a gas to be measured within a predetermined range and reducing the temperature dependency of the resistance.

[0010] (1) A sensor element for detecting a target gas in a measurement gas, The sensor element is a long plate-shaped substrate including an oxygen ion conductive solid electrolyte layer; a measurement gas flow space having a gas inlet opening on the surface of the base portion and a first internal space communicating with the gas inlet through a first diffusion-controlling passage; an oxygen pump cell including an in-void oxygen pump electrode disposed in the first internal space of the measurement gas flow space, and an outside-void oxygen pump electrode disposed at a position of the base portion different from the measurement gas flow space and corresponding to the in-void oxygen pump electrode; Including, A sensor element in which, with regard to the impedance between the oxygen pump electrode inside the cavity and the oxygen pump electrode outside the cavity of the oxygen pump cell, when the real part of the impedance at a frequency of 1 Hz is expressed as the resistance value at 1 Hz R1 and the real part of the impedance at a frequency of 0.1 Hz is expressed as the resistance value at 0.1 Hz R0, the ratio R1 / R0 of the resistance value R1 at 1 Hz to the resistance value R0 at 0.1 Hz is 93% or less.

[0011] (2) The sensor element described in (1) above, wherein the resistance value R0 between the oxygen pump electrode inside the cavity and the oxygen pump electrode outside the cavity of the oxygen pump cell at 0.1 Hz is such that the resistance increase per 1°C of the temperature decrease of the oxygen pump cell is less than 1 Ω / °C.

[0012] (3) The sensor element according to (1) or (2) above, wherein at least one of the oxygen pump electrode inside the cavity and the oxygen pump electrode outside the cavity has a thickness of 4 μm or more and 40 μm or less.

[0013] (4) At least one of the intra-cavity oxygen pump electrode and the extra-cavity oxygen pump electrode is a cermet electrode made of a metal and an oxygen ion conductive solid electrolyte, The sensor element according to any one of (1) to (3) above, wherein in a spectrum obtained when the surface of at least one of the electrodes is measured by X-ray photoelectron spectroscopy (XPS), the peak intensity ratio of the silicon peak intensity to the zirconium peak intensity is 20 or less.

[0014] (5) A sensor element according to any one of (1) to (4) above, wherein when a measurement gas having an oxygen concentration of 20.5% is introduced into the first internal space, the current value of the pump current flowing through the oxygen pump cell is 0.5 mA or more and 4.0 mA or less when the oxygen pump cell pumps out oxygen from the first internal space.

[0015] (6) a second internal space communicating with the first internal space via a second diffusion-controlled passage; an auxiliary pump cell including an in-void auxiliary pump electrode disposed in the second inner void of the measurement gas flow space, and an outside-void auxiliary pump electrode disposed at a position of the base portion different from the measurement gas flow space and corresponding to the in-void auxiliary pump electrode; a measurement pump cell including: an in-void measurement electrode disposed in the second internal cavity of the measurement gas flow space at a position farther from the second diffusion-controlling passage than the in-void auxiliary pump electrode, or in the third internal cavity that communicates with the second internal cavity via a third diffusion-controlling passage; and an out-of-void measurement electrode disposed in a position different from the measurement gas flow space of the base portion, the out-of-void measurement electrode corresponding to the in-void measurement electrode; a reference gas chamber formed inside the base portion and separated from the measurement gas flow space; a reference electrode disposed in the reference gas chamber; The sensor element according to any one of (1) to (5) above, further comprising:

[0016] a second internal space communicating with the first internal space via a second diffusion-controlled passage; a third internal space communicating with the second internal space via a third diffusion-controlling passage; an auxiliary pump cell including an in-void auxiliary pump electrode disposed in the second inner void of the measurement gas flow space, and an outside-void auxiliary pump electrode disposed at a position of the base portion different from the measurement gas flow space and corresponding to the in-void auxiliary pump electrode; a measurement pump cell including an in-space measurement electrode disposed in the third internal space of the measurement target gas flow space, and an out-space measurement electrode disposed at a position of the base portion different from the measurement target gas flow space, the out-space measurement electrode corresponding to the in-space measurement electrode; a reference gas chamber formed inside the base portion and separated from the measurement gas flow space; a reference electrode disposed in the reference gas chamber; The sensor element according to any one of (1) to (5) above, further comprising:

[0017] a second internal space communicating with the first internal space via a second diffusion-controlled passage; an auxiliary pump cell including an in-void auxiliary pump electrode disposed in the second inner void of the measurement gas flow space, and an outside-void auxiliary pump electrode disposed at a position of the base portion different from the measurement gas flow space and corresponding to the in-void auxiliary pump electrode; a measurement pump cell including an in-space measurement electrode disposed in the second internal space of the measurement gas flow space at a position farther from the second diffusion-controlling passage than the in-space auxiliary pump electrode, and an outside-space measurement electrode disposed in a position different from the measurement gas flow space of the base portion and corresponding to the in-space measurement electrode; a reference gas chamber formed inside the base portion and separated from the measurement gas flow space; a reference electrode disposed in the reference gas chamber; The sensor element according to any one of (1) to (5) above, further comprising:

[0018] (7) A gas sensor comprising the sensor element according to any one of (1) to (6) above and a control device for controlling the sensor element, for detecting a target gas in a gas to be measured. [Effects of the Invention]

[0019] According to the present invention, the temperature dependency of the resistance value of the oxygen pump cell can be reduced, and high measurement accuracy can be maintained even when the temperature of the sensor element changes. [Brief explanation of the drawings]

[0020] [Figure 1] 1 is a schematic vertical cross-sectional view of a sensor element 101 in the longitudinal direction, showing an example of the schematic configuration of a gas sensor 100. FIG. [Figure 2] 1 is a block diagram showing the electrical connection relationship between a control device 90, pump cells 21, 50, 41, sensor cells 80, 81, 82, 83, and a heater section 70 of a sensor element 101 in a gas sensor 100. FIG. [Figure 3] This is a Bode plot showing an example of the frequency characteristics of the real part (resistance component) of impedance, where the horizontal axis represents the frequency f [Hz] of the applied AC voltage in logarithm, and the vertical axis represents the real part (resistance component) R [Ω] of the impedance. [Figure 4] 1 is a Bode diagram showing an example of changes in the frequency characteristics of the real part (resistance component) of impedance when the temperature of the sensor element 101 (the temperature of the main pump cell 21) is changed. The horizontal axis represents the frequency f [Hz] of the applied AC voltage in logarithm, and the vertical axis represents the real part (resistance component) R [Ω] of the impedance. DETAILED DESCRIPTION OF THE INVENTION

[0021] The sensor element of the present invention is a sensor element for detecting a measurement target gas in a measurement target gas, a long plate-shaped substrate including an oxygen ion conductive solid electrolyte layer; a measurement gas flow space having a gas inlet opening on the surface of the base portion and a first internal space communicating with the gas inlet through a first diffusion-controlling passage; an oxygen pump cell including an in-void oxygen pump electrode disposed in the first internal space of the measurement gas flow space, and an outside-void oxygen pump electrode disposed at a position of the base portion different from the measurement gas flow space and corresponding to the in-void oxygen pump electrode; Including, With regard to the impedance between the oxygen pump electrode inside the cavity and the oxygen pump electrode outside the cavity of the oxygen pump cell, when the real part of the impedance at a frequency of 1 Hz is expressed as the resistance value at 1 Hz R1 and the real part of the impedance at a frequency of 0.1 Hz is expressed as the resistance value at 0.1 Hz R0, the ratio R1 / R0 of the resistance value R1 at 1 Hz to the resistance value R0 at 0.1 Hz is 93% or less.

[0022] The gas sensor of the present invention includes the sensor element and a control device that controls the sensor element.

[0023] [Outline of gas sensor configuration] A gas sensor including a sensor element of the present invention will be described below with reference to the drawings. Fig. 1 is a schematic vertical cross-sectional view in the longitudinal direction of a sensor element 101, showing an example of the general configuration of a gas sensor 100 including a sensor element 101. In the following, with Fig. 1 as the reference, the upper side of Fig. 1 will be referred to as the top, the lower side as the bottom, the left side of Fig. 1 as the front end side, and the right side as the rear end side.

[0024] In the embodiment of FIG. 1, the gas sensor 100 is an example of a limiting current type NOx sensor that detects NOx in a measurement gas by a sensor element 101 and measures its concentration.

[0025] (sensor element) The sensor element 101 is a long, plate-like element including a base portion 102 having a structure in which multiple oxygen-ion conductive solid electrolyte layers are stacked. The long, plate-like shape is also referred to as a long plate shape or a strip shape. The base portion 102 has a structure in which six layers are stacked in this order from bottom to top as viewed in the drawing: a first substrate layer 1, a second substrate layer 2, a third substrate layer 3, a first solid electrolyte layer 4, a spacer layer 5, and a second solid electrolyte layer 6, each of which is made of an oxygen-ion conductive solid electrolyte layer such as zirconia (ZrO). The solid electrolyte forming these six layers is dense and airtight. The six layers may all have the same thickness or may have different thicknesses. The layers are bonded together via adhesive layers made of solid electrolyte, and the base portion 102 includes the adhesive layers. While FIG. 1 illustrates a layer structure consisting of six layers, the layer structure of the present invention is not limited to this and any number and layer structure may be used.

[0026] The sensor element 101 is manufactured by, for example, laminating ceramic green sheets corresponding to each layer after performing predetermined processing and printing a circuit pattern on them, and then firing the sheets to integrate them.

[0027] The measurement gas flow space 15 has a gas inlet 10 opening on the surface of the base portion 102 and a first internal space 20 communicating with the gas inlet 10 via a first diffusion-controlling passage 11 (first diffusion-controlling section). In this embodiment, the measurement gas flow space 15 further has a second internal space 40 communicating with the first internal space 20 via a second diffusion-controlling passage 30 (second diffusion-controlling section), and a third internal space 61 communicating with the second internal space 40 via a third diffusion-controlling passage 60 (third diffusion-controlling section). That is, this embodiment shows an example of a configuration having three internal spaces.

[0028] A gas inlet 10 is formed at one longitudinal end (hereinafter referred to as the tip) of the sensor element 101, between the lower surface of the second solid electrolyte layer 6 and the upper surface of the first solid electrolyte layer 4. The measurement gas flow space 15, i.e., the measurement gas flow portion, is formed in a manner such that, from the gas inlet 10 in the longitudinal direction, a first diffusion-controlling passage 11, a buffer space 12, a fourth diffusion-controlling passage 13, a first internal cavity 20, a second diffusion-controlling passage 30, a second internal cavity 40, a third diffusion-controlling passage 60, and a third internal cavity 61 are connected in this order.

[0029] The gas inlet 10, the buffer space 12, the first internal space 20, the second internal space 40, and the third internal space 61 are spaces inside the sensor element 101, which are defined by hollowing out the spacer layer 5, with an upper portion defined by the underside of the second solid electrolyte layer 6, a lower portion defined by the upper surface of the first solid electrolyte layer 4, and sides defined by the side surfaces of the spacer layer 5.

[0030] The first diffusion-controlled passage 11, the fourth diffusion-controlled passage 13, and the second diffusion-controlled passage 30 are each provided as two horizontally elongated slits (with the openings extending in the direction perpendicular to the plane of the drawing in FIG. 1). The first diffusion-controlled passage 11, the fourth diffusion-controlled passage 13, and the second diffusion-controlled passage 30 may have any configuration that provides a desired diffusion resistance, and the configuration is not limited to the slits.

[0031] The third diffusion-controlling passage 60 is provided as a single horizontally elongated slit (the opening has its longitudinal direction perpendicular to the plane of the drawing in FIG. 1 ) between the spacer layer 5 and the second solid electrolyte layer 6. The third diffusion-controlling passage 60 may have any shape that provides a desired diffusion resistance, and the shape is not limited to the slit.

[0032] Furthermore, a reference gas introduction space 43 is provided at a position farther from the tip side than the measurement gas flow space 15, between the upper surface of the third substrate layer 3 and the lower surface of the spacer layer 5, and at a position defined at its side by the side surface of the first solid electrolyte layer 4. The reference gas introduction space 43 has an opening at the other end (hereinafter referred to as the rear end) of the sensor element 101. For example, air is introduced into the reference gas introduction space 43 as a reference gas when measuring the NOx concentration.

