Gas sensor

Abstract

To provide a gas sensor that can be favorably used even under rich atmosphere.SOLUTION: The sensor element comprises: a plurality of inner voids sequentially communicating with each other from a gas introduction port under predetermined diffusion resistance, and each including an inner electrode; an outer electrode arranged in a site other than the inner void; a porous body region covering the outer electrode; and a plurality of electrochemical pump cells capable of taking in or discharging oxygen between the corresponding inner void and the outside of the element. A ratio A / B is 0.07 or more, where A denotes the amplitude of limit current when a main pump cell takes in oxygen into the first inner void in a case where a pump-in current evaluation gas with known oxygen concentration is introduced, the main pump cell including a main pump electrode as the inner electrode included in a first inner void positioned at a tip of the introduction port and an out-of-void pump electrode; and B denotes the amplitude of limit current when oxygen is taken out from the first inner void in a case where a pump-out current evaluation gas with known oxygen concentration is introduced.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a limiting current type gas sensor, and more particularly to a gas sensor used in a rich atmosphere.

Background Art

[0002] For example, a limiting current type gas sensor (e.g., a NOx sensor, an oxygen sensor) using a sensor element mainly composed of an oxygen ion conductive solid electrolyte such as yttria-stabilized zirconia is already known. In such a gas sensor, the gas to be measured is introduced into a cavity (internal cavity) provided inside the sensor element. Then, control is performed to maintain the potential difference between an inner electrode provided facing the internal cavity and a reference electrode provided inside the element and in contact with a reference gas at a predetermined value corresponding to a desired oxygen concentration in the cavity.

[0003] Such control is generally performed in an electrochemical pump cell composed of an inner electrode, an outer electrode (outer cavity electrode) provided outside the cavity, and a solid electrolyte region existing between the two electrodes, by applying a pumping voltage between the two electrodes and performing oxygen pumping in or out between the internal cavity and the outside. By applying such a pumping voltage, an oxygen pumping current having a magnitude and direction corresponding to the oxygen concentration in the cavity flows between the inner electrode and the outer electrode.

[0004] As an example of such a gas sensor, a gas sensor provided with an outer electrode on the outer surface of a sensor element and provided with a ceramic layer such that a slit portion for imparting a predetermined diffusion resistance is formed around the outer electrode, and further a gas sensor having enhanced mechanical strength of the slit portion by embedding a porous body are already known (see, for example, Patent Document 1).

[0005] Also, a gas sensor including a sensor element configured such that an oxygen concentration detection cell and an oxygen pump cell are laminated in the element thickness direction with an insulating layer interposed therebetween, and the detection gas is introduced therein through a diffusion rate limiting portion composed of a porous body provided in a part of the insulating layer is already known (see, for example, Patent Document 2).

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0007] There are cases where the above-mentioned limiting current type gas sensor is used in an environment where rich gas with an air-fuel ratio smaller than the theoretical air-fuel ratio can be introduced into the element, such as in the middle of the exhaust path from a gasoline engine.

[0008] In such a case, when rich gas is introduced into the internal cavity, in the electrochemical pump cell, usually, an operation (pumping operation) is performed to draw oxygen from outside the element into the internal cavity in order to keep the oxygen concentration in the cavity constant. That is, a pumping voltage is applied so that oxygen is drawn into the internal cavity (so that oxygen ions move from outside the element to the internal cavity), and an oxygen pumping current corresponding thereto flows between the inner electrode and the outer electrode.

[0009] When this oxygen is drawn in, the larger the amount of rich gas introduced into the internal cavity, the greater the pumping voltage and the greater the tendency for the oxygen pump current to increase. However, if the richness of the gas to be measured becomes excessively large, it becomes difficult to draw in oxygen from the outside in response to the increase in the pumping voltage, and instead, oxygen in the solid electrolyte may be extracted, resulting in so-called blackening (darkening). Blackening is an irreversible phenomenon, and once it occurs, the gas sensor can no longer be used.

[0010] Such blackening is more likely to occur in gas sensors with a large diffusion resistance around the outer electrode, such as the gas sensor disclosed in Patent Document 1 having a configuration in which the outer electrode is covered with a ceramic layer or the gas sensor having a configuration in which the periphery of the outer electrode is covered with a porous body.

[0011] The present invention has been made in view of the above problems, and an object thereof is to provide a gas sensor that can be suitably used even in a rich atmosphere.

Means for Solving the Problems

[0012] In order to solve the above problems, a first aspect of the present invention is a gas sensor configured to be able to detect a predetermined gas component in a gas to be measured, comprising a sensor element composed of an oxygen ion conductive solid electrolyte, and a controller for controlling the operation of the gas sensor. The sensor element includes a plurality of internal cavities that communicate sequentially from an inlet of the gas to be measured under a predetermined diffusion resistance, and each of which is provided with an inner electrode, a cavity outer pump electrode disposed at a location other than the plurality of internal cavities, a porous body region covering the cavity outer pump electrode, and a plurality of electrochemical pump cells configured to be able to pump in or pump out oxygen between a corresponding internal cavity of the plurality of internal cavities and the outside of the sensor element by applying a pump voltage with a predetermined pump power supply between each of the inner electrodes and the cavity outer pump electrode. The plurality of internal cavities include a first internal cavity located at the foremost position from the inlet and provided with a main pump electrode as the inner electrode, and a measurement internal cavity located at the innermost position from the inlet and provided with a measurement electrode as the inner electrode. The plurality of electrochemical pump cells include a main pump cell including the main pump electrode and the cavity outer pump electrode, and a measurement pump cell including the measurement electrode and the cavity outer pump electrode. The controller adjusts the oxygen concentration in the corresponding internal cavity of the plurality of internal cavities by controlling the operation of the plurality of electrochemical pump cells except the measurement pump cell, controls the operation of the measurement pump cell so that a measurement pump current corresponding to the concentration of the predetermined gas component flows between the measurement electrode and the cavity outer pump electrode, and specifies the concentration of the predetermined gas component based on the magnitude of the measurement pump current. When a pump-in current evaluation gas with a known oxygen concentration is introduced from the inlet into the plurality of internal cavities, the magnitude of the reference pump-in current, which is the limiting current when the main pump cell pumps in oxygen into the first internal cavity based on the control of the controller, is defined as A. When a pump-out current evaluation gas with a known oxygen concentration is introduced from the inlet into the plurality of internal cavities, the magnitude of the reference pump-out current, which is the limiting current when the main pump cell pumps out oxygen from the first internal cavity based on the control of the controller, is defined as B. Then, the ratio A / B is 0.It is characterized by being 0.7 or more.

