Gas sensor

Abstract

A limiting current type gas sensor (100) capable of indicating a NOx concentration in a measurement gas, comprising: a sensor element (101) formed from an oxygen ion-conducting solid electrolyte (1, 2, 3, 4, 5, 6), the sensor element (101) comprising: a gas inlet (10) into which a measuring gas is introduced from the outside; a first interior space (20) connected to the gas inlet (10) at a predetermined diffusion resistance; a second interior space (40) connected to the first interior space (20) at a predetermined diffusion resistance; a main pumping cell (21) which is an electrochemical pumping cell consisting of an inner pumping electrode (22) facing the first inner space (20), an outer pumping electrode (23) provided on a surface of the sensor element (101), and the solid electrolyte (4, 5, 6) located between the inner pumping electrode (22) and the outer pumping electrode (23); a measuring electrode (44) facing the second inner space (40) and covered with a porous protective film (45) providing a predetermined diffusion resistance, the measuring electrode (44) serving as a reduction catalyst for NOx; an atmospheric air introduction layer (48) into which atmospheric air is introduced from outside the sensor element (101) as a reference gas; a reference electrode (42) covered with the atmospheric air introduction layer (48); and a measuring pump cell (41), which is an electrochemical pump cell consisting of the measuring electrode (44), the outer pump electrode (23) and the solid electrolyte (4, 5, 6) located between the measuring electrode (44) and the outer pump electrode (23); and a concentration specifying element (110) that specifies a concentration of NOx based on a magnitude of a NOx current (Ip2) flowing between the measuring electrode (44) and the outer pumping electrode (23) in the measuring pumping cell (41), wherein the main pumping cell (21) is configured and arranged to pump out oxygen in the first internal space (20) when a predetermined main pumping voltage (Vp0) is applied between the inner pumping electrode (22) and the outer pumping electrode (23), and pumps out oxygen in the measurement gas introduced to the first internal space (20) to lower the oxygen partial pressure of the measurement gas in the first internal space (20), the measuring pumping cell (41) is configured and arranged to pump out oxygen in the vicinity of the measuring electrode (44) when a predetermined pumping voltage (Vp0) is applied between the inner pumping electrode (22) and the outer pumping electrode (23), and to pump out oxygen generated by a reduction of NOx in the measuring gas that reaches the vicinity of the measuring electrode (44) at the measuring electrode (44), a diffusion resistance from the gas inlet (10) to the inner pump electrode (22) equal to or greater than 200 cm -1 and equal to or less than 1000 cm -1 an electrical resistance of the main pump cell (21) is equal to or less than 150Ω, and a shortest distance from the inner pump electrode (22) to the outer pump electrode (23) is equal to or greater than 0.1 mm and equal to or less than 0.6 mm and the inner pumping electrode (22) is a cermet electrode made of an Au-Pt alloy and ZrO2 with an area equal to or greater than 5 mm 2 and equal to or less than 20 mm 2 is formed.

Description

BACKGROUND OF THE INVENTIONField of the InventionThe present invention relates to a gas sensor that obtains a nitrogen oxide concentration (NOx), and more particularly, to ensuring accuracy in a high NOx concentration range.Description of Technical BackgroundA limit current type gas sensor (NOx sensor) having a sensor element mainly containing an oxygen ion conductive solid electrolyte as a constituent element has been already known (for example, see JP 3 050 781 B2). In order to obtain the NOx concentration in such a gas sensor, first, a measurement gas is introduced into a space inside the sensor element (an internal space) under a predetermined diffusion resistance, and the oxygen in the measurement gas is pumped out in an electrochemical pumping cell provided in two stages such as a main pumping cell and an auxiliary pumping cell (first and second electrochemical pumping cells in JP 3 050 781 B2) to sufficiently lower the oxygen concentration in the measurement gas beforehand. Subsequently, NOx in the measurement gas is reduced or dissolved in a measurement electrode functioning as a reduction catalyst (a third inner pump electrode in JP 3 050 781 B2), and the oxygen generated by the reduction or the dissolution is pumped out in an electrochemical pump cell including a measurement electrode other than the above-described pump cell, which is referred to as a measurement pump cell, for example (a third electrochemical pump cell in JP 3 050 781 B2). The NOx concentration is obtained by the fact that the current (NOx current) flowing in the measurement pump cell has a certain functional relationship with the NOx concentration.Also, it is already known that in the gas sensor (NOx sensor), Pt to which Au is added (Au-Pt alloy) is used as a metal component of an inner pump electrode provided in an internal space to form a main pump cell, to suppress the dissolution of NOx when the main pump cell pumps oxygen from the internal space, and to increase a detection accuracy of NOx (for example, see JP 2014-190 940 A and JP 2014-209 128 A).In the gas sensor described above, the NOx concentration is obtained based on an amount of oxygen generated by a reduction of NOx in the measurement gas that reaches the measurement electrode due to the catalytic action of the measurement electrode. At this time, the oxygen in the measurement gas is pumped out by the electrochemical pumping cell until the measurement gas reaches the measurement electrode, and this pumping out of the oxygen is performed so that the partial pressure of oxygen (oxygen concentration) of the measurement gas is lowered to be not dissolved. Namely, when NOx is dissolved before the measurement gas reaches the measurement electrode, the amount of NOx that reaches the measurement electrode decreases, whereby the concentration cannot be accurately determined.However, when the oxygen concentration of the measurement gas introduced into the internal space is high, NOx may be dissolved at the time of exhausting oxygen. After a severe examination by the inventor of the present invention, results are obtained that, due to a tendency in which the oxygen concentration of the measurement gas in the internal space is higher in a portion closer to an upstream side (in a side closer to a gas inlet of the sensor element), a high pumping voltage tends to be locally applied in a portion closer to an upstream side of the inner pumping electrode to perform oxygen pumping out of the measurement gas whose oxygen concentration is high, and NOx is dissolved in such a portion.In addition, DE 10 2017 008 086 A1 and DE 11 