[0033] The air introduction layer 48 is a layer made of porous alumina, and a reference gas is introduced into the air introduction layer 48 through the reference gas introduction space 43. The air introduction layer 48 is also formed so as to cover the reference electrode 42.

[0034] Reference electrode 42 is an electrode formed in a manner sandwiched between the upper surface of third substrate layer 3 and first solid electrolyte layer 4, and as described above, is surrounded by air introduction layer 48 that connects to reference gas introduction space 43. That is, reference electrode 42 is disposed so as to come into contact with the reference gas via porous air introduction layer 48 and reference gas introduction space 43. Furthermore, as will be described later, reference electrode 42 can be used to measure the oxygen concentrations (oxygen partial pressures) in first internal space 20, second internal space 40, and third internal space 61.

[0035] In the measurement gas flow space 15, the gas inlet 10 is a portion that opens to the external space, and the measurement gas is taken into the sensor element 101 from the external space through the gas inlet 10.

[0036] In this embodiment, the measurement gas is introduced into the measurement gas flow space 15 through the gas inlet 10 opening at the tip end surface of the sensor element 101, but the present invention is not limited to this. For example, the measurement gas flow space 15 does not need to have a recess for the gas inlet 10. In this case, the first diffusion-controlling passage 11 serves as both the gas inlet and the first diffusion-controlling passage.

[0037] Furthermore, for example, the measurement gas flow space 15 may have an opening in a side surface along the longitudinal direction of the base part 102, the opening communicating with the buffer space 12 or a position in the first internal space 20 close to the buffer space 12. In this case, the measurement gas is introduced from the side surface along the longitudinal direction of the base part 102 through the opening.

[0038] Furthermore, for example, the measurement gas flow space 15 may be configured so that the measurement gas is introduced through a porous body.

[0039] The buffer space 12 is a space provided to mitigate the influence of pressure fluctuations on the detected value when the pressure of the gas to be measured fluctuates. The sensor element 101 may not have a structure including the buffer space 12.

[0040] The fourth diffusion-controlling passage 13 is a portion that imparts a predetermined diffusion resistance to the measurement gas introduced from the buffer space 12 into the first internal space 20. The fourth diffusion-controlling passage 13 is provided in association with the provision of the buffer space 12.

[0041] When the buffer space 12 and the fourth diffusion-controlling passage 13 are not provided, the first diffusion-controlling passage 11 directly communicates with the first internal space 20 .

[0042] The first internal space 20 is provided as a space for adjusting the oxygen partial pressure in the measurement gas introduced through the fourth diffusion-controlled passage 13. The oxygen partial pressure is adjusted by the operation of the main pump cell 21. That is, the main pump cell 21 functions as the oxygen pump cell of the present invention.

[0043] The main pump cell 21 is an electrochemical pump cell including an inner main pump electrode 22, which is an intra-cavity oxygen pump electrode, disposed in the first inner cavity 20 of the measurement gas flow cavity 15, and an outer pump electrode 23, which is an extra-cavity oxygen pump electrode, disposed at a position on the base 102 different from the measurement gas flow cavity 15 (on the outer surface of the base 102 in FIG. 1 ) and corresponds to the inner main pump electrode 22. The phrase "corresponding to the inner main pump electrode 22" means that the outer pump electrode 23 is provided between the inner main pump electrode 22 and the inner main pump electrode 22 with a second solid electrolyte layer 6 interposed therebetween.

[0044] That is, the main pump cell 21 is an electrochemical pump cell including an inner main pump electrode 22 having a ceiling electrode portion 22a provided on almost the entire lower surface of the second solid electrolyte layer 6 facing the first internal space 20, an outer pump electrode 23 provided on the upper surface of the second solid electrolyte layer 6 in a region corresponding to the ceiling electrode portion 22a so as to be exposed to the external space, and the second solid electrolyte layer 6 sandwiched between these electrodes.

[0045] The inner main pump electrode 22 is disposed facing the first internal space 20. That is, the inner main pump electrode 22 is formed across the upper and lower solid electrolyte layers (the second solid electrolyte layer 6 and the first solid electrolyte layer 4) that define the first internal space 20 and the spacer layer 5 that provides the side walls. Specifically, a ceiling electrode portion 22a is formed on the lower surface of the second solid electrolyte layer 6 that provides the ceiling surface of the first internal space 20, and a bottom electrode portion 22b is formed on the upper surface of the first solid electrolyte layer 4 that provides the bottom surface. Side electrode portions (not shown) are formed on the side wall surfaces (inner surfaces) of the spacer layer 5 that constitute both side wall portions of the first internal space 20 so as to connect the ceiling electrode portion 22a and the bottom electrode portion 22b, and are disposed in a tunnel-like structure at the locations where the side electrode portions are disposed.

[0046] The inner main pump electrode 22 and the outer pump electrode 23 are porous cermet electrodes (electrodes in which a metal component and a ceramic component are mixed). The ceramic component is not particularly limited, but it is preferable to use an oxygen ion conductive solid electrolyte, similar to the base portion 102. For example, ZrO2 can be used as the ceramic component.

[0047] The inner main pump electrode 22 that comes into contact with the gas under measurement is made of a material that has a reduced ability to reduce the NOx component in the gas under measurement.

[0048] The inner main pump electrode 22 and the outer pump electrode 23 will be described in detail later.

[0049] In the main pump cell 21, by applying a desired pump voltage Vp0 between the inner main pump electrode 22 and the outer pump electrode 23 using a variable power supply 24 and flowing a pump current Ip0 in a positive or negative direction between the inner main pump electrode 22 and the outer pump electrode 23, it is possible to pump oxygen from the first internal space 20 out to the external space or pump oxygen from the external space into the first internal space 20.

[0050] In addition, in order to detect the oxygen concentration (oxygen partial pressure) in the atmosphere in the first internal space 20, an electrochemical sensor cell, i.e., an oxygen partial pressure detection sensor cell 80 for controlling the main pump, is configured by the inner main 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.

[0051] The oxygen concentration (oxygen partial pressure) in the first internal space 20 can be determined by measuring the electromotive force (voltage V0) in the main pump control oxygen partial pressure detection sensor cell 80. Furthermore, the pump current Ip0 is controlled by feedback controlling the pump voltage Vp0 so that the voltage V0 is constant. This makes it possible to maintain the oxygen concentration in the first internal space 20 at a predetermined constant value.

[0052] The second diffusion-controlling passage 30 is a section that imparts a predetermined diffusion resistance to the measurement gas whose oxygen concentration (oxygen partial pressure) has been controlled by the operation of the main pump cell 21 in the first internal space 20, and guides the measurement gas to the second internal space 40.

[0053] The second internal space 40 is provided as a space for adjusting with higher precision the oxygen partial pressure in the measurement gas introduced through the second diffusion-controlled passage 30. The oxygen partial pressure is adjusted by the operation of the auxiliary pump cell 50.

[0054] In the second internal space 40, the oxygen concentration (oxygen partial pressure) is adjusted in advance in the first internal space 20, and then the oxygen partial pressure of the measurement gas introduced through the second diffusion-controlling passage 30 is further adjusted by the auxiliary pump cell 50. This makes it possible to keep the oxygen concentration in the second internal space 40 constant with high precision, thereby enabling the gas sensor 100 to measure the NOx concentration with high precision.

[0055] The auxiliary pump cell 50 is an electrochemical pump cell including an auxiliary pump electrode 51, which is an intra-cavity auxiliary pump electrode, disposed in the second inner cavity 40 of the measurement gas flow cavity 15, and an extra-cavity auxiliary pump electrode disposed at a position of the base 102 different from the measurement gas flow cavity 15 (on the outer surface of the base 102 in FIG. 1 ) corresponding to the auxiliary pump electrode 51. In this embodiment, the outer pump electrode 23 disposed on the outer surface of the base 102 also functions as the extra-cavity auxiliary pump electrode. "Corresponding to the auxiliary pump electrode 51" means that the outer pump electrode 23 is provided on the auxiliary pump electrode 51 with a second solid electrolyte layer 6 interposed therebetween.

[0056] That is, the auxiliary pump cell 50 is an auxiliary electrochemical pump cell including an auxiliary pump electrode 51 having a ceiling electrode portion 51a provided on substantially the entire lower surface of the second solid electrolyte layer 6 facing the second internal space 40, an outer pump electrode 23 (not limited to the outer pump electrode 23, but any suitable electrode disposed at a position different from the measurement gas flow space 15, for example, outside the sensor element 101, will suffice), and the second solid electrolyte layer 6.

[0057] The auxiliary pump electrode 51 is disposed in the second internal space 40 in a tunnel-shaped structure similar to that of the inner main pump electrode 22 provided in the first internal space 20. That is, a ceiling electrode portion 51a is formed on the second solid electrolyte layer 6 that provides the ceiling surface of the second internal space 40, and a bottom electrode portion 51b is formed on the first solid electrolyte layer 4 that provides the bottom surface of the second internal space 40. Side electrodes (not shown) connecting the ceiling electrode portion 51a and the bottom electrode portion 51b are formed on both wall surfaces of the spacer layer 5 that provide the side walls of the second internal space 40, forming a tunnel-shaped structure.

[0058] Like the inner main pump electrode 22, the auxiliary pump electrode 51 is also formed using a material with a weakened ability to reduce the NOx component in the measurement gas.

[0059] In the auxiliary pump cell 50, by applying a desired voltage Vp1 between the auxiliary pump electrode 51 and the outer pump electrode 23 using a variable power supply 52, it is possible to pump oxygen from the atmosphere in the second internal space 40 into the external space or pump oxygen from the external space into the second internal space 40.

[0060] In addition, in order to control the oxygen partial pressure in the atmosphere within the second internal space 40, an electrochemical sensor cell, i.e., an oxygen partial pressure detection sensor cell 81 for controlling the auxiliary pump, is configured by 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.

[0061] The auxiliary pump cell 50 performs pumping using a variable power supply 52 whose voltage is controlled based on the electromotive force (voltage V1) detected by the auxiliary pump control oxygen partial pressure detection sensor cell 81. This allows the oxygen partial pressure in the atmosphere within the second internal space 40 to be controlled to a low level that does not substantially affect the measurement of NOx.

[0062] At the same time, the pump current Ip1 is used to control the voltage V0 of the main pump control oxygen partial pressure detection sensor cell 80. Specifically, the pump current Ip1 is input as a control signal to the main pump control oxygen partial pressure detection sensor cell 80, and by controlling the voltage V0, the gradient of the oxygen partial pressure in the measurement gas introduced into the second internal space 40 from the second diffusion-limited passage 30 is controlled to be always constant. When 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 action of the main pump cell 21 and the auxiliary pump cell 50.

[0063] The third diffusion-controlling passage 60 is a section that imparts a predetermined diffusion resistance to the measurement gas, the oxygen concentration (oxygen partial pressure) of which has been further controlled to a lower level by the operation of the auxiliary pump cell 50 in the second internal space 40, and guides the measurement gas to the third internal space 61.

[0064] The third internal space 61 is provided as a space for measuring the concentration of nitrogen oxides (NOx) in the measurement gas introduced through the third diffusion-controlled passage 60. The NOx concentration is measured by the operation of the measuring pump cell 41.

[0065] The measurement pump cell 41 measures the NOx concentration in the measurement gas in the third internal space 61. The measurement pump cell 41 is an electrochemical pump cell including a measurement electrode 44, which is an in-space measurement electrode, disposed in the third internal space 61 of the measurement gas flow space 15, and an extra-space measurement electrode corresponding to the measurement electrode 44, disposed at a position on the base 102 different from the measurement gas flow space 15. In this embodiment, the outer pump electrode 23 disposed on the outer surface of the base 102 also functions as an extra-space measurement electrode. "Corresponding to the measurement electrode 44" means that the outer pump electrode 23 is disposed relative to the measurement electrode 44 via the second solid electrolyte layer 6, the spacer layer 5, and the first solid electrolyte layer 4.

