Gas Sensor

Abstract

To provide a gas sensor for suppressing deterioration of measurement precision even if a contamination gas is generated in a reference gas space side.SOLUTION: A casing of a gas sensor having a reference gas space inside has an outer cylinder in which the one end side of a sensor element having a detection part in the other end side projects and a sealing member that is fitted to the end of the outer cylinder and seals the reference gas space, the sensor element has a first electrode lead part having a first electrode lead for electrically connecting the outside and at least one out-of-void pump electrode and a first lead insulating layer for covering it, and a second electrode lead part having a second electrode lead capable of electrically connecting the outside and a measurement electrode and a second lead insulation layer for covering it, A / B<1.00 is established when diffusion resistance in the first electrode lead part is set to A(cm-1) and diffusion resistance in the second electrode lead part is set to B(cm-1).SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention relates to a gas sensor having a ceramic sensor element, and particularly to protection of a measuring electrode and suppression of output fluctuations when used in a rich gas atmosphere. [Background technology]

[0002] Gas sensors in which a sensor element is formed using an oxygen ion conductive solid electrolyte ceramic such as zirconia (ZrO2) have been known for some time as devices for measuring the concentration of a specific gas component in a measurement gas such as combustion gas or exhaust gas from an internal combustion engine such as an automobile engine.

[0003] Such gas sensors typically have a main body in which a long, plate-shaped ceramic sensor element (detection element) is fixed inside (hollow portion of) a metal housing member by a ceramic supporter and a ceramic powder compact such as talc, and the powder compact hermetically seals between one end side, which is equipped with a gas inlet for introducing the gas to be measured into the element interior equipped with a measurement electrode, etc., and the other end side, through which a reference gas (atmosphere) is introduced (see, for example, Patent Document 1).

[0004] The other end of the housing member is a cylindrical member also called an outer cylinder, and at the tip of the cylindrical member is a sealing member. Grommet The space enclosed by the outer cylinder and grommet is the reference gas space. The grommet has through holes through which several lead wires are inserted to electrically connect the center element to the outside. The reference gas is usually the air that is present in the outer cylinder when the grommet is fitted into the outer cylinder during the gas sensor manufacturing process. However, air that subsequently enters the outer cylinder through the gap between the through holes in the grommet and the lead wires can also serve as the reference gas.

[0005] On the other end of the sensor element, a reference gas is introduced from the reference gas space into the element interior, and a reference electrode for providing a reference potential is provided so as to be in contact with the reference gas. By the reference electrode coming into contact with the reference gas having a constant oxygen concentration, the reference potential is kept constant, and a potential difference corresponding to the atmosphere around the electrode is generated between the reference electrode and other electrodes provided in the sensor element such as the measurement electrode.

[0006] Various electrodes provided in the sensor element are electrically connected to connection terminals provided at the element end by electrode leads disposed inside the element or on the element side surface. However, a sensor element in which the reference electrode lead connecting the reference electrode and an electrode pad as such a connection terminal is made porous is already known (see, for example, Patent Document 2).

[0007] Also, a gas sensor provided with an outer electrode on the outer surface of the sensor element and provided with a ceramic layer such that a slit portion for imparting a predetermined diffusion resistance is formed around the outer electrode, and further, a gas sensor in which the mechanical strength of the slit portion is increased by embedding a porous body is already known (see, for example, Patent Document 3).

[0008] Furthermore, a gas sensor including a sensor element in which an oxygen concentration detection cell and an oxygen pump cell are laminated in the element thickness direction via an insulating layer, and the detection gas is introduced into the interior through a diffusion rate-limiting portion made of a porous body provided in a part of the insulating layer is already known (see, for example, Patent Document 4).

[0009] In addition, a gas sensor element in which the width W1 of the measurement gas chamber into which the measurement gas is introduced and the width W2 of the porous diffusion resistance layer disposed at the gas inlet to the measurement gas chamber satisfy the relationship W1 < W2 is already known (see, for example, Patent Document 5).

Prior Art Documents

Patent Documents

[0010]

Patent Document 1

[0011] The gas sensor disclosed in Patent Document 1 is used in a high-temperature environment, with its main body disposed in the exhaust path from the engine and exposed to exhaust gases. Furthermore, the sensor element itself is heated by a heater. Therefore, when the main body is heated to a high temperature after use, the reference gas may be contaminated by contaminant gases generated by the evaporation of oil adhering to the inner surface of the outer tube or the generation of gas from the rubber stopper. If such contaminant gases reach the reference electrode, the reference potential, which should be kept constant, may change, potentially disrupting the measurement accuracy of the gas sensor. This situation is considered more likely to occur when the reference electrode lead is porous, as disclosed in Patent Document 2.

[0012] In the gas sensor disclosed in Patent Document 1, an electrochemical pump cell is composed of an outer electrode, a reference electrode, and a solid electrolyte between the two. By applying a predetermined voltage between the two electrodes, oxygen can be pumped from outside the element into the reference gas space. By pumping oxygen in this way, the reference potential can be kept constant even if the reference gas is contaminated.

[0013] However, the above-mentioned contaminant gas may reach not only the reference electrode but also the measurement electrode or even an internal space closer to the gas inlet via an electrode lead such as a measurement electrode lead. In this case, the measurement gas, which is supposed to be introduced through the gas inlet and then reach the measurement electrode after its oxygen concentration has been adjusted to a predetermined value, may be contaminated by the contaminant gas that has infiltrated through the lead, resulting in a deterioration in the measurement accuracy of the gas sensor. Even if the amount of infiltration is small, its effect cannot be ignored if the concentration of the measurement target gas component is low.

[0014] For example, Patent Document 5 discloses a configuration in which, when the width W1 of the measurement gas chamber is reduced compared to conventional configurations and the width of the electrodes provided in the gas sensor element is also reduced, the width of the electrode leads connected to each electrode is set relatively large to prevent a large change in the impedance of the electrode portion including the lead portions, thereby adjusting the electrode impedance to a predetermined value. If the width of the lead portions is increased in this way, the diffusion resistance of the lead portions decreases, increasing the risk of an increase in the amount of contaminant gas reaching the electrode portion via the lead portions.

[0015] Furthermore, adopting porous electrode leads such as those disclosed in Patent Document 2 for electrodes other than the reference electrode lead can reduce the amount of platinum used, leading to cost savings, but there is a risk that the amount of contaminant gas that penetrates through the electrode lead may increase.

[0016] The present invention has been made in consideration of the above-mentioned problems, and aims to provide a gas sensor in which deterioration of measurement accuracy is suppressed even when contaminant gas is generated on the reference gas space side. [Means for solving the problem]

[0017] In order to solve the above problems, a first aspect of the present invention is a gas sensor capable of detecting a predetermined gas component in a gas to be measured, comprising: a sensor element having a long, plate-shaped base portion made of an oxygen ion conductive solid electrolyte and having a detection portion on one end side; and a casing in which the sensor element is housed and fixed, the casing having a reference gas space in which a reference gas is present, an outer cylinder in which the other end side of the sensor element protrudes into the reference gas space; and a gas sensor fitted onto the end of the outer cylinder, the reference gas being present. and a seal member for sealing the space between the sensor element and the measurement gas inlet provided at the one end side. The sensor element comprises at least one oxygen partial pressure adjusting internal cavity that communicates with the measurement gas inlet at a predetermined diffusion resistance, a measurement internal cavity that further communicates with the at least one oxygen partial pressure adjusting internal cavity, an external pump electrode disposed at a location other than the at least one oxygen partial pressure adjusting internal cavity and the measurement internal cavity, and a pump electrode disposed facing the at least one oxygen partial pressure adjusting internal cavity and between the external pump electrode and the at least one inner electrode capable of pumping oxygen between the at least one oxygen partial pressure adjusting inner cavity and the outside of the sensor element by applying a predetermined pump voltage thereto; a measurement electrode provided facing the measurement inner cavity and capable of pumping oxygen between the measurement inner cavity and the outside of the sensor element by applying a predetermined pump voltage therebetween; and an outer electrode of the sensor element extending from the outer pump electrode. a first electrode lead portion including a first electrode lead for electrically connecting a portion of the sensor element to the outside-space pump electrode and a first lead insulating layer covering the first electrode lead; a ceramic layer covering at least the first electrode lead portion; a second electrode lead extending from the measurement electrode and capable of electrically connecting the outside of the sensor element to the measurement electrode; and a second lead insulating layer covering the second electrode lead; and -1 ), and the diffusion resistance in the second electrode lead portion is B (cm -1 ) A / B<1.00.