2017 004 028 T5 each describe a gas sensor.SUMMARYThe present invention relates to a gas sensor that obtains a nitrogen oxide concentration (NOx), and more particularly, to ensuring accuracy in a high NOx concentration range.According to the present invention, a limiting current type gas sensor capable of indicating a concentration of NOx in a measurement gas includes: a sensor element formed of an oxygen ion conductive solid electrolyte, the sensor element including: a gas inlet into which a measurement gas is introduced from the outside; a first internal space connected to the gas inlet under a predetermined diffusion resistance; a second internal space connected to the first internal space under a predetermined diffusion resistance; a main pump cell that is an electrochemical pump cell consisting of an inner pump electrode facing the first internal space, an outer pump electrode provided on a surface of the sensor element, and the solid electrolyte located between the inner pump electrode and the outer pump electrode; a measurement electrode facing the second internal space and covered with a porous protective film providing a predetermined diffusion resistance, the measurement electrode serving as a reduction catalyst for NOx; an atmospheric air introduction layer into which atmospheric air is introduced from the outside of the sensor element as a reference gas; a reference electrode covered with the atmospheric air introduction layer; and a measurement pump cell that is an electrochemical pump cell composed of the measurement electrode, the outer pump electrode, and the solid electrolyte located between the measurement electrode and the outer pump electrode; and a concentration specification element that indicates a concentration of the NOx based on a magnitude of a NOx current flowing between the measurement electrode and the outer pump electrode in the measurement pump cell, wherein the main pump cell is configured and arranged, to pump off oxygen in the first internal space when a predetermined main pump voltage is applied between the inner pump electrode and the outer pump electrode, and pump off oxygen in the measurement gas introduced to the first internal space to decrease the oxygen partial pressure of the measurement gas in the first internal space, the measurement pump cell is configured and arranged to pump off oxygen in the vicinity of the measurement electrode when a predetermined pump voltage is applied between the inner pump electrode and the outer pump electrode, and pump off oxygen generated by a reduction of NOx in the measurement gas reaching the vicinity of the measurement electrode in the measurement electrode, a diffusion resistance from the gas inlet to the inner pump electrode is equal to or greater than 200 cm -1 and equal to or less than 1000 cm -1, an electric resistance of the main pump cell is equal to or less than 150Ω, a shortest distance from the inner pump electrode to the outer pump electrode is equal to or greater than 0.1 mm and equal to or less than 0.6 mm, and the inner pump electrode is a cermet electrode formed of an Au-Pt alloy and ZrO 2 having an area equal to or greater than 5 mm 2 and equal to or less than 20 mm 2.According to the present invention, even when the oxygen concentration of the measurement gas is high, dissolution of NOx in the first internal space is preferably suppressed, and thus the NOx concentration in the measurement gas can be accurately obtained.Accordingly, it is an object of the present invention to provide a gas sensor capable of accurately measuring NOx even when the oxygen concentration in the measurement gas is high.BRIEF DESCRIPTION OF THE DRAWINGSFIG. 1 is a schematic drawing illustrating an example of a configuration of a gas sensor 100. FIG. 2 is a drawing showing an influence of a magnitude of a main pump voltage Vp 0 on a relationship between an oxygen concentration of a measurement gas and a NOx current Ip 2. FIG. 3 is a drawing illustrating a processing flow in manufacturing a sensor element 101.DESCRIPTION OF THE PREFERRED EMBODIMENTS< Configuration of Gas Sensor>First, a schematic configuration of a gas sensor 100 including a sensor element 101 according to the present embodiment will be described. In the present embodiment, the gas sensor 100 is a limit current type NOx sensor that detects NOx using the sensor element 101 for measuring a NOx concentration.FIG. 1 is a schematic drawing illustrating an example of a configuration of the gas sensor 100 with a vertical section through the sensor element 101 along a longitudinal direction.The sensor element 101 is a plate-like (elongated plate-like) element having a structure consisting of six solid electrolyte layers of a first substrate layer 1, a second substrate layer 2, a third substrate layer 3, a first solid electrolyte layer 4, a spacer layer 5, and a second solid electrolyte layer 6, each of which is formed of zirconia (ZrO 2) which is an oxygen ion conductive solid electrolyte (e.g., yttrium stabilized zirconia (YSZ)), laminated from a bottom side in this order when viewing a sign sheet of FIG. 1. The solid electrolyte constituting these six layers is dense and airtight. In the following description, a surface on an upper surface of each of these six layers in FIG. 1 is simply referred to as an upper surface, and a surface on a lower surface thereof is simply referred to as a lower surface in some cases. An entire part of the solid electrolyte in the sensor element 101 is collectively referred to as a base part.The sensor element 101 is manufactured by performing predetermined processing and printing a circuit pattern on a ceramic green sheet corresponding to each layer, then laminating the green sheets and further firing them to integrate them with each other, for example.A gas inlet 10, a first diffusion-limiting part 11, a buffer space 12, a second diffusion-limiting part 13, a first internal space 20, a third diffusion-limiting part 30, and a second internal space 40 are adjacently formed to be connected to each other in this order between a lower surface of the second solid electrolyte layer 6 and an upper surface of the first solid electrolyte layer 4 in one end of the sensor element 101.The gas inlet 10, the buffer space 12, the first internal space 20, and the second internal space 40 are spaces in the sensor element 101 that look as if they were provided by cavitying the spacer layer 5, an upper part thereof defined by the lower surface of the second solid electrolyte layer 6, a lower part thereof defined by the upper surface of the first solid electrolyte layer 4, and a side part thereof defined by the side surface of the spacer layer 5.Each of the first diffusion-limiting part 11, the second diffusion-limiting part 13, and the third diffusion-limiting part 30 is provided as two horizontally long slits (having an opening with a longitudinal direction perpendicular