SENSOR ELEMENT AND GAS SENSOR

Abstract

A sensor element 101 includes an element body 102 including an oxygen ion-conductive solid electrolyte layer, a sensing electrode 44 disposed inside the element body 102, and a conductive portion 74 connected to the sensing electrode 44 and in which an inner lead 77, a peripheral lead 78, and a connector electrode 75a are connected in this order. The peripheral lead 78 covers a left end of a fourth segment 77d of the inner lead 77. When a porosity of the peripheral lead 78 is Rp [%] and a thickness of the peripheral lead 78 is Dc [mm], the peripheral lead 78 satisfies Rp / Dc ≤ 145% / mm.

Description

Technical field

[0001] The present invention relates to a sensor element and a gas sensor. State of the art

[0002] A known gas sensor in the prior art includes a sensor element configured to detect the concentration of a specific gas, such as NOx, in a measurement object gas such as exhaust gas from an internal combustion engine (see, for example, PTL 1). The sensor element disclosed in PTL 1 includes an element body, an internal electrode, a terminal, and a lead. The element body includes an oxygen ion-conductive solid electrolyte layer and has a columnar shape extending in a longitudinal direction. The element body has a front end and a rear end, which are opposite ends in the longitudinal direction, and a peripheral surface, which is a surface extending in the longitudinal direction. A part of the element body located at the front end is allowed to be exposed to a measurement object gas. The internal electrode is disposed inside the element body.The terminal is disposed on a portion of the peripheral surface of the element body near the rear end. The lead is arranged to connect the inner electrode and the terminal. The lead includes an inner portion disposed within the element body and a conductive peripheral surface portion disposed on the peripheral surface and exposed from the element body. Document listPatent documents

[0003] PTL 1: JP 4628920 B Summary of the inventionTechnical problem

[0004] In such a sensor element, a gas outside the sensor element (element body) may penetrate into the interior of the element body through the end on the conductive peripheral surface portion side of the inner portion of the lead or through a gap between the inner portion and the element body, move along the inner portion or the gap between the inner portion and the element body to the inner electrode, and reach the inner electrode. If such a gas reaches the inner electrode, the accuracy of detecting the concentration of the specific gas may deteriorate.

[0005] The main object of the sensor element and the gas sensor according to the present invention is to reduce the probability of deterioration of the accuracy in detecting the concentration of the specific gas. Solution to the problem

[0006] To achieve the above-mentioned main object, the sensor element and the gas sensor according to the present invention employ the following structure.

[0007] [1] A sensor element according to the present invention is a sensor element configured to detect a concentration of a specific gas in a measurement object gas, the sensor element comprising: an element body including an oxygen ion-conductive solid electrolyte layer and having a columnar shape extending in a longitudinal direction, the element body having a front end and a rear end which are opposite ends in the longitudinal direction, and further having a peripheral surface which is a surface extending in the longitudinal direction, wherein a part of the element body located at the front end is allowed to be exposed to the measurement object gas; an internal electrode arranged inside the element body;and a conductive portion comprising an inner conductive portion and an outer conductive portion, wherein the inner conductive portion is disposed within the element body and is electrically continuous with the inner electrode, the outer conductive portion comprising a connector electrode disposed on a part of the peripheral surface located near the rear end, the outer conductive portion further comprising a segment disposed on the peripheral surface and / or a segment disposed on the peripheral surface and exposed to an outside of the sensor element, the outer conductive portion being electrically continuous with the inner conductive portion, wherein, when a porosity is Rp [%] and a thickness is Dc [mm], a covering portion of the outer conductive portion covering the inner conductive portion satisfies Rp / Dc ≤ 145% / mm;

[0008] In the sensor element according to the present invention, when a porosity is Rp [%] and a thickness is Dc [mm], a covering portion of the outer conductive portion covering the inner conductive portion satisfies Rp / Dc ≤ 145% / mm. Such a structure reduces the passage of a gas outside the sensor element (element body) through a part of the outer conductive portion covering the inner conductive portion, reduces the ingress of a gas into the element body through the inner conductive portion or the gap between the inner conductive portion and the element body, and reduces the probability of the gas reaching the inner electrode. Consequently, the probability of deterioration of the detection accuracy of the concentration of the specific gas is reduced. The present inventors have demonstrated these facts through experiments, analyses, and the like.

[0009] [2] In the sensor element according to the present invention (the sensor element according to the above [1]), the covering portion can satisfy Rp / Dc ≤ 120% / mm. Such a structure further reduces the permeation of a gas outside the sensor element (element body) through the part of the outer conductive portion covering the inner conductive portion.

