Gas sensor

Abstract

Gas sensor, comprising: an element body having an oxygen ion-conductive solid electrolyte layer and provided inside with a measurement object gas flow segment that introduces a measurement object gas and allows the measurement object gas to flow; a measuring electrode arranged on an inner surface of the measuring object gas flow segment; a measurement object gas-side electrode disposed in a portion of the element body, the portion being exposed to the measurement object gas; a reference electrode arranged inside the element body; a measurement object gas introduction segment that introduces the measurement object gas and allows the measurement object gas to flow to the measurement object gas-side electrode; a reference gas introduction segment that introduces a reference gas serving as a reference for detecting a specific gas concentration in the measurement object gas and flows the reference gas to the reference electrode; a detection device that detects the specific gas concentration in the measurement object gas based on an electromotive force generated between the reference electrode and the measuring electrode; and a reference gas adjusting device that carries an oxygen pumping current between the reference electrode and the measuring object gas-side electrode and pumps oxygen from the vicinity of the measuring object gas-side electrode into the vicinity of the reference electrode, where A [µA] is a limiting current when oxygen is pumped from the vicinity of the measuring object gas-side electrode to the vicinity of the reference electrode, the measuring object gas introduction segment being exposed to an atmosphere with an oxygen concentration of 1000 ppm, and B [µA] is a limiting current when oxygen is pumped from the vicinity of the reference electrode to the vicinity of the measuring object gas-side electrode, the reference gas introduction segment being exposed to an air atmosphere, then a ratio A / B is greater than or equal to 0.005.

Description

TECHNICAL FIELD

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

[0002] A conventional gas sensor is known that detects a concentration of a specific gas such as NOx in a measurement target gas, such as automobile exhaust gas. For example, PTL 1 describes a gas sensor including: a laminated body in which a plurality of oxygen ion-conductive solid electrolyte layers are laminated; a reference electrode formed inside the laminated body and into which a reference gas is introduced from the reference gas introduction space; a measuring electrode disposed in a measurement target gas flow segment inside the laminated body; and a measurement target gas-side electrode disposed in a portion of the laminated body exposed to a measurement target gas. The gas sensor detects a specific gas concentration in the measurement target gas based on an electromotive force generated between the reference electrode and the measuring electrode.In addition, the gas sensor includes a reference gas adjusting device that supplies a control current between the reference electrode and the measurement-object gas-side electrode and pumps oxygen into the vicinity of the reference electrode. PTL 1 describes that pumping oxygen into the vicinity of the reference electrode by the reference gas adjusting device makes it possible to compensate for the reduction in the oxygen concentration in the reference gas in the vicinity of the reference electrode and reduces the reduction in the detection accuracy of the specific gas concentration. It should be noted that the reduction in the oxygen concentration in the reference gas in the vicinity of the reference electrode applies to the case where, for example, the measurement-object gas slightly enters the reference gas introduction space. PTL 2 describes a gas sensor. PTL 3 discloses a sensor element and a gas sensor. Citation listPatent literature PTL 1: JP 2015 - 200 643 A PTL 2: DE 10 2018 002 576 A1 PTL 3: DE 10 2019 007 353 A1 DISCLOSURE OF THE INVENTION

[0003] However, even if the oxygen in the vicinity of the measurement object gas side electrode is pumped to the vicinity of the reference electrode, the amount of pumped oxygen may be insufficient and the oxygen concentration in the reference gas in the vicinity of the reference electrode may decrease.

[0004] The present invention has been developed to solve such a problem, and its main object is to reduce the reduction of oxygen concentration in the vicinity of the reference electrode.

[0005] The present invention applies the following solution to achieve the main object described above.

[0006] A gas sensor comprising: an element body having an oxygen ion-conductive solid electrolyte layer and provided inside with a measurement object gas flow segment which introduces a measurement object gas and allows the gas to flow; a measuring electrode arranged on an inner surface of the measuring object gas flow segment; a measurement object gas-side electrode arranged in a portion of the element body, the portion being exposed to the measurement object gas; a reference electrode arranged inside the element body; a measurement object gas introduction segment that introduces the measurement object gas and allows the measurement object gas to flow to the measurement object gas-side electrode; a reference gas introduction segment that introduces a reference gas serving as a reference for detecting a specific gas concentration in the measurement object gas and flows the reference gas to the reference electrode; a detection device that detects the specific gas concentration in the measurement object gas based on an electromotive force generated between the reference electrode and the measuring electrode; and a reference gas adjusting device that supplies an oxygen pumping current between the reference electrode and the measurement object gas-side electrode and pumps oxygen from the vicinity of the measurement object gas-side electrode to the vicinity of the reference electrode.

[0007] If A[µA] is to be a limiting current when oxygen is pumped from the vicinity of the measuring object gas-side electrode to the vicinity of the reference electrode, with the measuring object gas introduction segment being exposed to an atmosphere with an oxygen concentration of 1000 ppm, and B[µA] is to be a limiting current when oxygen is pumped from the vicinity of the reference electrode to the vicinity of the measuring object gas-side electrode, with the reference gas introduction segment being exposed to an air atmosphere, then a ratio A / B is greater than or equal to 0.005.

[0008] In the gas sensor, oxygen is pumped into the vicinity of the reference electrode by passing an oxygen pumping current between the reference electrode and the measured-object gas-side electrode. This can compensate, for example, for the reduction in oxygen concentration in the vicinity of the reference electrode when the measured-object gas enters the reference gas introduction segment. In the gas sensor, the ratio A / B is greater than or equal to 0.005, where A is the limiting current when oxygen is pumped from the vicinity of the measured-object gas-side electrode to the vicinity of the reference electrode, and B is the limiting current when oxygen is pumped from the vicinity of the reference electrode to the vicinity of the measured-object gas-side electrode. The limiting current A has a negative correlation with the diffusion resistance of the measured-object gas introduction segment.In addition, the limiting current B has a negative correlation with the diffusion resistance of the reference gas introduction segment. Since the ratio A / B of the gas sensor is greater than or equal to 0.005, the diffusion resistance of the measured object gas introduction segment is not too high, and the diffusion resistance of the reference gas introduction segment is not too low, so the oxygen concentration around the reference electrode is unlikely to be reduced.

[0009] In this case, the measuring object gas side electrode may be arranged outside the element body or on the inner surface of the measuring object gas flow segment and on the upstream side of the measuring object gas relative to the measuring electrode.

[0010] In the gas sensor of the present invention, the ratio A / B can be greater than or equal to 0.4. With this setting, the oxygen concentration in the reference gas around the reference electrode in the gas sensor is unlikely to be reduced.

[0011] In the gas sensor of the present invention, the A / B ratio can be less than or equal to 125. With this setting, an excessively high oxygen concentration in the reference gas in the vicinity of the reference electrode can be avoided due to excessive accumulation of oxygen pumped into the vicinity of the reference electrode. In this case, the A / B ratio can be less than or equal to 25.

[0012] In the gas sensor of the present invention, the limit current A can be from 1 µA to 10,000 µA. When the limit current A is greater than or equal to 1 µA, the A / B ratio can be easily adjusted to 0.005 or higher. When the limit current A is less than or equal to 10,000 µA, the A / B ratio can be easily adjusted to 125 or less.

[0013] In the gas sensor of the present invention, the limit current B can be between 8 µA and 200 µA. When the limit current B is greater than or equal to 8 µA, the A / B ratio can be easily adjusted to 125 or less. When the limit current B is less than or equal to 200 µA, the A / B ratio can be easily adjusted to 0.005 or higher.

[0014] In the gas sensor of the present invention, C[mm 2 ] the area of ​​the measuring object gas side electrode and D[mm 2 ] be the area of ​​the reference electrode, then the area C can be greater than or equal to 1.0 mm 2, the area D greater than or equal to 0.5 mm 2 and the ratio C / D must be greater than or equal to 1 and less than or equal to 20. The area C has a negative correlation with the resistance value of the electrode on the measurement object gas side. Furthermore, the area D has a negative correlation with the resistance value of the reference electrode. Since the area C is greater than or equal to 1.0 mm 2 the resistance value of the electrode on the measuring object gas side is not too high. Since the area D is greater than or equal to 0.5 mm 2, the resistance value of the reference electrode is not too high. Since the ratio C / D is greater than or equal to 1, the resistance value of the measured object gas-side electrode is not too high with respect to the resistance value of the reference electrode. Since the ratio C / D is less than or equal to 20, the resistance value of the reference electrode is not too high with respect to the resistance value of the measured object gas-side electrode. Since the area C is greater than or equal to 1.0 mm 2 , the area D greater than or equal to 0.5 mm 2 and the ratio C / D is greater than or equal to 1 and less than or equal to 20, the reference gas setting device can easily supply a suitable oxygen pumping current.

[0015] In a gas sensor in one aspect of the present invention, wherein the ratio C / D is greater than or equal to 1 and less than or equal to 20, the area C may be less than or equal to 15.0 mm 2 If the area C is less than or equal to 15.0 mm2 the C / D ratio can easily be adjusted to 20 or less.

