Gas sensor

Abstract

A limiting current type gas sensor (100), wherein the gas sensor (100) includes a sensor element (101) formed from an oxygen ion conductive solid electrolyte (1, 2, 3, 4, 5, 6), the gas sensor (100) being capable of indicating a NOx concentration in a measurement gas, the sensor element (101) including: a gas inlet (10) into which the measurement 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 pump cell (21) which is an electrochemical pump cell consisting of an inner pump electrode (22) including one or two unit electrode parts (22a, 22b) and facing the first inner space (20), an outer pump electrode (23) provided on a surface of the sensor element (101), and the solid electrolyte (6) located between the inner pump electrode (22) and the outer pump 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) introduced into the atmospheric air 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 (6) located between the measuring electrode (44) and the outer pump electrode (23); where when the inner pumping electrode (22) includes the two of the unit electrode parts (22a, 22b), the two unit electrode parts (22a, 22b) are arranged to face each other, the main pump cell (21) pumps out oxygen in the measurement gas introduced into the first inner space (20) by applying a predetermined main pump voltage (Vp0) between the inner pump electrode (22) and the outer pump electrode (23), so that the oxygen partial pressure of the measurement gas in the first inner space (20) is reduced, the measuring pump cell (41) pumps out oxygen produced by a reduction of NOx in the measuring gas reaching the vicinity of the measuring electrode (44) by applying a predetermined pump voltage (Vp2) between the inner pump electrode (22) and the outer pump electrode (23), wherein the gas sensor (100) further comprises: a concentration specification 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), a diffusion resistance from the gas inlet (10) to the first interior space (20) which is 200 cm-1 or greater and 1000 cm-1 or less, and when the first interior space (20) has a space length (L1) as a size in a longitudinal direction of the sensor element (101), a space thickness (t1) as a size in a thickness direction of the sensor element (101), and a space width (w1) as a size in a width direction perpendicular to both the longitudinal direction and the thickness direction, and the unit electrode part (22a, 22b) has an electrode length (L2) as a size in the longitudinal direction and an electrode width (w2) as a size in the width direction, the space length (L1) is 2.5 mm or greater and 10 mm or less, the space thickness (t1) is 50 µm or greater and 300 µm or less, a ratio of the electrode length (L2) relative to the space length (L1) is 0.5 or more and 1.0 or less and a ratio of the electrode width (w2) relative to the space width (w1) is 0.5 or more and 1.0 or less.

Description

BACKGROUND OF THE INVENTIONField of the invention

[0001] The present invention relates to a gas sensor that obtains a nitrogen oxide (NOx) concentration, and more particularly to ensuring accuracy in a high NOx concentration range. Description of the technical background

[0002] A limiting current type gas sensor (NOx sensor) using a sensor element mainly containing an oxygen ion-conductive solid electrolyte as a component is already known (e.g., see JP 3 050 781 B2). To obtain the NOx concentration in such a gas sensor, a measurement gas is first introduced into a space within the sensor element (an inner space) under a predetermined diffusion resistance, and the oxygen in the measurement gas is pumped out in an electrochemical pump cell provided in two stages such as a main pump cell and an auxiliary pump cell (first and second electrochemical pump cells in JP 3 050 781 B2) to sufficiently reduce the oxygen concentration in the measurement gas beforehand.Subsequently, NOx in the measurement gas is reduced or dissolved in a measuring electrode acting as a reduction catalyst (a third inner pumping electrode in JP 3 050 781 B2), and the oxygen generated by the reduction or dissolution is pumped out in an electrochemical pump cell including a different measuring electrode than the pump cell described above, which is referred to as a measuring 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 flowing in the measuring pump cell (NOx current) has a certain functional relationship with the NOx concentration.

[0003] Also already known is an embodiment in which, in the gas sensor (NOx sensor), Pt to which Au is added (Au-Pt alloy) is used as a metal component of an inner pumping electrode provided in an interior space to form a main pumping cell, with the purpose of suppressing the dissolution of NOx when the main pumping cell pumps oxygen from the interior space and increasing a detection accuracy of NOx (e.g., see JP 2014 - 190 940 A and JP 2014 - 209 128 A).

[0004] In the gas sensor described above, the NOx concentration is obtained based on the amount of oxygen generated by the 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. This oxygen pumping is performed in such a way that the oxygen partial pressure (oxygen concentration) of the measurement gas is lowered to the point where NOx is not dissolved. The reason for this is that if NOx is dissolved before the measurement gas reaches the measurement electrode, the amount of NOx reaching the measurement electrode decreases, so the concentration cannot be accurately obtained.

[0005] However, when the oxygen concentration of the measurement gas introduced into the interior space is high, NOx may be dissolved at the time of oxygen pumping. After serious investigation by the inventor of the present invention, results have been obtained that, due to a tendency in which the oxygen concentration of the measurement gas in the interior space is higher in a portion closer to an upstream side (on a side closer to a gas inlet of the sensor element), a high pumping voltage tends to be applied locally in a portion closer to an upstream side of the inner pumping electrode to perform oxygen pumping from the measurement gas whose oxygen concentration is high, and NOx is also dissolved in such a portion.

[0006] Furthermore, US 2003 / 0 062 904 A1 describes an error-minimizing structure of a gas sensor. Furthermore, US 2011 / 0 036 715 A1 discloses a gas sensor. SUMMARY

[0007] The present invention relates to a gas sensor that obtains a nitrogen oxide (NOx) concentration, and more particularly to ensuring accuracy in a high NOx concentration range.

[0008] According to the present invention, in a limiting current type gas sensor including a sensor element formed of an oxygen ion conductive solid electrolyte, the gas sensor capable of indicating a NOx concentration in a measurement gas, the sensor element includes: a gas inlet into which a measurement gas is introduced from the outside; a first internal space connected to the gas inlet at a predetermined diffusion resistance; a second internal space connected to the first internal space at a predetermined diffusion resistance; a main pump cell, which is an electrochemical pump cell composed of an inner pump electrode including one or two unit electrode parts and 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 pumping electrode and the outer pumping electrode; a measuring electrode facing the second internal space and covered by a porous protective film providing a predetermined diffusion resistance, the measuring electrode serving as a reduction catalyst for NOx; an atmospheric air introduction layer into which atmospheric air from outside the sensor element is introduced as a reference gas; a reference electrode covered with the atmospheric air introduction layer; and a measuring pump cell, which is an electrochemical pump cell consisting of the measuring electrode, the outer pumping electrode, and the solid electrolyte located between the measuring electrode and the outer pumping electrode. When the inner pumping electrode includes the two unit electrode parts,The two unit electrode parts are arranged facing each other. The main pump cell pumps out oxygen in the measurement gas introduced into the first internal space by applying a predetermined main pump voltage between the inner pump electrode and the outer pump electrode, thereby reducing the oxygen partial pressure of the measurement gas in the first internal space. The measuring pump cell pumps out the oxygen generated by the reduction of NOx in the measurement gas, which arrives near the measuring electrode in the measuring electrode, by applying a predetermined pump voltage between the inner pump electrode and the outer pump electrode. The gas sensor further includes: a concentration specification element that specifies a concentration of NOx based on a magnitude of a NOx current flowing between the measuring electrode and the outer pump electrode in the measuring pump cell. A diffusion resistance from the gas inlet to the first internal space is 200 cm. -1or larger and 1000 cm -1 or smaller. When the first internal space has a space length L1 as a size in a longitudinal direction of the sensor element, a space thickness t1 as a size in a thickness direction of the sensor element, and a space width w1 as a size in a width direction perpendicular to both the longitudinal direction and the thickness direction, and the unit electrode part has an electrode length L2 as a size in the longitudinal direction and an electrode width w2 as a size in the width direction, the space length L1 is 2.5 mm or larger and 10 mm or smaller, the space thickness t1 is 50 μm or larger and 300 μm or smaller, a ratio of the electrode length to the space length is 0.5 or more and 1.0 or less, and a ratio of the electrode width relative to the space width is 0.5 or more and 1.0 or less.