[0013] Further, a second aspect of the present invention is the gas sensor according to the first aspect, wherein the ratio A / B is 0.20 or more.

[0014] Further, a third aspect of the present invention is the gas sensor according to the first or second aspect, wherein the magnitude A of the reference pump current is 5 mA or less.

[0015] Further, a fourth aspect of the present invention is the gas sensor according to any one of the first to third aspects, wherein the plurality of internal cavities are the first internal cavity, a second internal cavity that communicates with the first internal cavity and includes an auxiliary pump electrode as the inner electrode, and a measurement internal cavity that is a third internal cavity communicating with the second internal cavity; the plurality of electrochemical pump cells include the main pump cell, an auxiliary pump cell including the auxiliary pump electrode and the outer-pump electrode outside the cavity, and the measurement pump cell; and the controller adjusts the oxygen concentration in the first internal cavity by controlling the operation of the main pump cell, and adjusts the oxygen concentration in the second internal cavity by controlling the operation of the auxiliary pump cell.

Advantages of the Invention

[0016] According to the first to fourth aspects of the present invention, a gas sensor capable of performing good measurement can be realized for at least a rich atmosphere in a range where the λ value is from the stoichiometric composition to 0.97, in addition to a lean atmosphere.

Brief Description of the Drawings

[0017]

Figure 1

Figure 2

Figure 3

Figure 4

Mode for Carrying Out the Invention

[0018] <Schematic Configuration of Gas Sensor> FIG. 1 is a diagram schematically showing an example of the configuration of the gas sensor 100 according to the present embodiment. The gas sensor 100 is a limit current type NOx sensor that detects NOx by the sensor element 101 and measures its concentration. Further, the gas sensor 100 further includes a controller 110 that controls the operation of each part and specifies the NOx concentration based on the NOx current flowing through the sensor element 101. FIG. 1 includes a vertical cross-sectional view along the longitudinal direction of the sensor element 101.

[0019] The sensor element 101 mainly has a structure in which six solid electrolyte layers, namely, a first substrate layer 1, a second substrate layer 2, a third substrate layer 3, a first solid electrolyte layer 4, a spacer layer 5, and a second solid electrolyte layer 6, each made of zirconia (ZrO2) which is an oxygen ion conductive solid electrolyte (for example, made of yttria stabilized zirconia (YSZ)), are laminated in this order from the lower side in the drawing view. The sensor element 101 is a flat plate-shaped (long plate-shaped) ceramic element body. Further, the solid electrolytes forming these six layers are dense and airtight. Hereinafter, the upper surface of each of these six layers in FIG. 1 may be simply referred to as the upper surface, and the lower surface may be simply referred to as the lower surface. Further, the entire portion of the sensor element 101 made of solid electrolyte is collectively referred to as the base portion.

[0020] Such a sensor element 101 is manufactured, for example, by performing predetermined processing and printing of a circuit pattern on ceramic green sheets corresponding to each layer, then laminating them, and further firing and integrating them.

[0021] At one 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, there are a first diffusion rate-limiting part 11 that also serves as a gas inlet 10, a buffer space 12, a second diffusion rate-limiting part 13, a first internal cavity 20, a third diffusion rate-limiting part 30, a second internal cavity 40, a fourth diffusion rate-limiting part 60, and a third internal cavity 61, which are adjacent and formed in communication in this order.

[0022] The buffer space 12, the first internal cavity 20, the second internal cavity 40, and the third internal cavity 61 are spaces (regions) inside the sensor element 101 that are defined such that the upper part, which is provided by cutting out the spacer layer 5, is the lower surface of the second solid electrolyte layer 6, the lower part is the upper surface of the first solid electrolyte layer 4, and the side part is the side surface of the spacer layer 5. Regarding the gas inlet 10 as well, it may be provided in a manner of cutting out the spacer layer 5 at the tip surface (left end in the drawing view) of the sensor element 101 separately from the first diffusion rate-limiting part 11. In such a case, the first diffusion rate-limiting part 11 will be formed adjacent to the inside of the gas inlet 10.

[0023] All of the first diffusion rate-limiting part 11, the second diffusion rate-limiting part 13, the third diffusion rate-limiting part 30, and the fourth diffusion rate-limiting part 60 are provided as two horizontally long slits (the opening has a longitudinal direction in a direction perpendicular to the drawing). The part from the gas inlet 10 to the innermost internal cavity, the third internal cavity 61, is also referred to as the gas flow part.

[0024] Also, at a position farther from the tip side than the gas flow part, a reference gas introduction space 43 is provided between the upper surface of the third substrate layer 3 and the lower surface of the spacer layer 5, and the side part is defined by the side surface of the first solid electrolyte layer 4. For example, air is introduced into the reference gas introduction space 43 as a reference gas when measuring the NOx concentration.

[0025] The air introduction layer 48 is a layer made of porous alumina, and the reference gas is introduced into the air introduction layer 48 through the reference gas introduction space 43. Also, the air introduction layer 48 is formed so as to cover the reference electrode 42.

[0026] The reference electrode 42 is an electrode formed in a manner sandwiched between the upper surface of the third substrate layer 3 and the first solid electrolyte layer 4. As described above, an air introduction layer 48 connected to the reference gas introduction space 43 is provided around it. Further, as will be described later, it is possible to measure the oxygen concentration (oxygen partial pressure) in the first internal cavity 20 and the second internal cavity 40 using the reference electrode 42.

[0027] In the gas flow portion, the gas inlet 10 (the first diffusion rate-limiting portion 11) is a portion that opens to the external space, and the gas to be measured is taken into the sensor element 101 from the external space through the gas inlet 10.

[0028] The first diffusion rate-limiting portion 11 is a portion that imparts a predetermined diffusion resistance to the taken-in gas to be measured.

[0029] The buffer space 12 is a space provided to guide the gas to be measured introduced from the first diffusion rate-limiting portion 11 to the second diffusion rate-limiting portion 13.

[0030] The second diffusion rate-limiting portion 13 is a portion that imparts a predetermined diffusion resistance to the gas to be measured introduced from the buffer space 12 into the first internal cavity 20.