to the sign sheet of FIG. 1 ). A region from the gas inlet 10 to the second interior 40 is also referred to as a gas distribution part.A reference gas introduction space 43 is provided at a position farther from an end side with respect to the gas introduction part between the upper surface of the third substrate layer 3 and the lower surface of the spacer layer 5, a side part thereof defined by a side surface of the first solid electrolyte layer 4. atmospheric air is introduced into the reference gas introduction space 43 as reference gas in the measurement of the NOx concentration, for example.An atmospheric air introducing layer 48 is a porous alumina layer, and the reference gas is introduced into the atmospheric air introducing layer 48 through the reference gas introduction space 43. The atmospheric air introduction layer 48 is formed to cover a reference electrode 42.The reference electrode 42 is an electrode having a configuration of being sandwiched between the upper surface of the third substrate layer 3 and the first solid electrolyte layer 4, and the atmospheric air introduction layer 48 leading to the reference gas introduction space 43 is provided around the reference electrode 42 as described above. An oxygen concentration (oxygen partial pressure) in the first internal space 20 and the second internal space 40 can be measured with the reference electrode 42 as described below.The gas inlet 10 is a portion having an opening to an outside in the gas introduction part, and the measurement gas is taken into the sensor element 101 from the outside through the gas inlet 10.The first diffusion limiting part 11 is a part for supplying the measurement gas from the gas inlet 10 of the predetermined diffusion resistance.The buffer space 12 is a space provided for guiding the measurement gas introduced from the first diffusion limiting part 11 to the second diffusion limiting part 13.The second diffusion limiting part 13 is a part for supplying the measurement gas introduced from the buffer space 12 to the first internal space 20 of the predetermined diffusion resistance.In the introduction of the measurement gas from the outside of the sensor element 101 into the first internal space 20, the measurement gas corresponding to a pressure change of the measurement gas in the external region (pulsation of an exhaust pressure in the case where the measurement gas is an exhaust gas of a vehicle) is not directly introduced into the first internal space 20, but is introduced into the first internal space 20 after a concentration variation of the measurement gas is canceled by the first diffusion limiting part 11, the buffer space 12, and the second diffusion limiting part 13. Thus, the concentration variation of the measurement gas introduced into the first internal space 20 is substantially negligible.The first internal space 20 is provided as a space for adjusting the oxygen partial pressure in the measurement gas introduced through the second diffusion-limiting part 13. The oxygen partial pressure is adjusted by the operation of a main pump cell 21.The main pump cell 21 is an electrochemical pump cell consisting of the inner pump electrode 22 having a ceiling electrode part 22 aprovided on almost the entire lower surface of the second solid electrolyte layer 6 facing the first internal space 20, an outer pump electrode 23 exposed to the outside in a region corresponding to the ceiling electrode part 22 aon the upper surface of the second solid electrolyte layer 6 (a main surface of the sensor element 101), and the second solid electrolyte layer 6 sandwiched between the electrodes 22 and 23.The inner pump electrode 22 is formed on the solid electrolyte layers on an upper side and a lower side defining the first internal space 20 (the second solid electrolyte layer 6 and the first solid electrolyte layer 4). Specifically, the ceiling electrode part 22 ais formed on the lower surface of the second solid electrolyte layer 6 forming a ceiling surface of the first internal space 20, and a lower electrode part 22 bis formed on the upper surface of the first solid electrolyte layer 4 forming a lower surface of the first internal space 20. The ceiling electrode part 22 aand the lower electrode part 22 bare connected to each other in a lead part provided on a side wall surface (an inner surface) of the spacer layer 5 that form both side wall parts of the first internal space 20 (illustration is omitted).The ceiling electrode part 22 aand the lower electrode part 22 bare formed to be rectangular in plan view. However, a configuration may be possible in which only the ceiling electrode part 22 aor only the lower electrode part 22 bis provided.Each of the inner pump electrode 22 and the outer pump electrode 23 is formed as a porous cermet electrode. Specifically, the inner pump electrode 22 contacting the measurement gas is formed of a material whose reducing ability on a NOx component in the measurement gas is weakened. For example, the inner pump electrode 22 is formed to have a porosity in the range of 5% to 40% and a thickness in the range of 5 μm to 20 μm as an Au-Pt alloy cermet electrode containing Au substantially equal to or greater than 0.6% by weight and equal to or less than 1.4% by weight and ZrO 2. A weight ratio of the Au-Pt alloy to ZrO 2 may be about Pt:ZrO 2= 7,0 : 3,0 to 5.0:5.0.Meanwhile, for example, the outer pump electrode 23 is formed into a rectangular shape in a plan view as a cermet electrode of Pt or a Pt alloy and ZrO 2.In the main pump cell 21, a desired pump voltage Vp 0 is applied between the inner pump electrode 22 and the outer pump electrode 23 by a variable source 24, and a pump current Ip 0 will flow in a positive or negative direction between the inner pump electrode 22 and the outer pump electrode 23, so that oxygen in the first internal space 20 can be pumped out to the outside or oxygen in the outside can be pumped into the first internal space 20. The pump voltage Vp 0 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 Vp 0.