[0066] That is, the measurement pump cell 41 is an electrochemical pump cell including a measurement electrode 44 provided on the upper surface of the first solid electrolyte layer 4 facing the third internal space 61, an outer pump electrode 23 (not limited to the outer pump electrode 23, but any suitable electrode disposed at a position different from the measurement gas flow space 15, for example, outside the sensor element 101), the second solid electrolyte layer 6, the spacer layer 5, and the first solid electrolyte layer 4.

[0067] The measurement electrode 44 is a porous cermet electrode. The measurement electrode 44 also functions as a NOx reduction catalyst that reduces NOx present in the atmosphere in the third internal space 61. The measurement electrode 44 is an electrode containing a catalytically active noble metal (e.g., at least one of Pt, Rh, Ir, Ru, and Pd). It is preferable that the measurement electrode 44 does not contain a noble metal (e.g., Au, Ag, etc.) that reduces the catalytic activity of the catalytically active noble metal with respect to the measurement target gas (NOx in this embodiment). The measurement electrode 44 may be, for example, a porous cermet electrode of Pt, Rh, and ZrO2.

[0068] In the measuring pump cell 41, oxygen produced by decomposition of nitrogen oxides in the atmosphere surrounding the measuring electrode 44 is pumped out, and the amount of oxygen produced can be detected as a pump current Ip2.

[0069] Furthermore, in order to detect the oxygen partial pressure around the measurement electrode 44, the first solid electrolyte layer 4, the third substrate layer 3, the measurement electrode 44, and the reference electrode 42 constitute an electrochemical sensor cell, i.e., an oxygen partial pressure detection sensor cell 82 for controlling the measurement pump. The variable power supply 46 is controlled based on the electromotive force (voltage V2) detected by the oxygen partial pressure detection sensor cell 82 for controlling the measurement pump.

[0070] The measurement gas introduced into the second internal space 40 reaches the measuring electrode 44 in the third internal space 61 through the third diffusion-limited passage 60 under conditions in which the oxygen partial pressure is controlled. Nitrogen oxides in the measurement gas around the measuring electrode 44 are reduced (2NO → N2 + O2) to generate oxygen. This generated oxygen is then pumped by the measuring pump cell 41, and the voltage Vp2 of the variable power supply 46 is controlled so that the voltage V2 detected by the measurement pump control oxygen partial pressure detection sensor cell 82 remains constant. Because the amount of oxygen generated around the measuring electrode 44 is proportional to the nitrogen oxide concentration in the measurement gas, the pump current Ip2 in the measuring pump cell 41 can be used to calculate the nitrogen oxide concentration in the measurement gas.

[0071] Furthermore, by combining the measurement electrode 44, the first solid electrolyte layer 4, the third substrate layer 3 and the reference electrode 42 to form an oxygen partial pressure detection means as an electrochemical sensor cell, it is possible to detect an electromotive force corresponding to the difference between the amount of oxygen generated by reduction of the NOx components in the atmosphere around the measurement electrode 44 and the amount of oxygen contained in the reference atmosphere, thereby making it possible to determine the concentration of the NOx components in the measured gas.

[0072] In addition, the second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, the third substrate layer 3, the outer pump electrode 23, and the reference electrode 42 constitute an electrochemical sensor cell 83, and the electromotive force (voltage Vref) obtained by this sensor cell 83 makes it possible to detect the oxygen partial pressure in the measurement gas outside the sensor.

[0073] In the gas sensor 100 having such a configuration, the measurement gas, in which the oxygen partial pressure is always kept at a constant low value (a value that does not substantially affect the measurement of NOx) by operating the main pump cell 21 and the auxiliary pump cell 50, is supplied to the measurement pump cell 41. Therefore, the NOx concentration in the measurement gas can be determined based on the pump current Ip2 that flows when oxygen generated by the reduction of NOx is pumped out of the measurement pump cell 41, which is approximately proportional to the NOx concentration in the measurement gas.

[0074] Furthermore, in order to enhance the oxygen ion conductivity of the solid electrolyte, the sensor element 101 is provided with a heater section 70 that adjusts the temperature by heating and maintaining the temperature of the sensor element 101. The heater section 70 includes a heater electrode 71, a heater 72, a heater lead 76, a through-hole 73, a heater insulating layer 74, and a pressure release hole 75.

[0075] The heater electrode 71 is an electrode formed in a manner to contact the lower surface of the first substrate layer 1. By connecting the heater electrode 71 to a heater power supply 77, it becomes possible to supply power to the heater section 70 from outside.

[0076] The heater 72 is an electrical resistor sandwiched between the second substrate layer 2 and the third substrate layer 3. The heater 72 is connected to the heater electrode 71 via a heater lead 76 that is connected to the heater 72 and extends to the rear end side of the sensor element 101 in the longitudinal direction, and a through hole 73. The heater 72 generates heat when power is supplied from the outside through the heater electrode 71, thereby heating and keeping warm the solid electrolyte that forms the sensor element 101.

[0077] The heater 72 is embedded throughout the entire area from the first internal space 20 to the third internal space 61, and can adjust the temperature of the sensor element 101 to a temperature at which the solid electrolyte is activated. The temperatures need only be adjusted so that the main pump cell 21, the auxiliary pump cell 50, and the measurement pump cell 41 can operate. It is not necessary to adjust all of these areas to the same temperature; there may be a temperature distribution in the sensor element 101. For example, it is preferable that the temperature of the main pump cell 21 (the temperature of the outer pump electrode 23) be approximately 600°C to 800°C.

[0078] In the sensor element 101 of this embodiment, the heater 72 is embedded in the base portion 102, but the present invention is not limited to this. The heater 72 may be disposed so as to heat the base portion 102. That is, the heater 72 may be capable of heating the sensor element 101 to an extent that the sensor element 101 exhibits oxygen ion conductivity that enables the main pump cell 21, the auxiliary pump cell 50, and the measurement pump cell 41 to operate. For example, the heater 72 may be embedded in the base portion 102 as in this embodiment. Alternatively, the heater portion 70 may be formed as a heater substrate separate from the base portion 102 and disposed adjacent to the base portion 102.

[0079] The heater insulating layer 74 is an insulating layer made of an insulator such as alumina and formed on the upper and lower surfaces of the heater 72 and heater lead 76. The heater insulating layer 74 is formed for the purpose of obtaining electrical insulation between the second substrate layer 2 and the heater 72 and heater lead 76, and between the third substrate layer 3 and the heater 72 and heater lead 76.

[0080] The pressure release hole 75 penetrates the third substrate layer 3 and is formed so as to connect the heater insulating layer 74 and the reference gas introduction space 43. The pressure release hole 75 can mitigate an increase in internal pressure that accompanies an increase in temperature within the heater insulating layer 74. Note that the pressure release hole 75 may be omitted.

[0081] Furthermore, a predetermined length of the surface of the sensor element 101 from the tip in the longitudinal direction may be covered with a porous protective layer (not shown). The porous protective layer is formed to protect the internal space and the area of ​​the sensor element 101 where the electrodes are present from thermal shock due to splashing with water, etc. The porous protective layer is made of ceramics such as alumina, and may have a thickness of about 10 μm to 2000 μm. It is also preferable that the porous protective layer is formed so as to be able to withstand a force of up to about 50 N.

[0082] The above-described sensor element 101 is incorporated into the gas sensor 100 in such a manner that the front end of the sensor element 101 contacts the gas to be measured and the rear end of the sensor element 101 contacts the reference gas.

[0083] (Control device) The gas sensor 100 of this embodiment includes the above-described sensor element 101 and a control device 90 that controls the sensor element 101. In the gas sensor 100, the electrodes 22, 23, 51, 44, and 42 of the sensor element 101 are electrically connected to the control device 90 via lead wires (not shown). FIG. 2 is a block diagram showing the electrical connections between the control device 90 and the pump cells 21, 50, and 41 of the sensor element 101, the sensor cells 80, 81, 82, and 83, and the heater unit 70. The control device 90 includes the above-described variable power supplies 24, 52, and 46, the heater power supply 77, and a control unit 91. The control unit 91 includes a heater control unit 92, a pump control unit 93, and a concentration calculation unit 94.

[0084] The control unit 91 is realized by a general-purpose or dedicated computer, and functions as a heater control unit 92, a pump control unit 93, and a concentration calculation unit 94 are realized by a CPU, memory, etc. mounted on the computer. Note that when the gas sensor 100 measures NOx contained in exhaust gas from an automobile engine and the sensor element 101 is attached to an exhaust path, some or all of the functions of the control device 90 (particularly the control unit 91) may be realized by an ECU (Electronic Control Unit) mounted on the automobile.

[0085] The control unit 91 is configured to acquire the electromotive forces (voltages V0, V1, V2, Vref) in the sensor cells 80, 81, 82, 83 of the sensor element 101, the pump currents (Ip0, Ip1, Ip2) in the pump cells 21, 50, 41, and the heater voltage Vh and heater current Ih in the heater unit 70. The control unit 91 is also configured to output control signals to the variable power supplies 24, 52, 46 and the heater power supply 77.

[0086] The heater control unit 92 may be configured to control the heater unit 70 (particularly the heater 72). The heater control unit 92 heats the heater 72 and maintains the temperature of the heater 72 at a desired operating temperature. The operating temperature may be a temperature at which the gas sensor 100 can detect the measurement target gas, that is, a temperature at which the solid electrolyte of the sensor element 101 is activated to the extent that each of the pump cells 21, 50, and 41 can operate. The operating temperature may be, for example, a temperature of the main pump cell 21 (the temperature of the outer pump electrode 23) of approximately 600°C to 800°C.

[0087] Various known control methods can be used to heat the heater 72. For example, the heater 72 can be heated by applying a constant voltage to it. The output of the heater power supply 77 can also be controlled based on the resistance value of the heater 72. Alternatively, the output of the heater power supply 77 can be controlled based on at least one of the resistance values ​​of the main pump cell 21, the auxiliary pump cell 50, and the measurement pump cell 41. The output of the heater power supply 77 can also be controlled based on at least one of the resistance values ​​of the main pump control oxygen partial pressure detection sensor cell 80, the auxiliary pump control oxygen partial pressure detection sensor cell 81, the measurement pump control oxygen partial pressure detection sensor cell 82, and the sensor cell 83.

[0088] For example, the heater control unit 92 feedback-controls the control signal output to the heater power supply 77 based on the heater resistance value Rh (=Vh / Ih) calculated from the heater voltage Vh and heater current Ih in the heater 72 so that the heater 72 reaches the target temperature.

[0089] The pump control unit 93 is configured to control the oxygen pump cell (in this embodiment, the main pump cell 21) so as to measure the measurement target gas (NOx in this embodiment) in the measurement gas. In this embodiment, the pump control unit 93 is further configured to control the operations of the auxiliary pump cell 50 and the measurement pump cell 41.

[0090] The pump control unit 93 may be configured to adjust the pump voltage Vp0 applied between the intra-void oxygen pump electrode (inner main pump electrode 22) and the extra-void oxygen pump electrode (outer pump electrode 23) of the oxygen pump cell (main pump cell 21) to adjust the oxygen partial pressure in the first internal space 20. In addition, the pump control unit 93 may be configured to adjust the oxygen partial pressure in the first internal space 20 based on the electromotive force (voltage V0) generated between the intra-void oxygen pump electrode (inner main pump electrode 22) and the reference electrode 42.

[0091] The pump control unit 93 may adjust the pump voltages (Vp0, Vp1, Vp2) applied to the pump cells 21, 50, 41 to adjust the oxygen partial pressure in each of the internal spaces 20, 40, 61. Furthermore, the pump control unit 93 may adjust the oxygen partial pressure in each of the internal spaces 20, 40, 61, for example, by performing the following feedback control based on the electromotive forces (voltages V0, V1, V2) generated in the sensor cells 80, 81, 82. Specifically, in this embodiment, the control is performed as follows.