[0031] When the gas to be measured is introduced from outside the sensor element 101 into the first internal cavity 20, the gas to be measured that is rapidly taken into the sensor element 101 from the gas inlet 10 due to the pressure fluctuation of the gas to be measured in the external space (if the gas to be measured is the exhaust gas of an automobile, the pulsation of the exhaust pressure) is not directly introduced into the first internal cavity 20. Instead, after the concentration fluctuation of the gas to be measured is canceled through the first diffusion rate-limiting portion 11, the buffer space 12, and the second diffusion rate-limiting portion 13, it is introduced into the first internal cavity 20. As a result, the concentration fluctuation of the gas to be measured introduced into the first internal cavity 20 becomes negligible.

[0032] The first internal space 20 is the internal space that is located first when viewed from the gas inlet 10, and is provided as a space for adjusting the oxygen partial pressure in the gas to be measured introduced through the second diffusion rate-determining section 13. Such oxygen partial pressure is adjusted by operating the main pump cell 21.

[0033] The main pump cell 21 includes an inner pump electrode 22 having a ceiling electrode portion 22a provided on substantially the entire lower surface of the second solid electrolyte layer 6 facing the first internal space 20, an outer (outside the space) pump electrode 23 provided in a manner exposed to the external space in a region corresponding to the ceiling electrode portion 22a on the upper surface (one main surface of the sensor element 101) of the second solid electrolyte layer 6, and the second solid electrolyte layer 6 sandwiched between these electrodes, and is an electrochemical pump cell.

[0034] The inner pump electrode 22 is formed on the upper and lower solid electrolyte layers (the second solid electrolyte layer 6 and the first solid electrolyte layer 4) partitioning the first internal space 20. Specifically, a ceiling electrode portion 22a is formed on the lower surface of the second solid electrolyte layer 6 providing 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 providing the bottom surface. The ceiling electrode portion 22a and the bottom electrode portion 22b are connected by a conduction portion provided on the side wall surface (inner surface) of the spacer layer 5 constituting both side wall portions of the first internal space 20 (not shown).

[0035] The ceiling electrode portion 22a and the bottom electrode portion 22b are provided in a rectangular shape in plan view. However, a mode in which only the ceiling electrode portion 22a or only the bottom electrode portion 22b is provided may also be possible.

[0036] The inner pump electrode 22 and the outer pump electrode 23 are formed as porous cermet electrodes. In particular, the inner pump electrode 22 that contacts the gas to be measured is formed using a material with weakened reduction ability for NOx components in the gas to be measured. For example, it is formed to a thickness of 5 μm to 20 μm as a cermet electrode of an Au-Pt alloy containing 0.6 wt% to 1.4 wt% of Au and ZrO2, having a porosity of 5% to 40%. The weight ratio of the Au-Pt alloy to ZrO2 may be about Pt:ZrO2 = 7.0:3.0 to 5.0:5.0.

[0037] On the other hand, the outer pump electrode 23 is formed in a rectangular shape in plan view, for example, as a cermet electrode of Pt or its alloy and ZrO2.

[0038] In the main pump cell 21, a desired pump voltage Vp0 is applied between the inner pump electrode 22 and the outer pump electrode 23 by a variable power source 24, and a main pump current Ip0 is passed in the positive or negative direction between the inner pump electrode 22 and the outer pump electrode 23, so that oxygen in the first internal cavity 20 can be pumped out to the external space, or oxygen in the external space can be pumped into the first internal cavity 20. Note that the pump voltage Vp0 applied between the inner pump electrode 22 and the outer pump electrode 23 in the main pump cell 21 is also referred to as the main pump voltage Vp0.

[0039] Also, in order to detect the oxygen concentration (oxygen partial pressure) in the atmosphere in the first internal cavity 20, a main sensor cell 80, which is an electrochemical sensor cell, is constituted by the inner pump electrode 22, the second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, the third substrate layer 3, and the reference electrode 42.

[0040] By measuring the electromotive force V0, which is the potential difference between the inner pump electrode 22 and the reference electrode 42 in the main sensor cell 80, the oxygen concentration (oxygen partial pressure) in the first internal cavity 20 can be known.

[0041] Furthermore, the controller 110 controls the main pump current Ip0 by feedback - controlling the main pump voltage Vp0 so that the electromotive force V0 becomes constant. As a result, the oxygen concentration in the first internal cavity 20 is maintained at a predetermined constant value.

[0042] The third diffusion - rate - determining section 30 is a part that imparts a predetermined diffusion resistance to the measurement gas whose oxygen concentration (oxygen partial pressure) is controlled by the operation of the main pump cell 21 in the first internal cavity 20 and guides the measurement gas to the second internal cavity 40.

[0043] The second internal cavity 40 is provided as a space for further adjusting the oxygen partial pressure in the measurement gas introduced through the third diffusion - rate - determining section 30. Such oxygen partial pressure is adjusted by the operation of the auxiliary pump cell 50. In the second internal cavity 40, the oxygen concentration of the measurement gas is adjusted with higher precision.

[0044] In the second internal cavity 40, after the oxygen concentration (oxygen partial pressure) is adjusted in the first internal cavity 20 in advance, the oxygen partial pressure of the measurement gas introduced through the third diffusion - rate - determining section 30 is further adjusted by the auxiliary pump cell 50.

[0045] The auxiliary pump cell 50 is an auxiliary electrochemical pump cell composed of an auxiliary pump electrode 51 having a ceiling electrode portion 51a provided substantially entirely on the lower surface of the second solid electrolyte layer 6 facing the second internal cavity 40, an outer pump electrode 23 (not limited to the outer pump electrode 23, and any appropriate electrode outside the sensor element 101 is sufficient), and the second solid electrolyte layer 6.

[0046] The auxiliary pump electrode 51 is disposed in the second internal cavity 40 in the same form as the inner pump electrode 22 provided in the previous first internal cavity 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 cavity 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 cavity 40. These ceiling electrode portion 51a and bottom electrode portion 51b are rectangular in plan view and are connected by a conduction portion provided on the side wall surface (inner surface) of the spacer layer 5 that constitutes both side wall portions of the second internal cavity 40 (not shown).

[0047] Note that the auxiliary pump electrode 51 is also formed using a material with a reduced reduction ability for the NOx component in the gas to be measured, similar to the inner pump electrode 22.