[0009] Accordingly, even when a measurement gas having a high oxygen concentration is introduced into the first internal space, oxygen is pumped out of the first internal space while suppressing the dissolution of NOx in the first internal space in a range where it is possible to ensure NOx measurement accuracy, thereby achieving a gas sensor in which the influence of the introduction on the NOx measurement accuracy is small.

[0010] It is preferable that the space length L1 is 3.0 mm or more and 3.5 mm or less, the space thickness t1 is 100 µm or more and 200 µm or less, the ratio of the electrode length relative to the space length is 0.8 or more and 1.0 or less, the ratio of the electrode width relative to the space width is 0.9 or more and 1.0 or less.

[0011] In such a case, even if a measurement gas having a high oxygen concentration is introduced into the first internal space, the oxygen partial pressure in the first internal space is substantially maintained at a target value and NOx dissolution does not occur in the first internal space, thereby achieving a gas sensor in which deterioration of NOx measurement accuracy hardly occurs.

[0012] 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 DRAWINGS Fig. 1 is a schematic drawing illustrating an example of a configuration of a gas sensor 100, including a vertical sectional view of the sensor element 101 along a longitudinal direction. Fig.2 is a drawing for describing the sizes of a first internal space 20 and an internal pumping electrode 22 provided at the top and bottom of the first internal space 20. Fig. 3 is a drawing illustrating a processing flow in manufacturing a sensor element 101. DESCRIPTION OF THE PREFERRED EMBODIMENTS<Schematische Konfiguration des Gassensors>

[0013] 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 limiting current type NOx sensor that detects NOx by using the sensor element 101 to measure a NOx concentration.

[0014] Fig.1 is a schematic drawing illustrating an example of a configuration of the gas sensor 100 with a vertical section of the sensor element 101 along a longitudinal direction.

[0015] 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 made of zirconium dioxide (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 drawing a sheet of the Fig.1. The solid electrolyte forming these six layers is dense and airtight. In the following description, a surface on top of each of these six layers is Fig. 1 is simply referred to as the upper surface, and a surface on a lower side thereof is in some cases simply referred to as the lower surface. An entire portion of the solid electrolyte in the sensor element 101 is collectively referred to as the base portion.

[0016] 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.

[0017] A gas inlet 10, a first diffusion-limiting part 11, a buffer space 12, a second diffusion-limiting part 13, a first inner space 20, a third diffusion-limiting part 30, and a second inner 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.

[0018] 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 appear to be provided by hollowing out 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.

[0019] Each of the first diffusion-limiting part 11, the second diffusion-limiting part 13 and the third diffusion-limiting part 30 is formed as two horizontally long slits (having an opening with a longitudinal direction perpendicular to the drawing sheet of Fig. 1). An area from the gas inlet 10 to the second interior space 40 is also referred to as the gas distribution section.

[0020] A reference gas introduction space 43 is provided at a position farther from one 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, for example, is introduced into the reference gas introduction space 43 as a reference gas in measuring the NOx concentration.

[0021] An atmospheric air introduction layer 48 is a layer of porous alumina, and the reference gas is introduced into the atmospheric air introduction layer 48 through the reference gas introduction space 43. The atmospheric air introduction layer 48 is formed to cover a reference electrode 42.

[0022] The reference electrode 42 is an electrode configured to be 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.

[0023] The gas inlet 10 is a portion having an opening to an outside area in the gas introduction part, and the measurement gas is taken in from the outside area through the gas inlet 10 into the sensor element 101.

[0024] The first diffusion-limiting part 11 is a part for providing the measurement gas from the gas inlet 10 of the predetermined diffusion resistance.

[0025] 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.

[0026] The second diffusion-limiting part 13 is a part for providing the measurement gas introduced from the buffer space 12 to the first inner space 20 of the predetermined diffusion resistance.

[0027] When introducing the measurement gas from outside the sensor element 101 into the first internal space 20, the measurement gas is not directly introduced into the first internal space 20 according to a pressure change of the measurement gas in the external space (pulsation of exhaust pressure in the case where the measurement gas is exhaust gas from a vehicle), but is introduced into the first internal space 20 after a concentration fluctuation of the measurement gas has been canceled by the first diffusion-limiting part 11, the buffer space 12, and the second diffusion-limiting part 13. Thus, the concentration fluctuation of the measurement gas introduced into the first internal space 20 is substantially negligible.

[0028] The sensor element 101 of the gas sensor 100 according to the present preferred embodiment is configured such that a diffusion resistance (hereinafter referred to as pre-indoor diffusion resistance) from the gas inlet 10 to the first interior space 20 has a value in the range of 200 cm -1 up to 1000 cm -1This is achieved by combining the diffusion resistance of the first diffusion-limiting part 11 and the diffusion resistance of the second diffusion-limiting part 13. The area of ​​a portion of the first interior space 20 orthogonal to the longitudinal direction (hereinafter referred to as the element longitudinal direction) of the sensor element 101 is larger than the area of ​​a portion of the second diffusion-limiting part 13, and thus the first interior space 20 does not act as a diffusion-limiting part on a measurement gas flowing through the second diffusion-limiting part 13 into the first interior space 20. Accordingly, the pre-interior diffusion resistance is essentially equivalent to a diffusion resistance from the gas inlet 10 to an inner pumping electrode 22.

[0029] The first interior space 20 is provided as a space for adjusting the oxygen partial pressure in the measurement gas, which is introduced through the second diffusion-limiting part 13. The oxygen partial pressure is adjusted by the operation of a main pump cell 21.

[0030] The main pump cell 21 is an electrochemical pump cell consisting of the inner pump electrode 22 provided on the lower surface of the second solid electrolyte layer 6 facing the first internal space 20 and on the upper surface of the first solid electrolyte layer 4 facing the first internal space 20, an outer pump electrode 23 provided to be exposed to the outside in a region corresponding to the inner pump electrode 22 on 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.