[0018] A second aspect of the present invention is the gas sensor according to the first aspect, characterized in that 0.03≦A / B≦0.29.

[0019] A third aspect of the present invention is the gas sensor according to the first or second aspect, wherein B≧700×10 -3 (cm -1 ) is characterized in that

[0020] A fourth aspect of the present invention is a gas sensor according to any one of the first to third aspects, further comprising at least one third electrode lead portion including at least one third electrode lead extending from the at least one inner electrode for electrically connecting the outside of the sensor element to the at least one inner electrode, and at least one third lead insulating layer covering the at least one third electrode lead, wherein the diffusion resistance of the at least one third electrode lead portion is equal to or greater than the diffusion resistance of the second electrode lead portion.

[0021] A fifth aspect of the present invention is a gas sensor according to any one of the first to fourth aspects, characterized in that the extra-void pump electrode and the first electrode lead portion are provided on one main surface of the base portion, the gas sensor further comprises a porous region covering the extra-void pump electrode, and the ceramic layer covers the first electrode lead portion and the porous region.

[0022] A sixth aspect of the present invention is a gas sensor according to any one of the first to fifth aspects, characterized in that the at least one oxygen partial pressure adjusting internal space is a first internal space and a second internal space which are sequentially connected to the inlet through a predetermined diffusion resistance, the measurement internal space is a third internal space which is connected to the second internal space through a predetermined diffusion resistance, and the at least one inner electrode is a main pump electrode provided in the first internal space and an auxiliary pump electrode provided in the second internal space. [Effects of the Invention]

[0023] According to the first to sixth aspects of the present invention, it is possible to suppress the amount of temporary fluctuation in the output of the NOx concentration when pollutant gas is generated in the reference gas space, thereby realizing a gas sensor in which the deterioration of measurement accuracy caused by the generation of pollutant gas in the reference gas space is suitably suppressed. [Brief explanation of the drawings]

[0024] [Figure 1] 1 is a cross-sectional view of a main part of a gas sensor 100 taken along the longitudinal direction. [Figure 2] 1 is a vertical cross-sectional view taken along the longitudinal direction of a sensor element 101, schematically illustrating an example of the configuration of the sensor element 101. FIG. [Figure 3] 10 is an example of a graph showing a change in NOx current Ip2 from the start of operation, together with a change in temperature of a rubber plug 106, when the gas sensor is operated in a model gas atmosphere with a constant NOx concentration. [Figure 4] 1 is a plan view showing the arrangement of the electrode lead portion including the electrode leads extending from each of the main pump electrode 22, the auxiliary pump electrode 51, and the measurement electrode 44. FIG. [Figure 5] FIG. 2 is a plan view showing the arrangement of the electrode lead portion including the electrode lead extending from the outer pump electrode 23. [Figure 6] FIG. 10 is a diagram for explaining a method for evaluating an output fluctuation amount. DETAILED DESCRIPTION OF THE INVENTION

[0025] <Gas sensor configuration> FIG. 1 shows an embodiment of the present invention. relating to 1 is a longitudinal cross-sectional view of a main portion of a gas sensor 100 (more specifically, of its main body). In this embodiment, the gas sensor 100 detects a predetermined gas component (e.g., NOx) using a sensor element 101 provided therein. In FIG. 1, the vertical direction is shown as the z-axis direction, and the longitudinal direction of the gas sensor 100 coincides with the z-axis direction (this also applies to the following figures).

[0026] The gas sensor 100 mainly comprises a sensor element 101, a ring part 120 mounted around the sensor element 101, a cylindrical body 130 which is further mounted around the ring part 120 and houses the ring part 120, and these are covered with a protective cover 102, a fixing bolt 103, and an outer cylinder 104. In other words, the gas sensor 100 is roughly configured such that the sensor element 101 passes through the cylindrical body 130 in the axial direction at the axial center position inside the cylindrical body 130, and the ring part 120 is mounted around the sensor element 101 inside the cylindrical body 130. The cylindrical body 130, the protective cover 102, and the outer cylinder 104 mainly form a casing (housing member) for the sensor element 101.

[0027] The sensor element 101 is a long, columnar or thin-plate member whose main constituent material is an element body made of oxygen-ion conductive solid electrolyte ceramics such as zirconia. The sensor element 101 is disposed on the central axis along the longitudinal direction of the cylindrical body 130. Hereinafter, the extending direction of the central axis, which coincides with the longitudinal direction of the cylindrical body 130, will also be referred to as the axial direction. In FIG. 1 and the following figures, the axial direction coincides with the z-axis direction.

[0028] The sensor element 101 has a detection section including a gas inlet and an internal cavity on the side of the first tip portion 101a, and is configured with various electrodes and wiring patterns on the surface and inside of the element body. In the sensor element 101, a test gas introduced into the internal cavity is reduced or decomposed in the internal cavity to generate oxygen ions. In the gas sensor 100, the amount of oxygen ions flowing inside the element is proportional to the concentration of the gas component in the test gas, and therefore the concentration of the gas component can be determined.

[0029] A predetermined area of ​​the surface of the sensor element 101 in the longitudinal direction from the first tip portion 101a is covered with a protective film 111. The protective film 111 is provided to protect the area around the first tip portion 101a of the sensor element 101, which includes an internal cavity and electrodes, from thermal shock due to exposure to water, and is also referred to as a thermal shock-resistant protective layer. The protective film 111 is a porous film made of, for example, Al2O3 and having a thickness of approximately 10 μm to 2000 μm. In light of its purpose, the protective film 111 is preferably formed to withstand a force of up to approximately 50 N. However, the formation area of ​​the protective film 111 in FIG. 1 and the subsequent figures is merely an example, and the actual formation area is determined appropriately depending on the specific structure of the sensor element 101.

[0030] The protective cover 102 is a substantially cylindrical exterior member that protects the first tip portion 101a, which is the portion of the sensor element 101 that comes into direct contact with the test gas during use. The protective cover 102 is fixed by welding to the outer circumferential end portion (the outer periphery of a reduced diameter portion 131, which will be described later) on the lower side (negative side in the z-axis direction) of the cylindrical body 130 as viewed in the drawing.

[0031] 1, the protective cover 102 has a two-layer structure consisting of an outer cover 102a and an inner cover 102b. The outer cover 102a and the inner cover 102b are each provided with a plurality of through-holes H1, H2, H3, and H4 through which gas can pass. Note that the type, number, position, and shape of the through-holes shown in FIG. 1 are merely examples and may be determined appropriately taking into account the manner in which the measurement gas flows into the protective cover 102.

[0032] The fixing bolt 103 is an annular member used to fix the gas sensor 100 to a measurement position. The fixing bolt 103 includes a threaded bolt portion 103a and a holding portion 103b that is held when the bolt portion 103a is screwed. The bolt portion 103a is screwed into a nut provided at the installation position of the gas sensor 100. For example, by screwing the bolt portion 103a into a nut provided in an exhaust pipe of an automobile, the gas sensor 100 is fixed to the exhaust pipe in a manner that the protective cover 102 side is exposed inside the exhaust pipe.

[0033] The outer cylinder 104 is a cylindrical member having one end (lower end as viewed in the drawing) welded to the outer peripheral end of the cylindrical body 130 on the upper side as viewed in the drawing (positive side in the z-axis direction). A connector 105 is disposed inside the outer cylinder 104. A rubber stopper 106 is fitted into the other end (upper end as viewed in the drawing) of the outer cylinder 104 as a sealing member. In the gas sensor 100, the space between the cylindrical body 130 and the rubber stopper 106 and surrounded by the outer cylinder 104 serves as a reference gas space SP. In other words, the reference gas space SP is sealed by the rubber stopper 106. A second tip portion 101b of the sensor element 101 protrudes into the reference gas space SP. For example, air is introduced into the reference gas space SP as a reference gas when measuring the NOx concentration.