[0010] [3] In the sensor element according to the present invention (the sensor element according to the above [1] or [2]), the cover portion can satisfy Rp / Dc ≥ 1.5% / mm. Such a structure increases the adhesive strength between the cover portion and the element body. The present inventors have demonstrated these facts through experiments, analyses, and the like.

[0011] [4] In the sensor element according to the present invention (the sensor element according to the above [3]), the cover portion can satisfy Rp / Dc ≥ 4.5% / mm. Such a structure further improves the adhesive strength between the cover portion and the element body.

[0012] [5] In the sensor element according to the present invention (the sensor element according to any one of the above [1] to [4]), the porosity Rp may be 0.01% or more and 9.0% or less.

[0013] [6] In the sensor element according to the present invention (the sensor element according to any one of the above [1] to [5]), the thickness Dc may be 0.001 mm or more and 0.080 mm or less.

[0014] [7] In the sensor element according to the present invention (the sensor element according to any one of the above [1] to [6]), the inner electrode may be a measuring electrode used in detecting the concentration of the specific gas.

[0015] [8] In the sensor element according to the present invention (the sensor element according to any one of the above [1] to [7]), the element body may be a laminated body obtained by stacking a plurality of layers including the solid electrolyte layer in a stacking direction orthogonal to the longitudinal direction, the laminated body may have a first surface and a second surface which are opposite end surfaces in the stacking direction;and a third surface and a fourth surface which are opposite end surfaces in a direction orthogonal to the longitudinal direction and to the lamination direction, wherein the first to fourth surfaces are each the circumferential surface, the inner conductive portion may comprise an inner lead led to the third surface or the fourth surface, and the cover portion may be a circumferential surface lead disposed on the third surface or the fourth surface and covering the inner lead.;

[0016] [9] In the sensor element according to the present invention (the sensor element according to any one of the above [1] to [7]), the element body may be a laminated body obtained by stacking a plurality of layers including the solid electrolyte layer in a stacking direction orthogonal to the longitudinal direction, the laminated body may have a first surface and a second surface which are opposite end surfaces in the stacking direction and are each the circumferential surface, the element body may have a through-hole with an opening provided in a part of the circumferential surface located near the rear end, the circumferential surface being the first surface or the second surface, the through-hole extending through one or more of the plurality of layers in the stacking direction, the inner conductive portion may include a through-hole conductor arranged in the through-hole,and the cover portion may be the connector electrode and covers the through-hole conductor.,

[0017]

[10] In the sensor element according to the present invention (the sensor element according to any one of the above [1] to [1], [2], and [5] to [7]), the element body may be a laminated body obtained by stacking a plurality of layers, including the solid electrolyte layer, in a stacking direction orthogonal to the longitudinal direction, the laminated body may have a first surface and a second surface which are opposite end surfaces in the stacking direction and are each the circumferential surface, the element body may have a through-hole with an opening provided in a part of the circumferential surface located near the rear end, the circumferential surface being the first surface or the second surface, the through-hole extending through one or more of the plurality of layers in the stacking direction, and the cover portion may comprise a part of a through-hole conductor,exposed to the outside at the opening, wherein the through-hole conductor is disposed in the through-hole, includes the part exposed to the outside, and is connected to the connector electrode. In this case, the cover portion may be part of the through-hole conductor or the entire through-hole conductor.

[0018]

[11] The sensor element according to the present invention (the sensor element according to any one of the above [1] to

[10] ) may be a sensor element for insertion into a gas sensor, the gas sensor comprising the sensor element; a housing having a cylindrical shape extending in the longitudinal direction of the sensor element, the housing having a second front end and a second rear end which are opposite ends in the longitudinal direction, the sensor element being arranged inside the housing; and a sealing member which seals the second rear end of the housing.

[0019]

[12] A gas sensor according to the present invention comprises the sensor element according to any one of [1] to

[10] ; a housing having a cylindrical shape extending in the longitudinal direction of the sensor element, the housing having a second front end and a second rear end which are opposite ends in the longitudinal direction, the sensor element being arranged inside the housing; and a sealing member which seals the second rear end of the housing.