[0016] In the gas sensor according to an aspect of the present invention, in which the ratio C / D is greater than or equal to 1 and less than or equal to 20, the area D may be less than or equal to 4.0 mm 2 If the area D is less than or equal to 4.0 mm 2 the C / D ratio can easily be adjusted to 1 or greater.

[0017] In the gas sensor of this invention, the measurement object gas introduction segment may have a porous protective layer covering a part of the element body, the reference gas introduction segment may have a porous reference gas introduction layer, the porosity of the porous protective layer may be greater than or equal to 20% and less than or equal to 60%, and the porosity of the reference gas introduction layer may be greater than or equal to 15% and less than or equal to 50%. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a vertical cross-sectional view of a gas sensor 100. Fig. 2 is a schematic cross-sectional view showing an example of the configuration of a sensor device 101. Fig. 3 is a schematic cross-sectional view showing the configuration of the surroundings of an air introduction layer 248. Fig. 4 is a schematic cross-sectional view of a sensor device 201 in a modification. DETAILED DESCRIPTION OF THE INVENTION

[0018] Next, an embodiment of the present invention will be described with reference to the drawings. Fig. 1 is a vertical cross-sectional view of a gas sensor 100 of the present invention and Fig.Fig. 2 is a schematic cross-sectional view schematically illustrating an example of the configuration of a sensor device 101 included in the gas sensor 100. The sensor device 101 has an elongated rectangular parallelepiped shape, the longitudinal direction (the horizontal direction of Fig. 2) of the sensor device 101 is the front-to-back direction and the depth direction (the vertical direction from Fig. 2) of the sensor device 101 is the vertical direction. The width direction (the direction perpendicular to the front-to-back direction and the vertical direction) is also the horizontal direction.

[0019] As in Fig.As shown in Figure 1, the gas sensor 100 includes the sensor device 101, a protective cover 130 that protects the front end of the sensor device 101, and a sensor assembly 140 having a connector 150 that is electrically connected to the sensor device 101. The gas sensor 100 is mounted, as shown, on a pipe 190, such as an exhaust pipe of a vehicle, and is used to measure the concentration of a specific gas (NOx in this embodiment) contained in an exhaust gas provided as a measurement target gas. The sensor device 101 includes an element body 101a and a porous protective layer 95 that covers a part of the element body 101a.

[0020] The protective cover 130 includes a cylindrical inner-side protective cover 131 with a bottom that covers the front end of the sensor device 101, and a cylindrical outer-side protective cover 132 with a bottom that covers the inner-side protective cover 131. A plurality of holes are formed in the inner-side protective cover 131 and the outer-side protective cover 132 to allow a measurement object gas to flow into the protective cover 130. A sensor device chamber 133 is formed as the space surrounded by the inner-side protective cover 131, and the front end of the sensor device 101 is arranged in the sensor device chamber 133.

[0021] The sensor assembly 140 includes a device sealing body 141 that seals and fixes the sensor device 101, a nut 147, an outer tube 148 mounted on the device sealing body 141, and a connector 150 that is in contact with and electrically connected to the connector electrodes (only the heater connector 71 described later is shown in Fig. 2), wherein the connector electrodes are formed on the surface (vertical surface) of the rear end of the sensor device 101 and are not shown.

[0022] The device sealing body 141 includes a tubular main metal fitting 142, a tubular inner tube 143 coaxially welded and fixed to the main metal fitting 142, ceramic supports 144a to 144c, green pellets 145a, 145b, and a metal ring 146 sealed in through holes inside the main metal fitting 142 and the inner tube 143. The sensor device 101 is located on the central axis of the device sealing body 141 and penetrates the device sealing body 141 in the front-to-back direction. In the inner tube 143, a reduced diameter segment 143a for pressing the green pellet 145b in the central axial direction of the inner tube 143 and a reduced diameter segment 143b for pressing the ceramic carriers 144a to 144c, the green pellets 145a, 145b via the metal ring 146 forward are formed.The green pellets 145a, 145b are compressed between the main metal fitting 142, the inner tube 143 and the sensor device 101 by a pressing force from the reduced diameter segments 143a, 143b, so that the green pellets 145a, 145b seal between the sensor device chamber 133 within the protective cover 130 and the space 149 within the outer tube 148 and fix the sensor device 101.

[0023] The nut 147 is coaxially fixed to the main metal fitting 142, and an external thread segment is formed on the outer peripheral surface. The external thread segment of the nut 147 is inserted into a fastener 191 welded to the tube 190 and having an internal thread segment on the inner peripheral surface. Thus, the gas sensor 100 is fixed to the tube 190, with the front end of the sensor device 101 and the portion of the protective cover 130 of the gas sensor 100 projecting into the tube 190.

[0024] The outer tube 148 covers the periphery of the inner tube 143, the sensor device 101, and the connector 150, and a plurality of lead wires connected to the connector 150 are drawn outward from the rear end. The lead wires 155 are electrically connected to the electrodes (described later) of the sensor device 101 via the connector 150. The gap between the outer tube 148 and the lead wires 155 is sealed with a rubber plug 157. The space 149 inside the outer tube 148 is filled with a reference gas (air in this embodiment). The rear end of the sensor device 101 is disposed in the space 149.

[0025] As in Fig.As shown in Figure 2, the sensor device 101 is a device having a laminated body in which six layers: a first substrate layer 1, a second substrate layer 2, a third substrate layer 3, a first solid electrolyte layer 4, a spacer layer 5, and a second solid electrolyte layer 6 are laminated in this order in the drawing view from the bottom side. Each of the layers is an oxygen ion conductive solid electrolyte layer such as zirconium dioxide (ZrO2). In addition, the solid electrolyte constituting these six layers is extremely airtight. Such a sensor device 101 is manufactured, for example, by performing predetermined processing and printing a circuit pattern on each of the ceramic green sheets corresponding to the layers, laminating the sheets, and further calcining and integrating the sheets.

[0026] At a front end (an end toward the front side) of the sensor device 101, between the lower surface of the second solid electrolyte layer 6 and the upper surface of the first solid electrolyte layer 4, a gas introduction port 10, a first diffusion control segment 11, a buffer space 12, a second diffusion control segment 13, a first inner space 20, a third diffusion control segment 30, a second inner space 40, a fourth diffusion control segment 60, and a third inner space 61 are formed side by side so as to be able to communicate with each other in this order.

[0027] The gas introduction port 10, the buffer space 12, the first internal space 20, the second internal space 40, and the third internal space 61 are internal spaces of the sensor device 101, wherein the internal space is provided such that the spacer layer 5 is drilled, and wherein the upper part, the lower part, and the side part are defined by the lower surface of the second solid electrolyte layer 6, the upper surface of the first solid electrolyte layer 4, and the side surface of the spacer layer 5, respectively.

[0028] The first diffusion control segment 11, the second diffusion control segment 13 and the third diffusion control segment 30 are each provided as two horizontally long slits (their opening has a longitudinal direction in the direction perpendicular to Fig. 2). The fourth diffusion control segment 60 is formed as a horizontally long slit (its opening has a longitudinal direction in the direction perpendicular to Fig.2) is provided, which is formed as a gap with the lower surface of the second solid electrolyte layer 6. Note that the segment from the gas introduction port 10 to the third inner space 61 is also referred to as the measurement object gas flow segment.

[0029] An air introduction layer 48 is provided between the upper surface of the third substrate layer 3 and the lower surface of the first solid electrolyte layer 4. The air introduction layer 48 is a porous body made of ceramic, such as aluminum oxide. The air introduction layer 48 has an inlet segment 48c at the rear end face, and the inlet segment 48c is exposed to the rear end face of the sensor device 101. The inlet segment 48c is exposed to the space 149 of the Fig. 1 suspended (see Fig.1). A reference gas for measuring the NOx concentration is introduced into the air introduction layer 48c through the inlet segment 48c. In this embodiment, the reference gas is air (the atmosphere in the space 149 of Fig. 1). In addition, the air introduction layer 48 is formed to cover the reference electrode 42. The air introduction layer 48 imparts a predetermined diffusion resistance to the reference gas introduced through the inlet segment 48c and introduces the reference gas to the reference electrode 42.

[0030] The reference electrode 42 is an electrode formed between the upper surface of the third substrate layer 3 and the first solid electrolyte layer 4, and as described above, the vicinity of the electrode is provided with the air introduction layer 48. The reference electrode 42 is formed directly on the upper surface of the third substrate layer 3, and all but the portion of the reference electrode 42 in contact with the upper surface of the third substrate layer 3 is covered by the air introduction layer 48. In addition, as described later, it is possible to measure the oxygen concentration (oxygen partial pressure) in each of the first internal space 20, the second internal space 40, and the third internal space 61 using the reference electrode 42. The reference electrode 42 is formed as a porous cermet electrode (e.g., a cermet electrode composed of Pt and ZrO2).Without being particularly limited thereto, the reference electrode 42 has a length of, for example, 0.2 to 2 mm in the front-to-back direction, a width of, for example, 0.2 to 2.5 mm in the horizontal direction and a thickness of, for example, 5 to 30 mm.