[0034] The connector 105 is provided with a plurality of contact members 151 that come into contact with a plurality of terminal electrodes (not shown) provided on the second tip portion 101b of the sensor element 101. The contact members 151 are connected to lead wires 107 that are inserted through a rubber plug 106. The lead wires 107 are connected to a controller 110 and various power sources (not shown in FIG. 1) outside the gas sensor 100.

[0035] Although only two contact members 151 and two lead wires 107 are shown in FIG. 1, this is merely an example.

[0036] The cylindrical body 130 is a metallic cylindrical member also called a metal shell. The sensor element 101 and the ring part 120 are housed inside the cylindrical body 130. In other words, the cylindrical body 130 is mounted around the ring part 120 which is mounted around the sensor element 101.

[0037] The cylindrical body 130 mainly comprises a thick-walled main portion 130M having a cylindrical inner space formed by a cylindrical inner surface 130a parallel to the axial direction, a tapered portion 131 which is located at the lower axial end (negative side in the z-axis direction) as viewed in the drawing and is thicker than the main portion 130M, a thin-walled crimped portion 132 which extends further upward from the end face 130c of the main portion 130M which is located at the upper end in the axial direction as viewed in the drawing and is bent in a direction toward the axial center, and a locking portion 133 which protrudes outward in the circumferential direction.

[0038] The crimping portion 132 is bent to press and fix (restrain) the ring component 120 (specifically, the second ceramic supporter 123) disposed inside from above in the drawing. As will be described later, the crimping portion 132 is against The ring member 120 is bent after being mounted on the ring member 120.

[0039] The annular component 120 is made up of a first ceramic supporter 121 , a powder compact 122 , and a second ceramic supporter 123 .

[0040] The first ceramic supporter 121 and the second ceramic supporter 123 are insulators made of ceramic. More specifically, a rectangular through-hole (not shown) corresponding to the cross-sectional shape of the sensor element 101 is provided at the axial center position of the first ceramic supporter 121 and the second ceramic supporter 123, and the sensor element 101 is inserted into the through-hole, whereby the first ceramic supporter 121 and the second ceramic supporter 123 are annularly mounted on the sensor element 101. The first ceramic supporter 121 is engaged with the tapered surface 130b of the cylindrical body 130 at the bottom as viewed in the drawing.

[0041] On the other hand, the powder compact 122 is made by molding ceramic powder such as talc, and like the first ceramic supporter 121 and the second ceramic supporter 123, the sensor element 101 is inserted into the through-hole, and the powder compact 122 is disposed inside the cylindrical body 130 in a state of being annularly mounted around the sensor element 101, and then further compressed to form an integrated body. More specifically, the ceramic particles that make up the powder compact 122 are surrounded by the first ceramic supporter 121, the second ceramic supporter 123, and the cylindrical body 130, and are densely packed in the space through which the sensor element 101 passes.

[0042] In the gas sensor 100, the sensor element 101 and the enclosure part 120 are fixed inside the cylindrical body 130 by engagement with the tapered surface 130b of the first ceramic supporter 121 and pressure from the upper side in the drawing by the crimping portion 132 of the second ceramic supporter 123. In addition, airtight sealing is achieved between the first tip end portion 101a side and the second tip end portion 101b side of the sensor element 101 by compressing and filling the powder compact 122.

[0043] <Schematic configuration of the sensor element> Fig. 2 is a vertical cross-sectional view taken along the longitudinal direction of the sensor element 101, schematically illustrating an example of the configuration of the sensor element 101. In Fig. 2, the protective film 111 provided on the first tip portion 101a side of the sensor element 101 is not shown, but a controller 110 that controls the operation of each part of the gas sensor 100 and determines the NOx concentration based on the NOx current flowing through the sensor element 101 is also shown.

[0044] The sensor element 101 is a flat (long plate-like) ceramic element having a structure in which six solid electrolyte layers—a first substrate layer 1, a second substrate layer 2, a third substrate layer 3, a first solid electrolyte layer 4, a spacer layer 5, and a second solid electrolyte layer 6—are stacked in this order from bottom to top as viewed in the drawing, each of which is made of zirconia (ZrO2), an oxygen-ion conductive solid electrolyte (e.g., yttria-stabilized zirconia (YSZ)). The solid electrolyte forming these six layers is dense and airtight. Hereinafter, the upper and lower surfaces of these six layers in FIG. 2 may be simply referred to as the upper and lower surfaces, respectively. The entire portion of the sensor element 101 made of the solid electrolyte is collectively referred to as the base portion.

[0045] The sensor element 101 is manufactured, for example, by laminating ceramic green sheets corresponding to each layer, performing predetermined processing and printing circuit patterns (e.g., electrodes, electrode leads, lead insulating layers, etc.), and then firing the sheets to integrate them.

[0046] On the first tip portion 101a side of the sensor element 101, between the lower surface of the second solid electrolyte layer 6 and the upper surface of the first solid electrolyte layer 4, a first diffusion-controlling section 11 also serving as a gas inlet 10, a buffer space 12, a second diffusion-controlling section 13, a first internal space 20, a third diffusion-controlling section 30, a second internal space 40, a fourth diffusion-controlling section 60, and a third internal space 61 are formed adjacent to each other and communicate with each other in this order.

[0047] The buffer space 12, the first internal space 20, the second internal space 40, and the third internal space 61 are spaces (regions) inside the sensor element 101, defined by an upper portion cut out from the spacer layer 5 by the underside of the second solid electrolyte layer 6, a lower portion cut out from the upper surface of the first solid electrolyte layer 4, and a side portion cut out from the side surface of the spacer layer 5. Similarly, the gas inlet 10 may be defined by a portion cut out from the spacer layer 5 at the first tip portion 101a, separately from the first diffusion rate-controlling portion 11. In this case, the first diffusion rate-controlling portion 11 is formed adjacent to and inside the gas inlet 10.

[0048] The first diffusion rate-controlling section 11, the second diffusion rate-controlling section 13, the third diffusion rate-controlling section 30, and the fourth diffusion rate-controlling section 60 are each provided as two horizontally elongated slits (with the openings extending longitudinally in the direction perpendicular to the drawing). The region extending from the gas inlet 10 to the third internal space 61, which is the innermost internal space, is also referred to as the gas flow section.

[0049] Furthermore, a reference gas introduction space 43 is provided on the second tip portion 101b side of the sensor element 101 between the upper surface of the third substrate layer 3 and the lower surface of the spacer layer 5, at a position defined on the side by the side surface of the first solid electrolyte layer 4. The reference gas introduction space 43 opens to the reference gas space SP in the outer cylinder 104, and air serving as a reference gas is introduced from the reference gas space SP.

[0050] The air introduction layer 48 is a layer made of porous alumina, and a reference gas is introduced into the air introduction layer 48 through the reference gas introduction space 43. The air introduction layer 48 is also formed so as to cover the reference electrode 42.

[0051] Reference electrode 42 is an electrode formed in a manner sandwiched between the upper surface of third substrate layer 3 and first solid electrolyte layer 4, and as described above, is surrounded by air introduction layer 48 that connects to reference gas introduction space 43. Furthermore, as will be described later, reference electrode 42 can be used to measure the oxygen concentration (oxygen partial pressure) in first internal space 20 and second internal space 40.

[0052] In the gas flow section, the gas inlet 10 (first diffusion rate-controlling section 11) is a section that opens to the external space, and the measurement gas is taken into the sensor element 101 from the external space through the gas inlet 10.

[0053] The first diffusion rate-controlling part 11 is a part that applies a predetermined diffusion resistance to the taken-in measurement gas.

[0054] The buffer space 12 is a space provided for guiding the measurement gas introduced from the first diffusion rate-controlling part 11 to the second diffusion rate-controlling part 13 .

[0055] The second diffusion rate-controlling portion 13 is a portion that applies a predetermined diffusion resistance to the measurement gas introduced from the buffer space 12 into the first internal space 20 .