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 constitute an electrochemical sensor cell, i.e., a main pump-controlled oxygen partial pressure detection sensor cell 80 for detecting the oxygen concentration (oxygen partial pressure) in the atmosphere in the first internal space 20.The oxygen concentration (oxygen partial pressure) in the first internal space 20 can be obtained by measuring an electromotive force V 0 in the main pump-controlled oxygen partial pressure detection sensor cell 80.Further, feedback control is performed on the main pump voltage Vp 0 so that the electromotive force V 0 is set to constant, thereby controlling the pump current Ip 0. Accordingly, the oxygen concentration in the first internal space 20 is maintained at a predetermined constant value.The third diffusion limiting part 30 is a part for supplying the measurement gas whose oxygen concentration (oxygen partial pressure) is controlled by an operation of the main pump cell 21 in the first internal space 20 of a predetermined diffusion resistance and for guiding the measurement gas to the second internal space 40.The second internal space 40 is provided as a space for performing processing according to measurement of nitrogen oxide (NOx) in the measurement gas introduced through the third diffusion limiting part 30. The NOx concentration is mainly measured in the second internal space 40, where the oxygen concentration is adjusted by an auxiliary pump cell 50 through operation of a measurement pump cell 41.The oxygen concentration (oxygen partial pressure) is previously adjusted in the first internal space 20, and then, in the second internal space 40, the adjustment of the oxygen partial pressure by the auxiliary pump cell 50 is further performed on the measurement gas introduced through the third diffusion limiting part 30. Accordingly, the oxygen concentration in the second internal space 40 can be accurately maintained constant, so that the gas sensor 100 enables the highly accurate NOx concentration measurement.The auxiliary pump cell 50 is an auxiliary electrochemical pump cell consisting of an auxiliary pump electrode 51 having a ceiling electrode part 51 aprovided on almost the entire lower surface of the second solid electrolyte layer 6 facing the second internal space 40, the outer pump electrode 23 (not limited to the outer pump electrode 23, but an appropriate electrode outside the sensor element 101 is also applicable), and the second solid electrolyte layer 6.The auxiliary pump electrode 51 is disposed in the second internal space 40 similarly to the inner pump electrode 22 in the first internal space 20 described above. In other words, the ceiling electrode part 51 ais formed on the second solid electrolyte layer 6 forming a ceiling surface of the second internal space 40, and a lower electrode part 51 bis formed on the first solid electrolyte layer 4 forming a lower surface of the second internal space 40. Each of the ceiling electrode part 51 aand the lower electrode part 51 bhas a rectangular shape in a plan view and is connected to each other in a lead part provided on a side wall surface (an inner surface) of the spacer layer 5 that forms both side wall parts of the second internal space 40 (illustration is omitted).In the same manner as in the inner pump electrode 22, the auxiliary pump electrode 51 is also formed of a material whose reducing ability on a NOx component in the measurement gas is weakened.In the auxiliary pump cell 50, a desired pump voltage Vp 1 is applied between the auxiliary pump electrode 51 and the outer pump electrode 23, so that oxygen in the atmosphere in the second internal space 40 can be pumped out to the outside or oxygen can be pumped out from the outside to the second internal space 40.The auxiliary pump electrode 51, the reference electrode 42, the second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, and the third substrate layer 3 constitute an electrochemical sensor cell, i.e., an auxiliary pump control oxygen partial pressure detection sensor cell 81 for controlling the oxygen partial pressure in the atmosphere in the second internal space 40.The auxiliary pump cell 50 performs the variable source pumping 52 in which voltage control is performed based on an electromotive force V 1 detected in the auxiliary pump control oxygen partial pressure detection sensor cell 81. Accordingly, the oxygen partial pressure in the atmosphere in the second internal space 40 is controlled to be low enough not to significantly influence measurement of NOx.Accordingly, a pump current Ip 1 thereof is used for controlling the electromotive force of the main pump control oxygen partial pressure detection sensor cell 80. Specifically, the pump current Ip 1 is input as a control signal to the main pump control oxygen partial pressure detection sensor cell 80, and by the control of the electromotive force V 0 thereof, the oxygen partial pressure in the measurement gas introduced into the second internal space 40 through the third diffusion limiting part 30 is controlled so as to have an always constant gradient. When the gas sensor 100 is used as the NOx sensor, the oxygen concentration in the second internal space 40 is maintained at a constant value of about 0.001 ppm by the functions of the main pump cell 21 and the auxiliary pump cell 50.The measurement pump cell 41 measures the NOx concentration in the measurement gas in the second internal space 40. the measurement pump cell 41 is an electrochemical pump cell that is composed of a measurement electrode 44 provided on the upper surface of the first solid electrolyte layer 4 opposite to the second internal space 40 at a position separated from the third diffusion-limiting part 30, the outer pump electrode 23, the second solid electrolyte layer 6, the spacer layer 5, and the first solid electrolyte layer 4.The measuring electrode 44 is a porous cermet electrode. For example, the measuring electrode 44 is formed as a cermet electrode of Pt or an alloy of Pt and ZrO 2. The measurement electrode 44 also functions as a NOx reduction catalyst for reducing NOx in the atmosphere in the second internal space 40.The fourth diffusion-limiting part 45 is a film formed of a porous material mainly containing alumina (Al 2 O 3). The fourth diffusion limiting part 45 has a function of limiting an amount of NOx flowing into the measurement electrode 44, and at the same time, functions as a protective film of the measurement electrode 44.The measurement pump cell 41 can pump out oxygen generated by dissolution of NOx in the atmosphere around the measurement electrode 44 and detect a generated amount of oxygen as the pump current Ip 2.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 constitute an electrochemical sensor cell, i.e., a measurement pump control oxygen partial pressure detection sensor cell 82 for detecting the oxygen partial pressure around the measurement electrode 44.The measurement gas introduced into the second internal space 40 reaches the measurement electrode 44 through the fourth diffusion limiting part 45 under a condition in which the oxygen partial pressure is controlled. NOxin the measurement gas is reduced around the measurement electrode 44 (2NO→N 2+ O 2), and oxygen is generated. The generated oxygen is pumped by the measurement pump cell 41. At this time, a voltage Vp 2 of the variable source 46 is controlled so that a control voltage V 2 detected in the measurement pump control oxygen partial pressure detection sensor cell 82 is set to constant. Since the amount of oxygen generated around the measurement electrode 44 is proportional to the NOx concentration in the measurement gas, the NOx concentration in the measurement gas is calculated with the pumping current Ip 2 