[0048] In the auxiliary pump cell 50, under the control of the controller 110, by applying a desired voltage (auxiliary pump voltage) Vp1 between the auxiliary pump electrode 51 and the outer pump electrode 23, oxygen in the atmosphere in the second internal cavity 40 can be pumped out to the external space or pumped into the second internal cavity 40 from the external space.

[0049] Also, in order to control the oxygen partial pressure in the atmosphere in the second internal cavity 40, an auxiliary sensor cell 81, which is an electrochemical sensor cell, is constituted 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. In the auxiliary sensor cell 81, an electromotive force V1, which is the potential difference generated between the auxiliary pump electrode 51 and the reference electrode 42 according to the oxygen partial pressure in the second internal cavity 40, is detected.

[0050] The auxiliary pump cell 50 performs pumping with a variable power supply 52 whose voltage is controlled based on the electromotive force V1 detected by this auxiliary sensor cell 81. As a result, the oxygen partial pressure in the atmosphere in the second internal cavity 40 is feedback-controlled to a low partial pressure that has substantially no influence on the measurement of NOx.

[0051] At the same time, the auxiliary pump current Ip1 is used to control the electromotive force of the main sensor cell 80. Specifically, the auxiliary pump current Ip1 is input to the main sensor cell 80 as a control signal, and by controlling its electromotive force V0, the gradient of the oxygen partial pressure in the measurement gas introduced from the third diffusion rate-limiting section 30 into the second internal cavity 40 is always kept constant. When used as a NOx sensor, due to the functions of the main pump cell 21 and the auxiliary pump cell 50, the oxygen concentration in the second internal cavity 40 is maintained at a constant value of about 0.001 ppm.

[0052] The fourth diffusion rate-limiting section 60 is a part that imparts a predetermined diffusion resistance to the measurement gas whose oxygen concentration (oxygen partial pressure) is controlled by the operation of the auxiliary pump cell 50 in the second internal cavity 40, and guides the measurement gas to the third internal cavity 61.

[0053] The third internal cavity 61 is an internal cavity located at the innermost part as seen from the gas inlet 10, and is provided as a space (measurement internal cavity) for performing processes related to the measurement of the nitrogen oxide (NOx) concentration in the measurement gas introduced through the fourth diffusion rate-limiting section 60. The measurement of the NOx concentration is performed by the operation of the measurement pump cell 41 in the third internal cavity 61. Since the measurement gas with the oxygen concentration precisely adjusted in the second internal cavity 40 is introduced into the third internal cavity 61, high-precision NOx concentration measurement is possible in the gas sensor 100.

[0054] The measurement pump cell 41 is for measuring the NOx concentration of the measurement gas introduced into the third internal cavity 61. The measurement pump cell 41 is an electrochemical pump cell composed of a measurement electrode 44 provided on the upper surface of the first solid electrolyte layer 4 facing the third internal cavity 61 and spaced apart from the third diffusion rate-limiting section 30, an outer pump electrode 23, a second solid electrolyte layer 6, a spacer layer 5, and the first solid electrolyte layer 4.

[0055] The measurement electrode 44 is a porous cermet electrode composed of a noble metal and a solid electrolyte. For example, it is formed as a cermet electrode of Pt or an alloy of Pt and other noble metals such as Rh and ZrO2 which is the constituent material of the sensor element 101. The measurement electrode 44 also functions as a NOx reduction catalyst for reducing NOx present in the atmosphere within the third internal cavity 61.

[0056] In the measurement pump cell 41, under the control of the controller 110, oxygen generated by the decomposition of NOx in the atmosphere within the third internal cavity 61 can be pumped out, and the generated amount can be detected as the pump current Ip2.

[0057] Also, in order to detect the oxygen partial pressure around the measurement electrode 44, a measurement sensor cell 82 which is an electrochemical sensor cell is constituted by the second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, the third substrate layer 3, the measurement electrode 44, and the reference electrode 42. The variable power supply 46 is feedback-controlled based on the electromotive force V2 which is the potential difference generated between the measurement electrode 44 and the reference electrode 42 according to the oxygen partial pressure within the third internal cavity 61 detected by the measurement sensor cell 82.

[0058] NOx in the gas to be measured introduced into the third internal cavity 61 is reduced by the measurement electrode 44 (2NO → N2 + O2) to generate oxygen. Then, the generated oxygen is pumped by the measurement pump cell 41. At this time, the voltage (measurement pump voltage) Vp2 of the variable power supply 46 is controlled so that the electromotive force V2 detected by the measurement sensor cell 82 becomes constant. Since the amount of oxygen generated around the measurement electrode 44 is proportional to the concentration of NOx in the gas to be measured, the concentration of NOx in the gas to be measured is calculated using the pump current Ip2 in the measurement pump cell 41. Hereinafter, such a pump current Ip2 is also referred to as the NOx current Ip2.

[0059] Further, if the measurement electrode 44, the first solid electrolyte layer 4, the third substrate layer 3, and the reference electrode 42 are combined to form oxygen partial pressure detection means as an electrochemical sensor cell, an electromotive force corresponding to the difference between the amount of oxygen generated by the reduction of the NOx component in the atmosphere around the measurement electrode 44 and the amount of oxygen contained in the reference atmosphere can be detected, and thereby the concentration of the NOx component in the gas to be measured can also be determined.

[0060] Further, an electrochemical sensor cell 83 is composed of 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, and the electromotive force Vref obtained by this sensor cell 83 enables the detection of the oxygen partial pressure in the gas to be measured outside the sensor.

[0061] The sensor element 101 further includes a heater unit 70 that plays a role in temperature adjustment for heating and maintaining the temperature of the sensor element 101 in order to enhance the oxygen ion conductivity of the solid electrolyte constituting the base portion.

[0062] The heater unit 70 mainly includes a heater electrode 71, a heater element 72, a heater lead 72a, a through hole 73, a heater insulating layer 74, a pressure dissipation hole 75, and a heater resistance detection lead (not shown in FIG. 1). Further, except for the heater electrode 71, the heater unit 70 is embedded in the base portion of the sensor element 101.

[0063] The heater electrode 71 is an electrode formed in a manner that contacts the lower surface of the first substrate layer 1 (the other main surface of the sensor element 101).