[0031] In the measurement object gas flow segment, the gas introduction port 10 is a part open to the outside, and a measurement object gas is designed to be introduced from the outside into the sensor device 101 through the gas introduction port 10. The first diffusion control segment 11 is a segment that imparts a predetermined diffusion resistance to the measurement object gas taken out through the gas introduction port 10. The buffer space 12 is the space provided for introducing the measurement object gas introduced from the first diffusion control segment 11 into the second diffusion control segment 13. The second diffusion control segment 13 is a segment that imparts a predetermined diffusion resistance to the measurement object gas introduced from the buffer space 12 into the first internal space 20.When the measurement object gas is introduced into the first internal space 20 from the outside of the sensor device 101, the measurement object gas is not directly introduced into the first internal space 20 due to the pressure change (pulsation of exhaust pressure when the measurement object gas is exhaust gas from a motor vehicle) of the measurement object gas in the external space, but the pressure fluctuation of the measurement object gas is canceled by the first diffusion control segment 11, the buffer space 12, and the second diffusion control segment 13, and then the measurement object gas is introduced into the first internal space 20. Consequently, the pressure fluctuation of the measurement object gas introduced into the first internal space 20 is almost negligible. The first internal space 20 is provided as a space for adjusting the oxygen partial pressure in the measurement object gas introduced through the second diffusion control segment 13. This oxygen partial pressure is adjusted by the operation of a main pumping cell 21.

[0032] The main pumping cell 21 is an electrochemical pumping cell comprising an inside pumping electrode 22 having a ceiling electrode segment 22a provided on substantially the entire lower surface of the second solid electrolyte layer 6 facing the first internal space 20; an outside pumping electrode 23 provided to face the outside space (the sensor device chamber 133 of Fig. 1) is exposed to the upper surface of the second solid electrolyte layer 6 in a region corresponding to the ceiling electrode segment 22a; and contains the second solid electrolyte layer 6 disposed between these electrodes.

[0033] The inside pumping electrode 22 is formed via the upper and lower solid electrolyte layers (the second solid electrolyte layer 6 and the first solid electrolyte layer 4) defining the first inner space 20 and the spacer layer 5 forming the side wall.Specifically, the ceiling electrode segment 22a is formed in the lower surface of the second solid electrolyte layer 6 constituting the ceiling surface of the first internal space 20, a lower electrode segment 22b is directly formed in the upper surface of the first solid electrolyte layer 4 constituting the lower surface, a side electrode segment (not shown) is formed in the side wall surface (inner surface) of the spacer layer 5 constituting the two side wall segments of the first internal space 20 so that the ceiling electrode segment 22a and the lower electrode segment 22b are connected, and these segments are arranged in a structure of a tunnel shape at the arrangement position of the side electrode segment.

[0034] The inside pumping electrode 22 and the outside pumping electrode 23 are each formed as a porous cermet electrode (e.g., a cermet electrode composed of Pt with 1% Au and ZrO2). Note that the inside pumping electrode 22, in contact with the measurement target gas, is formed using a material that has a weakened reduction ability against the NOx content in the measurement target gas.

[0035] In the main pumping cell 21, a desired pumping voltage Vp0 is applied between the inside pumping electrode 22 and the outside pumping electrode 23 to cause a pumping current Ip0 to flow in the positive or negative direction between the inside pumping electrode 22 and the outside pumping electrode 23, whereby it is possible to pump out the oxygen in the first inside space 20 to the outside space or to pump the oxygen in the outside space into the first inside space 20.

[0036] In order to detect the oxygen concentration (oxygen partial pressure) in the atmosphere in the first inner space 20, an electrochemical sensor cell, in particular an oxygen partial pressure detection sensor cell 80 for main pump control is also provided, which comprises the inside pump electrode 22, the second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, the third substrate layer 3 and the reference electrode 42.

[0037] The oxygen concentration (oxygen partial pressure) in the first internal space 20 is known by measuring an electromotive force V0 in the oxygen partial pressure detection sensor cell 80 for main pump control. Furthermore, the pump current Ip0 is controlled by feedback control of the pump voltage Vp0 of a variable power supply 25 so that the electromotive force is constant. Thus, the oxygen concentration in the first internal space 20 can be maintained at a predetermined constant value.

[0038] The third diffusion control segment 30 is a segment that imparts a predetermined diffusion to the measurement object gas whose oxygen concentration (oxygen partial pressure) is controlled by the operation of the main pump cell 21 in the first internal space 20 and introduces the measurement object gas into the second internal space 40.

[0039] The second internal space 40 is provided as a space for further adjusting the oxygen partial pressure of a measurement target gas through an auxiliary pumping cell 50, which gas is introduced through the third diffusion control segment 30 after the oxygen concentration (oxygen partial pressure) in the first internal space 20 has been pre-adjusted. Consequently, the oxygen concentration in the second internal space 40 can be kept constant with high accuracy, so that the NOx concentration can be measured with high accuracy by the gas sensor 100.

[0040] The auxiliary pumping cell 50 is an auxiliary electrochemical pumping cell including an auxiliary pumping electrode 51 having a ceiling electrode segment 51a provided on substantially the entire lower surface of the second solid electrolyte layer 6 facing the second internal space 40; the outside pumping electrode 23 (not limited to the outside pumping electrode 23, but a suitable electrode outside the sensor device 101 will suffice); and the second solid electrolyte layer 6.

[0041] Such an auxiliary pumping electrode 51 is arranged in the second internal space 40 in a tunnel-shaped structure similar to the inside pumping electrode 22 provided in the first internal space 20. Specifically, the ceiling electrode segment 51a is formed for the second solid electrolyte layer 6, which forms the ceiling surface of the second internal space 40; a lower electrode segment 51b is formed directly on the upper surface of the first solid electrolyte layer 4, which forms the lower surface of the second internal space 40; and a side electrode segment (not shown) connecting the ceiling electrode segment 51a and the lower electrode segment 51b is formed on each of the two wall surfaces of the spacer layer 5, which forms the side wall of the second internal space 40, and has a tunnel-shaped structure.It should be noted that, similarly to the inside pumping electrode 22, the auxiliary pumping electrode 51 is also formed using a material having a weakened reducing ability against the NOx content in the measurement object gas.

[0042] In the auxiliary pump cell 50, it is possible to pump the oxygen in the atmosphere in the second internal space 40 to the external space or to pump the oxygen in the external space to the second internal space 40 by applying a desired voltage Vp1 across the auxiliary pump electrode 51 and the external pump electrode 23.

[0043] In addition, for controlling the oxygen partial pressure in the atmosphere in the second interior space 40, an electrochemical sensor cell, in particular an oxygen partial pressure detection sensor cell 81 for auxiliary pump control is provided, which contains the auxiliary pump electrode 51, the reference electrode 42, the second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4 and the third substrate layer 3.

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

[0045] In addition, a pumping current Ip1 is used to control the electromotive force of the oxygen partial pressure detection sensor cell 80 for the main pump control. Specifically, the pumping current Ip1 is supplied to the oxygen partial pressure detection sensor cell 80 for the main pump control as a control signal, and the electromotive force V0 is controlled so that the gradient of the oxygen partial pressure of the measurement target gas introduced from the third diffusion control segment 30 into the second internal space 40 is always controlled constant. When the oxygen partial pressure detection sensor cell 80 for the main pump control is used as a NOx sensor, the oxygen concentration in the second internal space 40 is maintained at a constant value of approximately 0.001 ppm by the function of the main pump cell 21 and the auxiliary pump cell 50.

[0046] The fourth diffusion control segment 60 is a segment that imparts a predetermined diffusion resistance to the measurement target gas, whose oxygen concentration (oxygen partial pressure) is controlled by the operation of the auxiliary pump cell 50 in the second internal space 40, and introduces the measurement target gas into the third internal space 61. The fourth diffusion control segment 60 functions to limit the amount of NOx that has flowed into the third internal space 61.

[0047] The third internal space 61 is provided as a space for performing processing related to the measurement of the nitrogen oxide (NOx) concentration in a measurement target gas, which gas is introduced through the fourth diffusion control segment 60 after the oxygen concentration (oxygen partial pressure) in the second internal space 40 is adjusted in advance. The NOx concentration is measured by operating a pump cell 41 for measurement primarily in the third internal space 61.

[0048] The measurement pump cell 41 measures the NOx concentration in a measurement target gas in the third internal space 61. The measurement pump cell 41 is an electrochemical pump cell including a measurement electrode 44 disposed directly on the upper surface of the first solid electrolyte layer 4 facing the third internal space 61, the outside pump electrode 23, the second solid electrolyte layer 6, the spacer layer 5, and the first solid electrolyte layer 4. The measurement electrode 44 is a porous cermet electrode. The measurement electrode 44 also functions as a NOx reduction catalyst that reduces the NOx present in the atmosphere in the third internal space 61.