in the measurement pump cell 41. The pump current Ip 2 is also referred to as NOx current Ip 2 below.When the measurement electrode 44, the first solid electrolyte layer 4, the third substrate layer 3, and the reference electrode 42 are combined into an oxygen partial pressure detection means as an electrochemical sensor cell, an electromotive force corresponding to a difference between an amount of oxygen generated by the reduction of the NOx component in the atmosphere around the measurement electrode 44 and an amount of oxygen contained in a reference atmosphere can be obtained, and accordingly, a concentration of the NOx component in the measurement gas can also be obtained.The second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, the third substrate layer 3, the outer pump electrode 23, and the reference electrode 42 constitute an electrochemical sensor cell 83, and the partial pressure of oxygen in the measurement gas outside the sensor can be detected by an electromotive force V ref obtained from the sensor cell 83.The sensor element 101 further includes a heater part 70 having a function of adjusting a temperature for heating the sensor element 101 and maintaining the temperature to increase the oxygen ion conductivity of the solid electrolyte constituting the base part.The heater part 70 basically includes a heating electrode 71, a heater element 72, a heater lead 72 a, a through hole 73, and a heating insulating layer 74. the heater part 70 is embedded in the base part of the sensor element 101 except for the heater electrode 71.The heater electrode 71 is an electrode configured to contact the lower surface of the first substrate layer 1 (the other main surface of the sensor element 101).The heater element 72 is a resistance heating element provided between the second substrate layer 2 and the third substrate layer 3. The heater element 72 generates the heat by supplying current from the outside of the sensor element 101 via the heater electrode 71, the through hole 73, and the heater lead 72 afunctioning as a power path. The heater element 72 is formed of Pt or mainly of Pt. The heater element 72 is embedded in a predetermined area in the sensor element 101 on a side with the gas introduction part so as to face the gas introduction part in a thickness direction of the element. The heater element 72 is provided with a thickness of about 10 μm to 20 μm.In the sensor element 101, the current is supplied into the heater element 72 via the heater electrode 71, whereby the heater element 72 generates the heat, so that each part of the sensor element 101 can be heated to a predetermined temperature and can be maintained at temperature. Specifically, the sensor element 101 is heated so that the temperature of the solid electrolyte and the electrode near the gas introduction part increases to about 700° C. to 900° C. The heating increases the oxygen ion conductivity of the solid electrolyte constituting the base part in the sensor element 101. The heating temperature at the time of heating by the heater element 72 when the gas sensor 100 is used (when the sensor element 101 is driven) is referred to as the driving temperature of the sensor element.The gas sensor 100 further includes a controller 110 (controller 110) that controls the operation of each part and specifies the NOx concentration based on the NOx current Ip 2.In the gas sensor 100 having such a configuration, the oxygen contained in the measurement gas is pumped out by the operation of the main pump cell 21 and further the auxiliary pump cell 50, and the measurement gas whose oxygen partial pressure is lowered to such an extent that measurement of NOx (e.g., 0.0001 ppm to 1 ppm) is not significantly affected reaches the measurement electrode 44. The generated oxygen is pumped out by the measurement pump cell 41. The NOx current Ip 2 flowing at the time of oxygen pumping has a certain functional relationship with the NOx concentration in the measurement gas (hereinafter, sensitivity characteristics).The sensitivity characteristics are previously specified by a plurality of model gas types whose NOx concentrations are already known before the actual use of the gas sensor 100, and whose data are stored in the controller 110. In actual use of the gas sensor 100, signals indicating a value of the NOx current Ip 2 flowing according to the NOx concentration in the measurement gas are provided to the controller 110 from moment to moment, and the NOx concentration is continuously calculated based on the value and the sensitivity characteristics indicated and output to the controller 110. According to the gas sensor 100, the NOx concentration in the measurement gas can be obtained almost in real time.< Between Main Pump Voltage and Resolution of NOx>FIG. 2 is a drawing showing an influence of a magnitude of a main pump voltage Vp 0 on a relationship between the oxygen concentration of the measurement gas and a NOx current Ip 2. Specifically, measurement is performed by two different gas sensors 100 on four model gas types having the same constant NO concentration of 500 ppm, the oxygen concentration being varied in four stages of 0%, 5%, 10%, and 18%, respectively (all with the balance of N 2), and in FIG. 2, the NOxcurrent Ip2of the four model gas types is plotted with respect to the oxygen concentration of the model gas. The sensor element driving temperature was set to 830° C.FIG. 2 shows the graphs G 1 and G 2 corresponding to each of the two gas sensors 100. The graph G 1 shows a linear change of the monotone increase in a relationship between the NOxcurrent Ip 2 and the oxygen concentration, but the graph G 2 shows a tendency of the monotone increase in a range in which the oxygen concentration is equal to or less than 10% but remains at the same level in a range in which the oxygen concentration is 10% to 18%.After a severe examination by the inventor of the present invention, the results are confirmed that when the gas sensor 100 in which the magnitude of the main pump voltage Vp 0 during operation is maintained equal to or less than 650 mV is used therefor, a linear change of the monotone increase in which a determination factor (a square value of a correlation function) R 2 is equal to or greater than 0.975 as the graph G 1 is obtained, but when the gas sensor 100 in which the magnitude of the main pump voltage Vp 0 during operation is maintained greater than 650 mV is used therefor, NOx current Ip 2 tends to remain at the same level as the graph G 2 in a range in which the oxygen concentration of the measurement gas is high. The determination factor R2 in the latter case falls below 0.975.The results indicate that in the gas sensor 100 configured such that the magnitude of the main pump voltage Vp 0 exceeds 650 mV, NOx is dissolved in the measurement gas before reaching the measurement electrode 44 (in the first internal space 20, for example) when the oxygen concentration in the measurement gas is high.Further, it indicates that the sensor element 101 needs to have the configuration that the magnitude of the main pump voltage Vp 0 is kept equal to or less than 650 mV in order to accurately maintain the NOx concentration even at a high oxygen concentration in the measurement gas.Graph G 1 in FIG. 2 shows that the value of the NOx current Ip 2 tends to depend on the oxygen concentration in the measurement gas. It indicates that the correction by the oxygen concentration is effective to obtain the NOx concentration based on the sensitivity characteristics to more accurately determine the NOx concentration. It can be achieved by, for example, correcting the NOx current Ip 2 based on information indicating the oxygen concentration in the measurement gas (for example, the pump current Ip 0 or the electromotive force V ref).