[0064] The heater element 72 is a resistive heating element provided between the second substrate layer 2 and the third substrate layer 3. The heater element 72 is powered from a heater power source (not shown) provided outside the sensor element 101, which is omitted in FIG. 1, through a heater electrode 71, a through hole 73, and a heater lead 72a, which are the current conduction paths, and thus generates heat. The heater element 72 is formed of Pt or mainly composed of Pt. The heater element 72 is embedded in a predetermined range on the side where the gas flow portion of the sensor element 101 is provided so as to face the gas flow portion in the element thickness direction. The heater element 72 is provided to have a thickness of about 10 μm to 20 μm.

[0065] In the sensor element 101, by passing a current through the heater element 72 through the heater electrode 71 to heat the heater element 72, each part of the sensor element 101 can be heated and kept at a predetermined temperature. Specifically, the sensor element 101 is heated so that the temperature of the solid electrolyte and the electrode near the gas flow portion becomes about 700°C to 900°C. By such heating, the oxygen ion conductivity of the solid electrolyte constituting the base portion in the sensor element 101 is enhanced. Note that the heating temperature by the heater element 72 when the gas sensor 100 is used (when the sensor element 101 is driven) is referred to as the sensor element driving temperature.

[0066] The degree of heat generation (heater temperature) by the heater element 72 is grasped by the magnitude of the resistance value (heater resistance) of the heater element 72.

[0067] In addition to the above components, the sensor element 101 of the gas sensor 100 according to the present embodiment further includes a ceramic layer 7 and a porous region 8 on the second solid electrolyte layer 6. FIG. 2 is a cross-sectional view of a main part perpendicular to the longitudinal direction of the sensor element 101 for explaining the arrangement of the ceramic layer 7 and the porous region 8. In FIG. 2, it is assumed that the direction from the paper surface toward the front is the direction toward the tip of the sensor element 101 provided with the gas inlet 10.

[0068] The ceramic layer 7 is provided on the second solid electrolyte layer 6 so as to cover the entire surface thereof. The ceramic layer 7 is adjacent to most of the second solid electrolyte layer 6, but is separated from the outer pump electrode 23 and a region (hereinafter referred to as the electrode side region) located laterally (left and right in the drawing view) of the outer pump electrode 23 in the element short side direction (the left and right direction in FIG. 2) on the upper surface of the second solid electrolyte layer 6. The regions between the outer pump electrode 23 and the ceramic layer 7, and between the electrode side region of the second solid electrolyte layer 6 and the ceramic layer 7 are porous regions 8. The porous region 8 is provided so as to cover the outer pump electrode 23 and to be exposed at both ends in the element short side direction (the left and right ends in FIG. 2), and the ceramic layer 7 is provided so as to cover the entire upper surface of the second solid electrolyte layer 6 including such a porous region 8.

[0069] More specifically, the ceramic layer 7 is made of ceramics (for example, zirconia, alumina, etc.) having the same degree of density as that of the second solid electrolyte layer 6, for example.

[0070] On the other hand, the porous region 8 is made of a porous body (for example, alumina, etc.) having a porosity of about 30% to 60%. The porous region 8 is configured such that the thickness above the outer pump electrode 23 (in other words, the distance in the element thickness direction between the outer pump electrode 23 and the ceramic layer 7) is about 25 μm to 40 μm.

[0071] The ceramic layer 7 and the porous region 8 are formed, for example, by a known method such as printing after the sensor element 101 excluding these is previously configured. Alternatively, a material that will finally become the ceramic layer 7 and the porous region 8 may be further laminated on the green sheet laminate forming the six solid electrolyte layers described above by a known method such as printing, and such a laminate may be integrally fired.

[0072] By having the ceramic layer 7 and the porous region 8 configured in this way, in the sensor element 101, when oxygen is drawn in and out between the inside and outside of the sensor element 101 through the outer pump electrode 23, a predetermined diffusion resistance is imparted to the oxygen passing through the porous region 8.

[0073] Further, an aspect may be provided in which a thermal shock protection layer, which is a single-layer or multi-layer porous layer covering the sensor element 101, is further provided on the outer periphery of a predetermined range on one tip side (left end side in the drawing view) of the sensor element 101. Such a thermal shock protection layer is provided for the purpose of preventing cracks from occurring in the sensor element 101 due to a thermal shock generated when moisture contained in the gas to be measured adheres to and condenses on the sensor element 101 during use of the gas sensor 100, and for the purpose of preventing poisoning substances mixed in the gas to be measured from entering the inside of the sensor element 101. Note that an aspect may be such that a layered gap (gap layer) is formed between the sensor element 101 and the thermal shock protection layer.

[0074] Further, the sensor element 101 is housed in a metal housing member (casing), not shown, in such a manner that the space between the gas inlet 10 side and the reference gas introduction space 43 side is hermetically sealed. The sensor element 101 and the housing member constitute the main body of the gas sensor 100. When the gas sensor 100 is actually used, such a main body is attached to a use location such as an engine exhaust pipe. Also, from the housing member, wirings are drawn out in which electrical connections with each part of the sensor element 101 are ensured inside, and these wirings are appropriately connected to a controller 110, various power sources, and the like.

[0075] When measuring the NOx concentration in the gas sensor 100 having the above configuration, by operating the main pump cell 21 and further the auxiliary pump cell 50, feedback control is executed to make the oxygen concentration constant in the first internal cavity 20 and further the second internal cavity 40, and the measured gas with a constant oxygen concentration is introduced into the third internal cavity 61 and reaches the measurement electrode 44. For example, when the measured gas is in a lean atmosphere, the measured gas with its oxygen partial pressure sufficiently lowered to a level where it has substantially no influence on the measurement of NOx (for example, 0.0001 ppm to 1 ppm) is introduced into the third internal cavity 61.

[0076] Then, at the measurement electrode 44, oxygen is generated by reducing NOx in the reached measured gas. Such oxygen is pumped out by the measurement pump cell 41, and the NOx current Ip2 flowing during such pumping has a certain functional relationship (hereinafter referred to as the sensitivity characteristic) with the concentration of NOx in the measured gas.

[0077] Such sensitivity characteristics are specified in advance using a plurality of types of model gases with known NOx concentrations prior to the actual use of the gas sensor 100, and the data is stored in the controller 110. Then, during the actual use of the gas sensor 100, a signal representing the value of the NOx current Ip2 flowing according to the NOx concentration in the measured gas is given to the controller 110 at all times. In the controller 110, the NOx concentration is continuously calculated based on the value and the specified sensitivity characteristics, and is output as the NOx sensor detection value. Thereby, the gas sensor 100 can grasp the NOx concentration in the measured gas almost in real time.