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

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

[0051] The measurement target gas introduced into the second internal space 40 passes through the fourth diffusion control segment 60 to the measuring electrode 44 of the third internal space 61, where the oxygen partial pressure is controlled. The nitrogen oxide in the measurement target gas in the vicinity of the measuring electrode 44 is reduced (2NO → N2 + O2), and oxygen is generated. The generated oxygen is pumped by the pump cell 41 for measurement, and at this time, a voltage Vp2 of the variable power supply 46 is controlled so that the electromotive force V2 detected by the oxygen partial pressure detection sensor cell 82 for measurement pump control is kept constant. The amount of oxygen generated in the vicinity of the measuring electrode 44 is proportional to the concentration of nitrogen oxide in the measurement target gas; therefore, the nitrogen oxide concentration in the measurement target gas is calculated using the pump current Ip2 in the pump cell 41 for measurement.

[0052] An electrochemical sensor cell 83 further includes the second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, the third substrate layer 3, the outside pumping electrode 23, and the reference electrode 42. The oxygen partial pressure in the measurement object gas outside the sensor is detectable by an electromotive force Vref obtained from the sensor cell 83.

[0053] In addition, an electrochemical reference gas adjusting pumping cell 90 includes the second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, the third substrate layer 3, the outside pumping electrode 23, and the reference electrode 42. The electrochemical reference gas adjusting pumping cell 90 performs pumping by passing a control current (oxygen pumping current) Ip3 caused by a control voltage Vp3 applied by a power supply circuit 92 connected between the outside pumping electrode 23 and the reference electrode 42. Thus, the reference gas adjusting pumping cell 90 pumps oxygen from the space (the sensor device chamber 133 of Fig. 1) in the vicinity of the external pump electrode 23 into the vicinity of the reference electrode 42.

[0054] In the gas sensor 100 having such a configuration, a measurement object gas having an oxygen partial pressure maintained at a constant low value all the time (a value that has substantially no influence on the measurement of NOx) is supplied to the pumping cell 41 for measurement by operating the main pumping cell 21 and the auxiliary pumping cell 50. Thus, the NOx concentration in the measurement object gas can be known based on the pumping current Ip2 carried by pumping oxygen through the pumping cell 41 for measurement, where oxygen is substantially proportional to the concentration of NOx in the measurement object gas and is generated by the reduction of NOx.

[0055] Furthermore, the sensor device 101 includes a heating unit 70, which has a function of adjusting the temperature by heating the sensor device 101 and maintaining the temperature to improve the oxygen ion conductivity of the solid electrolyte. The heating unit 70 includes a heater connector electrode 71, a heater 72, a through-hole 73, a heater insulating layer 74, a pressure diffusion hole 75, and a lead wire 76.

[0056] The heater connector electrode 71 is an electrode shaped to allow connection to the bottom surface of the first substrate 1. It is possible to externally supply electrical power to the heater unit 70 by connecting the heater connector electrode 71 to an external power supply.

[0057] The heater 72 is an electrical resistor configured to be vertically disposed between the second substrate layer 2 and the third substrate layer 3. The heater 72 is connected to the heater connector electrode 71 via the lead wire 76 and the through-hole 73, is heated by the external electrical supply from the electrolyte through the heater connector electrode 71, and performs heating and temperature maintenance of the solid body constituting the sensor device 101.

[0058] In addition, the heater 72 is embedded over the entire area from the first inner space 20 to the third inner space 61 and can adjust the entire sensor device 101 to a temperature that causes activation of the solid electrolyte.

[0059] The heater insulating layer 74 is an insulating layer containing porous alumina, which is made of an insulator such as alumina. The heater insulating layer 74 is formed to provide electrical insulation between the second substrate layer 2 and the heater 72, and to provide electrical insulation between the third substrate layer 3 and the heater 72.

[0060] The pressure diffusion hole 75 is a segment provided to penetrate the third substrate layer 3 and the air introduction layer 48, and formed for the purpose of reducing the increase of an internal pressure in association with the temperature increase in the heater insulating layer 74.

[0061] It should be noted that the Fig. 2 are actually supplied via lead wires (not shown) provided in the sensor device 101 and the connector 150 and the lead wires 155 of Fig.1 are formed, are connected to electrodes.

[0062] As in Fig. 1, Fig. 2, a part (here, the front end part of the element body 101a) of the element body 101a is covered by the porous protective layer 95. As shown in Fig.2, the porous protective layer 95 covers part of the upper and lower surfaces of the element body 101a, and although one illustration is omitted, the porous protective layer 95 also covers part of the left surface and right surface of the element body 101a. The porous protective layer 95 covers the front surface of the front end. The porous protective layer 95 covers the outside pumping electrode 23. The porous protective layer 95 also covers the gas introduction port 10. Since the porous protective layer 95 is a porous body, the measurement object gas in the sensor device chamber 133 can reach the outside pumping electrode 23 or the gas introduction port 10 by flowing through the inside of the porous protective layer 95.Thus, the porous protective layer 95 functions as a measurement object gas introduction segment that introduces a measurement object gas from the outside (here, the sensor device chamber 133) and allows the gas to flow to the external pumping electrode 23. The porous protective layer 95 covers and protects a portion of the element body 101a. The porous protective layer 95 functions, for example, to prevent cracks in the element body 101a due to the adhesion of water in the measurement object gas to the element body 101a. Furthermore, the porous protective layer 95 functions to prevent poisonous substances, such as oil content in the measurement object gas, from adhering to the external pumping electrode 23 and to reduce degradation of the external pumping electrode 23.

[0063] The porous protective layer 95 is a porous body containing ceramic particles as constituent particles. In this embodiment, the porous protective layer 95 is assumed to be made of a porous aluminum oxide material. The porosity of the porous protective layer 95 is, for example, greater than or equal to 20% and less than or equal to 60%. The thickness of the porous protective layer 95 is, for example, greater than or equal to 100 µm and less than or equal to 800 µm.

[0064] The above-described air introduction layer 48 functions as a reference gas introduction segment, introducing a reference gas (here, air) serving as a reference for detecting the NOx concentration in a measurement target gas and allowing the reference gas to flow to the reference electrode 42. The porosity of the air introduction layer 48 is, for example, greater than or equal to 15% and less than or equal to 50%.

[0065] Next, an example of a manufacturing method for the gas sensor 100 is described below. First, six uncalcined ceramic green sheets containing an oxygen ion-conductive solid electrolyte, such as zirconium oxide, are prepared as the ceramic content. A plurality of plate holes and necessary through-holes are formed in the green sheets for positioning at the time of printing or at the time of lamination. In addition, a green sheet for the spacer layer 5 is provided with a space serving as a measurement target gas flow segment by performing punching or the like.For each of the first substrate layer 1, the second substrate layer 2, the third substrate layer 3, the first solid electrolyte layer 4, the spacer layer 5, and the second solid electrolyte layer 6, pattern printing and dry processing are performed to form various patterns on a corresponding ceramic green sheet. Specifically, the formed patterns include, for example, the above-described electrodes and the lead wires connected to the electrodes, the air introduction layer 48, and the heating unit 70. Pattern printing is performed by applying pattern-forming paste to a green sheet using a known screen printing technique, and the pattern-forming paste is prepared according to the properties required for an article to be molded. Dry processing is also performed using a known drying device.After pattern printing and drying are completed, printing and dry processing are performed on a bonding paste for layering and bonding the green sheets corresponding to the layers. The green sheets on which the bonding paste is formed are positioned with the plate holes, layered in a predetermined order, and bonded under pressure by applying predetermined temperature and pressure conditions. Thus, pressure bonding is performed to obtain a single-layered body. The thus obtained layered body contains a plurality of device bodies 101a. The layered body is cut and divided into pieces the size of the element body 101a. The divided layered body is calcined at a predetermined calcination temperature to obtain the individual device bodies 101a.

[0066] Subsequently, the porous protective layer 95 is formed in the element body 101a to obtain the sensor device 101. The porous protective layer 95 can be formed using, for example, plasma spraying, screen printing, gel casting, or dipping. When the porous protective layer 95 is formed by a method involving calcination, such as screen printing or dipping, the porous protective layer 95 can be formed in the element body 101a before calcination, and the sensor device 101 can be obtained by calcining both.

[0067] When the sensor device 101 is manufactured in this way, the sensor structure 140 (see Fig. 1) including the sensor device 101 is manufactured, and the gas sensor 100 is manufactured by mounting the protective cover 130 and a rubber plug 157 on the sensor assembly 140.