[0020] Since this gas sensor according to the present invention includes the above sensor element, the gas sensor produces the same advantageous effects as those produced by the above sensor element. For example, one advantageous effect is to reduce the likelihood of a gas outside the sensor element (element body) reaching the internal electrode, which in turn reduces the likelihood of deterioration in the detection accuracy of the concentration of the specific gas. Here, examples of the gas outside the sensor element include a gas generated from the sealing member (for example, a volatile organic gas generated from the rubber plug serving as the sealing member). Short description of the drawings Fig. 1 is a vertical sectional view of a gas sensor 100. Fig. 2 is a schematic sectional view of a sensor element 101 showing an exemplary structure thereof. Fig. 3 is a block diagram showing an electrical connection relationship between a control device 95 and relevant elements including cells. Fig. 4 is a perspective view of a rear end portion of an element body 102. Fig. 5 is an enlarged sectional view of a part including a conductive portion 74 provided for a measuring electrode 44. Fig. 6 is a sectional view of a part of a sensor element 201 according to a modification. Fig. 7 is a sectional view of a part of a sensor element 301 according to another modification. Fig. 8 is a sectional view of a part of a sensor element 401 according to another modification. Fig. 9 is a sectional view of a part of a sensor element 501 according to another modification. Fig. 10 is a schematic sectional view of a sensor element 601 according to another modification. Fig. Figure 11 is an explanatory diagram showing specifications in each of experimental examples. Fig. 12 is a diagram for describing a method of examining whether there is any peeling of a circumferential surface lead 78 from the element body 102. Fig. 13 is another diagram for describing the method of examining whether there is any peeling of the circumferential surface lead 78 from the element body 102. Detailed description of the invention.

[0021] Embodiments of the present invention will be described below with reference to the drawings. Fig. 1 is a vertical sectional view of a gas sensor 100 according to an embodiment of the present invention. Fig. 2 is a schematic sectional view of a sensor element 101 incorporated in the gas sensor 100, showing an exemplary structure thereof. Fig. 3 is a block diagram showing an electrical connection relationship between a control device 95 and relevant elements of the sensor element 101, including cells and a heater 71a. Fig. 4 is a perspective view of a rear end portion of an element body 102 incorporated in the sensor element 101. The Fig. Fig. 5 is an enlarged sectional view of a part of the sensor element 101, showing a measuring electrode 44 and a conductive portion 74 provided therefor. The element body 102 of the sensor element 101 has an elongated, rectangular parallelepiped shape. The longitudinal direction of the element body 102 (the left-right direction in the Fig. 2) is defined as the front-back direction. The thickness direction of the element body 102 (the top-bottom direction in the Fig. 2) is set as the up-down direction. The width direction of the element body 102 (the direction perpendicular to the front-back direction and the up-down direction) is set as the left-right direction. Since the element body 102 is a rectangular parallelepiped body, as shown in Fig. 2, Fig. 4 and Fig. 5, the element body 102 has the following six surfaces, which are the outer surfaces of solid electrolyte layers constituting the element body 102: a first surface 102a (upper surface), a second surface 102b (lower surface), a third surface 102c (left surface), a fourth surface 102d (right surface), a fifth surface 102e (front end surface), and a sixth surface 102f (rear end surface).

[0022] As it is in the Fig. 1, the gas sensor 100 includes the sensor element 101 with the element body 102, a protective cover 130 covering a part of the element body 102 located at the front end, and a sensor assembly 140 including a connector 150 electrically continuous with the sensor element 101. As shown in the drawing, the gas sensor 100 is attached to a pipe 190, such as an exhaust pipe of an internal combustion engine (diesel engine, gasoline engine, or the like) incorporated in a vehicle. The exhaust gas from the internal combustion engine is used as the measurement object gas. The gas sensor 100 is used to measure (detect) the concentration of a specific gas, which relates to the concentration of a specific gas, such as NOx, O 2or ammonia contained in the measurement object gas. In the present embodiment, the concentration of a specific gas to be measured by the gas sensor 100 is the NOx concentration.

[0023] The protective cover 130 includes a bottomed cylindrical inner protective cover 131 covering the front end portion of the element body 102, and a bottomed cylindrical outer protective cover 132 covering the inner protective cover 131. The inner protective cover 131 and the outer protective cover 132 each have a plurality of holes that allow the measurement object gas to flow into the protective cover 130. The space enclosed by the inner protective cover 131 serves as the sensor element chamber 133. The front end portion of the element body 102 is disposed in the sensor element chamber 133.

[0024] The sensor assembly 140 includes an element sealing unit 141 that seals and fixes the sensor element 101, a screw 147 and an outer cylinder 148 attached to the element sealing unit 141, and the connector 150 that is in contact with and electrically connected to connector electrodes 75 arranged on surfaces (upper and lower surfaces) of the element body 102 of the sensor element 101 at a rear end part.