[0056] When the measurement gas is introduced from the outside of the sensor element 101 into the first internal space 20, the measurement gas is suddenly taken into the sensor element 101 from the gas inlet 10 due to pressure fluctuations of the measurement gas in the external space (exhaust pressure pulsations if the measurement gas is automobile exhaust gas), but is not introduced directly into the first internal space 20, but passes through the first diffusion rate-controlling section 11, buffer space 12, and second diffusion rate-controlling section 13, where the concentration fluctuations of the measurement gas are canceled out, before being introduced into the first internal space 20. As a result, the concentration fluctuations of the measurement gas introduced into the first internal space 20 become almost negligible.

[0057] 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-controlling part 13. The oxygen partial pressure is adjusted by the operation of the main pump cell 21.

[0058] The main pump cell 21 is an electrochemical pump cell comprising an inner pump electrode (also referred to as a main pump electrode) 22 having a ceiling electrode portion 22a provided on almost the entire lower surface of the second solid electrolyte layer 6 facing the first internal space 20, an outer pump electrode (outside the space) 23 provided on the upper surface of the second solid electrolyte layer 6 (one of the main surfaces of the sensor element 101) in a region corresponding to the ceiling electrode portion 22a, and the second solid electrolyte layer 6 sandwiched between these electrodes.

[0059] The inner pump electrode 22 is formed on the upper and lower solid electrolyte layers (the second solid electrolyte layer 6 and the first solid electrolyte layer 4) that define the first internal space 20. Specifically, a ceiling electrode portion 22a is formed on the lower surface of the second solid electrolyte layer 6 that provides the ceiling surface of the first internal space 20, and a bottom electrode portion 22b is formed on the upper surface of the first solid electrolyte layer 4 that provides the bottom surface. The ceiling electrode portion 22a and the bottom electrode portion 22b are connected by conductive portions (not shown) that are provided on the side wall surfaces (inner surfaces) of the spacer layer 5 that form the both side wall portions of the first internal space 20.

[0060] The ceiling electrode portion 22a and the bottom electrode portion 22b are provided in a rectangular shape in a plan view, but may be provided with only the ceiling electrode portion 22a or only the bottom electrode portion 22b.

[0061] The inner pump electrode 22 and the outer pump electrode 23 are formed as porous cermet electrodes. In particular, the inner pump electrode 22, which comes into contact with the measurement gas, is formed using a material with a weakened ability to reduce the NOx components in the measurement gas. For example, it is formed as a cermet electrode of ZrO2 and an Au-Pt alloy containing approximately 0.6 wt% to 1.4 wt% Au, with a porosity of 5% to 40%, and a thickness of 5 μm to 20 μm. The weight ratio of the Au-Pt alloy to ZrO2 may be approximately Pt:ZrO2=7.0:3.0 to 5.0:5.0.

[0062] On the other hand, the outer pump electrode 23 is formed as a cermet electrode of, for example, Pt or an alloy thereof and ZrO2, and has a rectangular shape in plan view.

[0063] In the main pump cell 21, a desired pump voltage Vp0 is applied between the inner pump electrode 22 and the outer pump electrode 23 by a variable power supply 24, and a main pump current Ip0 is caused to flow between the inner pump electrode 22 and the outer pump electrode 23 in a positive or negative direction, thereby pumping oxygen from the first inner space 20 out to the external space or pumping oxygen from the external space 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.

[0064] In addition, in order to detect the oxygen concentration (oxygen partial pressure) in the atmosphere in the first internal space 20, 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 a main sensor cell 80, which is an electrochemical sensor cell.

[0065] By measuring the electromotive force V0, which is the potential difference between the inner pump electrode 22 and the reference electrode 42 in the main sensor cell 80, the oxygen concentration (oxygen partial pressure) in the first inner space 20 can be determined.

[0066] Furthermore, the controller 110 controls the main pump current Ip0 by feedback controlling the main pump voltage Vp0 so that the electromotive force V0 is constant, thereby maintaining the oxygen concentration in the first internal space 20 at a predetermined constant value.

[0067] The third diffusion control section 30 is a section that imparts a predetermined diffusion resistance to the measurement gas whose oxygen concentration (oxygen partial pressure) has been controlled by the operation of the main pump cell 21 in the first internal space 20, and guides the measurement gas to the second internal space 40.

[0068] The second internal space 40 is provided as a space for further adjusting the oxygen partial pressure in the measurement gas introduced through the third diffusion-controlling part 30. The oxygen partial pressure is adjusted by operating the auxiliary pump cell 50. In the second internal space 40, the oxygen concentration of the measurement gas is adjusted with even higher precision.

[0069] In the second internal space 40, the oxygen concentration (oxygen partial pressure) is adjusted in advance in the first internal space 20, and then the oxygen partial pressure of the measurement gas introduced through the third diffusion-controlling section 30 is further adjusted by the auxiliary pump cell 50.

[0070] The auxiliary pump cell 50 is an auxiliary electrochemical pump cell that includes an auxiliary pump electrode 51 having a ceiling electrode portion 51a provided on substantially the entire lower surface of the second solid electrolyte layer 6 facing the second internal space 40, an outer pump electrode 23 (not limited to the outer pump electrode 23, but any suitable electrode between the sensor element 101 and an outer electrode will suffice), and the second solid electrolyte layer 6.

[0071] The auxiliary pump electrode 51 is disposed in the second internal space 40 in a similar manner to the inner pump electrode 22 disposed in the first internal space 20. That is, a ceiling electrode portion 51a is formed on the second solid electrolyte layer 6 that provides the ceiling surface of the second internal space 40, and a bottom electrode portion 51b is formed on the first solid electrolyte layer 4 that provides the bottom surface of the second internal space 40. The ceiling electrode portion 51a and the bottom electrode portion 51b are rectangular in plan view, and are connected by conductive portions (not shown) that are provided on the side wall surfaces (inner surfaces) of the spacer layer 5 that constitute both side wall portions of the second internal space 40.

[0072] Like the inner pump electrode 22, the auxiliary pump electrode 51 is also formed using a material with a weakened ability to reduce the NOx component in the measurement gas.

[0073] In the auxiliary pump cell 50, under the control of the controller 110, by applying a desired voltage (auxiliary pump voltage) Vp1 between the auxiliary pump electrode 51 and the outer pump electrode 23, it is possible to pump oxygen in the atmosphere within the second internal space 40 out to the external space or pump oxygen from the external space into the second internal space 40.

[0074] In order to control the oxygen partial pressure in the atmosphere in the second internal space 40, an auxiliary sensor cell 81, which is an electrochemical sensor cell, is configured by the auxiliary pump electrode 51, the reference electrode 42, the second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, and the third substrate layer 3. In the auxiliary sensor cell 81, an electromotive force V1, which is a potential difference generated between the auxiliary pump electrode 51 and the reference electrode 42 in accordance with the oxygen partial pressure in the second internal space 40, is detected.

[0075] The auxiliary pump cell 50 performs pumping using a variable power supply 52 whose voltage is controlled based on the electromotive force V1 detected by the auxiliary sensor cell 81. As a result, the oxygen partial pressure in the atmosphere within the second internal space 40 is feedback-controlled to a low partial pressure that does not substantially affect the measurement of NOx.

[0076] At the same time, the auxiliary pump current Ip1 is used to control the electromotive force of the main sensor cell 80. Specifically, the auxiliary pump current Ip1 is input as a control signal to the main sensor cell 80, and the electromotive force V0 is controlled to always maintain a constant gradient of the oxygen partial pressure in the measurement gas introduced from the third diffusion-controlling part 30 into the second internal space 40. When used as a NOx sensor, the oxygen concentration in the second internal space 40 is maintained at a constant value of approximately 0.001 ppm by the action of the main pump cell 21 and the auxiliary pump cell 50.

[0077] The fourth diffusion rate-controlling section 60 is a section that imparts a predetermined diffusion resistance to the measurement gas whose oxygen concentration (oxygen partial pressure) has been controlled by the operation of the auxiliary pump cell 50 in the second internal space 40, and guides the measurement gas to the third internal space 61.