[0068] Here, the functions of the reference gas adjustment pump cell 90 are described in detail. A measurement object gas is drawn from the Fig. 1 is introduced into the measurement object gas flow segment, such as the gas introduction port 10, of the sensor device 101. In contrast, a reference gas (air) is introduced within the Fig.1 into the air introduction layer 48 of the sensor device 101. The sensor device chamber 133 and the space 149 are delimited and sealed by the sensor assembly 140 (specifically, the green pellets 145a, 145b), so that no gas flows from one to the other. However, a small amount of gas may penetrate from the sensor device chamber 133 into the space 149. Therefore, when the oxygen concentration in the vicinity of the reference electrode 42 is reduced, the reference potential, which is the potential of the reference electrode 42, changes. This changes an electromotive force relative to the reference electrode 42, such as the electromotive force V2 of the oxygen partial pressure detection sensor cell 82 for measuring pump control, and reduces the detection accuracy of the NOx concentration in the measurement target gas. The reference gas adjusting pump cell 90 has the function of reducing such a reduction in detection accuracy.The reference gas adjustment pump cell 90 supplies the control current Ip3 (oxygen pump current) by applying the control voltage Vp3 across the reference electrode 42 and the outside pump electrode 23, thereby pumping oxygen from the vicinity of the outside pump electrode 23 to the vicinity of the reference electrode 42. Thus, the reduction in oxygen concentration caused by supplying oxygen to the vicinity of the reference electrode 42 can be reduced, and the detection accuracy can be reduced. As described above, the measurement object gas in the sensor device chamber 133 is introduced through the porous protective layer 95 into the vicinity of the outside pump electrode 23. Thus, the reference gas adjustment pump cell 90 pumps the oxygen of the measurement object gas that has reached the vicinity of the outside pump electrode 23 through the porous protective layer 95 into the vicinity of the reference electrode 42.

[0069] In this embodiment, the ratio A / B is greater than or equal to 0.005, where A[µA] is the limiting current when oxygen is pumped from the vicinity of the external pumping electrode 23 to the vicinity of the reference electrode 42 with the porous protective layer 95 exposed to an atmosphere with an oxygen concentration of 1000 ppm, and B[µA] is the limiting current when oxygen is pumped from the vicinity of the reference electrode 42 to the vicinity of the external pumping electrode 23 with the air introduction layer 48 exposed to the air atmosphere. The ratio A / B is set to 0.005 or greater, so the amount of oxygen pumped by the reference gas adjustment pumping cell 90 to the vicinity of the reference electrode 42 is likely to be insufficient. Consequently, in the gas sensor 100 of this embodiment, the oxygen concentration in the reference gas in the vicinity of the reference electrode 42 is unlikely to be reduced.

[0070] The measurement procedure for the limiting current A is as follows: First, the porous protective layer 95 of the sensor device 101 is exposed to an atmospheric gas with nitrogen as the base gas and an oxygen concentration of 1000 ppm. For example, the gas sensor 100 is mounted on the tube 190 as shown in Fig.1, and the atmospheric gas is passed through the pipe 190. Thus, the front end portion, including the porous protective layer 95, of the sensor device 101 is exposed to the atmospheric gas. Although the oxygen concentration in the vicinity of the air introduction layer 48 has substantially no influence on a measured value of the limiting current A, the rear end portion, including the air introduction layer 48, of the sensor device 101 is exposed to the atmospheric air. Next, the sensor device 101 is heated to a predetermined temperature (e.g., 800°C) by turning on the heater 72. Each of the variable power supplies 25, 46, 52 and the power supply circuit 92 is in a state where no voltage is applied thereto.After the temperature of the sensor device 101 has stabilized, the control voltage Vp3 is applied across the external pumping electrode 23 and the reference electrode 42, so that oxygen is pumped from the vicinity of the external pumping electrode 23 to the vicinity of the reference electrode 42. The control current Ip3 (oxygen pumping current) flowing between the two electrodes 23, 42 at this time is measured. The control voltage Vp3 is assumed to be a DC voltage. Subsequently, with the gradual increase of the control voltage Vp3, the control current Ip3 also gradually increases. However, ultimately, even with the increase of the control voltage Vp3, the control current Ip3 does not increase and reaches an upper limit. The upper limit at this time is measured as the limit current A.The flow volume of the gas that passes through the measurement object gas introduction segment (here, the porous protective layer 95) into the vicinity of the outside pumping electrode 23 depends on the diffusion resistance of the porous protective layer 95. Specifically, the flow volume of the gas that reaches the vicinity of the outside pumping electrode 23 depends on the diffusion resistance (hereinafter simply referred to as the "diffusion resistance of the porous protective layer 95") of the portion of the porous protective layer 95 that serves as the gas path from the outside to the outside pumping electrode 23. What particularly affects the diffusion resistance of the porous protective layer 95 is the diffusion resistance of the surrounding portion of the outside pumping electrode 23, such as the portion immediately above the outside pumping electrode 23, the porous protective layer 95.The limiting current A has a negative correlation with the diffusion resistance of the porous protective layer 95, so it has a smaller value for a larger diffusion resistance. The limiting current A can be adjusted by changing the porosity of the porous protective layer 95, by changing the length (the thickness of the porous protective layer 95) of the gas flowing from the outside of the sensor device 101 to the outside pumping electrode 23 of the porous protective layer 95, or by changing the cross-sectional area of ​​the porous protective layer 95 when the porous protective layer 95 is cut by a plane perpendicular to the gas flowing from the outside of the sensor device 101 to the outside pumping electrode 23 of the porous protective layer 95.

[0071] The measurement method for the limiting current B is as follows: First, the air-introducing layer 48 is exposed to the air atmosphere. For example, similar to the measurement method for the limiting current A described above, the gas sensor 100 is mounted on the tube 190, and the rear end portion containing the air-introducing layer 48 of the sensor device 101 is exposed to the air atmosphere. Although the oxygen concentration in the vicinity of the porous protective layer 95 has substantially no influence on a measured value of the limiting current B, the interior of the tube 190 is placed in the air atmosphere, and the porous protective layer 95 is exposed to the air atmosphere. Next, the limiting current B is measured in the same manner as the limiting current A, except that the control voltage Vp3 is applied so that oxygen is pumped from the vicinity of the reference electrode 42 to the vicinity of the outside pumping electrode 23.In particular, even when the control voltage Vp3 is increased, the control current Ip3 (oxygen pumping current) does not increase, and an upper limit of the control current Ip3 is reached, and the limit current B is measured as the upper limit value. The flow volume of the gas that enters the vicinity of the reference electrode 42 via the reference gas introduction segment (here, the air introduction layer 48) depends on the diffusion resistance of the air introduction layer 48. Thus, the limit current B has a negative correlation with the air introduction layer 48 and has a smaller value for a larger diffusion resistance.The limiting current B can be adjusted by changing the porosity of the air introduction layer 48, by changing the length of the gas in the flow direction (here, the front-to-back direction) from the outside of the sensor device 101 to the reference electrode 42 of the air introduction layer 48, or by changing the cross-sectional area of ​​the air introduction layer 48 when the air introduction layer 48 is cut by a plane perpendicular to the flow direction of the gas from the outside of the sensor device 101 to the reference electrode 42 of the air introduction layer 48.

[0072] Here, if the limit current A is too small, that is, if the diffusion resistance of the porous protective layer 95 is too high, it is difficult for the measurement object gas to enter the vicinity of the external pumping electrode 23, so that the oxygen in the vicinity of the external pumping electrode 23 is likely to be insufficient. In this case, the reference gas adjusting pumping cell 90 may not be able to pump a sufficient amount of oxygen into the vicinity of the reference electrode 42. Also, if the limit current is too large, that is, the diffusion resistance of the air introducing layer 48 is too high, the oxygen pumped by the reference gas adjusting pumping cell 90 into the vicinity of the reference electrode 42 is likely to flow out via the air introducing layer 48, and the oxygen concentration in the vicinity of the reference electrode 42 may not be maintained.To address this, in the gas sensor 100 in this embodiment, the ratio A / B is greater than or equal to 0.005, so that the diffusion resistance of the porous protective layer 95 is not too high and the diffusion resistance of the air introduction layer 48 is not too low. Therefore, the oxygen concentration in the vicinity of the reference electrode 42 is unlikely to be reduced.

[0073] As described above, the limiting current A is measured in an atmosphere with an oxygen concentration of 1000 ppm (= 0.1%), and the limiting current B is measured in an atmosphere (oxygen concentration 20.5%). Therefore, when the diffusion resistance of the porous protective layer 95 is equal to the diffusion resistance of the air introduction layer 48, the ratio A / B = (0.1%) / (20.5%) is obtained, which is approximately 0.005. In other words, the ratio A / B greater than 0.005 means that the diffusion resistance of the porous protective layer 95 is less than or equal to the diffusion resistance of the air introduction layer 48. The ratio A / B is preferably greater than or equal to 0.4. In this case, the oxygen concentration in the vicinity of the reference electrode 42 is less likely to be reduced.

[0074] If the limiting current A is relatively large, i.e., the diffusion resistance of the porous protective layer 95 is relatively low, and if the limiting current B is too small, i.e., the diffusion resistance of the air introduction layer 48 is too high, the oxygen pumped into the vicinity of the reference electrode 42 becomes too enriched, and the oxygen concentration in the vicinity of the reference electrode 42 may become too high. Therefore, the ratio A / B is preferably less than or equal to 125, and more preferably less than or equal to 25.

[0075] The limit current A is preferably 1 µA to 10,000 µA. When the limit current A is greater than or equal to 1 µA, the A / B ratio can be easily adjusted to 0.005 or higher. When the limit current A is less than or equal to 10,000 µA, the A / B ratio can be easily adjusted to 125 or less. The limit current A can be greater than or equal to 10 µA, or greater than or equal to 20 µA. The limit current A can be less than or equal to 1,000 µA, less than or equal to 500 µA, or less than or equal to 400 µA.