[0078] Note that the target values of the electromotive forces V0, V1, and V2 in the main sensor cell 80, the auxiliary sensor cell 81, and the measurement sensor cell 82 when performing feedback control on each of the main pump cell 21, the auxiliary pump cell 50, and the measurement pump cell 41 may be appropriately set according to the configuration, size of each part of the specific sensor element 101, and further the usage situation and usage mode of the gas sensor 100.

[0079] <Relationship between Pump Limiting Current Ratio and Measured Gas Atmosphere> The gas sensor 100 according to the present embodiment is mainly assumed to operate in the above-described normal mode in a situation where the measured gas contains oxygen relatively sufficiently, such as in a lean atmosphere, that is, to specify the concentration of NOx in the measured gas.

[0080] More specifically, when the gas sensor 100 operates in the normal mode, the main pump cell 21 operates so that the electromotive force V0 generated in the main sensor cell 80 becomes a predetermined value corresponding to a desired value as the oxygen concentration value (or oxygen partial pressure value) in the first internal cavity 20. However, since the oxygen concentration in the measured gas introduced from the external space into the first internal cavity 20 changes every moment, in the main pump cell 21, both pumping out and pumping in of oxygen can be performed even in a lean atmosphere.

[0081] On the other hand, although the auxiliary pump cell 50 and the measurement pump cell 41 are configured to be able to pump in oxygen, the set values of the electromotive force V1 in the auxiliary sensor cell 81 and the electromotive force V2 in the measurement sensor cell 82, which are control target values when operating each pump cell, are set on the premise that oxygen pumping out is performed based on the measurement principle of the NOx concentration. That is, when the gas sensor 100 operates in the normal mode, only oxygen pumping out is performed in the auxiliary pump cell 50 and the measurement pump cell 41.

[0082] However, the gas sensor 100 is not always used in an atmosphere sufficiently containing oxygen. For example, when the main body of the gas sensor 100 is attached to the exhaust path of a gasoline engine and the exhaust gas from the engine is used as the measured gas, it may be used in an environment where the ambient gas can be a rich gas with a small air-fuel ratio or λ value (air ratio). In such a case, the measured gas introduced into the sensor element 101 also becomes a rich gas, and the main pump cell 21 tries to maintain the oxygen concentration value in the first internal cavity 20 by pumping in oxygen from the outside.

[0083] Therefore, the higher the oxygen pumping ability of the main pump cell 21 in a rich gas atmosphere, the more possible it is for the gas sensor 100 to be used in a richer (lower air-fuel ratio) gas atmosphere.

[0084] In the present embodiment, the ratio of the magnitude of the main pump current Ip0 (referred to as the reference pump-out current) flowing through the main pump cell 21 to pump out oxygen from the first internal cavity 20 when the gas to be measured is in a lean atmosphere, to the magnitude of the main pump current Ip0 (referred to as the reference pump In current) flowing through the main pump cell 21 to pump in oxygen to the first internal cavity 20 when the gas to be measured is in a rich atmosphere, which is the pump limit current ratio, is used as an index of the usage adaptability of the gas sensor 100 with respect to a rich gas atmosphere.

[0085] Specifically, the ratio of the magnitude of the reference pump-in current when an evaluation gas (gas for evaluating pump-in current) with a known oxygen concentration, which can be regarded as a sufficiently rich gas atmosphere, is introduced as the gas to be measured, to the magnitude of the reference pump-out current when an evaluation gas (gas for evaluating pump-out current) with a known oxygen concentration, which can be regarded as a sufficiently lean gas atmosphere, is introduced as the gas to be measured, is defined as the pump limit current ratio.

[0086] That is, when the magnitude of the reference pump-in current is A and the magnitude of the reference pump-out current is B, the degree of suitability for use of the gas sensor 100 in a rich gas atmosphere is represented by the magnitude of the pump limit current ratio A / B. The magnitude of the reference pump-in current, which is the limit current during oxygen pumping-in, is the same for the sizes of the inner pump electrode 22 and the outer pump electrode 23, and when the oxygen concentration in the gas to be measured is constant, it corresponds to the magnitude of the diffusion resistance provided by the porous body region 8 to the gas to be measured outside the sensor element 101 introduced into the outer pump electrode 23 through the porous body region 8. Similarly, the magnitude of the reference pump-out current, which is the limit current during oxygen pumping-out, also corresponds to the magnitude of the diffusion resistance provided by the gas flow portion to the gas to be measured introduced from the gas inlet 10 to the first internal cavity 20 when the oxygen concentration in the gas to be measured is constant.

[0087] In this embodiment, as the gas for evaluating the pump-out current, a mixed gas having an oxygen concentration of 20.5%, containing 2% H2O, and the remainder being N2 is used. As the gas for evaluating the pump-in current, a mixed gas having an oxygen concentration of 5%, containing 3% H2O, and the remainder being N2 is used. However, other embodiments in which gases for evaluating the pump-out current and gases for evaluating the pump-in current with different oxygen concentrations are used may also be possible.

[0088] The reason for considering the reference pump-out current under a lean atmosphere in the evaluation of the degree of suitability for use in a rich atmosphere is that the magnitude of the limit current flowing through the main pump cell 21 depends on the diffusion resistance imparted to the gas to be measured flowing from the gas inlet to the internal cavity for both the pump-out current and the pump-in current, and there is a correlation between the pump-out current value under a lean atmosphere and the pump-in current value under a rich atmosphere. Therefore, by measuring the reference pump-out current under a lean atmosphere, the influence of the above diffusion resistance on the pump-in current under a rich atmosphere can be taken into account.

[0089] Also, although the magnitude of the pump limit current depends on the shape, size, etc. of the electrodes and the gas flow path, regarding the evaluation of the pump limit current ratio A / B, which is the ratio of the reference pump in-current and the reference pump out-current flowing through the same main pump cell 21, that is, flowing between the same inner pump electrode 22 and outer pump electrode 23, it may be considered that these effects can cancel each other out. That is, the value of the pump limit current ratio A / B can be compared even between gas sensors 100 with different sizes of the inner pump electrode 22 and the outer pump electrode 23.

[0090] On the other hand, the determination of to what extent each gas sensor 100 can actually be used in a rich gas atmosphere is performed by evaluating the controllable λ threshold value.