[0078] The third internal space 61 is provided as a space (measurement internal space) for performing processing related to measurement of the nitrogen oxide (NOx) concentration in the measurement gas introduced through the fourth diffusion-controlling part 60. The NOx concentration is measured by operating the measurement pump cell 41 in the third internal space 61. Since the measurement gas, the oxygen concentration of which has been adjusted with high precision in the second internal space 40, is introduced into the third internal space 61, the gas sensor 100 can measure the NOx concentration with high precision.

[0079] The measurement pump cell 41 is used to measure the NOx concentration of the measurement gas introduced into the third internal space 61. The measurement pump cell 41 is an electrochemical pump cell including a measurement electrode 44 provided on the upper surface of the first solid electrolyte layer 4 facing the third internal space 61 and spaced apart from the third diffusion-controlling part 30, an outer pump electrode 23, the second solid electrolyte layer 6, a spacer layer 5, and the first solid electrolyte layer 4.

[0080] The measurement electrode 44 is a porous cermet electrode of a precious metal and a solid electrolyte. For example, it is formed as a cermet electrode of Pt or an alloy of Pt and another precious metal such as Rh, and ZrO2, which is a constituent material of the sensor element 101. The measurement electrode 44 also functions as a NOx reduction catalyst that reduces NOx present in the atmosphere in the third internal space 61.

[0081] In the measuring pumping cell 41, under the control of the controller 110, oxygen generated by the decomposition of NOx in the atmosphere in the third internal space 61 is pumped out, and the amount of oxygen generated can be detected as a pump current Ip2.

[0082] Furthermore, in order to detect the oxygen partial pressure around the measurement electrode 44, 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 a measurement sensor cell 82, which is an electrochemical sensor cell. The variable power supply 46 is feedback-controlled based on the electromotive force V2, which is the potential difference generated between the measurement electrode 44 and the reference electrode 42 in response to the oxygen partial pressure in the third internal space 61, detected by the measurement sensor cell 82.

[0083] The NOx in the measurement gas introduced into the third internal space 61 is reduced by the measurement electrode 44 (2NO → N2 + O2) to generate oxygen. The generated oxygen is then pumped by the measurement pump cell 41, and the voltage (measurement pump voltage) Vp2 of the variable power supply 46 is controlled so that the electromotive force V2 detected by the measurement sensor cell 82 remains 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 using the pump current Ip2 in the measurement pump cell 41. Hereinafter, this pump current Ip2 will also be referred to as the NOx current Ip2.

[0084] Furthermore, by combining the measurement electrode 44, the first solid electrolyte layer 4, the third substrate layer 3 and the reference electrode 42 to form an oxygen partial pressure detection means as an electrochemical sensor cell, it is possible to detect an electromotive force corresponding to the difference between the amount of oxygen generated by reduction of the NOx components in the atmosphere around the measurement electrode 44 and the amount of oxygen contained in the reference atmosphere, thereby making it possible to determine the concentration of the NOx components in the measured gas.

[0085] In addition, the second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, the third substrate layer 3, the outer pump electrode 23, and the reference electrode 42 constitute an electrochemical sensor cell 83, and the electromotive force Vref obtained by this sensor cell 83 makes it possible to detect the oxygen partial pressure in the measurement gas outside the sensor.

[0086] 2, for the sake of simplicity, wiring for electrically connecting the electrodes constituting the various pump cells and sensor cells to various parts outside the sensor element 101 is shown only schematically. However, in the actual sensor element 101, electrode leads that connect the electrodes of the respective pump cells and sensor cells to terminal electrodes are provided inside or on the side surfaces. Details of the electrode lead portion including such electrode leads will be described later.

[0087] The sensor element 101 further includes a heater portion 70 that serves to adjust the temperature by heating and keeping the sensor element 101 warm in order to increase the oxygen ion conductivity of the solid electrolyte that constitutes the base portion.

[0088] The heater section 70 mainly includes a heater electrode 71, a heater element 72, a heater lead 72a, a through hole 73, a heater insulating layer 74, a pressure release hole 75, and a heater resistance detection lead (not shown in Fig. 2). The heater section 70, except for the heater electrode 71, is embedded in the base of the sensor element 101.

[0089] The heater electrode 71 is an electrode formed in a manner to contact the lower surface of the first substrate layer 1 (the other main surface of the sensor element 101).

[0090] 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 when power is supplied from a heater power source (not shown in FIG. 2) provided outside the sensor element 101 through a current path including the heater electrode 71, the through hole 73, and the heater lead 72a. The heater element 72 is made of Pt or contains Pt as its main component. The heater element 72 is embedded in a predetermined area on the gas flow section side of the sensor element 101 so as to face the gas flow section in the element thickness direction. The heater element 72 is provided to have a thickness of approximately 10 μm to 30 μm.

[0091] In the sensor element 101, current is passed through the heater electrode 71 to the heater element 72, causing the heater element 72 to generate heat, thereby heating and maintaining each part of the sensor element 101 at a predetermined temperature. Specifically, the sensor element 101 is heated so that the temperature of the solid electrolyte and electrodes near the gas flow portion reaches approximately 700°C to 900°C. This heating increases the oxygen ion conductivity of the solid electrolyte that constitutes the base portion of the sensor element 101. The heating temperature by the heater element 72 when the gas sensor 100 is used (when the sensor element 101 is driven) is referred to as the sensor element driving temperature.

[0092] The degree of heat generation by the heater element 72 (heater temperature) is determined by the magnitude of the resistance value of the heater element 72 (heater resistance).

[0093] In addition to the above-mentioned components, the sensor element 101 of the gas sensor 100 according to this embodiment further includes a ceramic layer 7 and a porous region 8 on the second solid electrolyte layer 6.

[0094] The ceramic layer 7 is provided on the second solid electrolyte layer 6 so as to cover substantially the entire surface thereof. The ceramic layer 7 is adjacent to most of the second solid electrolyte layer 6 but is separated from the outer pump electrode 23 and regions of the upper surface of the second solid electrolyte layer 6 that are located to the sides of the outer pump electrode 23 in the transverse direction of the element (hereinafter referred to as electrode side regions). The regions between the outer pump electrode 23 and the ceramic layer 7 and the regions between the electrode side regions of the second solid electrolyte layer 6 and the ceramic layer 7 form porous regions 8. The porous region 8 is provided so as to cover the outer pump electrode 23 and to be exposed at both end portions (not shown) in the transverse direction of the element. The ceramic layer 7 is provided so as to cover the entire upper surface of the second solid electrolyte layer 6, including the porous region 8 and an outer pump electrode lead portion 23P (described later).

[0095] More specifically, the ceramic layer 7 is made of ceramics (for example, zirconia, alumina, etc.) that is as dense as the second solid electrolyte layer 6, for example.

[0096] On the other hand, the porous region 8 is made of a porous material (such as alumina) with a porosity of about 30% to 60%. The porous region 8 is configured so that its thickness above the outer pump electrode 23 (in other words, the distance between the outer pump electrode 23 and the ceramic layer 7 in the element thickness direction) is about 25 μm to 40 μm.

[0097] The ceramic layer 7 and the porous region 8 may be formed by, for example, a known method such as printing after the sensor element 101, excluding these components, has been constructed in advance. Alternatively, the ceramic layer 7 and the porous region 8 may be formed by further stacking materials that will ultimately become the ceramic layer 7 and the porous region 8 on a green sheet laminate that forms the six solid electrolyte layers described above by a known method such as printing, and then firing the resulting laminate together.

[0098] By having the ceramic layer 7 and porous region 8 configured in this manner, the sensor element 101 is able to provide a predetermined diffusion resistance to oxygen passing through the porous region 8 when oxygen is pumped in and out between the inside and outside of the sensor element 101 through the outer pump electrode 23.

[0099] When the concentration of NOx is measured in the gas sensor 100 having the above configuration, feedback control is performed to maintain constant oxygen concentrations in the first internal space 20 and the second internal space 40 by operating the main pump cell 21 and then the auxiliary pump cell 50, and the measurement gas with the constant oxygen concentration is introduced into the third internal space 61 and reaches the measurement electrode 44. For example, when the measurement gas is a lean atmosphere, the measurement gas is introduced into the third internal space 61 with its oxygen partial pressure sufficiently reduced to a level that does not substantially affect the measurement of NOx (for example, 0.0001 ppm to 1 ppm).