[0025] The element sealing unit 141 includes a cylindrical main metal member 142 and a cylindrical inner cylinder 143 coaxially welded to the main metal member 142. The element sealing unit 141 further includes ceramic supports 144a to 144c, compacts 145a and 145b, and a metal ring 146 encapsulated in a through hole provided on the inside of the main metal member 142 and the inner cylinder 143. The sensor element 101 is located on the central axis of the element sealing unit 141 and extends through the element sealing unit 141 in the front-to-back direction. The inner cylinder 143 has a diameter reducing portion 143a for pressing the compact 145b toward the center axis of the inner cylinder 143 and a diameter reducing portion 143b for pressing the ceramic carriers 144a to 144c and the compacts 145a and 145b forward over the metal ring 146.The compressive forces generated by the diameter-reducing portions 143a and 143b compress the compression members 145a and 145b between the sensor element 101 and the combination of the main metal member 142 and the inner cylinder 143. Consequently, the compression members 145a and 145b seal between the sensor element chamber 133 in the protective cover 130 and a space 149 in the outer cylinder 148, and also secure the sensor element 101.

[0026] The screw 147 is coaxially attached to the main metal member 142 and has an external thread portion around its outer peripheral surface. The external thread portion of the screw 147 is screwed into a fastener 191 welded to the conduit 190 and having an internal thread portion around its inner peripheral surface. In this way, the gas sensor 100 is attached to the conduit 190, with a part of the gas sensor 100, including the front end portion of the element body 102 of the sensor element 101 and the protective cover 130, projecting into the conduit 190.

[0027] The outer cylinder 148 surrounds the inner cylinder 143, the sensor element 101, and the connector 150. A plurality of lead wires 155 are connected to the connector 150 and led outward from the rear end of the outer cylinder 148. The lead wires 155 are electrically continuous with electrodes (described below) of the sensor element 101 via the connector 150. At the rear end of the outer cylinder 148, the gap between the outer cylinder 148 and the lead wires 155 is sealed by a rubber plug 157. The outer cylinder 148 has swaged portions 148a and 148b that restrict the front-to-back movement of the rubber plug 157 relative to the outer cylinder 148. The space 149 in the outer cylinder 148 is filled with a reference gas. The rear end part of the element body 102 of the sensor element 101 is arranged in the space 149.

[0028] As it is in the Fig. 2, the sensor element 101 includes the element body 102; cells 21, 41, 50, and 80 to 83; and a heater portion 70. The element body 102 is a laminated body obtained by stacking six layers in the following order from the bottom in the drawing: 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 an oxygen ion-conductive solid electrolyte layer made of zirconium oxide (ZrO 2) or the like. The solid electrolyte constituting each of these six layers has a high density and is hermetic. The element body 102 is manufactured, for example, by performing predetermined processing and printing circuit patterns on ceramic green sheets to become the individual layers, then stacking the sheets, and then uniting the sheets by performing calcination.

[0029] At the front end part of the element body 102 and between the lower surface of the second solid electrolyte layer 6 and the upper surface of the first solid electrolyte layer 4, a gas inlet 10, a first diffusion adjusting portion 11, a buffer space 12, a second diffusion adjusting portion 13, a first internal cavity (oxygen concentration adjusting chamber) 20, a third diffusion adjusting portion 30, a second internal cavity (oxygen concentration adjusting chamber) 40, a fourth diffusion adjusting portion 60, and a third internal cavity (measuring chamber) 61 are provided, which are juxtaposed in this order and communicate with each other.

[0030] The gas inlet 10, the buffer space 12, the first inner cavity 20, the second inner cavity 40, and the third inner cavity 61 are spaces provided inside the sensor element 101 by hollowing out the spacer layer 5, and each have a top defined by the bottom surface of the second solid electrolyte layer 6, a bottom defined by the top surface of the first solid electrolyte layer 4, and lateral sides defined by the side surfaces of the spacer layer 5.

[0031] The first diffusion adjusting portion 11, the second diffusion adjusting portion 13, and the third diffusion adjusting portion 30 are each provided as two horizontally elongated slits (i.e., openings extending in the direction perpendicular to the plane of the drawing). The fourth diffusion adjusting portion 60 is provided as a single horizontally elongated slit (i.e., an opening extending in the direction perpendicular to the plane of the drawing), so that a gap is provided with respect to the lower surface of the second solid electrolyte layer 6. A portion extending from the gas inlet 10 to the third internal cavity 61, inclusive, is also referred to as a measurement-object gas flow portion.

[0032] The element body 102 includes a reference gas introduction portion 49, which allows the reference gas used in NOx concentration measurement to flow from outside the element body 102 to a reference electrode 42. The reference gas introduction portion 49 includes a reference gas introduction space 43 and a reference gas introduction layer 48. The reference gas introduction space 43 is a space extending from the sixth surface 102f (rear end surface) of the element body 102 to the fifth surface 102e (front end surface). The reference gas introduction space 43 is provided at a position between the upper surface of the third substrate layer 3 and the lower surface of the spacer layer 5 and has lateral sides defined by the side surfaces of the first solid electrolyte layer 4. The reference gas introduction space 43 has an opening at the rear end surface of the element body 102.This opening acts as the entrance 49a of the reference gas introduction section 49. The reference gas is to be introduced into the reference gas introduction space 43 through the entrance 49a. The reference gas introduction section 49 introduces the reference gas to the reference electrode 42 while applying a predetermined diffusion resistance to the reference gas received through the entrance 49a. In the present embodiment, the reference gas is ambient air.