[0091] Specifically, with the gas sensor 100 operating in a control mode of Ip1 constant control in which an auxiliary pump current Ip1 of a certain magnitude flows through the auxiliary pump cell 50, when the λ value of the gas to be measured is gradually changed from the lean side to the rich side, as a result of the oxygen pumping ability in the main pump cell 21 reaching its limit, the value of the auxiliary pump current Ip1 cannot maintain the set predetermined control target value, and the λ value immediately before a deviation of more than a predetermined threshold occurs is specified as the controllable λ threshold value. When Ip1 constant control is performed well in the auxiliary pump cell 50 41, it becomes possible to accurately measure the NOx current corresponding to the NOx concentration in the gas to be measured. From the viewpoint of being able to preferably perform Ip1 constant control, the sensor element 101 is preferably configured such that an auxiliary pump current Ip1 of 3 μA to 10 μA flows through it.

[0092] FIG. 3 is a diagram showing an example of the evaluation of the controllable λ threshold value. Specifically, FIG. 3 shows the state when Ip1 constant control with a control target value of the auxiliary pump current Ip1 of 7 μA is performed for a certain gas sensor 100 while gradually decreasing the λ value of the gas to be measured in steps over a span of approximately 60 seconds with the initial value being λ = 1.05 in a lean atmosphere.

[0093] In the example shown in FIG. 3, until the λ value reached 0.99, the value of the auxiliary pump current Ip1 remained approximately 7 μA. However, when the time elapsed approximately 170 seconds and the λ value stepped down from 0.99 to 0.97, the auxiliary pump current Ip1 fluctuated greatly from 7 μA. In such a case, the controllable λ threshold is determined to be 0.99.

[0094] The controllable λ threshold evaluated in this way has a negative correlation with the pump limit current ratio A / B, and specifically, there is the following correspondence.

[0095] A / B < 0.07 → Controllable λ threshold = 0.97; 0.07 ≤ A / B ≤ 0.20 → 0.95 ≤ Controllable λ threshold ≤ 0.97; 0.20 < A / B (< 1.0) → Controllable λ threshold < 0.95; Such a correspondence indicates that, for example, when using the gas sensor 100 with a pump limit current ratio A / B of 0.07 or more, not only in a lean atmosphere but also in a rich atmosphere with a λ value ranging from at least the stoichiometric composition to 0.97, the gas sensor 100 can measure the NOx concentration well.

[0096] Also, according to the above correspondence, the larger the value of the pump limit current ratio A / B, the more it is possible to measure even in a richer atmosphere. However, in the case of exhaust gas from a general gasoline engine, the λ value in a rich atmosphere remains at most about 0.95 - 0.97 at minimum. Therefore, it can be said that the gas sensor 100 with a pump limit current ratio A / B of 0.2 or more has sufficient usability that can be used without problems for measuring exhaust gas from a general gasoline engine. Note that if the sensor element 101 is configured such that the magnitude A of the reference pump-in current is 0.14 mA or more, it is possible to obtain the pump limit current ratio A / B.

[0097] On the one hand, when the magnitude B of the reference pump-out current is constant, the larger the pump limit current ratio A / B, the greater the value of the main pump voltage Vp0 applied to the main pump cell 21 when pumping in oxygen. However, the greater the value of the main pump voltage Vp0, the higher the risk of blackening. Therefore, it is desirable that the magnitude A of the reference pump-in current be 5 mA or less, and it is desirable that the value of the pump limit current ratio A / B be less than 1.0.

[0098] As described above, according to the present embodiment, by setting the pump limit current ratio in the gas sensor to 0.07 or more, in addition to the lean atmosphere, the NOx concentration can be measured favorably even in a rich atmosphere in the range where the λ value is from the stoichiometric composition to 0.97.

[0099] <Modification Example> In the above-described embodiment, the case where the gas sensor includes a sensor element having three cavities inside has been targeted. However, the configuration of the sensor element (for example, the mode of providing internal cavities) that can be suitably used even in a rich atmosphere by setting the pump limit current ratio to a predetermined value is not limited to that of the above-described embodiment. As long as it is possible to measure the pump limit current ratio and specify the controllable λ threshold under the Ip1 constant control, other configurations may be adopted.

[0100] A ceramic layer similar to the ceramic layer 7 may be provided in a manner of covering the first substrate layer 1 provided at the lower end in the drawing view of the sensor element 101 in FIG. 1.

Example

[0101] As an example, eight types (eight levels) of gas sensors 100 (Examples 1 to 8) having the same configuration were fabricated, except that the combination of the porosity of the porous region 8 provided around the outer pump electrode 23 and the thickness above the outer pump electrode 23 was varied. For each of them, the pump limit current ratio A / B and the controllable λ threshold were determined. Based on the obtained controllable λ threshold, the usability of the gas sensor 100 in a rich atmosphere was determined when exhaust gas from a general gasoline engine was assumed as the gas to be measured.

[0102] As the gas for evaluating the pump-out current, a mixed gas having an oxygen concentration of 20.5%, containing 2% H2O, and the balance being N2 was used. As the gas for evaluating the pump-in current, a mixed gas having an oxygen concentration of 5%, containing 3% H2O, and the balance being N2 was used.

[0103] As a comparative example, a gas sensor 100 having the same configuration as that of Examples 1 to 8 was also prepared, except that the portion where the porous region 8 is originally provided was a slit-shaped space, and the same evaluation as that of Examples 1 to 8 was performed. Note that the gas sensor 100 of such a comparative example virtually corresponds to one having a porosity of 100% in the porous region 8.

[0104] Table 1 shows a list of the type of the structure around the outer pump electrode 23, the porosity, the thickness above the outer pump electrode 23, the magnitude A of the reference pump-in current, the magnitude B of the reference pump-out current, the pump limit current ratio A / B, the controllable λ threshold, and the determination result of the usability in a rich atmosphere in the gas sensors 100 according to Examples 1 to 8 and the comparative example. In Table 1, the item names of the respective items are, in order, "structure around the outer electrode", "porosity around the outer electrode", "film thickness around the outer electrode", "pump-in limit current A", "pump-out limit current B", "ratio A / B", "threshold", and "determination". Further, when evaluating the controllable λ value, the control target value of the auxiliary pump current Ip1 was set to 7 μA.