[0100] At the measuring electrode 44, NOx in the measurement gas that has reached the electrode is reduced to generate oxygen. The oxygen is pumped out by the measuring pump cell 41, and the NOx current Ip2 that flows during this pumping has a certain functional relationship (hereinafter referred to as sensitivity characteristic) with the concentration of NOx in the measurement gas.

[0101] The sensitivity characteristic is determined in advance of actual use of the gas sensor 100 using a plurality of model gases with known NOx concentrations, and the data is stored in the controller 110. During actual use of the gas sensor 100, a signal representing the value of the NOx current Ip2 that flows in response to the NOx concentration in the measurement gas is continuously supplied to the controller 110. The controller 110 successively calculates the NOx concentration based on the value and the determined sensitivity characteristic, and outputs the calculated NOx concentration as a NOx sensor detection value. This allows the gas sensor 100 to grasp the NOx concentration in the measurement gas in almost real time.

[0102] <Generation of pollutant gas in the reference gas space> In the case of a gas sensor that has a configuration in which the reference gas space SP is surrounded by the outer cylinder 104 and the rubber stopper 106 and is used in a high-temperature environment, as in the gas sensor 100 according to this embodiment, contaminant gas may be generated in the reference gas space SP due to the evaporation of oil adhering to the inner surface 104a of the outer cylinder 104 or the generation of gas from the rubber stopper 106, resulting in a change in the NOx current Ip2. Such generation of contaminant gas is particularly likely to occur under high-temperature gas conditions or in a harsh environment in which heat is trapped around the main body.

[0103] 3 is an example of a graph showing the change in NOx current Ip2 from the start of operation when such a gas sensor is operated in a model gas atmosphere with a constant NOx concentration, along with the change in temperature of the rubber plug 106. In FIG. 3, the NOx current Ip2, which should be constant, temporarily decreases after the temperature of the rubber plug 106 increases. Then, after the temperature of the rubber plug 106 stabilizes, the NOx current Ip2 increases to about the same as the initial peak value and remains approximately constant.

[0104] 3 is thought to be caused by the intrusion of the above-mentioned pollutant gases, which are generated in the process of heating the gas sensor, into the third internal space 61, which contains the measurement electrode 44, through the measurement electrode lead portion (described later).The increase in the value of the NOx current Ip2 after a certain time has passed is thought to be caused by the outer tube 104 and the rubber plug 106 being sufficiently heated, resulting in the release of all the pollutant gases.

[0105] Naturally, such fluctuations in the NOx current Ip2 will, even if only temporarily, reduce the accuracy of measuring the NOx concentration in the gas sensor. Furthermore, the generation of pollutant gases does not necessarily occur only after the start of use, as shown in Figure 3.

[0106] <Electrode lead configuration> In the gas sensor 100 according to this embodiment, the electrode lead portion of the sensor element 101 is designed to meet specified conditions, thereby suppressing the effect on the accuracy of measuring the NOx concentration of fluctuations in the NOx current Ip2, as shown in Figure 3, caused by the generation of pollutant gas in the reference gas space SP.

[0107] Fig. 4 is a plan view showing, as an example, the arrangement of electrode lead portions including electrode leads extending from the main pump electrode 22 (more specifically, the bottom electrode portion 22b), the auxiliary pump electrode 51 (more specifically, the bottom electrode portion 51b), and the measurement electrode 44. Fig. 5 is a plan view showing the arrangement of electrode lead portions including the electrode leads extending from the outer pump electrode 23.

[0108] The bottom electrode portion 22b, the bottom electrode portion 51b, and the measurement electrode 44 are all provided on the first solid electrolyte layer 4. A main pump electrode lead 22L extends from the bottom electrode portion 22b toward the second tip portion 101b, an auxiliary pump electrode lead 51L extends from the bottom electrode portion 51b, and a measurement electrode lead 44L extends from the measurement electrode 44 toward the second tip portion 101b. On the other hand, the outer pump electrode 23 is provided on the second solid electrolyte layer 6. An outer pump electrode lead 23L extends from the outer pump electrode 23 toward the second tip portion 101b.

[0109] The main pump electrode lead 22L, the auxiliary pump electrode lead 51L, the measurement electrode lead 44L, and the outer pump electrode lead 23L are surrounded by a main pump electrode lead insulating layer 22I, an auxiliary pump electrode lead insulating layer 51I, a measurement electrode lead insulating layer 44I, and an outer pump electrode lead insulating layer 23I, respectively, thereby insulating each electrode lead from the solid electrolyte that constitutes the sensor element 101.

[0110] The outer pump electrode lead 23L is an interlayer wiring, with most of the outer pump electrode lead 23L being embedded between the second solid electrolyte layer 6 and the ceramic layer 7. Although FIG. 2 illustrates the ceramic layer 7 extending up to the second tip portion 101b, in reality, the second solid electrolyte layer 6 is exposed over a predetermined range from the second tip portion 101b, and the outer pump electrode lead 23L is connected to the terminal electrode 23T provided in this exposed portion. Although FIG. 5 illustrates the outer pump electrode lead insulating layer 23I provided linearly (in a line) along the outer pump electrode lead 23L, the outer pump electrode lead insulating layer 23I may be provided in a planar manner (in a layer) between the outer pump electrode 23 and the terminal electrode 23T.

[0111] Meanwhile, the main pump electrode lead 22L, the auxiliary pump electrode lead 51L, and the measurement electrode lead 44L are each interlayer wirings, with most of the lead buried between the first solid electrolyte layer 4 and the spacer layer 5, but are bent toward the side of the element near the second tip 101b. Furthermore, although not shown, the main pump electrode lead 22L, the auxiliary pump electrode lead 51L, and the measurement electrode lead 44L are surrounded by the main pump electrode lead insulating layer 22I, the auxiliary pump electrode lead insulating layer 51I, and the measurement electrode lead insulating layer 44I, respectively, and connected to each other via through holes in either the upper or lower direction in the thickness direction near the element-side end, and then connected to one of a plurality of terminal electrodes (not shown) similar to the terminal electrode 23T provided at the second tip 101b via other interlayer wirings. Note that in this embodiment, "surrounding each electrode lead with a lead insulating layer" includes a mode in which the electrode lead is covered with the lead insulating layer and a mode in which the electrode lead is sandwiched between the lead insulating layers.

[0112] The main pump electrode lead 22L, the auxiliary pump electrode lead 51L, the measurement electrode lead 44L, and the outer pump electrode lead 23L are preferably made of platinum, and the main pump electrode lead insulating layer 22I, the auxiliary pump electrode lead insulating layer 51I, the measurement electrode lead insulating layer 44I, and the outer pump electrode lead insulating layer 23I are preferably made of alumina.

[0113] Hereinafter, a region including the outer pump electrode lead 23L and the outer pump electrode lead insulating layer 23I and surrounded by the second solid electrolyte layer 6 and the ceramic layer 7 will be collectively referred to as the outer pump electrode lead portion 23P. Similarly, a region including the measurement electrode lead 44L and the measurement electrode lead insulating layer 44I and surrounded by the solid electrolyte (mainly the first solid electrolyte layer 4 and the spacer layer 5) will be collectively referred to as the measurement electrode lead portion 44P. Furthermore, a region including the main pump electrode lead 22L and the main pump electrode lead insulating layer 22I and surrounded by the solid electrolyte (mainly the first solid electrolyte layer 4 and the spacer layer 5) will be collectively referred to as the main pump electrode lead portion 22P. Similarly, a region including the auxiliary pump electrode lead 51L and the auxiliary pump electrode lead insulating layer 51I and surrounded by the solid electrolyte (mainly the first solid electrolyte layer 4 and the spacer layer 5) will be collectively referred to as the auxiliary pump electrode lead portion 51P.

[0114] In other words, the outer pump electrode lead portion 23P is formed by inserting the outer pump electrode lead 23L and the outer pump electrode lead insulating layer 23I therethrough. The measurement electrode lead portion 44P is formed by inserting the measurement electrode lead 44L and the measurement electrode lead insulating layer 44I therethrough. The main pump electrode lead portion 22P is formed by inserting the main pump electrode lead 22L and the main pump electrode lead insulating layer 22I therethrough. The auxiliary pump electrode lead portion 51P is formed by inserting the auxiliary pump electrode lead 51L and the auxiliary pump electrode lead insulating layer 51I therethrough.