[0092] The pump control unit 93 controls the voltage V0 of the oxygen partial pressure detection sensor cell 80 for controlling the main pump so that the voltage V0 is a constant value (set value V0 SET The pump voltage Vp0 of the variable power supply 24 in the main pump cell 21 is feedback-controlled so that the voltage Vp0 represents the oxygen partial pressure near the inner main pump electrode 22. Therefore, maintaining the voltage V0 constant means maintaining the oxygen partial pressure near the inner main pump electrode 22 constant. As a result, the pump current Ip0 in the main pump cell 21 changes depending on the oxygen concentration in the measurement gas.

[0093] When the oxygen partial pressure in the measurement gas reaches the set value V0SET On the other hand, when the oxygen partial pressure in the measurement gas is higher than the set value V0 SET When the oxygen partial pressure is lower than the oxygen partial pressure corresponding to (for example, when hydrocarbons HC, etc. are contained), the main pump cell 21 pumps oxygen from the space outside the sensor element 101 into the first internal space 20. Therefore, the pump current Ip0 can take either a positive or negative value.

[0094] The pump control unit 93 controls the auxiliary pump control oxygen partial pressure detection sensor cell 81 so that the voltage V1 is a constant value (set value V1 SET The pump voltage Vp1 of the variable power supply 52 in the auxiliary pump cell 50 is feedback-controlled so that the set value Vp1 is equal to the oxygen partial pressure in the vicinity of the auxiliary pump electrode 51. Since the voltage V1 indicates the oxygen partial pressure in the vicinity of the auxiliary pump electrode 51, maintaining the voltage V1 constant means maintaining the oxygen partial pressure in the vicinity of the auxiliary pump electrode 51 constant. This allows the oxygen partial pressure in the atmosphere in the second internal space 40 to be controlled to a low partial pressure that does not substantially affect the measurement of NOx. SET It is preferable to set the oxygen partial pressure in the atmosphere in the second internal space 40 to a value at which substantially all of the oxygen in the gas to be measured is pumped out in the second internal space 40.

[0095] At the same time, the pump current Ip1 in the auxiliary pump cell 50 is set to a constant value (set value Ip1 SET The set value V0 of the voltage V0 is set based on the pump current Ip1 so that the set value V0 of the voltage V0 is SET Specifically, the pump current Ip1 is input as a control signal to the oxygen partial pressure detection sensor cell 80 for controlling the main pump, and the voltage V0 of the sensor cell 80 is adjusted to a set value V0 based on the pump current Ip1. SETBy controlling the oxygen partial pressure gradient in the measurement gas introduced from the second diffusion-controlled passage 30 into the second internal space 40, the gradient is always kept constant. When used as a NOx sensor, the oxygen concentration in the second internal space 40 is kept constant at about 0.001 ppm by the actions of the main pump cell 21 and the auxiliary pump cell 50. In other words, it is considered that the oxygen concentration in the measurement gas introduced from the third diffusion-controlled passage 60 into the third internal space 61 is kept constant at about 0.001 ppm.

[0096] The pump control unit 93 controls the voltage V2 detected by the oxygen partial pressure detection sensor cell 82 for controlling the measurement pump to be a constant value (set value V2 SET The pump voltage Vp2 of the variable power supply 46 in the measuring pump cell 41 is feedback-controlled so that the voltage Vp2 is equal to the set value Vp2. At the measuring electrode 44, nitrogen oxides in the gas to be measured are reduced (2NO → N2 + O2) to generate oxygen. The generated oxygen is then converted to oxygen when the voltage V2 reaches the set value Vp2. SET The set value V2 is pumped out by the measuring pump cell 41. SET can be set to a value that substantially completely decomposes NOx at the measuring electrode 44. SET By setting the pump current Ip2 at the measuring electrode 44, substantially all of the NOx in the measurement gas is detected as the pump current Ip2. Therefore, the value of the pump current Ip2 corresponds to the NOx concentration in the measurement gas.

[0097] The concentration calculation unit 94 is configured to calculate the concentration of the measurement target gas (NOx in this embodiment) in the measurement gas. The concentration calculation unit 94 calculates the NOx concentration in the measurement gas based on the value of the current (pump current Ip2) flowing through the measurement pump cell 41.

[0098] The concentration calculation unit 94 acquires the pump current Ip2 in the measurement pump cell 41, calculates the NOx concentration in the measurement gas based on a pre-stored conversion parameter (current-NOx concentration conversion parameter) between the pump current Ip2 and the NOx concentration in the measurement gas, and outputs the calculated value as the detection value of the gas sensor 100. The current-NOx concentration conversion parameter is stored in advance in the memory of the control unit 91, which functions as the concentration calculation unit 94, as data representing the relationship (linear relationship) between the pump current Ip2 and the NOx concentration in the measurement gas. The current-NOx concentration conversion parameter can be appropriately determined for the gas sensor 100 in advance through experiments or the like by a person skilled in the art. The current-NOx concentration conversion parameter may be, for example, a coefficient of an approximate equation (such as a linear function) obtained through experiments, or may be a map showing the correspondence between the pump current Ip2 and the NOx concentration in the measurement gas. The current-NOx concentration conversion parameter may be a parameter unique to each gas sensor 100, or a parameter commonly used by multiple gas sensors.

[0099] (Use of gas sensors) When the gas sensor 100 measures a target gas (NOx in this embodiment), as described above, the heater 72 heats the sensor element 101 (base 102), maintaining the temperature of the sensor element 101 (base 102) at its operating temperature (e.g., approximately 800°C). However, due to external environmental factors such as a high flow rate or a low gas temperature, the sensor element 101 may be cooled by the target gas, temporarily lowering its temperature below its normal operating temperature. Furthermore, for example, when the gas sensor 100 is installed in the exhaust path of an automobile, the gas sensor 100 is constantly exposed to vibrations. Such vibrations may increase the resistance of electrical connections within the gas sensor 100 (e.g., contacts between the heater electrode 71 and wiring, contacts between the wiring and the control device 90, connections on the wiring, etc.). Such poor electrical contacts in the electrical circuit may increase the resistance of the connections. As a result, the power supplied to the heater 72 may change, causing the temperature control of the sensor element 101 to fluctuate, resulting in a change in the temperature of the sensor element.

[0100] When the temperature of the sensor element 101 changes due to such factors, the temperature of a pump cell such as the oxygen pump cell (in this embodiment, the main pump cell 21) (the temperature of a pair of electrodes and the solid electrolyte sandwiched between them) also changes. A high pump cell temperature tends to lower the pump cell's resistance, while a low pump cell temperature tends to increase the pump cell's resistance. The temperature of the oxygen pump cell (main pump cell 21) may be expressed as the surface temperature of the oxygen pump electrode in the cavity (the inner main pump electrode 22) or the surface temperature of the oxygen pump electrode outside the cavity (the outer pump electrode 23). The electrode surface temperature may be expressed as an average temperature or as a temperature at an arbitrary position, such as the center position. Alternatively, the temperature may be expressed as the temperature of the solid electrolyte (second solid electrolyte layer 6) sandwiched between the oxygen pump electrode in the cavity (the inner main pump electrode 22) and the oxygen pump electrode outside the cavity (the outer pump electrode 23). The temperature may also be expressed as an average value of two or more of the above temperatures.

[0101] As described above, the oxygen pump cell (main pump cell 21) discharges oxygen from the first internal space 20 so that the oxygen concentration in the first internal space 20 reaches a predetermined value. At this time, the greater the resistance value of the main pump cell 21, the higher the pump voltage Vp0 that must be applied to pump out the oxygen. Furthermore, the higher the oxygen concentration in the measured gas, the greater the amount of oxygen discharged by the main pump cell 21, and the greater the pump current Ip0 flowing through the main pump cell 21. Therefore, the higher the oxygen concentration in the measured gas, the greater the pump voltage Vp0 at the main pump cell 21.

[0102] If the pump voltage Vp0 becomes excessively high, NOx may be decomposed at the inner main pump electrode 22. This reduces the amount of NOx that reaches the measuring electrode 44. As a result, the value of the pump current Ip2 detected by the measuring pump cell 41 may become smaller than the value that should be detected. This may result in a decrease in the accuracy of NOx detection, particularly when the oxygen concentration in the measurement gas is high.

[0103] Furthermore, if the pump voltage Vp0 becomes excessively high, a portion of the inner main pump electrode 22 and / or the outer pump electrode 23 of the main pump cell 21 may peel off. This may reduce the oxygen pumping function of the main pump cell 21, causing the remaining oxygen to reach the measuring electrode 44. As a result, the pump current Ip2 detected by the measuring pump cell 41 may be higher than the intended value. This may result in a decrease in the accuracy of NOx detection, particularly when the oxygen concentration in the measurement gas is high. Alternatively, the peeling of the electrode may further increase the resistance of the main pump cell 21, causing the pump voltage Vp0 to become even more excessively high. In this case, the decomposition of NOx at the inner main pump electrode 22 may proceed further, further reducing the accuracy of NOx detection, particularly when the oxygen concentration in the measurement gas is high.

[0104] After extensive research, the inventors have found that by reducing the temperature dependence of the resistance of the oxygen pump cell, it is possible to prevent the pump voltage Vp0 from becoming excessively high even if the temperature of the sensor element 101 changes (particularly decreases) for some reason during use of the gas sensor 100. As a result, it has been found that even if the temperature of the sensor element 101 changes (particularly decreases) due to the influence of the external environment (temperature, flow rate, flow velocity, etc. of the gas to be measured) or due to poor contact (poor contact within the gas sensor 100 or poor contact between the gas sensor 100 and an external power source, etc.), it is possible to suppress the above-mentioned decrease in measurement accuracy. Note that the temperature dependence of the resistance of the oxygen pump cell refers to a change in the resistance value when the temperature of the oxygen pump cell changes. The oxygen pump cell and the intra-cavity oxygen pump electrode and extra-cavity oxygen pump electrode included in the oxygen pump cell are described in detail below.

[0105] (oxygen pump cell) In the oxygen pump cell of the present invention (in this embodiment, the main pump cell 21), with regard to the impedance between the inner main pump electrode 22 of the oxygen pump electrode inside the cavity and the outer pump electrode 23 of the oxygen pump electrode outside the cavity, when the real part of the impedance at a frequency of 1 Hz is expressed as the resistance value at 1 Hz R1 and the real part of the impedance at a frequency of 0.1 Hz is expressed as the resistance value at 0.1 Hz R0, the ratio R1 / R0 of the resistance value R1 at 1 Hz to the resistance value R0 at 0.1 Hz is 93% or less.

[0106] Impedance is the ratio of voltage to current in an AC circuit and is generally expressed as a complex number. It is also called complex impedance. In complex impedance, the real part represents the resistive component of the impedance, and the imaginary part represents the reactive component of the impedance. Impedance is measured by AC impedance measurement.

[0107] Specifically, the heater 72 heats the sensor element 101 to its operating temperature. Temperature control may be performed by the heater control unit 92. In this state, AC impedance measurement is performed in the air atmosphere. AC impedance measurement may be performed in an inert gas, a gas atmosphere simulating the air, for example, with an oxygen concentration of 20.5%, or an atmosphere simulating the target gas component in the measured gas, such as automobile exhaust gas. AC impedance measurement can be performed using a known measuring device such as an impedance analyzer. An AC voltage is applied between the inner main pump electrode 22 and the outer pump electrode 23 while changing the frequency, and the frequency characteristics of the impedance are obtained. Figure 3 is a Bode diagram showing an example of the frequency characteristics of the real part (resistance component) of the impedance. At 0.1 Hz (10 -1 Hz) to 1,000,000 Hz (10 6 3 shows an example of the frequency characteristics when the frequency is changed from 1000 to 1000 Hz. In Fig. 3, the horizontal axis represents the frequency f [Hz] of the applied AC voltage in logarithm, and the vertical axis represents the real part of the impedance (resistance component) R [Ω].

[0108] As shown in FIG. 3, the resistance component R of the oxygen pump cell (main pump cell 21) changes with frequency. The real part (resistance component) R of the impedance takes a maximum value at the lowest frequency of 0.1 Hz, and the resistance component R tends to decrease as the frequency increases. As the frequency increases further, the resistance component R decreases at a frequency of 100,000 Hz (10 5 Hz), the resistance component R tends to increase slightly as the frequency increases. Also, between frequencies of around 1 Hz and several hundred Hz, the change in the resistance component R with frequency tends to be small.