<Unterdrückung of Main Pump Voltage>In consideration of the above points, in the gas sensor 100 according to the present embodiment, the requirements for a diffusion resistance from the gas inlet 10 to the inner pump electrode 22, an electric resistance of the main pump cell 21, a portion of the inner pump electrode 22 that constitutes the main pump cell 21 and directly contacts the measurement gas, and a shortest distance between the electrodes in the main pump cell 21 (a shortest distance from the inner pump electrode 22 to the outer pump electrode 23) from the viewpoint of more reliably suppressing the dissolution of NOx in the first internal space 20 when the oxygen concentration of the measurement gas is high. Since a contribution of the conduction part to the dissolution of NOx may be negligible, the term "inner pump electrode 22" in the following description indicates a part other than the conduction part.In the gas sensor 100 according to the present embodiment, by satisfying requirements (a) to (d) described below, dissolution of NOx in the first internal space 20 is suppressed even when the oxygen concentration of the measurement gas is high. Specifically, the gas sensor 100 has the configuration that the value of the main pump voltage Vp 0 is kept equal to or less than 650 mV. (a) Diffusion resistance from the gas inlet 10 to the inner pump electrode 22: equal to or greater than 200 cm -1 and equal to or less than 1000 cm -1; ( b) Electrical resistance of the main pump cell 21: equal to or less than 150 Ω; (c) Area of the inner pump electrode 22: equal to or greater than 5 mm 2 and equal to or less than 20 mm 2; ( d) Shortest electrode distance of the main pump cell 21: equal to or greater than 0.1 mm and equal to or less than 0.6 mm.When the ceiling electrode part 22 ais provided, the shortest distance between the electrodes corresponds to a thickness of the second solid electrolyte layer 6.The diffusion resistance from the gas inlet 10 to the inner pump electrode 22 is equal to or greater than 200 cm -1 and equal to or less than 1000 cm -1 is achieved by a suitable combination of the diffusion resistance of the first diffusion-limiting part 11 and the diffusion resistance of the second diffusion-limiting part 13.The state in which the diffusion resistance exceeds 1000 cm -1 is not preferable because the oxygen detecting ability is lowered. Meanwhile, the state in which the diffusion resistance is less than 200 cm -1 is not preferable because NOx can be easily resolved with an increase in the value of the pump current Ip 0 and an increase in the value of the main pump voltage Vp 0, thereby lowering the detection accuracy.The state in which the electric resistance exceeds 150 Ω is not preferable because NOx can be easily resolved with the increase in the value of the main pump voltage Vp 0, thereby lowering the detection accuracy.The state where the area of the inner pump electrode 22 exceeds 20 mm 2 is not preferable because NOx in the first internal space 20 can be easily dissolved.Meanwhile, the state in which the area of the inner pump electrode 22 is less than 5 mm 2 or the state in which the shortest distance between the electrodes exceeds 0.6 mm is not preferable because an impedance of the main pump cell 21 increases and the value of the main pump voltage Vp 0 increases and NOx can be easily resolved.The state in which the shortest distance between the electrodes is less than 0.1 mm is not preferable because a thickness of the solid electrolyte between the electrodes is reduced, thereby easily creating a crack.Even in a case where the inner pump electrode 22 includes only one of the above-described ceiling electrode part 22 aand the lower electrode part 22 b, dissolution of NOx in the first internal space 20 in which the oxygen concentration of the measurement gas is high is suppressed as long as the above-described requirements (a) to (d) are satisfied.The gas sensor 100 configured to satisfy the above-described requirements (a) to (d) is used in a state where a temperature of the inner pump electrode 22 is equal to or higher than 700° C. and equal to or lower than 900° C. by setting a sensor element driving temperature to be equal to or higher than 700° C. and equal to or lower than 900° C. A state where the temperature of the inner pump electrode 22 exceeds 900° C. is not preferable because NOx in the first internal space 20 can be easily dissolved, thus an assumed linear set is not ensured. The state that the temperature is below 700° C. is not preferable because an impedance of the main pump cell 21 increases and the detection accuracy of the pump current Ip 0 is lowered and the value of the main pump voltage Vp 0 increases and NOx is easily resolved.<Production Method of Sensor Element>Next, a method of manufacturing the sensor element 101 having the configuration and the above-described feature will be described. In the present embodiment, the sensor element 101 is manufactured by forming a laminated body formed of green sheets with an oxygen ion conductive solid electrolyte such as zirconia as a ceramic component, and then cutting and firing the laminated body.Next, a case of manufacturing the sensor element 101 including the six layers illustrated in FIG. 1 will be described as an example. In this case, six green sheets are prepared, which correspond to 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. FIG. 3 is a drawing showing a processing flow in manufacturing a sensor element 101.In manufacturing the sensor element 101, first, a blank plate (not shown) which is a green sheet on which no pattern is formed is prepared (step S 1). When the sensor element 101 including the six layers is manufactured, six dummy plates corresponding to each layer are manufactured. Specifically, for the second solid electrolyte layer 6, the green sheet whose thickness satisfies the requirement (d) and further the requirement (b) at the end is used.The blank plates have a plurality of plate holes used for alignment