[0031] The inner pumping electrode 22 consists of two unit electrode parts formed on the solid electrolyte layers (the second solid electrolyte layer 6 and the first solid electrolyte layer 4) on an upper surface and a lower surface defining the first internal space 20. Specifically, the ceiling electrode part 22a is formed on the lower surface of the second solid electrolyte layer 6, which forms a ceiling surface of the first internal space 20, and a lower electrode part 22b is formed on the upper surface of the first solid electrolyte layer 4, which forms a lower surface of the first internal space 20. The ceiling electrode part 22a and the lower electrode part 22b are connected to each other in a narrow lead part extending from the electrode parts and provided along a sidewall surface (inner surface) of the spacer layer 5, both of which form the sidewall parts of the first internal space 20 (the figure is omitted).

[0032] The ceiling electrode part 22a and the lower electrode part 22b are formed in rectangular shapes in a plan view. However, it may be possible to provide only the ceiling electrode part 22a or only the lower electrode part 22b.

[0033] Each of the inner pumping electrode 22 and the outer pumping electrode 23 is formed as a porous cermet electrode. Specifically, the inner pumping electrode 22 contacting the measurement gas is formed from a material whose reducing ability to a NOx component in the measurement gas is weakened. For example, the inner pumping 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 a cermet electrode made of an Au-Pt alloy containing Au substantially equal to or greater than 0.6 wt% and equal to or less than 1.4 wt% and ZrO 2 A weight ratio of the Au-Pt alloy to ZrO 2can be Pt:ZrO 2 =7.0:3.0 to 5.0:5.0.

[0034] Meanwhile, for example, the outer pump electrode 23 is shown in a plan view as a cermet electrode made of Pt or a Pt alloy and ZrO 2 formed into a rectangular shape.

[0035] In the main pump cell 21, a desired pump voltage Vp0 is applied between the inner pump electrode 22 and the outer pump electrode 23 by a variable source 24, and a pump current Ip0 flows in a positive or negative direction between the inner pump electrode 22 and the outer pump electrode 23, so that oxygen in the first inner space 20 can be pumped out to the outer space or oxygen in the outer space can be pumped into the first inner space 20. The pump voltage Vp0 applied between the inner pump electrode 22 and the outer pump electrode 23 in the main pump cell 21 is also referred to as the main pump voltage Vp0.

[0036] The sizes of the first internal space 20 and the inner pumping electrode 22 are determined so that the measurement accuracy of the NOx concentration is ensured even when a measurement gas with a high oxygen concentration is introduced into the sensor element 101. Details will be described below.

[0037] The inner pumping 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, that is, 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 inner space 20.

[0038] The oxygen concentration (oxygen partial pressure) in the first interior space 20 can be determined by measuring an electromotive force V0 in the main pump-controlled oxygen partial pressure detection sensor cell 80.

[0039] Furthermore, feedback control is performed on the main pump voltage Vp0 so that the electromotive force V0 is set to a constant value, thereby controlling the pump current Ip0. Accordingly, the oxygen concentration in the first internal space 20 is maintained at a predetermined constant value.

[0040] The third diffusion-limiting part 30 is a part for providing the measurement gas whose oxygen concentration (oxygen partial pressure) is controlled by an operation of the main pump cell 21 in the first inner space 20 of a predetermined diffusion resistance and for guiding the measurement gas to the second inner space 40.

[0041] The second internal space 40 is provided as a space for performing processing according to the 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 measuring pump cell 41.

[0042] In the second internal space 40, the adjustment of the oxygen partial pressure of the measurement gas, whose oxygen concentration (oxygen partial pressure) was previously adjusted in the first internal space 20 and then introduced through the third diffusion-limiting part 30, is further performed by the auxiliary pump cell 50. Accordingly, the oxygen concentration in the second internal space 40 can be kept exactly constant, so that the gas sensor 100 enables highly accurate NOx concentration measurement.

[0043] The auxiliary pump cell 50 is an electrochemical auxiliary pump cell consisting of an auxiliary pump 51 having a ceiling electrode part 51a provided 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 a suitable electrode outside the sensor element 101 is also applicable), and the second solid electrolyte layer 6.

[0044] The auxiliary pumping electrode 51 is arranged in the second internal space 40 similarly to the inner pumping electrode 22 in the first internal space 20 described above. In other words, the ceiling electrode part 51a is formed on the second solid electrolyte layer 6, which forms a ceiling surface of the second internal space 40, and a lower electrode part 51b is formed on the first solid electrolyte layer 4, which forms a lower surface of the second internal space 40. Each of the ceiling electrode part 51a and the lower electrode part 51b has 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, which forms both side wall parts of the second internal space 40 (illustration is omitted).

[0045] In a manner similar to the inner pumping electrode 22, the auxiliary pumping electrode 51 is also formed of a material whose reducing ability to a NOx component in the measurement gas is weakened.

[0046] In the auxiliary pump cell 50, a desired pump voltage Vp1 is applied between the auxiliary pump electrode 51 and the outer pump electrode 23 so that oxygen in the atmosphere in the second inner space 40 can be pumped out to the outer space or oxygen can be pumped from the outer space into the second inner space 40.

[0047] 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, that is, 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.

[0048] The auxiliary pump cell 50 performs pumping with a variable source 52, in which voltage control is performed based on an electromotive force V1 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 affect the measurement of NOx.

[0049] Accordingly, a pumping current Ip1 thereof is used to control the electromotive force of the main pump control oxygen partial pressure detection sensor cell 80. Specifically, the pumping current Ip1 is input as a control signal to the main pump control oxygen partial pressure detection sensor cell 80, and by controlling the electromotive force V0 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 to maintain a constant gradient. When the gas sensor 100 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 functions of the main pump cell 21 and the auxiliary pump cell 50.

[0050] The measuring pump cell 41 measures the NOx concentration in the measurement gas in the second inner space 40. The measuring pump cell 41 is an electrochemical pump cell consisting of a measuring electrode 44 provided on the upper surface of the first solid electrolyte layer 4 opposite to the second inner space 40 in 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.

[0051] The measuring electrode 44 is a porous cermet electrode. For example, the measuring electrode 44 is a cermet electrode made of Pt or an alloy of Pt and ZrO 2 The measuring electrode 44 also functions as a NOx reduction catalyst for reducing NOx in the atmosphere in the second interior space 40. In addition, the measuring electrode 44 is covered with a fourth diffusion-limiting part 45.

[0052] The fourth diffusion-limiting part 45 is a film formed from a porous material mainly containing aluminum oxide (Al 2 O 3 ). The fourth diffusion-limiting part 45 has a function of limiting an amount of NOx flowing into the measuring electrode 44 and simultaneously functions as a protective film of the measuring electrode 44.

[0053] The measuring pump cell 41 can pump out oxygen generated by the dissolution of NOx in the atmosphere around the measuring electrode 44 and detect a generated amount of oxygen as a pumping current Ip2.

[0054] The second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, the third substrate layer 3, the measuring electrode 44, and the reference electrode 42 constitute an electrochemical sensor cell, i.e., a metering pump control oxygen partial pressure detection sensor cell 82 for detecting the oxygen partial pressure around the measuring electrode 44. A variable source 46 is controlled based on an electromotive force V2 detected in the metering pump control oxygen partial pressure detection sensor cell 82.