[0109] The real part (resistance component) of the impedance of the oxygen pump cell (main pump cell 21) is considered to include resistance components such as the resistance of each of the pair of electrodes (the inner main pump electrode 22 and the outer pump electrode 23) that make up the oxygen pump cell (main pump cell 21), the resistance of the solid electrolyte (second solid electrolyte layer 6) sandwiched between the pair of electrodes (the inner main pump electrode 22 and the outer pump electrode 23), and the diffusion resistance of oxygen gas that reaches the oxygen pump cell (main pump cell 21).The appearance of these resistance components is considered to differ depending on the frequency of the AC voltage applied to the oxygen pump cell (main pump cell 21) during impedance measurement, and therefore the real part (resistance component) of the impedance is considered to have frequency characteristics such as those shown in FIG.

[0110] When the gas sensor 100 measures the target gas (when the gas sensor 100 is driven), the pump control unit 93 applies the pump voltage Vp0 to the main pump cell 21 as described above to pump oxygen out of the first internal space 20 or pump oxygen into the first internal space 20. For example, in the gas sensor 100 of this embodiment, as shown in FIG. 1 , the main pump cell 21 is controlled by the pump voltage Vp0, which is a DC voltage. Therefore, the resistance value of the main pump cell 21 when driven is considered to roughly correspond to the resistance component when the frequency in AC impedance measurement is sufficiently low.

[0111] In the present invention, the real part (resistance component) of the impedance at 0.1 Hz is referred to as the resistance value R0 at 0.1 Hz. Because 0.1 Hz is considered to be a sufficiently low frequency relative to the oxygen pump cell, the resistance value R0 at 0.1 Hz can be considered to roughly represent the total resistance of the oxygen pump cell. This resistance value R0 at 0.1 Hz is considered to correspond to the resistance value of the main pump cell 21 when the gas sensor 100 is operating. In the present invention, the resistance value R0 at 0.1 Hz is decomposed into two components based on the frequency band. With the frequency of 1 Hz as the boundary, the resistance component appearing in the frequency range of 1 Hz or higher is referred to as resistance value R1, and the resistance component appearing in the frequency range of 0.1 Hz or higher but lower than 1 Hz is referred to as resistance value R2. That is, the real part of the impedance at 1 Hz is referred to as the resistance value R1 at 1 Hz, and the difference between the resistance value R0 at 0.1 Hz and the resistance value R1 at 1 Hz is referred to as resistance value R2. Resistance value R2 = R0 - R1. The resistance value R0 at 0.1 Hz and the resistance value R1 at 1 Hz may be obtained from the frequency characteristics of the impedance as shown in Fig. 3 by applying an AC voltage between the inner main pump electrode 22 and the outer pump electrode 23 while changing the frequency. Alternatively, an AC voltage with a frequency of 0.1 Hz may be applied between the inner main pump electrode 22 and the outer pump electrode 23 to measure the resistance value R0 at 0.1 Hz, and an AC voltage with a frequency of 1 Hz may be applied to measure the resistance value R1 at 1 Hz.

[0112] FIG. 4 is a Bode diagram showing an example of the change in frequency characteristics of the real part (resistance component) of impedance when the temperature of the sensor element 101 (the temperature of the main pump cell 21) is changed. The horizontal axis represents the frequency f [Hz] of the applied AC voltage in logarithm, and the vertical axis represents the real part (resistance component) R [Ω] of the impedance. The graph shows frequency characteristics when the electrode temperatures of the main pump cell 21 are set to 700°C (H), 660°C (M), and 620°C (L). The electrode temperature here refers to the surface temperature of the center of the outer pump electrode 23 when the sensor element 101 is viewed from above. In the sensor element 101 of this embodiment, the temperature of the heater 72 is set to 840°C (H), 800°C (M), and 760°C (L), thereby adjusting the electrode temperatures of the main pump cell 21 to the above temperatures.

[0113] At an electrode temperature of 700°C (H) of the main pump cell, R0H=67Ω, R1H=58Ω, and R2H=9Ω. At an electrode temperature of 660°C (M) in the main pump cell, R0M = 83Ω, R1M = 74Ω, and R2M = 8Ω. At an electrode temperature of 620°C (L) of the main pump cell, R0L = 112Ω, R1L = 105Ω, and R2L = 7Ω.

[0114] As shown in FIG. 4, the resistance R1 at 1 Hz (resistance component appearing in a frequency range of 1 Hz or higher) varies significantly with temperature, while the resistance R2 (resistance component appearing in a frequency range of 0.1 Hz or higher but less than 1 Hz) varies little with temperature. The resistance R1 at 1 Hz (resistance component appearing in a frequency range of 1 Hz or higher) is considered to include the resistance of the inner main pump electrode 22 and the outer pump electrode 23 themselves, as well as the resistance of the solid electrolyte (second solid electrolyte layer 6) sandwiched between the two electrodes. The resistance R2 (resistance component appearing in a frequency range of 0.1 Hz or higher but less than 1 Hz) is considered to include the resistance of the inner main pump electrode 22 and the outer pump electrode 23 themselves, as well as the diffusion resistance of oxygen gas reaching the oxygen pump cell (main pump cell 21). The diffusion resistance of oxygen gas reaching the oxygen pump cell (main pump cell 21) includes the diffusion resistance of oxygen gas reaching the intra-void oxygen pump electrode (inner main pump electrode 22) and the diffusion resistance of oxygen gas reaching the extra-void oxygen pump electrode (outer pump electrode 23). In this embodiment, since the outer pump electrode 23 is disposed on the outer surface of the sensor element 101, the diffusion resistance to oxygen gas reaching the oxygen pump cell (main pump cell 21) is mainly the diffusion resistance to oxygen gas reaching the inner main pump electrode 22, and is generally considered to be the diffusion resistance to oxygen gas in the measurement gas introduced into the first internal space 20. In this embodiment, this generally corresponds to the diffusion resistance from the gas inlet 10 to the fourth diffusion-controlling passage 13 (the gas inlet 10, the first diffusion-controlling passage 11, the buffer space 12, and the fourth diffusion-controlling passage 13).

[0115] Thus, the resistance R1 at 1 Hz (the resistance component appearing in the frequency range of 1 Hz or higher) and the resistance R2 at 1 Hz (the resistance component appearing in the frequency range of 0.1 Hz or higher but less than 1 Hz) have different degrees of temperature dependence. Therefore, by changing the ratio of the resistance R1 to the resistance R2 (e.g., the proportion of the resistance R1 in the total resistance R0 (= R1 + R2)), the temperature dependence of the real part (resistance component) of the impedance of the oxygen pump cell (main pump cell 21) as a whole can be reduced. The inventors have found that, as described above, the temperature dependence of the resistance of the oxygen pump cell (main pump cell 21) can be effectively reduced by setting the ratio R1 / R0 of the resistance R1 at 1 Hz to the resistance R0 at 0.1 Hz to 93% or less. That is, they have found that an increase in the resistance can be suppressed even when the temperature of the oxygen pump cell (main pump cell 21) decreases.

[0116] Setting the ratio R1 / R0 within this range reduces the temperature dependency of the resistance value of the oxygen pump cell (main pump cell 21), thereby suppressing a decrease in measurement accuracy due to temperature changes in the sensor element 101. The smaller the ratio R1 / R0, the more the temperature dependency of the resistance value of the oxygen pump cell (main pump cell 21) is expected to be reduced, so the lower limit of the ratio R1 / R0 is not particularly limited. However, due to the configuration of the oxygen pump cell (main pump cell 21), the lower limit of the ratio R1 / R0 will likely be approximately 30% or more.

[0117] The degree of temperature dependence of the resistance of the oxygen pump cell (main pump cell 21) can be expressed by the resistance increase [Ω / °C] per 1°C decrease in temperature of the oxygen pump cell (main pump cell 21). The resistance increase per 1°C decrease in temperature may be referred to as the resistance increase rate or resistance change rate hereinafter. The resistance increase rate [Ω / °C] per 1°C decrease in temperature may be, for example, less than 1 Ω / °C. That is, the increase in resistance when the temperature decreases by 1°C may be, for example, less than 1 Ω. As the temperature decreases, the resistance of the oxygen pump cell (main pump cell 21) typically increases. Although there is no particular lower limit to the resistance increase rate, the resistance increase per 1°C decrease in temperature is typically 0 Ω / °C or greater. Within this range, even if the temperature of the oxygen pump cell (main pump cell 21) decreases during use of the gas sensor 100, the increase in the resistance of the main pump cell 21 can be suppressed. Therefore, the pump voltage Vp0 does not become excessively high, and sufficient measurement accuracy is believed to be maintained. The magnitude of the resistance value may vary depending on the configuration of the gas sensor 100, but even if the temperature of the oxygen pump cell (main pump cell 21) drops by about 40°C, it is believed that the resistance value of the main pump cell 21 can be kept below, for example, about 120 Ω.

[0118] The resistance increase [Ω / °C] per 1°C decrease in temperature of the oxygen pump cell (main pump cell 21) can be calculated, for example, as follows: Regarding the impedance between the oxygen pump electrode (inner main pump electrode 22) inside the cavity and the oxygen pump electrode (outer pump electrode 23) outside the cavity of the oxygen pump cell (main pump cell 21), the impedance at a frequency of 0.1 Hz is measured when the temperature (electrode temperature) of the oxygen pump cell (main pump cell 21) is a first temperature T1 and a second temperature T2. The first temperature T1 may be, for example, the operating temperature of the gas sensor 100. The second temperature T2 may be, for example, about 5 to 50°C lower than the first temperature T1. For example, the first temperature T1 may be the operating temperature of the gas sensor 100, and the second temperature T2 may be 40°C lower than the operating temperature of the gas sensor 100. From the resistance value R0 [R0(T1)] at 0.1 Hz at the first temperature T1 and the resistance value R0 [R0(T2)] at 0.1 Hz at the second temperature T2, the resistance increase of the resistance value R0 at 0.1 Hz per 1°C temperature decrease of the oxygen pump cell can be calculated.

[0119] Resistance increase rate (Ω / ℃) = [R0(T2)-R0(T1)] / (T1-T2)

[0120] At least one of the intra-void oxygen pump electrode (inner main pump electrode 22) and the extra-void oxygen pump electrode (outer pump electrode 23) of the oxygen pump cell (main pump cell 21) may have a thickness (electrode film thickness) of 3.5 μm to 45 μm, or 4 μm to 40 μm. The thickness of either the intra-void oxygen pump electrode (inner main pump electrode 22) or the extra-void oxygen pump electrode (outer pump electrode 23) may be within the above-mentioned range. Preferably, the thickness of the intra-void oxygen pump electrode (inner main pump electrode 22) is within the above-mentioned range. Even when the oxygen concentration in the measurement gas is high, the possibility of NOx decomposition at the inner main pump electrode 22 can be further reduced. More preferably, the thickness of both the intra-void oxygen pump electrode (inner main pump electrode 22) and the extra-void oxygen pump electrode (outer pump electrode 23) of the oxygen pump cell (main pump cell 21) is within the above-mentioned range.

[0121] The thicker the electrode thickness, the lower the resistance value R1 at 1 Hz tends to be. If the electrode thickness is 3.5 μm or more, or 4 μm or more, the ratio R1 / R0 in the oxygen pump cell (main pump cell 21) is likely to be 93% or less. In particular, if the electrode thickness of the cavity oxygen pump electrode (inner main pump electrode 22) is 45 μm or less, or 40 μm or less, it is believed that decomposition of NOx at the inner main pump electrode 22 can be prevented even when the NOx concentration in the measurement gas is high.

[0122] In this embodiment, the oxygen pump electrode inside the cavity (inner main pump electrode 22) and the oxygen pump electrode outside the cavity (outer pump electrode 23) are rectangular in plan view. The area of ​​the inner main pump electrode 22 (in this embodiment, the total area of ​​the ceiling electrode portion 22a and the bottom electrode portion 22b) is, for example, 5 mm 2 More than 20mm 2 The same applies to the outer pump electrode 23. The larger the areas of the inner main pump electrode 22 and the outer pump electrode 23, the lower the resistance value R1 at 1 Hz tends to be.