in performing printing and laminating the plates. The plate hole is previously formed in the blank plate, for example, by punching processing with a punching apparatus at a stage before pattern formation. Green sheets corresponding to the layers including the interiors also include penetrating portions corresponding to the interiors also provided by the similar punching processing. The penetrating portions are formed so as to satisfy the requirement (a) in the finally obtained sensor element 101. A thickness of each blank plate corresponding to each layer of the sensor element 101 may not be the same.After the blank plate is prepared for each layer, pattern printing and dry processing are performed on each blank plate (step S2). Specifically formed are patterns of various electrode types, a pattern of the fourth diffusion-limiting part 45, patterns of the heater element 72 and the heater insulating layer 74, and a pattern of internal wiring that is not illustrated in the drawings. Application or placement of a sublimation material for forming the first diffusion-limiting part 11, the second diffusion-limiting part 13, and the third diffusion-limiting part 30 is also performed at a time of pattern printing. The application or the placement is performed so as to satisfy the requirement (a) in the finally obtained sensor element 101.The printing of each pattern is effected by applying a pattern formation paste which is produced on the blank plate in accordance with the properties required for each formation object in the known screen printing technique. A known drying agent can be used for drying the post-printing processing.Specifically, the paste for forming the inner pump electrode 22 is prepared such that the finally obtained inner pump electrode 22 satisfies at least the requirements (b) to (c) and is applied.After the pattern printing on each blank plate is completed, the processing of printing and drying of an adhesive paste for laminating and adhering the green sheet corresponding to the individual layers on and among each other are performed (step S3). For printing the adhesive paste, a known screen printing technique can be used, and for drying after printing, a known drying processing can be used.Subsequently, the green sheets to which an adhesive agent has been applied are stacked in a predetermined order, and the stacked green sheets are pressed into a laminated body under a predetermined temperature and pressure condition (step S 4). Specifically, crimping is performed by stacking and holding the green sheets to be laminated on a predetermined laminator, which is not illustrated, while aligning the green sheets with the plate holes, and then heating and pressurizing the green sheets together with the laminator of a laminator such as a known oil-hydraulic press machine. The pressure, temperature and time for heating and pressurizing depend on the laminating machine used, but an appropriate condition may be determined to achieve favorable lamination.When the laminated body is obtained as described above, the laminated body is then cut out at a plurality of locations to obtain a single unit (called element body) of the sensor element 101 (step S 5).The firing is performed on the element body at a firing temperature of about 1300° C. to 1500° C. (step S 6). At this time, the sensor element 101 is manufactured. In other words, the sensor element 101 is manufactured by integrally firing the solid electrolyte and the electrode. The firing temperature is preferably set to 1200° C. to 1500° C. (e.g., 1400° C.). The integrated firing is performed in the above-described manner, so that each electrode in the sensor element 101 has a sufficient adhesive force.The sensor element 101 thus obtained is accommodated in a predetermined housing and assembled into a main body (not illustrated) of the gas sensor 100.[Example]Twelve types of gas sensors 100 (No. 1 to No. 12) each having a different combination of a diffusion resistance from the gas inlet 10 to the inner pump electrode 22, an electric resistance of the main pump cell 21, a portion of the inner pump electrode 22, and a shortest distance from the inner pump electrode 22 to the outer pump electrode 23 (shortest electrode distance) were manufactured, and each gas sensor 100 performed measurement on a measurement gas having an oxygen concentration of 18% and an NO concentration of 500 ppm (with the balance of N 2). The presence or absence of the NOx resolution in the first internal space 20 was determined based on the value of the main pump voltage Vp 0 at this time. The sensor element driving temperature was set to 830° C.Specifically, the diffusion resistance, the electric resistance, the area, and the shortest distance between the electrodes are varied as described below.Diffusion resistance: eight steps of 100 cm -1, 180 cm -1, 200 cm -1, 300 cm -1, 500 cm -1, 600 cm -1, 700 cm -1 and 900 cm -1;Electrical resistance: eleven stages of 30Ω, 45Ω, 55Ω, 75Ω, 80Ω, 85Ω, 90Ω, 100Ω, 140Ω, 150Ω, and 200Ω;Area: eight steps of 4.0 mm 2, 5,0 mm 2, 6,0 mm 2, 7,3 mm 2, 7,5 mm 2, 9,0 mm 2, 15,0 mm 2 and 20.0 mm 2;Shortest distance between electrodes: four steps of 0.2 mm, 0.3 mm, 0.4 mm and 0.6 mm.Table 1 shows the state and a determination result of the presence or absence of the NOx resolution at each gas sensor 100. All the gas sensors 100 of No. 1 to No. 10 satisfy all the requirements (a) to (d). Meanwhile, the gas sensors 100 of No. 11 and No. 12 do not satisfy requirement (a), and the gas sensor 100 of No. 12 still does not satisfy requirements (b) and (c). [Table 1] Table 1] [Table 1] Table 1]1300857,50,3◯2500557,50,2◯37001006,00,3◯46001505,00,4◯5900807,50,3◯65003020,00,3◯73004515,00,6◯8200759,00,3◯95001007,50,4◯103001406,00,4◯11100907,30,3×121802004,00,4×The presence or absence of the NOx resolution in the first internal space 20 is determined as follows.The gas sensor 100 whose main pump voltage is indicated to be equal to or less than 650 mV is determined not to resolve NOx, and is denoted by "O" (circle) in the point of "determination" of the corresponding gas sensor 100 in Table 1.Meanwhile, the gas sensor 100 whose main pump voltage is greater than 650 mV is determined to be resolved NOx, and is denoted by "×" (cross mark) in "determination" of the corresponding gas sensor 100 in Table 1.As shown in Table 1, the gas sensors 100 of No. 1 to No. 10 satisfying all of the requirements (a) to (d) were determined not to have dissolved NOx. In contrast, the gas sensor 100 of No. 11 not satisfying requirement (a) and the gas sensor 100 of No. 12 not satisfying requirements (a) to (c) were determined to have dissolved NOx in the first internal space 20.The result shows that the gas sensor 100 capable of accurately determining the NOx concentration even at a high oxygen concentration in the measurement gas can be achieved by satisfying all the requirements (a) to (d).