[0115] In the gas sensor 100 according to this embodiment, of the three electrode lead portions excluding the outer pump electrode lead portion 23P, the diffusion resistance of the measurement electrode lead portion 44P connected to the measurement electrode 44 located farthest from the gas inlet 10 in the gas flow portion of the sensor element 101 is greater than the diffusion resistance between the measurement electrode lead portion 44P and the outer pump electrode lead portion 23P connected to the outer pump electrode 23.

[0116] That is, the gas sensor 100 according to this embodiment is configured so that, when the diffusion resistance of the outer pump electrode lead portion 23P is A and the diffusion resistance of the measurement electrode lead portion 44P is B, the ratio A / B between the two satisfies at least the relationship A / B<1.00.

[0117] Here, the diffusion resistance of each electrode lead portion (lead diffusion resistance) refers to the diffusion resistance that not only the internal structure of each electrode lead and electrode lead insulating layer, but also the entire space surrounded by the solid electrolyte present around each electrode lead portion, including these, provides to the gas moving through the space. In other words, not only minute gaps present inside the electrode lead and electrode lead insulating layer, but also gaps present between the electrode lead and the electrode lead insulating layer and between the electrode lead insulating layer and the surrounding solid electrolyte contribute to this diffusion resistance.

[0118] The magnitude of the diffusion resistance in the electrode lead portion can be adjusted by adjusting, for example, the porosity of the electrode lead or insulating layer, the overall length (wiring length) of the electrode lead portion, or the cross-sectional area of ​​the electrode lead portion.

[0119] The fact that A / B<1.00 is satisfied means that, compared to a gas sensor having a similar configuration except that this condition is not satisfied, leakage of contaminant gas through the measurement electrode lead portion 44P is less likely to occur than leakage through the outer pump electrode lead portion 23P.

[0120] Moreover, in the gas sensor 100 according to this embodiment, in which the measurement gas introduced through the gas inlet 10 has oxygen pumped out sequentially in the first internal space 20 and the second internal space 40 before reaching the third internal space 61, the main pump current Ip0 flowing through the main pump cell 21 is usually larger than the NOx current Ip2 flowing through the measurement pump cell 41. Therefore, even if the amount of intrusion of pollutant gas is the same, the rate of output fluctuation in the main pump current Ip0 caused by the intrusion of pollutant gas into the outer pump electrode lead portion 23P is smaller than the rate of output fluctuation in the NOx current Ip2 caused by the intrusion of pollutant gas into the measurement electrode lead portion 44P and further into the third internal space 61.

[0121] When A / B<1.00 is satisfied, output fluctuations in the NOx current Ip2 due to the intrusion of pollutant gases into the outer pump electrode lead portion 23P are further suppressed.

[0122] For the above reasons, in the gas sensor 100 according to this embodiment, A / B<1.00 is satisfied, so that even if contaminant gas is generated in the reference gas space SP at the start of use, the contaminant gas is prevented from flowing into the third internal space 61 where the measurement electrode 44 is located and contaminating the measurement electrode 44. Furthermore, the amount of temporary output fluctuation from the actual NOx concentration value in the gas sensor 100 is kept within an allowable range in light of measurement accuracy. For example, the amount of output fluctuation for an NOx concentration of 100 ppm is kept within a range of 20 ppm.

[0123] The output fluctuation amount can be roughly determined by converting the amount of change in the NOx current Ip2 (the difference between the final constant value and the value when it temporarily decreases) as shown in Figure 3, which is obtained when the gas sensor 100 starts operating in a model gas atmosphere with a constant NOx concentration, into the amount of change in the NOx concentration value using the sensitivity characteristics described above.

[0124] On the other hand, if A / B<0.001, the inflow and outflow of gases other than contaminant gases (e.g., the measured gas and oxygen in the reference gas space SP) through the outer pump electrode lead portion 23P cannot be ignored, resulting in a deterioration in measurement accuracy, so it is preferable that A / B≧0.001.

[0125] Also, B≧700×10 -3 (cm -1 ) is preferable. The value of B is 700 × 10 -3 (cm -1 ), even if A / B<1.00 is satisfied, contaminant gas is likely to flow into the third internal space 61 through the measuring electrode lead portion 44P. -3 (cm-1 ) is a configuration that is generally adopted in gas sensors having the configurations shown in FIGS.

[0126] Preferably, the gas sensor 100 is configured so that the ratio A / B satisfies the range of 0.03≦A / B≦0.29. In this case, the output fluctuation described above is further suppressed. For example, the output fluctuation for an NOx concentration of 500 ppm is kept within a range of 10 ppm.

[0127] Preferably, the diffusion resistance of the main pump electrode lead portion 22P and the auxiliary pump electrode lead portion 51P is set to be equal to or greater than the diffusion resistance B of the measurement electrode lead portion 44P. In this case, the diffusion resistance of the main pump electrode lead portion 22P and the auxiliary pump electrode lead portion 51P also becomes greater than the diffusion resistance between them and the outer pump electrode lead portion 23P. This effectively prevents contaminant gas from flowing into the first internal space 20 or the second internal space 40 through the main pump electrode lead portion 22P or the auxiliary pump electrode lead portion.

[0128] As described above, according to this embodiment, when the diffusion resistance of the outer pump electrode lead portion in the gas sensor is A and the diffusion resistance of the measurement electrode lead portion in the gas sensor is B, the gas sensor is configured to satisfy the relationship A / B<1.00, thereby making it possible to suppress the amount of temporary output fluctuation in the NOx concentration when pollutant gas is generated in the reference gas space. Preferably, the gas sensor is configured to satisfy the relationship 0.03≦A / B≦0.29, making it possible to further suitably suppress the amount of such output fluctuation.

[0129] This makes it possible to realize a gas sensor in which the deterioration of measurement accuracy due to the generation of contaminant gases in the reference gas space is suitably suppressed. [Example]

[0130] As an example, the combination of the magnitude of the diffusion resistance A of the outer pump electrode lead portion 23P and the magnitude of the diffusion resistance B of the measurement electrode lead portion 44P was varied within the range satisfying A / B<1.00. 7The gas sensor 100 (hereinafter referred to as Examples 1 to 3) 7 ) was prepared, and the amount of output fluctuation in the NOx concentration at the start of use was evaluated. The diffusion resistance of the main pump electrode lead portion 22P and the auxiliary pump electrode lead portion 51P was set to be equal to or greater than the diffusion resistance B of the measurement electrode lead portion 44P. Similar evaluations were also performed on two gas sensors (hereinafter referred to as Comparative Examples 1 and 2) that were prepared in the same manner as in the example except that they did not satisfy A / B<1.00.

[0131] Fig. 6 is a diagram for explaining a method for evaluating the amount of output fluctuation. Specifically, each gas sensor was started in a model gas atmosphere having a temperature of 1050°C, an air ratio λ = 1.05, and an NOx concentration of 100 ppm, and the NOx current Ip2 was continuously measured while controlling the heating by the heater unit 70 so that the temperature of the rubber plug 106 was ultimately kept constant at 300°C, thereby obtaining a profile Ip2(t) corresponding to the function of the NOx current Ip2 with respect to time, as shown in Fig. 6.

[0132] Although the degree of the profile varied, all of the gas sensors obtained a profile Ip2(t) similar to the example shown in Fig. 3. That is, the NOx current Ip2 becomes substantially constant soon after the gas sensor starts operating, then decreases, and becomes substantially constant again as pollutant gas is generated. After a while, the NOx current Ip2 starts to increase, and finally becomes substantially constant at a value that is substantially the same as when the operation started.

[0133] However, for ease of understanding, the profile Ip2(t) is shown as a solid polygonal line in FIG. 6, but in reality, the profile Ip2(t) had some fluctuations as shown schematically by the dashed line.