[0123] The porosity of either the intra-void oxygen pump electrode (inner main pump electrode 22) or the extra-void oxygen pump electrode (outer pump electrode 23) may be, for example, approximately 3% to 50% by volume. Within this range, the ratio R1 / R0 in the oxygen pump cell (main pump cell 21) is likely to be 93% or less. Regarding the porosity of the intra-void oxygen pump electrode (inner main pump electrode 22) and the extra-void oxygen pump electrode (outer pump electrode 23), the lower the porosity, the higher the resistance value R2 (the resistance component appearing in a frequency range of 0.1 Hz to 1 Hz), and the higher the porosity, the lower the resistance value R2.

[0124] At least one of the oxygen pump electrode inside the cavity (inner main pump electrode 22) and the oxygen pump electrode outside the cavity (outer pump electrode 23) of the oxygen pump cell (main pump cell 21) is a cermet electrode made of a metal and an oxygen ion conductive solid electrolyte, In a spectrum obtained when the surface of the at least one electrode is measured by X-ray photoelectron spectroscopy (XPS), the peak intensity ratio of silicon peak intensity to zirconium peak intensity (Si / Zr ratio) may be 20 or less.

[0125] Both the oxygen pump electrode inside the cavity (inner main pump electrode 22) and the oxygen pump electrode outside the cavity (outer pump electrode 23) are preferably cermet electrodes made of a metal and an oxygen ion conductive solid electrolyte. The ratio of the metal to the solid electrolyte in the cermet electrode can be determined appropriately by those skilled in the art, but may be, for example, about 10% by weight to 50% by weight of the solid electrolyte relative to the total weight of the metal and solid electrolyte. For example, when Pt is used as the metal and ZrO2 is used as the solid electrolyte, the weight ratio may be about Pt:ZrO2 = 9.0:1.0 to 5.0:5.0.

[0126] The inner main pump electrode 22 preferably contains, as a metal, a catalytically active noble metal (e.g., at least one of Pt, Rh, Ir, Ru, and Pd) and a noble metal (e.g., Au, Ag, etc.) that reduces the catalytic activity of the catalytically active noble metal for the measurement target gas (NOx in this embodiment). The metal component of the inner main pump electrode 22 may be, for example, Pt containing 0.1 wt.% to 30.0 wt.% Au. The outer pump electrode 23 may contain, as a metal, the above-mentioned catalytically active noble metal. The metal component of the outer pump electrode 23 may be, for example, Pt.

[0127] The oxygen ion conductive solid electrolyte may be, for example, zirconia (ZrO2), preferably stabilized zirconia (partially stabilized zirconia). Examples of stabilizers include yttria (Y2O3), calcia (CaO), and magnesia (MgO). For example, when yttria-stabilized zirconia is used, the molar concentration of yttria (Y2O3) in the yttria-stabilized zirconia may be approximately 10 mol% or less. Within this range, the higher the molar concentration of yttria (Y2O3), the lower the resistance value (R1) at 1 Hz tends to be. The molar concentration of yttria (Y2O3) may be, for example, approximately 1 mol% or more and 10 mol% or less. Within this range, the ratio (R1 / R0) in the oxygen pump cell (main pump cell 21) is likely to be 93% or less.

[0128] At least one of the oxygen pump electrode inside the cavity (inner main pump electrode 22) and the oxygen pump electrode outside the cavity (outer pump electrode 23) of the oxygen pump cell (main pump cell 21) may further contain silicon (Si). For example, the solid electrolyte of at least one of the electrodes may contain zirconium atoms and silicon atoms. Silicon (Si) functions as a sintering aid for the electrodes during the manufacture of the sensor element 101.

[0129] Preferably, in a spectrum obtained by measuring the surface of the electrode by X-ray photoelectron spectroscopy (XPS), the peak intensity ratio of the silicon (Si) peak intensity to the zirconium (Zr) peak intensity (Si / Zr ratio) is 20 or less. In other words, the abundance ratio of the silicon (Si) to the zirconium (Zr) on the surface of the electrode (Si / Zr ratio) is 20 or less. The resistance value R1 at 1 Hz tends to decrease as the Si concentration on the intra-cavity oxygen pump electrode (inner main pump electrode 22) and the extra-cavity oxygen pump electrode (outer pump electrode 23) decreases, i.e., as the Si / Zr ratio decreases. Therefore, it is preferable that the Si / Zr ratio is 20 or less in at least one of the intra-cavity oxygen pump electrode (inner main pump electrode 22) and / or extra-cavity oxygen pump electrode (outer pump electrode 23). Within this range, the ratio R1 / R0 in the oxygen pump cell (main pump cell 21) is likely to be 93% or less. The Si / Zr ratio may be, for example, 18 or less, 15 or less, 10 or less, etc. The lower limit of the Si / Zr ratio is not particularly limited, but may be equal to or greater than 0. A Si / Zr ratio of 0 means that no Si is contained.

[0130] As described above, the Si / Zr ratio is the abundance ratio of silicon Si to zirconium Zr on the electrode surface of the oxygen pump electrode inside the cavity (inner main pump electrode 22) and / or the oxygen pump electrode outside the cavity (outer pump electrode 23). When the abundance (element concentration) of zirconium Zr (zirconia ZrO2) is equal to the abundance (element concentration) of Si (SiO2) on the electrode surface (surface of the solid electrolyte present on the electrode surface), the Si / Zr ratio is 1.

[0131] The Si / Zr ratio is the peak intensity ratio of silicon to zirconium in a spectrum obtained when the surface of the electrode is measured by X-ray photoelectron spectroscopy (XPS). It can be determined by calculating the peak intensity ratio of Si to Zr using the relative sensitivity factor method from the peak intensities of the detected peaks for Si and Zr in a spectrum obtained when the surface of the cavity oxygen pump electrode (inner main pump electrode 22) is measured by XPS (X-ray photoelectron spectroscopy). The peak intensity ratio can be determined by calculating the peak area ratio of Si to Zr.

[0132] Furthermore, with regard to the diffusion resistance of oxygen gas reaching the oxygen pump cell (main pump cell 21), the resistance R2 (resistance component appearing in a frequency range of 0.1 Hz or more and less than 1 Hz) tends to increase as the diffusion resistance of oxygen gas reaching the inner main pump electrode 22 (i.e., the diffusion resistance of oxygen gas in the measurement gas introduced into the first internal space 20) increases, and the resistance R2 (resistance component appearing in a frequency range of 0.1 Hz or more and less than 1 Hz) tends to decrease as the diffusion resistance decreases. In this embodiment, this generally corresponds to the diffusion resistance from the gas inlet 10 to the fourth diffusion-controlling passage 13 (the gas inlet 10, the first diffusion-controlling passage 11, the buffer space 12, and the fourth diffusion-controlling passage 13). The magnitude of this diffusion resistance can be adjusted, for example, by changing the longitudinal length or the cross-sectional area (width / thickness) perpendicular to the longitudinal direction of the slits of the first diffusion-controlling passage 11 and / or the fourth diffusion-controlling passage 13.

[0133] The limiting current value in the oxygen pump cell (main pump cell 21) may be used as an indicator of the diffusion resistance. When a pump voltage Vp0 is applied between the inner main pump electrode 22 and the outer pump electrode 23 to pump oxygen from the first internal space 20 while the measurement gas, such as air (or a model gas with an oxygen concentration of 20.5%), is introduced into the first internal space 20, the pump current Ip0 increases as the pump voltage Vp0 increases while the pump voltage Vp0 is low. As the pump voltage Vp0 subsequently increases, the pump current Ip0 no longer increases even with an increase in the pump voltage Vp0 and reaches saturation. The saturated current value at this time is referred to as the limiting current value. The region where the pump current Ip0 becomes the limiting current value for the pump voltage Vp0 is referred to as the limiting current region. The limiting current value of the pump current Ip0 corresponds to the amount of gas reaching the inner main pump electrode 22. In other words, the larger the diffusion resistance for oxygen gas, the smaller the limiting current value. The smaller the diffusion resistance for oxygen gas, the larger the limiting current value. The limiting current value of the pump current Ip0 may be, for example, 0.4 mA to 4.5 mA, or 0.5 mA to 4.0 mA. Within this range, the ratio R1 / R0 in the oxygen pump cell (main pump cell 21) is likely to be 93% or less.

[0134] Furthermore, the value of the pump current Ip0 flowing through the oxygen pump cell (main pump cell 21) when the gas sensor 100 is driven can be used as an indicator of the diffusion resistance. The term "driving the gas sensor 100" refers to a state in which the control device 90 of the gas sensor 100 performs the above-described control and detects the NOx concentration in the measurement gas. As described above, the value of the pump current Ip0 varies depending on the oxygen concentration in the measurement gas. However, the value of the pump current Ip0 at a certain oxygen concentration depends on the magnitude of the diffusion resistance for the measurement gas introduced into the first internal space 20, and therefore can be used as an indicator of the diffusion resistance. For example, the pump current Ip0 when the measurement gas is air (or a model gas with an oxygen concentration of 20.5%) may be used. The value of the pump current Ip0 at this time is considered to be approximately equal to the limiting current value.

[0135] When a measurement gas having an oxygen concentration of 20.5% is introduced into the first internal space 20, the pump current Ip0 (referred to as the pump current Ip0 at an oxygen concentration of 20.5%) flowing through the oxygen pump cell (main pump cell 21) when the oxygen pump cell (main pump cell 21) pumps out oxygen from the first internal space 20 may be used as an index of the diffusion resistance. The pump current Ip0 at an oxygen concentration of 20.5% may be, for example, 0.4 mA to 4.5 mA, or 0.5 mA to 4.0 mA. Within these ranges, the ratio R1 / R0 in the oxygen pump cell (main pump cell 21) is likely to be 93% or less.

[0136] When the oxygen pump cell (main pump cell 21) is used to pump out oxygen from the first internal space 20, as described above, the pump control section 93 determines whether the voltage V0 at the main pump control oxygen partial pressure detection sensor cell 80 is equal to or greater than the set value V0. SET The pump voltage Vp0 of the variable power supply 24 in the main pump cell 21 is feedback-controlled so that the oxygen in the atmosphere introduced into the first internal space 20 is almost entirely pumped out. The pump voltage Vp0 applied at this time may vary depending on the configuration and individual gas sensor 100, but may be, for example, about 300 mV to 600 mV.

[0137] Similarly to the solid electrolyte in the electrodes, the solid electrolyte (second solid electrolyte layer 6) sandwiched between the intra-void oxygen pump electrode (inner main pump electrode 22) and the extra-void oxygen pump electrode (outer pump electrode 23) may be, for example, zirconia (ZrO2), preferably stabilized zirconia (partially stabilized zirconia). Examples of stabilizers include yttria (Y2O3), calcia (CaO), and magnesia (MgO). For example, when yttria-stabilized zirconia is used, the molar concentration of yttria (Y2O3) in the yttria-stabilized zirconia may be approximately 10 mol% or less. Within this range, the higher the molar concentration of yttria (Y2O3), the lower the resistance value R1 at 1 Hz tends to be. The molar concentration of yttria (Y2O3) may be, for example, approximately 1 mol% or more and 10 mol% or less. Within this range, the ratio R1 / R0 in the oxygen pump cell (main pump cell 21) is likely to be 93% or less.

[0138] As described above, in the oxygen pump cell (main pump cell 21) of the sensor element 101 of the present invention, the ratio R1 / R0 of the resistance value R1 at 1 Hz to the resistance value R0 at 0.1 Hz is 93% or less with respect to the impedance between the inner main pump electrode 22 of the cavity oxygen pump electrode and the outer pump electrode 23 of the cavity oxygen pump electrode. Setting the ratio R1 / R0 within this range reduces the temperature dependence of the resistance value of the oxygen pump cell (main pump cell 21). Therefore, even if the temperature of the sensor element 101 changes (particularly decreases) for some reason during use of the gas sensor 100, the pump voltage Vp0 can be prevented from becoming excessively high. As a result, even if the temperature of the sensor element 101 changes (particularly decreases) due to the influence of the external environment (such as the temperature, flow rate, or flow velocity of the measurement target gas) or contact failure (such as contact failure within the gas sensor 100 or contact failure between the gas sensor 100 and an external power source), the aforementioned degradation in measurement accuracy can be suppressed.