[0076] The limit current B is preferably between 8 µA and 200 µA. The limit current B can be greater than or equal to 10 µA or greater than or equal to 20 µA. The limit current B can be less than or equal to 100 µA or less than or equal to 80 µA.

[0077] Let C[mm 2 ] the area of ​​the external pump electrode 23, D[mm 2] the area of ​​the reference electrode 42, then it is preferred that at least one of the following three conditions is met. The area C is greater than or equal to 1.0 mm 2 , the area D is greater than or equal to 0.5 mm 2and the ratio C / D is greater than or equal to 1 and less than or equal to 20. It is more preferable if all three conditions are met. The area C of the outside pumping electrode 23 is the area (here, the area of ​​the upper surface of the outside pumping electrode 23) viewed in a direction perpendicular to the surface (here, the upper surface of the second solid electrolyte layer 6) on which the outside pumping electrode 23 is disposed. The area D of the reference electrode 42 is the area (here, the area of ​​the upper surface of the reference electrode 42) viewed in a direction perpendicular to the surface (here, the upper surface of the first solid electrolyte layer 4) on which the reference electrode 42 is disposed. Here, the area C has a negative correlation with the resistance value of the outside pumping electrode 23. In addition, the area D has a negative correlation with the resistance value of the reference electrode 42. Since the area C is greater than or equal to 1.0 mm 2the resistance value of the external pump electrode 23 is not too high. Since the area D is greater than or equal to 0.5 mm 2 , the resistance value of the reference electrode 42 is not too high. Since the ratio C / D is greater than or equal to 1, the resistance value of the external pumping electrode 23 is not too high compared to the resistance value of the reference electrode 42. Since the ratio C / D is less than or equal to 20, the resistance value of the reference electrode is not too high compared to the resistance value of the measurement object gas-side electrode. If the area C is greater than or equal to 1.0 mm 2 , the area D greater than or equal to 0.5 mm 2 and the ratio C / D is greater than or equal to 1 and less than or equal to 20, a corresponding control current Ip3 (oxygen pumping current) can therefore be easily supplied from the reference gas adjusting pumping cell 90.

[0078] The area C is preferably greater than or equal to 1.0 mm 2and less than or equal to 15.0 mm 2 If the area C is greater than or equal to 1.0 mm 2 the ratio C / D can easily be adjusted to 1 or greater. If the area C is less than or equal to 15.0 mm 2 it is easy to set the C / D ratio to 20 or less.

[0079] The area D is preferably greater than or equal to 0.5 mm 2 and less than or equal to 4.0 mm 2 If the area D is greater than or equal to 0.5 mm 2 it is easy to set the ratio C / D to 20 or less. If the area D is less than or equal to 4.0 mm 2 it is easy to set the C / D ratio to 1 or greater.

[0080] As the area D of the reference electrode 42 is increased, the electrostatic capacitance between the reference electrode 42 and another electrode also increases. Therefore, if the area D of the reference electrode 42 is too large, when the electromotive forces V0, V1, V2 measured relative to the reference electrode 42 change, it takes longer for each voltage to stabilize, which may reduce the responsiveness of the sensor device 101. Also, in this regard, the area D is preferably less than or equal to 4.0 mm. 2 .

[0081] The thickness of the external pumping electrode 23 is, for example, greater than or equal to 10 µm and less than or equal to 40 µm. The thickness of the reference electrode 42 is, for example, greater than or equal to 10 µm and less than or equal to 40 µm. The external pumping electrode 23 and the reference electrode 42 are each a thin, flat, plate-shaped electrode, so that the above-described areas C, D have more effect on the performance of the sensor device 101 than the thickness. However, in addition to setting the ratio C / D to 1 or greater, it is also preferable that a ratio E / F be set to 1 or greater, where E is the volume of the external pumping electrode 23 and F is the volume of the reference electrode 42.

[0082] The control current Ip3 (oxygen pumping current) supplied by the reference gas adjustment pump cell 90 can be determined in advance so that the oxygen concentration in the vicinity of the reference electrode 42 can be maintained at a suitable value (e.g., the air atmosphere, i.e., the oxygen concentration is 20.5%). For example, if P[µA] is the average value of the control current (oxygen pumping current) Ip3, the ratio B / P can be set to 0.8 to 10. The average value P can be 1 to 30 µA. The higher the limit current B, the more likely it is that oxygen will remain in the vicinity of the reference electrode 42. Thus, the oxygen concentration in the vicinity of the reference electrode 42 can be easily maintained at a suitable value by setting the average value P according to the limit current B. Even if the limit current A is too small when setting a suitable average value P, i.e.,If the diffusion resistance of the porous protective layer 95 is too high, the oxygen in the vicinity of the outside pumping electrode 23 may become insufficient over time, and the average value of the control current Ip3 may become smaller than a predetermined value (average value P). However, since the ratio A / B is greater than or equal to 0.005, the oxygen in the vicinity of the outside pumping electrode 23 in the gas sensor 100 of this embodiment is unlikely to be insufficient. Therefore, even if oxygen is pumped from the reference gas adjustment pumping cell 90 for a long time, the average value of the control current Ip3 is likely to be maintained at a predetermined value (average value P).

[0083] Herein, the correspondence between the components of this embodiment and the components of the present invention will be explained. The element body 101a of this embodiment corresponds to the element body of the present invention, the measuring electrode 44 corresponds to the measuring electrode, the external pumping electrode 23 corresponds to the measurement-object gas-side electrode, the reference electrode 42 corresponds to the reference electrode, the porous protective layer 95 corresponds to the measurement-object gas introduction segment, the air introduction layer 48 corresponds to the reference gas introduction layer, the pumping cell 41 for measurement corresponds to the detection device, and the reference gas adjustment pumping cell 90 corresponds to the reference gas adjustment device.

[0084] With the gas sensor 100 of this embodiment described in detail above, the reference gas adjustment pumping cell 90 pumps oxygen into the vicinity of the reference electrode 42, making it possible to compensate for the reduction in the oxygen concentration in the vicinity of the reference electrode 42. Since the ratio A / B is greater than or equal to 0.005, the diffusion resistance of the porous protective layer 95 is not too high, and the diffusion resistance of the air introduction layer 48 is not too low. Therefore, the oxygen concentration of the reference gas in the vicinity of the reference electrode 42 is unlikely to be reduced.

[0085] Furthermore, since the A / B ratio is greater than or equal to 0.4, it is unlikely that the oxygen concentration in the reference gas in the vicinity of the reference electrode 42 will be reduced. Since the A / B ratio is less than 125, it is also possible to suppress an excessive oxygen concentration in the reference gas in the vicinity of the reference electrode, which is caused by excessive oxygen being pumped into the vicinity of the reference electrode.

[0086] Furthermore, since the limit current A is 1 µA to 10,000 µA, it is easy to adjust the A / B ratio to 0.005 or more and 125 or less. Since the limit current B is 8 µA to 200 µA, it is easy to adjust the A / B ratio to 0.005 or more and 125 or less.

[0087] In addition, since the area C is greater than or equal to 1.0 mm 2 , the area D greater than or equal to 0.5 mm 2and the ratio C / D is greater than or equal to 1 and less than or equal to 20, the reference gas adjustment pump cell 90 easily carries a corresponding control current Ip3 (oxygen pump current). Since the area C is less than or equal to 15.0 mm 2 the ratio C / D can easily be adjusted to 20 or less. Since the area D is less than or equal to 4.0 mm 2 it is easy to set the C / D ratio to 1 or greater.

[0088] It should be noted that the present invention is not limited to the above-described embodiment. Of course, the present invention can be implemented in various ways within the technical scope of the present invention.

[0089] For example, in the above-described embodiment, the reference gas adjustment pumping cell 90 can pump oxygen into the vicinity of the reference electrode 42 by supplying a pulse current as the control current (oxygen pumping current) Ip3 between the reference electrode 42 and the outside pumping electrode 23, and the pulse current is turned on and off at a predetermined period. The pumping cell 41 for measurement can detect the NOx gas concentration in the measurement target gas during a period in which the control current (oxygen pumping current) Ip3 is turned off. In this way, since the control current (oxygen pumping current) Ip3 is turned off when the NOx gas concentration in the measurement target gas is detected, the decrease in the detection accuracy of the NOx gas concentration caused by the control current (oxygen pumping current) Ip3 can be reduced.It should be noted that even in a period in which the control current (oxygen pumping current) Ip3 is turned off, the current value is not necessarily zero due to the electrostatic capacitance between the reference electrode 42 and the outside pumping electrode 23.

[0090] In the embodiment described above, the reference gas introduction segment includes only the air introduction layer 48, but this is not always the case. The reference gas introduction segment refers to a segment that introduces the reference gas and allows the gas to flow to the reference electrode 42. For example, instead of the air introduction layer 48, an air introduction layer 248, as shown in Fig. 3 shown. In Fig.3, a cavity 43 is provided inwardly at the rear end face of the element body 101a, and the upper surface of the porous air introduction layer 248 is exposed to the cavity 43. The cavity 43 is formed in its shape by notching the first solid electrolyte layer 4, which is sandwiched between the third substrate layer 3 and the spacer layer 5, from the rear end face. In the example of Fig. 3, the cavity 43 and the air introduction layer 248 correspond to the reference gas introduction segment. Thus, not only the air introduction layer 248, but also the shape of the cavity 43 affects the limiting current B.