[0134] When evaluating the output fluctuation amount for such a profile Ip2(t), first, the average value av1 of the value of the NOx current Ip2 for a predetermined time Δt1 (e.g., 10 minutes) immediately before the NOx current Ip2 changes from a substantially constant state after decreasing to an increase is determined. The reason for taking the average value is that the value that appears to be substantially constant actually fluctuates.

[0135] Next, the difference value Δ1(t) of the value of the NOx current Ip2 after it has started to increase and then becomes substantially constant relative to the average value av1, and the difference value Δ2(t) of the maximum and minimum values ​​of the NOx current Ip2 in the substantially constant state after it has started to increase were obtained. Both the difference value Δ1(t) and the difference value Δ2(t) are dynamic values ​​that change according to the fluctuations in the value of the NOx current Ip2 after it has increased, and are therefore expressed as a function of time.

[0136] Then, between the difference value Δ1(t) and the difference value Δ2(t), Difference value Δ1(t)>2 × difference value Δ2(t) (1) When a predetermined observation time Δtz (for example, 60 minutes) has elapsed, it is determined that the fluctuations due to pollutant gases have been eliminated and the value of the NOx current Ip2 has become approximately constant at its original value. Then, the average value av2 of the value of the NOx current Ip2 for a predetermined time Δt2 (for example, 10 minutes) immediately before the determination time is specified, and the difference between the two is calculated as follows: Pump current fluctuation value = average value av2 - average value av1 (2) It was calculated as:

[0137] Finally, the pump current fluctuation value obtained by equation (2) was converted into a NOx concentration value to obtain the output fluctuation amount.

[0138] Table 1 shows Examples 1 to 3. 7 10 is a diagram showing a list of the diffusion resistance A of the outer pump electrode lead portion 23P, the diffusion resistance B of the measurement electrode lead portion 44P, the ratio A / B between the two, the output fluctuation amount, and the evaluation results (pass / fail judgment results) of the output fluctuation amount for the gas sensors according to Comparative Examples 1 and 2.

[0139] [Table 1]

[0140] In the judgment of the output fluctuation amount, gas sensors for which the obtained output fluctuation amount was within 10 ppm were judged to have adequately suppressed the output fluctuation caused by the generation of pollutant gases at the start of use (Judgment A). The corresponding gas sensors are marked with "A" in the "Judgment" column of Table 1.

[0141] Furthermore, for gas sensors whose output fluctuations were greater than 10 ppm and less than 20 ppm, it was determined that the output fluctuations caused by the generation of polluting gases at the start of use had been suppressed to an acceptable level in light of the measurement accuracy (Judgment B).The relevant gas sensors are marked with "B" in the "Judgment" column of Table 1.

[0142] On the other hand, for gas sensors where the obtained output fluctuation amount exceeded 20 ppm, it was determined that the output fluctuation caused by the generation of pollutant gases at the start of use was not suppressed (rating F). The corresponding gas sensors are marked with "F" in the "Rating" column of Table 1.

[0143] As shown in Table 1, the gas sensors of Examples 1, 2, and 5 were rated A. , Example 6, and Example 7 The gas sensor of Comparative Example 1 was rated B. On the other hand, the gas sensors of Comparative Example 1 and Comparative Example 2 were rated F.

[0144] The above results indicate that in a gas sensor in which the ratio A / B of the diffusion resistance A of the outer pump electrode lead to the diffusion resistance B of the measurement electrode lead satisfies the range of A / B<1.00, even if contaminated gas is generated in the reference gas space SP at the start of use, the amount of temporary output fluctuation from the actual value of the NOx concentration remains within an acceptable range in light of measurement accuracy.

[0145] Furthermore, it is also shown that when the range of 0.03≦A / B≦0.29 is satisfied, the amount of output fluctuation is more suitably suppressed. [Explanation of symbols]

[0146] 1~3 1st~3rd substrate layer 4 First solid electrolyte layer 5 Spacer layer 6 Second solid electrolyte layer 7 ceramic layer 8 Porous region 10 Gas inlet 11 First diffusion-controlled section 13 Second diffusion-controlled section 20 1st internal void 21 Main pump cell 22 Inner pump electrode 22P Main pump electrode lead 23 Outer pump electrode 23L Outer pump electrode lead 23T terminal electrode 30 Third diffusion-controlled section 40 Second internal void 41 Measuring pump cell 42 Reference electrode 43 Reference gas introduction space 44 Measuring electrode 44P Measurement electrode lead part 48 Atmospheric introduction layer 50 Auxiliary pump cell 51 Auxiliary pump electrode 51P Auxiliary pump electrode lead 60 4th diffusion-controlled section 61 3rd internal void 70 Heater section 100 Gas Sensor 101 Sensor element 102 Protective cover 103 Fixing bolt 104 Outer cylinder 105 Connector 106 Rubber stopper 107 Lead Wire 120 Ring Parts 151 Contact parts H1~H4 through hole SP Reference gas space

Claims

1. A gas sensor capable of detecting a predetermined gas component in a measurement gas, a sensor element having a long plate-shaped base portion made of an oxygen ion conductive solid electrolyte and having a detection portion on one end side; a casing in which the sensor element is housed and fixed; Equipped with The casing is an outer cylinder having a reference gas space in which a reference gas is present, the other end of the sensor element protruding into the reference gas space; a seal member fitted to an end of the outer cylinder and sealing the reference gas space; Equipped with The sensor element is at least one internal space for adjusting the oxygen partial pressure, which is in communication with the inlet for the gas under measurement provided at the one end side under a predetermined diffusion resistance; a measuring internal cavity further communicating with the at least one oxygen partial pressure adjusting internal cavity; an extra-void pump electrode disposed at a location other than the at least one oxygen partial pressure adjusting internal cavity and the measurement internal cavity; at least one inner electrode provided facing the at least one oxygen partial pressure adjusting internal space, and capable of pumping oxygen in and out between the corresponding at least one oxygen partial pressure adjusting internal space and the outside of the sensor element by applying a predetermined pump voltage between the inner electrode and the external pump electrode of the space; a measurement electrode provided facing the measurement internal space, capable of pumping oxygen in and out between the measurement internal space and the outside of the sensor element by applying a predetermined pump voltage between the measurement electrode and the external pump electrode; a first electrode lead portion including: a first electrode lead extending from the pump electrode outside the cavity for electrically connecting the outside of the sensor element to the pump electrode outside the cavity; and a first lead insulating layer covering the first electrode lead; a ceramic layer covering at least the first electrode lead portion; a second electrode lead portion including: a second electrode lead extending from the measurement electrode and capable of electrically connecting the outside of the sensor element to the measurement electrode; and a second lead insulating layer covering the second electrode lead; Equipped with The diffusion resistance in the first electrode lead portion is defined as A (cm －１ ), and the diffusion resistance in the second electrode lead portion is B (cm －１ ) in the case where A / B<1.00 A gas sensor characterized by:

2. 2. The gas sensor according to claim 1, 0.03≦A / B≦0.29 A gas sensor characterized by:

3. 3. The gas sensor according to claim 1, B≧700×10 －３ (cm －１ ) A gas sensor characterized by:

4. 4. The gas sensor according to claim 1, at least one third electrode lead portion including: at least one third electrode lead extending from the at least one inner electrode for electrically connecting an outside of the sensor element to the at least one inner electrode; and at least one third lead insulating layer covering the at least one third electrode lead; Furthermore, the diffusion resistance of the at least one third electrode lead portion is equal to or greater than the diffusion resistance of the second electrode lead portion; A gas sensor characterized by:

5. 5. The gas sensor according to claim 1, the outside-space pump electrode and the first electrode lead portion are provided on one main surface of the base portion, a porous region covering the extra-cavity pump electrode; Furthermore, the ceramic layer covers the first electrode lead portion and the porous body region; A gas sensor characterized by:

6. 6. The gas sensor according to claim 1, the at least one oxygen partial pressure adjusting internal space is a first internal space and a second internal space that are sequentially connected to the inlet through a predetermined diffusion resistance, the measurement internal space is a third internal space that communicates with the second internal space through a predetermined diffusion resistance, the at least one inner electrode is a main pump electrode provided in the first inner cavity and an auxiliary pump electrode provided in the second inner cavity; A gas sensor characterized by:

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