[0055] The measurement gas introduced into the second internal space 40 reaches the measuring electrode 44 through the fourth diffusion-limiting part 45 under a condition where the oxygen partial pressure is controlled. NOx in the measurement gas is reduced around the measuring electrode 44 (2NO → N 2 +O 2), and oxygen is generated. The generated oxygen is pumped by the metering pump cell 41. At this time, an electromotive force Vp2 of the variable source 46 is controlled so that a control voltage V2 detected in the metering pump control oxygen partial pressure detection sensor cell 82 is set to constant. Since the amount of oxygen generated around the measuring electrode 44 is proportional to the NOx concentration in the measurement gas, the NOx concentration in the measurement gas is calculated using the pumping current Ip2 in the metering pump cell 41. The pumping current Ip2 is also referred to as the NOx current Ip2 hereinafter.

[0056] When the measuring 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 detecting 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 measuring electrode 44 and an amount of oxygen contained in a reference atmosphere can be detected, and accordingly, a concentration of the NOx component in the measurement gas can also be obtained.

[0057] The second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, the third substrate layer 3, the outer pumping electrode 23 and the reference electrode 42 form an electrochemical sensor cell 83, and the oxygen partial pressure in the measurement gas outside the sensor can be detected by an electromotive force Vref obtained from the sensor cell 83.

[0058] The sensor element 101 further includes a heater part 70 having the function of setting 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.

[0059] The heater part 70 essentially includes a heating electrode 71, a heating element 72, a heating lead 72a, 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.

[0060] The heater electrode 71 is an electrode formed to contact the lower surface of the first substrate layer 1 (the other main surface of the sensor element 101).

[0061] 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 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 72a, which function 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 region 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 approximately 10 µm to 20 µm.

[0062] In the sensor element 101, the current is conducted into the heater element 72 via the heater electrode 71, causing the heater element 72 to generate heat, allowing each part of the sensor element 101 to be heated to a predetermined temperature and maintained at that temperature. Specifically, the sensor element 101 is heated so that the temperature of the solid electrolyte and the electrode near the gas introduction part rises to approximately 700°C to 900°C. This heating increases the oxygen ion conductivity of the solid electrolyte, which forms 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 drive temperature of the sensor element.

[0063] The gas sensor 100 further includes a controller 110 that controls the operation of each part and specifies the NOx concentration based on the NOx current Ip2.

[0064] 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 the measurement of NOx (e.g., 0.0001 ppm to 1 ppm) is not significantly affected, reaches the measuring electrode 44. In the measuring electrode 44, NOx in the measurement gas that has reached the measuring electrode 44 is reduced, and oxygen is generated. The generated oxygen is pumped out by the measuring pump cell 41. The NOx current Ip2 flowing at the time of oxygen pumping out has a certain functional relationship with the NOx concentration in the measurement gas (hereinafter referred to as sensitivity characteristics).

[0065] The sensitivity characteristics are specified in advance using 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. During actual use of the gas sensor 100, signals indicating a value of the NOx current Ip2 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 specified sensitivity characteristics 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.

[0066] Although the sensitivity characteristics should fundamentally have a perfectly proportional relationship between the NOx concentration and the NOx current Ip2, the measurement gas reaching the measurement pump cell 41 contains a small amount of oxygen, which is not pumped out in the main pump cell 21 and the auxiliary pump cell 50 when the gas sensor 100 is actually used. Thus, even if no NOx is contained in the measurement gas, the NOx current Ip2 is not completely zero. The NOx current Ip2 in this case is referred to as an offset current. Normally, in the gas sensor 100, the sensitivity characteristics are specified taking into account the presence of such an offset current, but from the viewpoint of ensuring measurement accuracy, the offset current is preferably smaller, and it is not preferable for the offset current to fluctuate. <Reduzierender Einfluss der Einführung von Messungsgas mit hoher Sauerstoffkonzentration>

[0067] The sensor element 101 of the gas sensor 100 according to the present preferred embodiment generally has the configuration described above, but specifically satisfies some configuration requirements from the perspective of suppressing NOx dissolution in the first internal space 20 to ensure measurement accuracy even when the measurement gas having a high oxygen concentration is continuously supplied inside. In the following description, the inner pumping electrode 22 includes both the ceiling electrode part 22a and the lower electrode part 22b in the same shape at the same position in the plan view unless otherwise stated. Since a contribution of the conducting part to the dissolution of NOx may be negligible, the term "inner pumping electrode 22" in the following description indicates a part other than the conducting part.

[0068] First, the sensor element 101 is configured as a prerequisite so that the diffusion resistance in front of the first interior space, as described above, covers the range of 200 cm -1 up to 1000 cm -1 fulfilled in order to adjust the flow rate of the measuring gases introduced into the first interior space 20 from the outside accordingly.

[0069] If the diffusion resistance in front of the pre-interior is less than 200 cm -1, the flow rate when introducing the oxygen-containing measurement gas into the first internal space 20 is large, and thus the absolute amount of oxygen in the first internal space 20 is large. Accordingly, the main pump voltage Vp0 and the pump current Ip0 inevitably become large because it is necessary to pump out this oxygen. In such a case, NOx contained in the measurement gas, which should normally reach the vicinity of the measuring electrode 44 in the second internal space 40, is likely to be dissolved in the first internal space 20 by the pumping of the main pump cell 21. In addition, if the flow rate of the measurement gas is too large, the measurement gas may flow out into the second internal space 40 without sufficiently discharging oxygen. This may lead to an increase in the offset current. Either of these measures results in a decrease in the detection accuracy of NOx in the gas sensor 100 and is therefore not preferable.

[0070] In case the pre-indoor diffusion resistance is greater than 1000 cm -1 , the absolute amount of NOx reaching the vicinity of the measuring electrode 44 decreases because the flow rate of the measurement gas introduced into the first internal space 20 is small. Such a case is not preferable because NOx cannot be accurately detected or the responsiveness is reduced.

[0071] When the flow rate of the measurement gas introduced into the first internal space 20 is appropriate, the quality of pumping oxygen from the first internal space 20 depends on the balance between the size of the first internal space 20 and the capacity of the main pumping cell 21. Since the capacity of the main pumping cell 21 greatly depends on the size of the inner pumping electrode 22 provided opposite the first internal space 20, the quality of pumping oxygen from the first internal space 20 mainly depends on the size of the first internal space 20 and the ratio of this size to the size of the inner pumping electrode 22.

[0072] Fig. 2 is a drawing for describing the dimensions of the first internal space 20 and the inner pumping electrode 22 provided on the upper and lower surfaces of the first internal space 20. In Fig.2, the right and left directions in the drawing are the element longitudinal directions, and a dimension drawing in a plane perpendicular to the thickness direction is shown on the upper side, and a dimension drawing in a plane perpendicular to the width direction is shown on the lower side.