[0139] Although the sensor element 101 for detecting the NOx concentration in a measurement gas has been described above as an example of an embodiment of the present invention, the present invention is not limited to this embodiment. The present invention can include various types of sensor elements as long as they achieve the object of the present invention of maintaining high measurement accuracy even when the temperature of the sensor element changes.

[0140] In the above-described embodiment, the gas sensor 100 detects the NOx concentration in the measurement gas, but the measurement target gas is not limited to NOx. The measurement target gas may be, for example, an oxide gas other than oxygen O or NOx (e.g., carbon dioxide CO, water HO, etc.). Alternatively, it may be a non-oxide gas such as ammonia NH.

[0141] In the above-described embodiment, the gas sensor 100 is a so-called limiting current type NOx sensor, but the present invention is not limited to this. It is sufficient that the gas sensor has an internal space into which the measurement gas is introduced via a diffusion-limited passage and an oxygen pump cell capable of pumping oxygen from the internal space. A limiting current type oxygen sensor may also be used. Furthermore, when the measurement target gas is NOx, for example, the NOx concentration may be detected by measuring the electromotive force V2 between the measurement electrode 44 and the reference electrode 42 instead of using the pump current Ip2 in the measurement pump cell 41. The same applies to other oxide gases (e.g., carbon dioxide CO2, water HO, etc.) and non-oxide gases such as ammonia NH3.

[0142] In the sensor element 101 of the above-described embodiment, the inner main pump electrode 22 is composed of a ceiling electrode portion 22a formed on the ceiling surface of the first internal space 20, a bottom electrode portion 22b formed on the bottom surface of the first internal space 20, and a side electrode portion formed on the side surface of the first internal space 20 so as to connect the ceiling electrode portion 22a and the bottom electrode portion 22b. However, this is not limited to this. The inner main pump electrode 22 may be formed only on the ceiling surface of the first internal space 20. Alternatively, it may be formed only on the bottom surface of the first internal space 20. Furthermore, for example, when the inner main pump electrode 22 has a ceiling electrode portion 22a and a bottom electrode portion 22b, the ceiling electrode portion 22a and the bottom electrode portion 22b may have the same size or different sizes. The same applies to the auxiliary pump electrode 51. Furthermore, in the sensor element 101 of the above-described embodiment, the measurement electrode 44 is formed on the bottom surface of the third internal space 61. However, this is not limited to this. It may be formed on the ceiling surface of the third inner space 61, or may be formed in a tunnel shape like the inner main pump electrode 22.

[0143] In the above-described embodiment, the sensor element 101 has three internal cavities, namely, the first internal cavities 20, the second internal cavities 40, and the third internal cavities 61, and the inner main pump electrode 22, the auxiliary pump electrode 51, and the measurement electrode 44 are respectively disposed in each internal cavities, as shown in FIG. 1 . However, the present invention is not limited to this. For example, the sensor element 101 may have two internal cavities, namely, the first internal cavities 20 and the second internal cavities 40, and the inner main pump electrode 22 is disposed in the first internal cavities 20, and the auxiliary pump electrode 51 and the measurement electrode 44 are disposed in the second internal cavities 40. In this case, for example, a porous protective layer covering the measurement electrode 44 may be formed as a diffusion rate-controlling part between the auxiliary pump electrode 51 and the measurement electrode 44. Furthermore, for example, the number of internal cavities may be one or four or more.

[0144] In the gas sensor 100 according to the above-described embodiment, the outer pump electrode 23 functions as three electrodes: the outer main pump electrode in the main pump cell 21, the outer auxiliary pump electrode in the auxiliary pump cell 50, and the outer measurement electrode in the measurement pump cell 41. However, this is not limiting. For example, the outer main pump electrode, the outer auxiliary pump electrode, and the outer measurement electrode may be formed as separate electrodes. For example, one or more of the outer main pump electrode, the outer auxiliary pump electrode, and the outer measurement electrode may be provided separately from the outer pump electrode 23 on the outer surface of the base portion 102 so as to be in contact with the gas to be measured. Furthermore, the reference electrode 42 may also function as one or two of the outer main pump electrode, the outer auxiliary pump electrode, and the outer measurement electrode.

[0145] In addition to the above configuration, the components of the sensor element 101, such as the measurement target gas flow space 15 and the electrodes, can take various forms depending on the type of gas to be measured, the purpose and environment of use of the gas sensor, etc.

[0146] [Gas sensor manufacturing method] Next, an example of a method for manufacturing the above-mentioned gas sensor will be described. After performing predetermined processing and printing a circuit pattern on a plurality of unfired sheet-shaped molded articles (so-called green sheets) containing an oxygen-ion conductive solid electrolyte such as zirconia (ZrO2) as a ceramic component, the plurality of sheets are stacked, cut, and then fired to produce a sensor element. The fabricated sensor elements can then be assembled into a gas sensor.

[0147] In the following, an example will be described in which the sensor element 101 made up of six layers as shown in FIG. 1 is fabricated.

[0148] First, six green sheets containing an oxygen-ion conductive solid electrolyte such as zirconia (ZrO2) as a ceramic component are prepared. A known forming method can be used to fabricate the green sheets. All six green sheets may be the same thickness, or the thickness may vary depending on the layer being formed. Holes and other properties used for positioning during printing and lamination are formed in each of the six green sheets in advance using a known method, such as punching with a punching device, to create blank sheets. In the blank sheet used for the spacer layer 5, through-holes such as internal voids are also formed in the same manner. Necessary through-holes are also formed in the other layers in advance.

[0149] Various patterns required for each layer are printed and dried on blank sheets used for the six layers: first substrate layer 1, second substrate layer 2, third substrate layer 3, first solid electrolyte layer 4, spacer layer 5, and second solid electrolyte layer 6. Known screen printing techniques can be used to print the patterns. Known drying means can also be used for the drying process.

[0150] A paste may be prepared for each pattern to be printed, and a coating film may be formed by screen printing. For example, an electrode paste to be used for forming the inner main pump electrode 22 is prepared to have a desired composition. The electrode paste is prepared by mixing, for example, metal powder (Pt and Au in this embodiment), solid electrolyte powder (yttria-stabilized zirconia in this embodiment), and, if necessary, a sintering aid (silica SiO2 in this embodiment), an organic binder, an organic solvent, and the like. Next, the electrode paste for the inner main pump electrode 22 is printed in a desired pattern at a desired position on the surface that will become the lower surface of the second solid electrolyte layer 6 and the surface that will become the upper surface of the first solid electrolyte layer 4, and then dried. The other electrodes are similarly printed in a desired pattern at a desired position and then dried. The order of these printing steps can be determined appropriately.

[0151] This process is repeated until various patterns have been printed and dried on each of the six blank sheets. The six printed blank sheets are then stacked in a predetermined order while being positioned using sheet holes, etc., and then pressed under predetermined temperature and pressure conditions to form a laminate. The pressing process is carried out by applying heat and pressure using a laminator such as a known hydraulic press. The temperature, pressure, and time for heating and pressing depend on the laminator used, but can be determined appropriately to achieve good lamination.

[0152] The obtained laminate contains a plurality of sensor elements 101. The laminate is cut into individual sensor elements 101. The cut laminate is fired at a predetermined firing temperature to obtain the sensor elements 101. That is, the sensor elements 101 are obtained by integrally firing the solid electrolyte layer and the electrodes. The firing temperature may be any temperature at which the solid electrolyte constituting the base portion 102 of the sensor element 101 is sintered into a dense body and the electrodes and the like maintain a desired porosity. For example, firing is performed at a firing temperature of about 1200°C or higher and 1500°C or lower.

[0153] The obtained sensor element 101 is incorporated into the gas sensor 100 in such a manner that the front end of the sensor element 101 contacts the gas to be measured and the rear end of the sensor element 101 contacts the reference gas.

[0154] [Gas sensor performance evaluation method] As described above, the inventors have discovered that by setting the ratio R1 / R0 of the resistance value R1 at 1 Hz to the resistance value R0 at 0.1 Hz in the oxygen pump cell (main pump cell 21) to 93% or less, the temperature dependence of the resistance value of the oxygen pump cell (main pump cell 21) can be reduced, and as a result, even if the temperature of the sensor element 101 changes (particularly decreases), the above-mentioned deterioration in measurement accuracy can be suppressed.

[0155] With reference to the sensor element of the present invention, by evaluating the temperature dependency of the resistance value of the oxygen pump cell (main pump cell 21), the performance of the sensor element (gas sensor) can be evaluated from the perspective of whether measurement accuracy can be maintained even when the temperature of the sensor element 101 changes (particularly decreases). As described above, there is a relationship between the ratio R1 / R0 of the resistance value R1 at 1 Hz to the resistance value R0 at 0.1 Hz and the temperature dependency of the resistance value of the oxygen pump cell (main pump cell 21). Therefore, for example, for gas sensors to be developed in the future, by measuring the value of the ratio R1 / R0, the performance of the sensor element (gas sensor) can be evaluated from the perspective of whether measurement accuracy can be maintained even when the temperature of the sensor element 101 changes (particularly decreases).

[0156] An embodiment of the present invention includes the following method for evaluating the performance of a gas sensor (sensor element).

[0157] (101) A method for evaluating the performance of a sensor element (or a gas sensor including the sensor element) that detects a target gas in a measurement gas, comprising: The sensor element is a long plate-shaped substrate including an oxygen ion conductive solid electrolyte layer; a measurement gas flow space having a gas inlet opening on the surface of the base portion and a first internal space communicating with the gas inlet through a first diffusion-controlling passage; an oxygen pump cell including an in-void oxygen pump electrode disposed in the first internal space of the measurement gas flow space, and an outside-void oxygen pump electrode disposed at a position of the base portion different from the measurement gas flow space and corresponding to the in-void oxygen pump electrode; Including, The evaluation method includes: With regard to the impedance between the oxygen pump electrode inside the cavity and the oxygen pump electrode outside the cavity of the oxygen pump cell, the impedance at a frequency of 1 Hz and the impedance at a frequency of 0.1 Hz are measured; When the real part of the impedance at a frequency of 1 Hz is represented as a resistance value R1 at 1 Hz and the real part of the impedance at a frequency of 0.1 Hz is represented as a resistance value R0 at 0.1 Hz, a ratio R1 / R0 of the resistance value R1 at 1 Hz to the resistance value R0 at 0.1 Hz is calculated; A performance evaluation method including determining that the sensor element has high performance (high measurement accuracy) when the ratio R1 / R0 is 93% or less.

[0158] (102) A method for evaluating the performance of a sensor element (or a gas sensor including the sensor element) that detects a target gas in a measurement gas, comprising: The sensor element is a long plate-shaped substrate including an oxygen ion conductive solid electrolyte layer; a measurement gas flow space having a gas inlet opening on the surface of the base portion and a first internal space communicating with the gas inlet through a first diffusion-controlling passage; an oxygen pump cell including an in-void oxygen pump electrode disposed in the first internal space of the measurement gas flow space, and an outside-void oxygen pump electrode disposed at a position of the base portion different from the measurement gas flow space and corresponding to the in-void oxygen pump electrode; Including, The evaluation method includes: Regarding the impedance between the oxygen pump electrode inside the cavity and the oxygen pump electrode outside the cavity of the oxygen pump cell, the impedance is measured at a frequency of 0.1 Hz when the temperature of the oxygen pump cell is at a first temperature T1 and a second temperature T2, respectively; When the real part of the impedance at the frequency of 0.1 Hz is expressed as a resistance value R0 at 0.1 Hz, an increase in resistance of the resistance value R0 at 0.1 Hz per 1°C decrease in temperature of the oxygen pump cell is calculated; A performance evaluation method including determining that the sensor element has high performance (high measurement accuracy) when the resistance increase is less than 1 Ω / °C.

[0159] The performance evaluation method may include both the above (101) and (102).