[0033] The reference gas introduction layer 48 is disposed between the upper surface of the third substrate layer 3 and the lower surface of the first solid electrolyte layer 4. The reference gas introduction layer 48 is a porous body composed of a ceramic material such as alumina. A portion of the upper surface of the reference gas introduction layer 48 is exposed in the reference gas introduction space 43. The reference gas introduction layer 48 is provided above the reference electrode 42. The reference gas introduction layer 48 allows the reference gas to flow from the reference gas introduction space 43 to the reference electrode 42.

[0034] The reference electrode 42 is disposed between the upper surface of the third substrate layer 3 and the first solid electrolyte layer 4 and is surrounded by the reference gas introduction layer 48, which is connected to the reference gas introduction space 43, as described above. Furthermore, as described below, the reference electrode 42 is intended to be used for measuring the oxygen concentration (oxygen partial pressure) in the first internal cavity 20, the second internal cavity 40, and the third internal cavity 61.

[0035] In the measurement object gas flow section, the gas inlet 10 is open to an outside space, so that the measurement object gas is taken in from the outside space through the gas inlet 10 into the sensor element 101. The first diffusion adjusting section 11 is provided for exerting a predetermined diffusion resistance on the measurement object gas taken in through the gas inlet 10. The buffer space 12 is provided for guiding the measurement object gas introduced through the first diffusion adjusting section 11 to the second diffusion adjusting section 13. The second diffusion adjusting section 13 is provided for exerting a predetermined diffusion resistance on the measurement object gas to be introduced from the buffer space 12 into the first internal cavity 20.When the measurement object gas is introduced into the first internal cavity 20 from outside the sensor element 101, the measurement object gas, which is rapidly taken into the sensor element 101 through the gas inlet 10 due to a pressure fluctuation of the measurement object gas (ie, a pulsation of the exhaust pressure when the measurement object gas is exhaust gas from an internal combustion engine) in the external space, is not directly introduced into the first internal cavity 20, but is introduced into the first internal cavity 20 after the pressure fluctuation of the measurement object gas has been eliminated by passing through the first diffusion adjusting section 11, the buffer space 12, and the second diffusion adjusting section 13. Accordingly, the pressure fluctuation of the measurement object gas to be introduced into the first internal cavity 20 becomes substantially negligible.The first internal cavity 20 is provided as a space for adjusting the oxygen partial pressure in the measurement object gas introduced thereinto via the second diffusion adjusting portion 13. The oxygen partial pressure is adjusted by actuating a main pump cell 21.

[0036] The main pumping cell 21 is an electrochemical pumping cell formed of an inner pumping electrode 22 including an upper electrode portion 22a disposed on the lower surface of the second solid electrolyte layer 6 over substantially the entire area facing the first inner cavity 20; an outer pumping electrode 23 disposed on the upper surface of the second solid electrolyte layer 6 over an area corresponding to the upper electrode portion 22a so as to be exposed to the outside of the element body 102; and the second solid electrolyte layer 6, the spacer layer 5, and the first solid electrolyte layer 4, which form a current path between the electrodes 22 and 23.

[0037] The inner pumping electrode 22 is provided over the upper and lower solid electrolyte layers (ie, the second solid electrolyte layer 6 and the first solid electrolyte layer 4) defining the first inner cavity 20 and the spacer layer 5 providing side walls of the first inner cavity 20.Specifically, the lower surface of the second solid electrolyte layer 6, which provides the upper surface of the first internal cavity 20, is provided with the upper electrode portion 22a, the upper surface of the first solid electrolyte layer 4, which provides the lower surface of the first internal cavity 20, is provided with a lower electrode portion 22b, and the upper electrode portion 22a and the lower electrode portion 22b are connected to each other by side electrode portions (not shown) provided on respective sidewalls (inner surfaces) of the spacer layer 5, which form opposite sidewalls of the first internal cavity 20. Consequently, the internal pumping electrode 22 has a tunnel-like structure in a region where the side electrode portions are provided.

[0038] In the main pumping cell 21, a desired voltage Vp0 is applied between the inner pumping electrode 22 and the outer pumping electrode 23, causing a pumping current Ip0 to flow in the positive direction or the negative direction between the inner pumping electrode 22 and the outer pumping electrode 23. Consequently, the oxygen in the first inner cavity 20 is pumped out to the outside, or the oxygen in the outside is pumped into the first inner cavity 20.