[0105]

Table 1

[0106] When determining the fitness for use, the evaluation is performed in three stages. Specifically, for the gas sensor 100 with a controllable λ threshold value of less than 0.95, when assuming the exhaust gas from a general gasoline engine as the gas to be measured, it is determined that the fitness for use in a rich atmosphere is sufficient, and a "◎" (double circle mark) is attached to the "Determination" item. Also, for the gas sensor 100 with a controllable λ threshold value of 0.95 or more and 0.97 or less, it is determined that it has generally good fitness for use in a rich atmosphere, and a "○" (circle mark) is attached to the "Determination" item. On the other hand, for the gas sensor 100 with a controllable λ threshold value exceeding 0.97, it is determined that it does not substantially have fitness for use in a rich atmosphere, and an "×" (cross mark) is attached to the "Determination" item.

[0107] Further, FIG. 4 is a graph plotting the controllable λ threshold values of the gas sensors 100 according to Examples 1 to 8 and the comparative example shown in Table 1 against the pump limit current ratio A / B (described as "pump-in / pump-out limit current ratio A / B" in FIG. 4).

[0108] As shown in Table 1, although the combinations of the porosity and thickness of the porous body region 8 in the gas sensors 100 of Examples 1 to 8 are various, it can be seen from FIG. 4 that the controllable λ threshold values of those gas sensors 100 have a negative linear correlation with the pump limit current ratio A / B.

[0109] More specifically, in FIG. 4, if the pump limit current ratio A / B is 0.07 or more, regardless of the combination of the porosity and thickness of the porous body region 8, the controllable λ threshold value is 0.97 or less. Such a result indicates that if the pump limit current ratio A / B of the gas sensor 100 is 0.07 or more, the gas sensor 100 can be generally used well even in a rich atmosphere.

[0110] Furthermore, in FIG. 4, if the pump limit current ratio A / B exceeds 0.20, the controllable λ threshold is less than 0.95 regardless of the combination of the porosity and thickness of the porous region 8. Such a result indicates that if the pump limit current ratio A / B of the gas sensor 100 exceeds 0.20, the gas sensor 100 can be used without problems in a rich atmosphere.

Description of the reference numerals

[0111] 1 First substrate layer 2 Second substrate layer 3 Third substrate layer 4 First solid electrolyte layer 5 Spacer layer 6 Second solid electrolyte layer 7 Ceramic layer 8 Porous region 10 Gas inlet 11 First diffusion rate-limiting section 13 Second diffusion rate-limiting section 20 First internal cavity 21 Main pump cell 22 Inner pump electrode 23 Outer pump electrode 24, 46, 52 Variable power source 30 Third diffusion rate-limiting section 40 Second internal cavity 41 Measurement pump cell 42 Reference electrode 43 Reference gas introduction space 44 Measurement electrode 50 Auxiliary pump cell 51 Auxiliary pump electrode 60 Fourth diffusion rate-limiting section 61 Third internal cavity 70 Heater section 80 Main sensor cell 81 Auxiliary sensor cell 82 Measurement sensor cell 100 Gas sensor 101 Sensor element Ip0 Main pump current Ip1 Auxiliary pump current Ip2 NOx current

Claims

1. A gas sensor configured to be capable of detecting a predetermined gas component in a gas to be measured, comprising a sensor element made of an oxygen ion conductive solid electrolyte, and a controller for controlling the operation of the gas sensor, wherein the sensor element includes a plurality of internal cavities that communicate sequentially from an inlet of the gas to be measured under a predetermined diffusion resistance, and each of which is provided with an inner electrode, an out-of-cavity pump electrode disposed at a location outside the plurality of internal cavities, a porous body region covering the out-of-cavity pump electrode, and a plurality of electrochemical pump cells configured such that by applying a pump voltage with a predetermined pump power source between each of the inner electrodes and the out-of-cavity pump electrode, oxygen can be pumped in or out between a corresponding internal cavity among the plurality of internal cavities and the outside of the sensor element, wherein the plurality of internal cavities include a first internal cavity located at the foremost position from the inlet and provided with a main pump electrode as the inner electrode, and a measurement internal cavity located at the innermost position from the inlet and provided with a measurement electrode as the inner electrode, wherein the plurality of electrochemical pump cells include a main pump cell including the main pump electrode and the out-of-cavity pump electrode, and a measurement pump cell including the measurement electrode and the out-of-cavity pump electrode, wherein the controller adjusts the oxygen concentration in a corresponding internal cavity among the plurality of internal cavities by controlling the operation of the plurality of electrochemical pump cells excluding the measurement pump cell, controls the operation of the measurement pump cell such that a measurement pump current corresponding to the concentration of the predetermined gas component flows between the measurement electrode and the out-of-cavity pump electrode, and identifies the concentration of the predetermined gas component based on the magnitude of the measurement pump current, when a pump-in current evaluation gas with a known oxygen concentration is introduced from the inlet into the plurality of internal cavities, the magnitude of the reference pump-in current, which is the limiting current when the main pump cell pumps oxygen into the first internal cavity based on the control of the controller, is defined as A, when a pump-out current evaluation gas with a known oxygen concentration is introduced from the inlet into the plurality of internal cavities, the magnitude of the reference pump-out current, which is the limiting current when the main pump cell pumps oxygen out of the first internal cavity based on the control of the controller, is defined as B, a gas sensor, characterized in that the ratio A / B is 0.07 or more.

2. ​ The gas sensor according to claim 1, wherein the ratio A / B is 0.20 or more, characterized in that it is a gas sensor.

3. The gas sensor according to claim 1 or claim 2, wherein the magnitude A of the reference pump current is 5 mA or less, characterized in that it is a gas sensor.

4. The gas sensor according to any one of claims 1 to 3, wherein the plurality of internal cavities are the first internal cavity, a second internal cavity that communicates with the first internal cavity and includes an auxiliary pump electrode as the inner electrode, and a measurement internal cavity that is a third internal cavity that communicates with the second internal cavity, wherein the plurality of electrochemical pump cells include the main pump cell, an auxiliary pump cell that includes the auxiliary pump electrode and the outer-pump electrode outside the cavity, and the measurement pump cell, wherein the controller adjusts the oxygen concentration in the first internal cavity by controlling the operation of the main pump cell, and adjusts the oxygen concentration in the second internal cavity by controlling the operation of the auxiliary pump cell, characterized in that it is a gas sensor.

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