Claims

A limiting current type gas sensor (100) capable of indicating a NOx concentration in a measurement gas, comprising: a sensor element (101) formed of an oxygen ion conductive solid electrolyte (1, 2, 3, 4, 5, 6), the sensor element (101) including: a gas inlet (10) into which a measurement gas is introduced from the outside; a first internal space (20) connected to the gas inlet (10) under a predetermined diffusion resistance; a second internal space (40) connected to the first internal space (20) under a predetermined diffusion resistance; a main pump cell (21) which is an electrochemical pump cell consisting of an inner pump electrode (22) facing the first internal space (20), an outer pump electrode (23) provided on a surface of the sensor element (101), and the solid electrolyte (4, 5, 6) located between the inner pump electrode (22) and the outer pump electrode (23); a measurement electrode (44) facing the second internal space (40) and covered with a porous protective film (45) providing a predetermined diffusion resistance, the measurement electrode (44) serving as a reduction catalyst for NOx; an atmospheric air introduction layer (48) into which atmospheric air is introduced from the outside of the sensor element (101) as a reference gas; a reference electrode (42) covered with the atmospheric air introduction layer (48); and a measurement pump cell (41) that is an electrochemical pump cell composed of the measurement electrode (44), the outer pump electrode (23), and the solid electrolyte (4, 5, 6) located between the measurement electrode (44) and the outer pump electrode (23); and a concentration specification element (110) that specifies a concentration of the NOx based on a magnitude of a NOx current (Ip 2) flowing between the measurement electrode (44) and the outer pump electrode (23) in the measurement pump cell (41), wherein the main pump cell (21) is configured and arranged to pump off oxygen in the first internal space (20) when a predetermined main pump voltage (Vp 0) is applied between the inner pump electrode (22) and the outer pump electrode (23), and pump off oxygen in the measurement gas introduced to the first internal space (20), in order to decrease the oxygen partial pressure of the measurement gas in the first internal space (20), the measurement pump cell (41) is configured and arranged to pump off oxygen in the vicinity of the measurement electrode (44) when a predetermined pump voltage (Vp0) is applied between the inner pump electrode (22) and the outer pump electrode (23), and to pump off oxygen generated by a reduction of NOx in the measurement gas reaching the vicinity of the measurement electrode (44) at the measurement electrode (44), a diffusion resistance from the gas inlet (10) to the inner pump electrode (22) is equal to or greater than 200 cm -1 and equal to or less than 1000 cm -1 an electric resistance of the main pump cell (21) is equal to or less than 150Ω, and a shortest distance from the inner pump electrode (22) to the outer pump electrode (23) is equal to or greater than 0.1 mm and equal to or less than 0.6 mm, and the inner pump electrode (22) is a cermet electrode formed of an Au-Pt alloy and ZrO 2 having an area equal to or greater than 5 mm 2 and equal to or less than 20 mm 2.The gas sensor (100) according to claim 1, wherein the sensor element (101) further includes: a main pump control sensor cell (80) that is an electrochemical sensor cell consisting of the inner pump electrode (22), the reference electrode (42), and the solid electrolyte (3, 4, 5, 6) located between the inner pump electrode (22) and the reference electrode (42); an auxiliary pump cell (50) that is an electrochemical pump cell consisting of an auxiliary pump electrode (51) provided to face the second internal space (40), the outer pump electrode (23), and the solid electrolyte (6) located between the auxiliary pump electrode (51) and the outer pump electrode (23); an auxiliary pump control sensor cell (81) which is an electrochemical sensor cell consisting of the auxiliary pump electrode (51), the reference electrode (42), and the solid electrolyte (3, 4, 5, 6) located between the auxiliary pump electrode (51) and the reference electrode (42); and a measurement pump control sensor cell (82) which is an electrochemical sensor cell consisting of the measurement electrode (44), the reference electrode (42), and the solid electrolyte (3, 4) located between the measurement electrode (44) and the reference electrode (42), wherein when the main pump cell (21) pumps oxygen in the measurement gas located in the first internal space (20), the main pump voltage (Vp0) is calculated according to an electromotive force (V0) generated between the inner pump electrode (22) and the reference electrode (42) in the main pump control sensor cell (80), between the inner pump electrode (22) and the outer pump electrode (23), the auxiliary pump cell (50) is configured and arranged to pump off oxygen in the measurement gas introduced into the second internal space (40) when a pump voltage (Vp1) according to an electromotive force (V1) generated between the auxiliary pump electrode (51) and the reference electrode (42) in the auxiliary pump control sensor cell (81) is applied between the auxiliary pump electrode (51) and the outer pump electrode (23), the measurement gas whose oxygen partial pressure has been further lowered compared to the oxygen partial pressure in the first internal space (20) by pumping off oxygen with the auxiliary pump cell (50) reaches the measurement electrode (44), and when the measurement pump cell (41) pumps off oxygen generated in the measurement electrode (44), a pump voltage (Vp2) according to an electromotive force (V2) generated between the measurement electrode (44) and the reference electrode (42) in the measurement pump control sensor cell (82) is applied between the measurement electrode (44) and the outer pump electrode (23).

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