[0091] In the embodiment described above, the external pumping electrode 23 serves as the outer electrode of the pumping cell 41 for measurement and the measurement-object gas-side electrode of the reference gas-adjusting pumping cell 90, but this is not always the case. The outer electrode of the pumping cell 41 for measurement and the measurement-object gas-side electrode of the reference gas-adjusting pumping cell 90 can be formed separately outside the element body 101a. Also, the measurement-object gas-side electrode of the reference gas-adjusting pumping cell 90 is not necessarily arranged outside the sensor body 101a, as long as the measurement-object gas-side electrode is arranged in a portion of the sensor device 101 exposed to the measurement-object gas. For example, the measurement-object gas-side electrode can be arranged within the measurement-object gas distribution segment. For example,The inside pumping electrode 22 can also serve as the measurement-object gas-side electrode of the reference gas adjustment pumping cell 90. In this case, the diffusion resistance of the porous protective layer 95 (specifically, a segment of the porous protective layer 95 serving as a path for the gas from the outside to the gas introduction port 10) and the path (specifically, the first diffusion control segment 11, the buffer space 12, the second diffusion control segment 13, and the first internal space 20) for the gas from the gas introduction port 10 to the inside pumping electrode 22 of the measurement-object gas distribution segment correspond to the measurement-object gas distribution segment. For example, the shapes of the first diffusion control segment 11 and the second diffusion control segment 13 also affect the limiting current A.Thus, the measurement object gas introduction segment means not only the porous protective layer 95, but also a segment that introduces the measurement object gas and allows the gas to flow to the measurement object gas-side electrode.

[0092] In the above-described embodiment, the sensor device 101 of the gas sensor 100 includes the first internal space 20, the second internal space 40, and the third internal space 61, but this is not always the case. For example, the sensor device 101 may not include the third internal space 61 as in the sensor device 201 in Fig. 4. In the sensor device 201, in a Fig.In the modification shown in Figure 4, between the lower surface of the second solid electrolyte layer 6 and the upper surface of the first solid electrolyte layer 4, the gas introduction port 10, the first diffusion control segment 11, the buffer space 12, the second diffusion control segment 13, the first inner space 20, the third diffusion control segment 30, and the second inner space 40 are arranged adjacent to each other in this order, allowing communication with each other. Furthermore, the measuring electrode 44 is arranged on the upper surface of the first solid electrolyte layer 4 in the second inner space 40. The measuring electrode 44 is covered by a diffusion control segment 45. The diffusion control segment 45 is a film made of a ceramic porous material such as alumina (Al2O3).Similar to the fourth diffusion control segment 60 of this embodiment described above, the diffusion control segment 45 functions to limit the amount of NOx flowing into the sensing electrode 44. Furthermore, the diffusion control segment 45 functions as a protective film for the sensing electrode 44. The ceiling electrode segment 51a of the auxiliary pumping electrode 51 is formed up to a position immediately above the sensing electrode 44. Also, with the sensor device 201 in this configuration, the NOx concentration can be detected by the pumping cell 41 for measurement in the same manner as in the above-described embodiment.

[0093] In the above-described embodiment, the voltage Vp2 of the variable power supply 46 is controlled so that the control voltage (electromotive force) V2 detected by the oxygen partial pressure detection sensor cell 82 for measuring pump control is constant, and the nitrogen oxide concentration in the measurement object gas is calculated using the pump current Ip2 present at that time. However, this is not always the case as long as a specific concentration is detected in the measurement object gas based on the voltage between the reference electrode 42 and the measuring electrode 44. For example, ifWhen an oxygen partial pressure detection device is configured as an electrochemical sensor cell by combining the measuring electrode 44, the first solid electrolyte layer 4, the third substrate layer 3, and the reference electrode 42, it is possible to detect an electromotive force corresponding to the difference between the amount of oxygen generated by reducing the NOx content in the atmosphere around the measuring electrode 44 and the amount of oxygen contained in the reference gas. Thus, the concentration of NOx content in the measurement target gas can be determined. In this case, the electrochemical sensor cell corresponds to the detection device of the present invention.

[0094] In the embodiment described above, the reference electrode 42 is formed directly on the upper surface of the third substrate layer 3, but this is not always the case. The reference electrode 42 can, for example, be formed directly on the lower surface of the first solid electrolyte layer 4.

[0095] In the embodiment described above, the reference gas is air. However, this is not always the case as long as the reference gas serves as a reference for detecting the concentration of a specific gas in the measurement object gas. For example, a gas with an adjusted predetermined oxygen concentration (> the oxygen concentration in the measurement object gas) can fill the space 149 as the reference gas.

[0096] In the above-described embodiment, the sensor device 101 detects the NOx concentration in the measurement target gas. However, this is not always the case as long as the sensor device 101 detects the concentration of a specific gas in the measurement target gas. For example, the oxygen concentration or ammonia concentration in the measurement gas can be detected. [Embodiment]

[0097] Below, an example in which the gas sensor is specifically manufactured will be described as one embodiment. Note that the present invention is not limited to the following embodiment. [Experiment Example 1]

[0098] The Fig. 1, Fig.The gas sensor 100 shown in Figure 2 was manufactured according to the above-described manufacturing method, which is referred to as Experimental Example 1. In the manufacture of the sensor device 101, zirconium dioxide particles were mixed with 4 mol% yttrium oxide as a stabilizer, an organic binder, and an organic solvent, and the green sheets were formed by tape casting. The green pellets 145a, 145b of Fig. 1 were obtained by molding talc particles. The porous protective layer 95 and the air introduction layer 48 are porous aluminum oxide materials. The porous protective layer 95 was formed by plasma spraying. The porosity of the porous protective layer 95 and the air introduction layer 48 was 20% each. The area C of the external pumping electrode 23 was set to 7.50 mm 2 and the area D of the reference electrode 42 to 1.85 mm 2The C / D ratio was 4.05. The thicknesses of the external pumping electrode 23 and the air introduction layer 48 were each 20 µm. When measuring the limiting currents A and B according to the measurement method described above, the limiting current A was 500 µA, the limiting current B was 20 µA, and the A / B ratio was 25.0. [Experiment examples 2 to 11]

[0099] The gas sensor 100 was manufactured in the same manner as in Experimental Example 1, except that the limit currents A, B were set as shown in Table 1, and Experimental Examples 2 to 11 were obtained. [Experiment examples 12 to 15]

[0100] The gas sensor 100 was manufactured in the same manner as in Experimental Example 1, except that the ratio A / B was set to 0.40 and the limiting currents A and B were set as shown in Table 2, and Experimental Examples 12 to 15 were obtained. Experimental Example 4 is also shown again in Table 2. [Experiment examples 16 to 23]

[0101] The gas sensor 100 was manufactured in the same manner as in Experimental Example 1, except that the ratio A / B was set to 25.0, the limiting currents A, B, the areas C, D, the thickness E of the external pumping electrode 23, and the thickness F of the reference electrode 42 were set as shown in Table 3, and Experimental Examples 16 to 23 were obtained. Experimental Example 1 is also shown again in Table 3. [Assessment of detection accuracy]

[0102] The gas sensor in Experimental Example 1 was mounted on a pipe. The temperature was set to 800°C by turning on the heater 72, and the sensor device 101 was heated. The control voltage Vp3 applied from the power supply circuit 92 of the reference gas adjustment pumping cell 90 was set to a pulse voltage (duty cycle of 20%) with a period T of 10 ms, an ON time T-on of 2 ms, and an OFF time T-off of 8 ms. The control voltage Vp3 applied from the power supply circuit 92 was set so that the control current Ip3 (oxygen pumping current) flowing through the reference electrode 42 at the time of voltage ON was 20 µA. The average value P of the control current Ip3 was 4 µA (= 20 µA × 20%). In this state, a model gas with nitrogen as the base gas, an oxygen concentration of 10% and a NOx concentration of 500 ppm was prepared and introduced into the tube as the measurement object gas.This state was maintained for 20 minutes, and the electromotive force Vref was measured during this time. The measurement was performed similarly in Experimental Examples 2 to 23. However, the control voltage Vp3 in Experimental Examples 16 to 23 was set to the same value as the control voltage Vp3 in Experimental Example 1. Specifically, in Experimental Examples 1 to 15, the average value P of the control current Ip3 was 4 μA each; however, in Experimental Examples 16 to 23, the average value P of the control current Ip3 was changed according to the resistance values ​​of the outside pumping electrode 23 and the reference electrode 42. Furthermore, in each of Experimental Examples 1 to 23, the average value P of the control current Ip3 was reduced as the oxygen around the outside pumping electrode 23 decreased over time.