[0039] Further, for detecting the oxygen concentration (oxygen partial pressure) in the atmosphere within the first inner cavity 20, 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, an oxygen partial pressure detection sensor cell for the main pumping controller 80.

[0040] The oxygen concentration (oxygen partial pressure) in the first internal cavity 20 is to be determined by measuring an electromotive force (voltage V0) in the oxygen partial pressure detection sensor cell for the main pump controller 80. Furthermore, feedback control is performed with the voltage Vp0 of a variable current source 24 such that the voltage V0 reaches a target value, thereby controlling the pump current Ip0. Consequently, the oxygen concentration in the first internal cavity 20 is maintained at a predetermined fixed value.

[0041] The third diffusion adjusting section 30 is provided for exerting a predetermined diffusion resistance on the measurement object gas having an oxygen concentration (an oxygen partial pressure) adjusted in the first inner cavity 20 by the operation of the main pumping cell 21, and then guiding the measurement object gas to the second inner cavity 40.

[0042] The second inner cavity 40 is provided as a space for an auxiliary pumping cell 50 for further adjusting the oxygen partial pressure in the measurement target gas with an oxygen concentration (an oxygen partial pressure) previously adjusted in the first inner cavity 20 and introduced into the second inner cavity 40 via the third diffusion adjusting portion 30. Consequently, the oxygen concentration in the second inner cavity 40 is maintained at a fixed level with high accuracy, whereby the gas sensor 100 can achieve highly accurate measurement of the NOx concentration.

[0043] The auxiliary pumping cell 50 is an auxiliary electrochemical pumping cell formed of an auxiliary pumping electrode 51 including an upper electrode portion 51a disposed on the lower surface of the second solid electrolyte layer 6 over substantially the entire area facing the second inner cavity 40; the outer pumping electrode 23 (but not limited to the outer pumping electrode 23, and it may be any suitable electrode disposed on the outer peripheral surface of the element body 102, as appropriate); and the second solid electrolyte layer 6, the spacer layer 5, and the first solid electrolyte layer 4.

[0044] The auxiliary pumping electrode 51 is disposed in the second internal cavity 40 and has a tunnel-like structure corresponding to the internal pumping electrode 22 disposed in the first internal cavity 20. Specifically, the second solid electrolyte layer 6, which provides the upper surface of the second internal cavity 40, is provided with the upper electrode portion 51a, the first solid electrolyte layer 4, which provides the lower surface for the second internal cavity 40, is provided with a lower electrode portion 51b, and the upper electrode portion 51a and the lower electrode portion 51b are connected to each other by side electrode portions (not shown) provided on opposite wall surfaces of the spacer layer 5, which provide respective side walls of the second internal cavity 40. Thus, the auxiliary pumping electrode 51 has a tunnel-like structure.

[0045] In the auxiliary pumping cell 50, a desired voltage Vp1 is applied between the auxiliary pumping electrode 51 and the outer pumping electrode 23 such that the oxygen in the atmosphere within the second inner cavity 40 can be pumped out to the outside space or the oxygen can be pumped from the outside space into the second inner cavity 40.

[0046] Further, in order to adjust the oxygen partial pressure in the atmosphere within the second inner cavity 40, the auxiliary pumping 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 oxygen partial pressure detection sensor cell for auxiliary pumping control 81.

[0047] A variable current source 52, whose voltage is controlled based on an electromotive force (voltage V1) detected by the oxygen partial pressure detection sensor cell for auxiliary pumping control 81, causes the auxiliary pumping cell 50 to perform pumping. Consequently, the oxygen partial pressure in the atmosphere in the second internal cavity 40 is set to a low partial pressure that has substantially no effect on NOx measurement.

[0048] Furthermore, a pumping current Ip1 generated by the auxiliary pumping cell 50 is used in controlling the electromotive force of the oxygen partial pressure detection sensor cell for the main pumping controller 80. Specifically, the pumping current Ip1 is input as a control signal to the oxygen partial pressure detection sensor cell for the main pumping controller 80, and the above target value for the voltage V0 of the oxygen partial pressure detection sensor cell for the main pumping controller 80 is set, thereby controlling the gradient of the oxygen partial pressure in the measurement object gas to be introduced into the second internal cavity 40 from the third diffusion adjusting section 30 to be set constant. To use the gas sensor 100 as a NOx sensor, the oxygen concentration in the second internal cavity 40 is maintained at a fixed value of about 0.001 ppm by the cooperation of the main pumping cell 21 and the auxiliary pumping cell 50.