[0073] In the gas sensor 100 according to the present preferred embodiment, the sensor element 101 is configured such that the relationship between these quantities is preferable. In particular, when the first internal space 20, as shown in Fig. 2, a space length L1 as a size in the element longitudinal direction, a space thickness t1 as a size in the thickness direction (stacking direction of the solid electrolyte layers) of the sensor element 101 and a space width w1 as a size in the width direction perpendicular to the element longitudinal direction and the thickness direction, the first interior space 20 satisfies the following requirements (a) to (b). (a) Space length L1: 2.5 mm or larger (10 mm or smaller) (b) Space thickness t1: 50 µm or larger and 300 µm or smaller

[0074] Requirements (a) and (b) are the size requirements of the first internal space 20, which influences the flow of the measurement gas from the upstream side (in particular the second diffusion-limiting part 13) to the downstream side (in particular the third diffusion-limiting part 30).

[0075] In the case where the space length L1 is less than 2.5 mm, the interval between the upstream and downstream sides of the first internal space 20 is small, and thus the pumping performance of the main pump cell 21 is insufficient even when the internal pump electrode 22 is provided at its maximum. Accordingly, when the measurement gas with a high oxygen concentration continuously flows in from the upstream side, the measurement gas accumulates in the first internal space 20 without sufficiently pumping out oxygen after the main pump voltage Vp0 is applied. Specifically, since the oxygen concentration in the first internal space 20 increases toward the upstream region where the measurement gas flows in, the main pump voltage Vp0 tends to increase near the upstream side of the internal pump electrode 22, which is likely to cause local NOx dissolution at that location.As accumulation progresses, the outflow to the downstream side occurs through the third diffusion-limiting part 30. This means that the measurement gas flows out to the downstream side without oxygen being pumped out to a set oxygen partial pressure. This is not preferable because it leads to an increase in the offset current.

[0076] These tendencies are of course more significant since the size of the inner pumping electrode 22 is smaller.

[0077] On the other hand, the case that the space length L1 is too long does not pose a particular problem in dealing with the situation that the measurement gas has a high oxygen concentration, but the space length L1 over 10 mm means that the sensor element 101 is long and thus cost-disadvantageous and that the response time is long and thus not preferable.

[0078] In the case where the space thickness t1 is less than 50 μm, the ceiling electrode part 22a and the lower electrode part 22b are close to each other and a gap at a part where the measurement gas flows decreases, and thus, oxygen pumping is preferably performed near the upstream side of the inner pumping electrode 22. At the time when the measurement gas starts flowing from the upstream side, significant problems are unlikely to occur, but when the measurement gas is continuously introduced with a high oxygen concentration, the oxygen concentration in the first inner space 20 gradually increases from the upstream side and thus closer to the upstream side, and the main pumping voltage Vp0 is more likely to increase locally and cause NOx dissolution.Furthermore, as a result of such remarkable resolution, the measurement gas accumulates in the first internal space 20 with insufficiently pumped oxygen and eventually flows out from the downstream side, possibly leading to an increase in the offset current. All of this is not preferable. Although the inner pumping electrode 22 needs to be thinner as the space thickness t1 decreases to ensure a gap through which the measurement gas flows, there is a limit to the dilution, and therefore, excessively reducing the space thickness t1 is not preferable.

[0079] In the case where the space thickness t1 exceeds 300 μm, the ceiling electrode part 22a and the lower electrode part 22b are separated, and thus a relatively small amount of the measurement gas introduced into the first internal space 20 contacts the inner pumping electrode 22. Accordingly, the pumping of oxygen by the main pumping cell 21 is difficult to realize, and most of the measurement gas accumulates without contacting the inner pumping electrode 22 and undergoing oxygen pumping, and then flows out from the downstream side. This undesirably increases the offset current.

[0080] The space width w1 represents a size in a direction perpendicular to the direction from the upstream side (specifically, the second diffusion-limiting part 13) to the downstream side (specifically, the third diffusion-limiting part 30). Furthermore, from the perspective of suppressing the increase in the oxygen concentration of the measurement gas in the first internal space 20, it is important that the electrode be provided to correspond to the value of the space width w1, as described later. It can be determined by considering the production circumstances of the sensor element 101, the balance with other parts, the cost, and the like. For example, the range from 1.5 mm to 3.5 mm inclusive is exemplified.

[0081] In addition to the above requirements (a) and (b), as set out in Fig.2, when the sensor element 101 has an electrode length L2 as the size of the ceiling electrode part 22a and the lower electrode part 22b of the inner pumping electrode 22 in the element longitudinal direction, an electrode thickness t2 as the size in the thickness direction, and an electrode width w2 as the size in the width direction, the size of the first internal space 20 and the size of the ceiling electrode part 22a and the lower electrode part 22b of the inner pumping electrode 22 satisfy the following requirements (c) to (d). (c) The ratio (length ratio) L2 / L1 of the electrode length L2 relative to the space length L1: 0.5 or more (1.0 or less) (d) The ratio (width ratio) w2 / w1 of the electrode width w2 relative to the space width w1: 0.5 or more (1.0 or less)

[0082] The requirements (c) and (d) relate to the pumping performance of the main pump cell 21. In the following description, the length L2, the width w2, and the thickness t2 of each of the ceiling electrode part 22a and the lower electrode part 22b are also simply referred to as the length L2, the width w2, and the thickness t2 of each unit electrode part of the inner pump electrode 22.

[0083] In the case where the value of the aspect ratio L2 / L1 is less than 0.5 and in the case where the value of the width ratio w2 / w1 is less than 0.5, the pumping performance of the main pump cell 21 is insufficient compared to the size of the first internal space 20. Thus, when the flowing measurement gas has a high oxygen concentration, the oxygen concentration in the first internal space 20 increases from the upstream side to the upstream side, the main pump voltage Vp0 is more likely to increase locally and cause NOx dissolution. Moreover, the measurement gas flows out from the downstream side with insufficiently pumped oxygen, and thus the offset current is likely to increase. All of this is not preferable.

[0084] From the definitions, the upper limits of the length ratio L2 / L1 and the width ratio w2 / w1 are both 1.0. In other words, the value of the length ratio L2 / L1 is 1.0 when the inner pumping electrode 22 is formed over the entire space length L1, and the value of the width ratio w2 / w1 is 1.0 when the inner pumping electrode 22 is formed over the entire space width w1.

[0085] In addition, the ceiling electrode part 22a and the lower electrode part 22b of the inner pumping electrode 22 are provided so that the requirements (c) and (d) are met, the thickness t2 is 5 µm or larger and 30 µm or smaller, preferably 10 µm or larger and 20 µm or smaller, and the area (flat surface) S2 = L2w2 in plan view 5 mm 2 or larger and 20 mm 2 or smaller.

[0086] The lower limit of the thickness t2 is 5 µm because it is difficult to control the thickness to a predetermined value of less than 5 µm when forming the inner pumping electrode 22.

[0087] In case the thickness t2 is greater than 30 µm and in case the area S2 is greater than 20 mm 2 the reactivity of NOx in the inner pumping electrode 22 increases and NOx dissolution is likely to occur, which is not preferable.