[0103] Since the oxygen concentration around the reference electrode 42 becomes higher than the oxygen concentration in the air, the electromotive force Vref tends to increase over time from the value at the beginning of the measurement. As the electromotive force Vref increases, the pumping current Ip2 tends to decrease from a proper value (value corresponding to a NOx concentration of 500 ppm). Conversely, since the oxygen concentration around the reference electrode 42 becomes lower than the oxygen concentration in the air, the electromotive force Vref tends to decrease over time from the value at the beginning of the measurement. As the electromotive force Vref decreases, the pumping current Ip2 tends to increase from the proper value.

[0104] Therefore, the value of the electromotive force Vref is assumed to be 100% at the start of measurement, and if a measured electromotive force Vref is within a predetermined range (80% or more and 120% or less), even after 20 minutes have elapsed, it is determined that the detection accuracy of the NOx concentration is significantly high ("A"). If a measured electromotive force Vref was outside the predetermined range after 15 minutes and before 20 minutes have elapsed, it is determined that the detection accuracy of the NOx concentration is high ("B"). If a measured electromotive force Vref was outside the predetermined range before 15 minutes have elapsed, it is determined that the detection accuracy of the NOx concentration is low ("F").

[0105] The results of the evaluation tests described above are shown in Tables 1 to 3. In Tables 1 to 3, there was no test example in which the electromotive force Vref exceeded the upper limit of the specified range. In other words, in each of the test examples in Tables 1 and 3 where the determination was B or F, the electromotive force Vref was lower than the lower limit of the specified range and was outside the specified range. Table 1 Limit current A [µA] Limit current B [µA] A / B ratio Determination Experimental example 1 500 20 25,0 A Experimental example 2 10000 80 125,0 A Experimental example 3 20 20 1,0 A Experimental example 4 10 25 0,40 A Experimental example 5 400 30 13,3 A Experimental example 6 18 50 0,36 B Experimental Example 7 10 65 0,15 B Experimental Example 8 5 100 0,05 B Experimental example 9 1,0 200 0,005 B Experimental Example 10 1,0 300 0,003 F Experimental Example 11 1,0 500 0,002 F Table 2 Limit current A [µA] Limit current B [µA] A / B ratio Determination Experimental example 4 10 25 0,40 A Experimental Example 12 20 50 0,40 A Experimental Example 13 6 15 0,40 A Experimental Example 14 30 75 0,40 A Experimental Example 15 50 125 0,40 A Table 3 Limit current A[µA] Limit current B[µA] Ratio A / B Area C[mm 2 ] Area D[mm] 2 ] Ratio C / D Thickness E [µm] Thickness F [µm] Determination Experimental example 1 500 20 25,0 7,5 1,85 4,05 20 20 A Experimental Example 16 1000 40 25,0 7,5 1,85 4,05 40 40 A Experimental Example 17 200 8 250 7,5 1,85 4,05 10 10 A Experimental Example 16 500 20 25,0 3,0 3,0 1,00 20 20 A Experimental Example 16 500 20 25,0 15,0 4,0 3,75 20 20 A Experimental Example 20 500 20 25,0 1,0 1,0 1,00 20 20 B Experimental Example 21 500 20 25,0 8,0 0,5 16,00 20 20 B Experimental Example 22 500 20 25,0 1,0 0,7 1,43 20 20 B Experimental Example 23 500 20 25,0 2,0 3,0 0,57 20 20 B

[0106] As shown in Tables 1 to 3, when the A / B ratio was greater than or equal to 0.005, the evaluation was "A" or "B" and the NOx concentration detection accuracy was high (Experimental Examples 1 to 9, 12 to 15, 16 to 23). On the other hand, when the A / B ratio was less than 0.005, the evaluation was "F" and the NOx concentration detection accuracy was poor (Experimental Examples 10, 11). As also shown in the result of Table 1, in which the same conditions except for the limit currents A and B apply, when the A / B ratio was greater than or equal to 0.4, the evaluation was "A" and the NOx concentration detection accuracy was significantly high (Experimental Examples 1 to 5).As can also be seen from the result in Table 2, in which the A / B ratio is fixed and the limit currents A, B are changed, even with different values ​​of the limit currents A, B, when the A / B ratio is the same, the detection accuracy of the NOx concentration was almost the same (Experimental Examples 4, 12 to 15). From this result, it can be seen that the A / B ratio, rather than the values ​​of the limit currents, probably has more influence on the detection accuracy of the NOx concentration. Even when the A / B ratio is less than or equal to 125, the electromotive force Vref does not exceed the upper limit of the specified range, that is, the oxygen concentration of the reference gas in the vicinity of the reference electrode 42 does not become excessively high (Experimental Examples 1 to 23).

[0107] In addition, as shown in Table 3, for larger values ​​of the area C greater than or equal to 1.0 mm 2 and less than or equal to 15.0 mm 2, the area D greater than or equal to 0.5 mm 2 and less than or equal to 4.0 mm 2and the C / D ratio is greater than or equal to 1 and less than or equal to 20 (Experimental Examples 1, 16 to 22), the detection accuracy of the NOx concentration tends to be higher. For example, in Experimental Examples 18, 20 to 22, at least one of the area C, the area D, and the C / D ratio has the same value as the lower limit of the above-mentioned range, and in Experimental Examples 20 to 22, the evaluation was "B." In contrast, in Experimental Examples 1, 16, 17, and 19, each of the area C, the area D, and the C / D ratio has a value higher than the lower limit of the above-mentioned range, and the evaluation for each was "A." Also, in Experimental Example 23, in which the C / D ratio falls within the range of 1 or more and 20 or less, the evaluation was "B."Although the evaluation for Experimental Example 23 is the same "B" as Experimental Examples 20 to 22, the time required for a measured electromotive force Vref to deviate from the predetermined range is shorter than in Experimental Examples 20 to 22. In other words, in Experimental Example 23, the detection accuracy of NOx concentration is high, but reduced compared to Experimental Examples 1, 16 to 22 in which the ratio C / D is greater than or equal to 1 and less than or equal to 20.

[0108] It should be noted that Experimental Examples 1 to 9, 12 to 23 correspond to the embodiments of the present invention, and Experimental Examples 10, 11 correspond to a comparative example. INDUSTRIAL APPLICABILITY

[0109] The present invention is applicable to a gas sensor that detects a concentration of a specific gas such as NOx in a measurement object gas such as the exhaust gas of an automobile.

Claims

[1] Gas sensor, comprising: an element body having an oxygen ion-conductive solid electrolyte layer and provided inside with a measurement object gas flow segment that introduces a measurement object gas and allows the measurement object gas to flow; a measuring electrode arranged on an inner surface of the measuring object gas flow segment; a measurement object gas-side electrode disposed in a portion of the element body, the portion being exposed to the measurement object gas; a reference electrode arranged inside the element body; a measurement object gas introduction segment that introduces the measurement object gas and allows the measurement object gas to flow to the measurement object gas-side electrode; a reference gas introduction segment that introduces a reference gas serving as a reference for detecting a specific gas concentration in the measurement object gas and flows the reference gas to the reference electrode; a detection device that detects the specific gas concentration in the measurement object gas based on an electromotive force generated between the reference electrode and the measuring electrode; and a reference gas adjusting device that carries an oxygen pumping current between the reference electrode and the measuring object gas-side electrode and pumps oxygen from the vicinity of the measuring object gas-side electrode into the vicinity of the reference electrode, where A [µA] is a limiting current when oxygen is pumped from the vicinity of the measuring object gas-side electrode to the vicinity of the reference electrode, the measuring object gas introduction segment being exposed to an atmosphere with an oxygen concentration of 1000 ppm, and B [µA] is a limiting current when oxygen is pumped from the vicinity of the reference electrode to the vicinity of the measuring object gas-side electrode, the reference gas introduction segment being exposed to an air atmosphere, then a ratio A / B is greater than or equal to 0.

005. [2] A gas sensor according to claim 1, wherein the ratio A / B is greater than or equal to 0.

4. [3] A gas sensor according to claim 1 or 2, wherein the ratio A / B is less than or equal to 125. [4] Gas sensor according to one of claims 1 to 3, wherein the limit current A is 1 µA to 10000 µA. [5] Gas sensor according to one of claims 1 to 4, wherein the limit current B is 8 µA to 200 µA. [6] Gas sensor according to one of claims 1 to 5, wherein C [mm 2 ] is a surface of the measuring object gas side electrode and D [mm 2 ] is to be an area of ​​the reference electrode, then the area C is greater than or equal to 1.0 mm 2 , the area D is greater than or equal to 0.5 mm 2 and a ratio C / D is greater than or equal to 1 and less than or equal to 20. [7] Gas sensor according to claim 6, wherein the area C is less than or equal to 15.0 mm 2 is. [8] Gas sensor according to claim 6 or 7, wherein the area D is less than or equal to 4.0 mm 2 is. [9] A gas sensor according to any one of claims 1 to 8, wherein the measurement object gas introduction segment has a porous protective layer covering a part of the element body, the reference gas introduction segment has a porous reference gas introduction layer, a porosity of the porous protective layer is greater than or equal to 20% and less than or equal to 60%, and a porosity of the reference gas introduction layer is greater than or equal to 15% and less than or equal to 50%.

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