[0160] (103) When a measurement gas having an oxygen concentration of 20.5% is introduced into the first internal space, a current value of a pump current flowing through the oxygen pump cell is measured when the oxygen pump cell pumps out oxygen from the first internal space; The performance evaluation method according to (101) or (102) above, further comprising determining that the sensor element has high performance (high measurement accuracy) when the current value of the pump current is 0.5 mA or more and 4.0 mA or less.

[0161] Moreover, the performance evaluation methods (101) to (103) above may further include the requirements described in (3) and / or (4) above as evaluation items. [Example]

[0162] The present invention will be further explained below using examples, but it should be noted that the present invention is not limited to the following examples.

[0163] [Fabrication of gas sensors] As examples, gas sensors 100 of Samples Nos. 1 to 23 were fabricated according to the method for manufacturing the gas sensor 100 described above.

[0164] In sample No. 1, the pump current Ip0 when measuring a measurement gas with an oxygen concentration of 20.5% (pump current Ip0 at an oxygen concentration of 20.5%) was set to 3.4 mA. As described above, the pump current Ip0 at an oxygen concentration of 20.5% is an index of the diffusion resistance of oxygen gas reaching the inner main pump electrode 22 of the main pump cell 21.

[0165] In Sample No. 1, the thickness of the inner main pump electrode 22 was 12 μm, and the Si / Zr ratio was 6.

[0166] The Si / Zr ratio was measured by X-ray photoelectron spectroscopy (XPS). In the obtained spectrum, it was calculated using the relative response factor method from the peak intensities (i.e., peak areas) of the detected peaks for Si and Zr. The atomic relative response factor (ARSF) was used as the relative response factor. The measurement conditions were as follows:

[0167] Measurement equipment: ULVAC-PHI Inc. ESCA-5600ci X-ray source: MgKα ray, 400W Detection area: 400 μmφ Detection angle: 70° Detected spectrum (detected peaks): Zr3d, Si2p

[0168] In Sample No. 1, the electrode temperature during operation of the gas sensor was set to 700° C. Here, the electrode temperature means the surface temperature at the center of the outer pump electrode 23 when the sensor element 101 is viewed from above.

[0169] Samples Nos. 2 to 23 were prepared in the same manner as Sample No. 1, except that the pump current Ip0 at an oxygen concentration of 20.5%, the thickness and Si / Zr ratio of the inner main pump electrode 22, and the electrode temperature during gas sensor operation were different. Table 1 below shows the respective values ​​for Samples Nos. 1 to 23.

[0170] In Samples Nos. 1 to 23, the pump current Ip0 at an oxygen concentration of 20.5% was set to a value in the range of 0.3 mA to 4.5 mA.

[0171] In Samples Nos. 1 to 23, the thickness of the inner main pump electrode 22 was set to a range of 3 μm to 40 μm, and the thickness of the outer pump electrode 23 was set to 12 μm in all of Samples Nos. 1 to 23.

[0172] In Samples No. 1 to 23, the Si / Zr ratio of the inner main pump electrode 22 was set to a value within the range of 0 to 21. Here, the Si / Zr ratio is the peak area ratio of the Si peak area to the Zr peak area in the spectrum obtained by XPS measurement. The Si / Zr ratio was varied by changing the amount of SiO2 added to the electrode paste of the inner main pump electrode 22. The Si / Zr ratio of the outer pump electrode 23 was set to 6 in all of Samples No. 1 to 23.

[0173] In Samples Nos. 1 to 23, the electrode temperature during operation of the gas sensor was set to 700° C., except for Samples Nos. 2 and 3. Samples Nos. 2 and 3 were set to 660° C. and 620° C., respectively.

[0174] Other than the above, the configurations of Samples Nos. 1 to 23 were all the same.

[0175] [AC impedance measurement] The AC impedance of each of the fabricated gas sensors 100 of Samples No. 1 to 23 was measured. A VersaSTAT4 Model-100 (manufactured by AMETEK) was used for the measurement. The measurement was performed by a two-terminal method, with WE / SE wires connected to the inner main pump electrode 22 and CE / RE wires connected to the outer pump electrode 23. The measurement was performed in an atmospheric environment. The frequencies ranged from 0.1 Hz to 10 6 Measurements were carried out by varying the frequency from 0.1 Hz to 100 Hz. From the frequency characteristics of the measured impedance, the resistance value R0 at 0.1 Hz, the resistance value R1 at 1 Hz, and the resistance value R2 (= R0 - R1) were calculated, and the ratio R1 / R0 was determined. Table 1 below shows the resistance values ​​R0, R1, R2, and the ratio R1 / R0 for Samples No. 1 to 23. Samples No. 1, 2, 4 to 10, and 12 to 23 are examples (ratio R1 / R0 is 93% or less), and Samples No. 3 and 11 are comparative examples (ratio R1 / R0 is more than 93%).

[0176] [Evaluation of resistance change due to temperature drop] For each of the gas sensors 100 samples 1 to 23, an evaluation test was conducted to simulate cooling of the sensor element 101 due to the external environment (temperature, flow rate, flow velocity, etc. of the gas to be measured). Specifically, the sensor element 101 was heated by the heater 72 so that the electrode temperature was 40°C lower than the electrode temperature when the gas sensor was operating, and the resistance value R0 at 0.1 Hz was measured using the same device as in the AC impedance measurement described above. The rate of increase in resistance per 1°C of temperature decrease was calculated using the following equation.

[0177] Resistance change rate (Ω / ℃) = [R0(cooled)-R0(normal)] / 40 Here, R0(cooled) is the resistance value R0 at an electrode temperature 40° C. lower than the electrode temperature when the gas sensor is in operation, and R0(normal) is the resistance value R0 at the electrode temperature when the gas sensor is in operation.

[0178] Table 1 shows the resistance value R0 (cooled) and the resistance change rate (Ω / °C) at an electrode temperature 40°C lower than the electrode temperature when the gas sensor is operated.

[0179] [Table 1]

[0180] As described above, it has been confirmed that when the ratio R1 / R0 is 93% or less, the rate of resistance change per 1°C temperature drop is less than 1 Ω / °C, and the temperature dependency of the resistance value of the main pump cell 21 can be reduced. Therefore, even if the temperature (electrode temperature) of the sensor element 101 drops during use of the gas sensor 100 due to the external environment or the like, an excessive increase in the pump voltage Vp0 can be prevented, and a decrease in measurement accuracy can be suppressed.

[0181] Furthermore, it was confirmed that the resistance value R0(cooled) itself at an electrode temperature 40° C. lower than the electrode temperature during gas sensor operation was further reduced (to 120Ω or less) by setting the thickness of the inner main pump electrode 22 to 4 μm or more, the Si / Zr ratio to 20 or less, or the pump current Ip0 to 0.5 mA or more. This further reduced the increase in the pump voltage Vp0 and further suppressed the deterioration of measurement accuracy.

[0182] As described above, according to the present invention, the temperature dependency of the resistance value of the oxygen pump cell (main pump cell 21) can be reduced, and therefore, even if the temperature of the sensor element 101 changes (particularly decreases) for some reason during use of the gas sensor 100, it is possible to prevent the pump voltage Vp0 from becoming excessively high. As a result, even if the temperature of the sensor element 101 changes (particularly decreases), high measurement accuracy can be maintained. [Explanation of symbols]

[0183] 1 First board layer 2 Second board layer 3 Third board layer 4 First solid electrolyte layer 5 Spacer layer 6 Second solid electrolyte layer 10 Gas inlet 11 First diffusion-controlled pathway 12 Buffer space 13 Fourth diffusion-controlled pathway 15 Measurement gas flow space 20 1st internal void 21 Main pump cell 22 Inner main pump electrode 22a (Inner main pump electrode) ceiling electrode part 22b Bottom electrode part (of inner main pump electrode) 23 Outer pump electrode 24 Variable power supply (for main pump cell) 30 Second diffusion-controlled passage 40 Second internal void 41 Measuring pump cell 42 Reference electrode 43 Reference gas introduction space 44 Measuring electrode 46 Variable power supply (for measuring pump cell) 48 Atmospheric introduction layer 50 Auxiliary pump cell 51 Auxiliary pump electrode 51a (auxiliary pump electrode) ceiling electrode part 51b (auxiliary pump electrode) bottom electrode part 52 Variable power supply (for auxiliary pump cell) 60 Third diffusion-controlled passage 61 3rd internal void 70 Heater section 71 Heater electrode 72 Heater 73 through holes 74 Heater insulator 75 Pressure relief hole 76 Heater lead 80 Oxygen partial pressure detection sensor cell for main pump control 81 Oxygen partial pressure detection sensor cell for auxiliary pump control 82 Oxygen partial pressure detection sensor cell for measuring pump control 83 Sensor Cell 100 Gas Sensor 101 Sensor element 102 Base

Claims

1. A sensor element for detecting a measurement target gas in a measurement gas, The sensor element is a long plate-shaped substrate including an oxygen ion conductive solid electrolyte layer; a measurement gas flow space having a gas inlet opening on the surface of the base portion and a first internal space communicating with the gas inlet through a first diffusion-controlling passage; an oxygen pump cell including an intra-void oxygen pump electrode disposed in the first internal space of the measurement gas flow space, and an extra-void oxygen pump electrode disposed at a position of the base portion different from the measurement gas flow space and corresponding to the intra-void oxygen pump electrode; Including, A sensor element in which, with regard to the impedance between the oxygen pump electrode inside the cavity and the oxygen pump electrode outside the cavity of the oxygen pump cell, when the real part of the impedance at a frequency of 1 Hz is expressed as the resistance value at 1 Hz R1 and the real part of the impedance at a frequency of 0.1 Hz is expressed as the resistance value at 0.1 Hz R0, the ratio R1 / R0 of the resistance value R1 at 1 Hz to the resistance value R0 at 0.1 Hz is 93% or less.

2. 2. The sensor element of claim 1, wherein the resistance R0 between the oxygen pump electrode inside the cavity and the oxygen pump electrode outside the cavity of the oxygen pump cell at 0.1 Hz increases by less than 1 Ω / °C per 1°C decrease in temperature of the oxygen pump cell.

3. 2. The sensor element according to claim 1, wherein at least one of the intra-space oxygen pump electrode and the extra-space oxygen pump electrode has a thickness of 4 [mu]m or more and 40 [mu]m or less.

4. at least one of the intra-cavity oxygen pump electrode and the extra-cavity oxygen pump electrode is a cermet electrode made of a metal and an oxygen ion conductive solid electrolyte, 2. The sensor element according to claim 1, wherein in a spectrum obtained when the surface of at least one of the electrodes is measured by X-ray photoelectron spectroscopy (XPS), the peak intensity ratio of a silicon peak intensity to a zirconium peak intensity is 20 or less.

5. 2. The sensor element according to claim 1, wherein when a measurement gas having an oxygen concentration of 20.5% is introduced into the first internal space, a current value of a pump current flowing through the oxygen pump cell when the oxygen pump cell pumps out oxygen from the first internal space is 0.5 mA or more and 4.0 mA or less.

6. a second internal space communicating with the first internal space via a second diffusion-controlled passage; an auxiliary pump cell including an in-cavity auxiliary pump electrode disposed in the second inner cavity of the measurement gas flow cavity, and an outside-cavity auxiliary pump electrode disposed at a position of the base portion different from the measurement gas flow cavity, the outside-cavity auxiliary pump electrode corresponding to the in-cavity auxiliary pump electrode; a measurement pump cell including: an in-space measurement electrode disposed in the second internal space of the measurement gas flow space at a position farther from the second diffusion-controlling passage than the in-space auxiliary pump electrode, or in the third internal space communicating with the second internal space via a third diffusion-controlling passage; and an out-of-space measurement electrode disposed in a position different from the measurement gas flow space of the base portion, the out-of-space measurement electrode corresponding to the in-space measurement electrode; a reference gas chamber formed inside the base portion and separated from the measurement gas flow space; a reference electrode disposed in the reference gas chamber; The sensor element of claim 1 further comprising:

7. 10. A gas sensor comprising: the sensor element according to claim 1; and a control device for controlling said sensor element, said gas sensor detecting a gas to be measured in a gas to be measured.

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