[0088] In case the area S2 is less than 5 mm 2 , the impedance in the main pump cell 21 increases and the value of the pump current Ip0 becomes too small, or the main pump voltage Vp0 slightly increases due to insufficient pump power and consequently the NOx dissolution is likely to occur, which is not preferable.

[0089] The thickness t2 of each electrode part of the inner pumping electrode 22 can be determined according to the thickness t1 of the first inner space 20. In particular, the ratio t2 alle / t1 of the sum of the thickness t2 of the inner pumping electrode 22 (the sum of the thicknesses of the ceiling electrode part 22a and the lower electrode part 22b) t2 alle based on the thickness t1 of the first inner space, be 0.06 or more and 0.60 or less.

[0090] In the gas sensor 100 according to the present preferred embodiment, the sensor element 101 satisfies the above-described requirements (a) to (d), and thus, even when the oxygen concentration in the measurement gas is high, the pumping of oxygen from the first internal space 20 is appropriately performed while suppressing the NOx dissolution in the first internal space 20, thereby ensuring the measurement accuracy.

[0091] The arranged positions of the ceiling electrode part 22a and the lower electrode part 22b in the first inner space 20 are not particularly limited when the electrode length L2 and the electrode width w2 are smaller than the space length L1 and the space width w1, respectively. For example, the center of gravity of the first inner space 20 may or may not coincide with the center of gravity positions of the ceiling electrode part 22a and the lower electrode part 22b in plan view. If the latter is the case, the parts may be arranged on the upstream side or the downstream side in the element longitudinal direction. It is preferable that the ceiling electrode part 22a and the lower electrode part 22b be arranged symmetrically in the element width direction, but this is not absolutely necessary. <Herstellungsverfahren des Sensorelements>

[0092] Next, a method for manufacturing the sensor element 101 having the configuration and feature described above will be described. In the present embodiment, the sensor element 101 is manufactured by forming a laminated body made 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.

[0093] The following is an example of a case of manufacturing the sensor element 101 including the Fig. 1 will be described. In this case, six green sheets are prepared, corresponding 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.

[0094] In manufacturing the sensor element 101, a blank sheet (not shown) is first prepared, which is a green sheet on which no pattern is formed (step S1). When manufacturing the sensor element 101 including six layers, six blank sheets are prepared corresponding to each layer. Specifically, for forming the spacer layer 5, a thickness of the blank sheet is determined so that requirement (b) is finally met.

[0095] The blank sheets have a plurality of plate holes used for alignment when performing printing and lamination of the sheets. The plate hole is previously formed in the blank sheet, for example, by punching with a punching machine in a stage before pattern formation. Green sheets corresponding to the layers including the interior spaces also include penetrating portions corresponding to the interior spaces, which are also provided by the similar preceding punching. The formation of the penetrating portion is carried out in such a manner that requirement (a) is satisfied in the finally obtained sensor element 101. A thickness of each blank sheet corresponding to each layer of the sensor element 101 need not be the same.

[0096] After the blank plate is prepared for each layer, pattern printing and dry processing are performed on each blank plate (step S2). Specifically, 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 not shown in the drawings are formed. 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 the time of pattern printing. The application or placement is performed such that the diffusion resistance in front of the interior reaches the range of 200 cm -1 up to 1000 cm -1 fulfilled as described above in the final sensor element 101.

[0097] Each pattern is printed by applying a pattern-forming paste, which is prepared according to the properties required for each formation object, to the blank plate using the well-known screen printing technique. A well-known desiccant can be used for post-printing drying.

[0098] In particular, the paste for forming the inner pumping electrode 22 is prepared and applied so that the finally obtained inner pumping electrode 22 satisfies at least the requirements (c) to (d).

[0099] After the pattern printing on each blank plate is completed, the printing and drying of an adhesive paste is performed to laminate and adhere the green plate to and between the individual layers (step S3). A well-known screen printing technique can be used for the adhesive paste printing, and a well-known drying process can be used for the post-printing drying process.

[0100] Subsequently, the green sheets to which an adhesive 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 S4). Specifically, crimping is performed by stacking and holding the green sheets to be laminated on a predetermined laminating jig (not shown), aligning the green sheets with the plate holes, and then heating and pressurizing the green sheets together with the laminating jig of a laminating machine such as a known oil-hydraulic press. The pressure, temperature, and time for heating and pressurizing depend on the laminating machine used, but an appropriate condition can be determined to achieve favorable lamination.

[0101] 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 an element body) of the sensor element 101 (step S5).

[0102] Firing is performed on the element body at a firing temperature of approximately 1300°C to 1500°C (step S6). This produces the sensor element 101. 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 manner described above so that each electrode in the sensor element 101 has sufficient adhesive strength.

[0103] The sensor element 101 thus obtained is housed in a predetermined casing and incorporated into a main body (not shown) of the gas sensor 100. <modifizierungen>

[0104] As described above, only one of the ceiling electrode part 22a and the lower electrode part 22b may be provided as the inner pumping electrode 22, but in this case, the thickness t2 may be 5 µm or larger and 30 µm or smaller, as in the case where both electrode parts are provided. The thickness ratio t2 alle However, / t1 must satisfy the range from 0.06 to 0.60 inclusive with only one of the unit electrode parts, as in the case where both electrode parts are provided. Accordingly, the ratio t2 alle / t1 of the sum of the thickness of the inner pumping electrode 22 relative to the room thickness must satisfy the range from 0.06 to 0.60 inclusive even if both the ceiling electrode part 22a and the lower electrode part 22b are provided.

[0105] As for the area of ​​the surface S2, considering the case where both electrode parts are provided, the upper limit can be 20 mm 2 as in the case where both electrode parts are provided, but the lower limit of it should be 10 mm 2 which is the sum of the lower limits in the case where both electrode parts are provided, with respect to the impedance in the main pump cell 21.

[0106] The thickness t2 and the area S2 between the ceiling electrode part 22a and the lower electrode part 22b do not have to be identical. [Example](Manufacturing a gas sensor)

[0107] A total of 16 types of gas sensors 100 (No. 1 to No. 16) with mutually different combinations of the pre-indoor diffusion resistance, the space length L1 and the space thickness t1 of the first interior space 20, and the length ratio L2 / L1 and the width ratio w2 / w1 of the first interior space 20 and the inner pumping electrode 22 were manufactured.

[0108] In particular, the pre-indoor diffusion resistance, the room length L1, the room thickness t1, the length ratio L2 / L1 and the width ratio w2 / w1 were different as follows.

[0109] The pre-interior diffusion resistance: nine steps of 150 cm -1 , 200 cm -1 , 300 cm -1 , 400 cm -1 , 500 cm -1 , 600 cm -1 , 700 cm -1 800 cm -1 and 1000 cm -1 ;

[0110] The space length L1: six levels of 2.0 mm, 2.5 mm, 3.0 mm, 3.5 mm, 7.0 mm and 8.0 mm: The spatial thickness t1: nine levels of 40 µm, 50 µm, 80 µm, 100 µm, 120 µm, 150 µm, 200 µm, 300 µm and 350 µm; The length ratio L2 / L1: seven steps of 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 and 1.0; and The width ratio W2 / W1: eight steps of 0.40, 0.50, 0.60, 0.75, 0.80, 0.85, 0.90 and 0.95.