[0049] The fourth diffusion adjusting section 60 is provided for exerting a predetermined diffusion resistance on the measurement object gas having an oxygen concentration (an oxygen partial pressure) adjusted in the second internal cavity 40 by the operation of the auxiliary pumping cell 50, and then guiding the measurement object gas to the third internal cavity 61. The fourth diffusion adjusting section 60 has a role of restricting the amount of NOx flowing into the third internal cavity 61.

[0050] The third inner cavity 61 is provided as a space in which an operation for measuring the concentration of nitrogen oxide (NOx) in the measurement object gas is performed with the measurement object gas having an oxygen concentration (an oxygen partial pressure) adjusted in advance in the second inner cavity 40 and introduced into the third inner cavity 61 via the fourth diffusion adjusting section 60. The NOx concentration is predominantly measured in the third inner cavity 61 by the operation of a measuring pump cell 41.

[0051] The measuring pumping cell 41 is configured to measure the NOx concentration in the measurement target gas in the third internal cavity 61. The measuring pumping cell 41 is an electrochemical pumping cell composed of the measuring electrode 44 disposed on the upper surface of the first solid electrolyte layer 4 in an area facing the third internal cavity 61; and the outer pumping electrode 23, the second solid electrolyte layer 6, the spacer layer 5, and the first solid electrolyte layer 4. The measuring electrode 44 also functions as a NOx reduction catalyst that reduces the NOx present in the atmosphere in the third internal cavity 61.

[0052] In the measuring pump cell 41, oxygen resulting from the decomposition of nitrogen oxide (NOx) in the atmosphere surrounding the measuring electrode 44 is pumped out, and the amount of oxygen thus generated is detected as the pump current Ip2.

[0053] Further, to detect the oxygen partial pressure around the measuring electrode 44, the first solid electrolyte layer 4, the third substrate layer 3, the measuring electrode 44, and the reference electrode 42 form an electrochemical sensor cell, that is, an oxygen partial pressure detection sensor cell for measuring pump control 82. A variable current source 46 is controlled based on an electromotive force (voltage V2) detected by the oxygen partial pressure detection sensor cell for measuring pump control 82.

[0054] The measurement object gas introduced into the second inner cavity 40 reaches the measuring electrode 44 in the third inner cavity 61 via the fourth diffusion adjustment section 60 under a condition where the oxygen partial pressure is adjusted. The nitrogen oxide (NOx) in the measurement object gas surrounding the measuring electrode 44 is reduced (2NO → N 2 + O 2), so that oxygen is generated. Then, the thus-generated oxygen is subjected to pumping by the measuring pumping cell 41. During the pumping of oxygen, a voltage Vp2 of the variable current source 46 is controlled so that the voltage V2 detected by the oxygen partial pressure detection sensor cell for measuring pumping control 82 reaches a fixed value (i.e., a target value). Since the amount of oxygen generated around the measuring electrode 44 is proportional to the concentration of nitrogen oxide in the measurement-object gas, the nitrogen oxide concentration in the measurement-object gas is calculated using the pumping current Ip2 of the measuring pumping cell 41.

[0055] Alternatively, 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 electrochemical sensor cell serving as an oxygen partial pressure detection device, an electromotive force corresponding to the difference between the amount of oxygen resulting from the reduction of the NOx component in the atmosphere around the measuring electrode 44 and the amount of oxygen contained in the reference air can be detected. Consequently, the concentration of the NOx component in the measurement target gas can be obtained.

[0056] Furthermore, 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. Using an electromotive force (voltage Vref) generated by the sensor cell 83, the oxygen partial pressure in the measurement object gas outside the element body 102 of the sensor element 101, particularly around the outer pumping electrode 23, can be detected.

[0057] In the gas sensor 100 having the above structure, the measuring pumping cell 41 receives the measurement object gas whose oxygen partial pressure is constantly maintained at a fixed low value (i.e., a value having substantially no effect on NOx measurement) as a result of the operation of the main pumping cell 21 and the auxiliary pumping cell 50. Consequently, the NOx concentration in the measurement object gas can be determined based on the pumping current Ip2, which flows as a result of oxygen generated by NOx reduction being pumped out by the measuring pumping cell 41, substantially proportional to the NOx concentration in the measurement object gas.