[0111] The gas sensors from No. 1 to No. 10 meet all requirements (a) to (d) and the gas sensors from No. 11 to No. 16 do not meet at least one of the requirements (a) to (d).

[0112] The space width w1 ranged from 1.5 mm to 3.5 mm. The total electrode thickness t2 alle was changed in the range from 10 µm to 60 µm so that the ratio t2 alle / t1 is in the range of 0.06 to 0.60.

[0113] The formation modes of the components other than those mentioned above were the same for all gas sensors 100. (Provision 1)

[0114] The measurement with a test gas with an oxygen concentration of 18% and the remainder of N 2 As the measurement gas, each gas sensor 100 manufactured as described above was tested. The offset current value was calculated, and the pumping quality from the first internal space 20 by the main pump cell 21 was determined from the value. The driving temperature of the sensor element was 830°C.

[0115] The offset current is the pumping current (NOx current) Ip2 flowing through the measuring pump cell 41 when NOx is not contained in the measurement gas, and is preferably close to zero as the ideal value. However, if the pumping of oxygen by the main pump cell 21 in the first internal space 20 is not performed excellently, the measurement gas flows out of the first internal space 20 to the downstream side with oxygen that is not sufficiently pumped out (not reaching the set partial pressure), and as a result, the value of the offset current becomes large. Thus, the value of the offset current can be regarded as an index indicating the degree of oxygen outflow from the first internal space 20, in other words, an index of the quality of the oxygen pumped from the first internal space 20 by the main pump cell 21.

[0116] Furthermore, the oxygen concentration of 18% in the test gas is a relatively high value compared to the oxygen concentration in an engine exhaust pipe, which is a primarily assumed application situation for the gas sensor 100. Thus, the provision 1 is for pumping oxygen out of the first internal space 20 when the oxygen concentration in the measurement gas is high.

[0117] In the present example, the quality of the oxygen pumped out of the first internal space 20 was determined by applying the value of the offset current obtained by measurement to a determination index consisting of three levels of equal to or less than 0.1 µA, more than 0.1 µA and equal to or less than 0.3 µA and more than 0.3 µA.

[0118] In particular, for the gas sensor 100 having an offset current of 0.1 µA or less, it was determined that the pumping of oxygen from the first internal space 20 by the main pump cell 21 was sufficiently performed, so that the oxygen partial pressure in the first internal space 20 is controlled to a value set in advance.

[0119] For the gas sensor 100 with an offset current of more than 0.1 µA and 0.3 µA or less, the oxygen outflow from the first internal space 20 is slightly larger than in the case described above, but its influence on the NOx measurement accuracy is small, and thus it was found that the measurement was possible.

[0120] For the gas sensor 100 with an offset current of more than 0.3 µA, oxygen flowed from the first inner space 20 to the downstream side, as far as it affects the measurement accuracy, which was determined to be not preferable. (Provision 2)

[0121] The measurement (first measurement) with a test gas with a NOx concentration of 500 ppm and the remainder of N 2 as the measurement gas and the measurement (second measurement) with a test gas with an oxygen concentration of 18%, a NOx concentration of 500 ppm and the remainder of N 2 as the measurement gas were performed using each gas sensor 100 manufactured as described above. The driving temperature of the sensor element was 830°C. The degree of NOx dissolution in the first internal space 20 was determined based on the decrease rate (hereinafter referred to as Ip2 decrease rate) of the NOx current value Ip2 in the second measurement relative to the NOx current value Ip2 in the first measurement.

[0122] The low Ip2 decrease rate means that even if a measurement gas with a high oxygen concentration is introduced into the first internal space 20, the impact on the NOx measurement accuracy is small. Thus, the Ip2 decrease rate can be regarded as an index that correlates with the NOx resolution in the first internal space 20, which is caused by the increase in the main pump voltage Vp0 when the measurement gas with a high oxygen concentration is introduced into the first internal space 20.

[0123] In the present example, the quality of NOx dissolution in the first internal space 20 was determined by applying the calculated value of the Ip2 decrease rate to a determination index consisting of three levels of equal to or less than 15%, more than 15%, equal to or less than 20%, and more than 20%. Specifically, for the gas sensor 100 with an Ip2 decrease rate of equal to or less than 15%, it was determined that NOx dissolution in the first internal space 20 was excellently suppressed.

[0124] For the gas sensor 100 with an Ip2 decrease rate of more than 15% and equal to or less than 20%, the NOx dissolution in the first interior space 20 partially occurred, but it was determined that the influence thereof on the measurement accuracy was small.

[0125] For the gas sensor 100 with an Ip2 decay rate of more than 20%, the NOx resolution in the first interior space 20 influenced the measurement accuracy, which was considered undesirable.

[0126] The Ip2 decay rate threshold of 15% or 20% may seem high at first glance. However, since the oxygen concentration of 18% in the test gas is a relatively high value, as described above, setting the threshold is reasonable. (Result)

[0127] Table 1 lists the pre-indoor diffusion resistance, the space length L1, the space thickness t1, the length ratio L2 / L1, the value of the width ratio w2 / w1, and the results of Determination 1 and Determination 2 for each gas sensor 100. Table 1 also lists the space width w1 and the electrode thickness sum t2. alle In Table 1, a double circle, a circle, or a cross is placed for each case in which the offset current value measured in Determination 1 is equal to or less than 0.1 µA, greater than 0.1 µA, equal to or less than 0.3 µA, or greater than 0.3 µA. In addition, a double circle, a circle, or a cross is placed in each case in which the Ip2 decay rate value calculated in Determination 2 is equal to or less than 15%, greater than 15%, equal to or less than 20%, or greater than 20%. [Table 1] Nr. Pre-interior diffusion resistance [cm -1 ] Room length L1 [mm] Room thickness t1 [µm] Electrode length L2 / room length L1 Electrode width w2 / Room width W1 Electrode thickness sum t2 alle [µm] Room width w1 [µm] Provision 1 Provision 2 1 700 2,5 80 0,8 0,50 30 3,5 ◯ ◯ 2 300 3,5 120 0,9 0,90 26 2,6 ⊚ ⊚ 3 400 3,0 200 0,8 0,90 30 2,5 ⊚ ⊚ 4 700 3,0 100 1,0 0,95 34 2,5 ⊚ ⊚ 5 500 3,5 300 0,9 0,80 18 2,9 ◯ ⊚ 6 600 3,0 50 0,9 0,50 10 2,0 ◯ ◯ 7 200 3,0 80 0,7 0,75 30 3,1 ◯ ◯ 8 800 2,5 100 0,5 0,60 60 1,5 ◯ ◯ 9 1000 8,0 100 0,7 0,90 13 2,5 ◯ ◯ 10 500 7,0 80 0,7 0,50 15 3,0 ◯ ◯ 11 700 3,5 350 0,9 0,80 30 2,9 × ◯ 12 600 2,5 40 0,8 0,90 10 2,6 ◯ × 13 500 3,0 100 0,6 0,40 20 2,5 ◯ × 14 150 3,0 100 0,9 0,80 20 2,9 ◯ × 15 400 2,0 200 0,9 0,85 26 2,7 × X 16 800 2,5 150 0,4 0,90 30 2,6 × ×

[0128] In Table 1, a double circle or a circle for both Determination 1 and Determination 2 is provided only for the gas sensors 100 from Nos. 1 to No. 10 that meet all of the requirements (a) to (d). This means that neither the oxygen outflow from the first internal space 20 to the downstream side nor the NOx dissolution in the first internal space 20, which affect the measurement accuracy of the NOx concentration, has occurred only in these gas sensors 100. Meanwhile, a cross is provided for at least one of Determination 1 and Determination 2 for the gas sensors Nos. 11 to 16 that do not meet at least one of the requirements (a) to (d).

[0129] The above results indicate that, by satisfying requirements (a) to (d), a gas sensor 100 in which, even when a measurement gas with a high oxygen concentration is introduced into the first internal space 20, the influence of the introduction on the NOx measurement accuracy is small is achieved. Specifically, in the gas sensor 100, even when a measurement gas with a high oxygen concentration is introduced into the first internal space 20, oxygen pumping from the first internal space 20 is performed while suppressing NOx dissolution in the first internal space 20, at least within a range where NOx measurement accuracy can be ensured.

[0130] Specifically, for the gas sensors 100 of No. 2 to No. 4, a double circuit is attached for both Determination 1 and Determination 2. This indicates that in a gas sensor 100 that satisfies the requirements (a) to (d) and additionally satisfies the following requirements (a') to (d'), even when a measurement gas having a high oxygen concentration is introduced into the first internal space 20, the oxygen partial pressure in the first internal space 20 is substantially maintained at a target value by the oxygen to be pumped out by the main pumping cell 21, and NOx dissolution in the first internal space 20 does not occur, and thus the NOx measurement accuracy hardly deteriorates. (a') Space length L1: 3.0 mm or larger and 3.5 mm or smaller (b') Space thickness t1: 100 µm or larger and 200 µm or smaller (c') Aspect ratio L2 / L1: 0.8 or more (1.0 or less) (d') Width ratio w2 / w1: 0.90 or more (1.0 or less)< / modifizierungen>

Claims

[1] A limiting current type gas sensor (100), the gas sensor (100) including a sensor element (101) formed from an oxygen ion conductive solid electrolyte (1, 2, 3, 4, 5, 6), the gas sensor (100) being capable of indicating a NOx concentration in a measurement gas, the sensor element (101) including: a gas inlet (10) into which the measurement 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 pump cell (21) which is an electrochemical pump cell consisting of an inner pump electrode (22) including one or two unit electrode parts (22a, 22b) and facing the first inner space (20), an outer pump electrode (23) provided on a surface of the sensor element (101), and the solid electrolyte (6) located between the inner pump electrode (22) and the outer pump 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) introduced into the atmospheric air 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 (6) located between the measuring electrode (44) and the outer pump electrode (23); where when the inner pumping electrode (22) includes the two of the unit electrode parts (22a, 22b), the two unit electrode parts (22a, 22b) are arranged to face each other, the main pump cell (21) pumps out oxygen in the measurement gas introduced into the first inner space (20) by applying a predetermined main pump voltage (Vp0) between the inner pump electrode (22) and the outer pump electrode (23), so that the oxygen partial pressure of the measurement gas in the first inner space (20) is reduced, the measuring pump cell (41) pumps out oxygen produced by a reduction of NOx in the measuring gas reaching the vicinity of the measuring electrode (44) by applying a predetermined pump voltage (Vp2) between the inner pump electrode (22) and the outer pump electrode (23), wherein the gas sensor (100) further comprises: a concentration specification 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), a diffusion resistance from the gas inlet (10) to the first interior space (20) which is 200 cm-1 or greater and 1000 cm-1 or less, and when the first interior space (20) has a space length (L1) as a size in a longitudinal direction of the sensor element (101), a space thickness (t1) as a size in a thickness direction of the sensor element (101), and a space width (w1) as a size in a width direction perpendicular to both the longitudinal direction and the thickness direction, and the unit electrode part (22a, 22b) has an electrode length (L2) as a size in the longitudinal direction and an electrode width (w2) as a size in the width direction, the space length (L1) is 2.5 mm or greater and 10 mm or less, the space thickness (t1) is 50 µm or greater and 300 µm or less, a ratio of the electrode length (L2) relative to the space length (L1) is 0.5 or more and 1.0 or less and a ratio of the electrode width (w2) relative to the space width (w1) is 0.5 or more and 1.0 or less. [2] Gas sensor (100) according to claim 1, wherein the space length (L1) is 3.0 mm or greater and 3.5 mm or less, the space thickness (t1) is 100 µm or greater and 200 µm or less, the ratio of the electrode length (L2) relative to the space length (L1) is 0.8 or more and 1.0 or less and the ratio of the electrode width (w2) relative to the space width (w1) is 0.9 or more and 1.0 or less. [3] Gas sensor (100) according to claim 1 or 2, wherein in the one or two unit electrode parts (22a, 22b), an electrode thickness (t2) as a size in the thickness direction is 5 µm or larger and 30 µm or smaller, an area equal to or less than 20 mm 2 is, a total area of ​​the inner pumping electrode (22) equal to or greater than 10 mm 2 is, and a ratio of a sum of the electrode thickness (t2) relative to the space thickness (t1) is 0.06 or more and 0.60 or less. [4] Gas sensor (100) according to one of claims 1 to 3, wherein the sensor element (101) further comprises: a main pump control sensor cell (80) which is an electrochemical sensor cell consisting of the inner pumping electrode (22), the reference electrode (42) and the solid electrolyte (4) located between the inner pumping electrode (22) and the reference electrode (42); an auxiliary pump cell (50) which 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 (4) located between the auxiliary pump electrode (51) and the reference electrode (42); and a measuring pump control sensor cell (82) which is an electrochemical sensor cell consisting of the measuring electrode (44), the reference electrode (42) and the solid electrolyte (4) located between the measuring electrode (44) and the reference electrode (42), wherein the main pump cell (21) pumps out oxygen in the measurement gas located in the first inner space (20) by applying the main pump voltage (Vp0) between the inner pump electrode (22) and the outer pump electrode (23) 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), the auxiliary pump cell (50) pumps out oxygen in the measurement gas introduced into the second internal space (40) by applying 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) between the auxiliary pump electrode (51) and the outer pump electrode (23), so that the measurement gas, whose oxygen partial pressure has been further reduced compared to the oxygen partial pressure in the first internal space (20), reaches the measurement electrode (44), and the measuring pump cell (41) pumps out oxygen generated in the measuring electrode (44) by applying a pump voltage (Vp2) between the measuring electrode (44) and the outer pump electrode (23) in accordance with an electromotive force (V2) generated between the measuring electrode (44) and the reference electrode (42) in the measuring pump control sensor cell (82).

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