[0058] The electrodes 22, 23, 42, 44, and 51 are described below. The inner pumping electrode 22, the auxiliary pumping electrode 51, and the measuring electrode 44 each contain a Class 1 noble metal having catalytic activity. The Class 1 noble metal can be, for example, at least one of Pt, Rh, Ir, Ru, and Pd. The outer pumping electrode 23 and the reference electrode 42 each also contain a Class 1 noble metal. The inner pumping electrode 22 and the auxiliary pumping electrode 51 each further contain a Class 2 noble metal, which reduces the catalytic activity of the Class 1 noble metal with respect to the specific gas (NOx). Consequently, the reducing ability of each of the inner pumping electrode 22 and the auxiliary pumping electrode 51 with respect to the NOx component in the measured gas is weakened. The Class 2 noble metal can be, for example, Au. The measuring electrode 44 does not contain any Class 2 noble metal.Consequently, the reducing ability of the measuring electrode 44 with respect to the NOx component in the measurement object gas is made higher than those of the inner pumping electrode 22 and the auxiliary pumping electrode 51. The measuring electrode 44 preferably contains at least one of Pt and Rh among the class 1 noble metals, or may contain both Pt and Rh. The outer pumping electrode 23 and the reference electrode 42 each preferably contain none of the class 2 noble metals. The electrodes 22, 23, 42, 44, and 51 are each preferably a cermet, the noble metal, and an oxygen ion-conductive oxide (such as ZrO). 2 ). Preferably, the electrodes 22, 23, 42, 44, and 51 are each a porous body. In the present embodiment, the inner pumping electrode 22 and the auxiliary pumping electrode 51 are each a porous cermet electrode made of Pt and ZrO 2with 1% Au. The outer pump electrode 23 and the reference electrode 42 are each a porous cermet electrode made of Pt and ZrO 2 The measuring electrode 44 is a porous cermet electrode made of Pt, Rh and ZrO 2 is composed.

[0059] To increase the oxygen ion conductivity of the solid electrolyte in the element body 102, the heater portion 70 has the role of a temperature adjuster for keeping the sensor element 101 warm by heating the sensor element 101. The heater portion 70 includes a heater 71a, a heater insulating layer 71b, and a pressure relief hole 71c.

[0060] The heater 71a is an electrical resistor disposed between the second substrate layer 2 and the third substrate layer 3 from above and below. The heater 71a generates heat when supplied with electricity from a heater power source 72, thereby heating and keeping warm the solid electrolyte in the element body 102 of the sensor element 101. Furthermore, the heater 71a is embedded in such a manner as to overlap the entire area from the first internal cavity 20 to the third internal cavity 61 and can adjust the entirety of the sensor element 101 to a temperature at which the solid electrolyte is activated.

[0061] The heater insulating layer 71b is provided on the upper and lower surfaces of the heater 71a and is an insulator composed of alumina or the like. The heater insulating layer 71b is provided to provide electrical insulation between the second substrate layer 2 and the heater 71a and electrical insulation between the third substrate layer 3 and the heater 71a.

[0062] The pressure relief hole 71c extends through the third substrate layer 3 and the reference gas introduction layer 48 to communicate with the reference gas introduction space 43. The pressure relief hole 71c is provided to reduce an increase in internal pressure that occurs with a temperature increase in the heater insulating layer 71b.

[0063] The gas sensor 100 further comprises the control device 95. As shown in the Fig. 3, the control device 95 includes the above-described variable power sources 24, 46, and 52, the above-described heater power source 72, and a controller 96. The controller 96 is a microprocessor including a CPU 97, a memory unit 98, etc. The memory unit 98 is an information-rewritable, non-volatile memory and can store, for example, various programs and various data.The control device 96 is configured to receive the voltage V0 of the oxygen partial pressure detection sensor cell for the main pump controller 80, the voltage V1 of the oxygen partial pressure detection sensor cell for the auxiliary pump controller 81, the voltage V2 of the oxygen partial pressure detection sensor cell for the measuring pump controller 82, the voltage Vref of the sensor cell 83, the pump current Ip0 flowing through the main pump cell 21, the pump current Ip1 flowing through the auxiliary pump cell 50, and the pump current Ip2 flowing through the measuring pump cell 41. The control device 96 is configured to output control signals to the variable current sources 24, 46 and 52 and consequently to control the voltages Vp0, Vp1 and Vp2 to be output by the variable current sources 24, 46 and 52, thereby controlling the main pump cell 21, the measuring pump cell 41 and the auxiliary pump cell 50.The controller 96 is configured to output a control signal to the heater power source 72, thereby controlling the electric current to be supplied from the heater power source 72 to the heater 71a. The storage unit 98 further stores target values ​​V0*, V1*, V2*, and the like, which will be described below. The CPU 97 of the controller 96 is configured to refer to the target values ​​V0*, V1*, and V2* and thus control the pump cells 21, 41, and 50.

[0064] The controller 96 is configured to perform an auxiliary pumping control operation, in which the auxiliary pumping cell 50 is controlled so that the oxygen concentration in the second internal cavity 40 reaches a target concentration. Specifically, the controller 96 performs closed-loop control of the voltage Vp1 of the variable current source 52 such that the voltage V1 is set to a fixed value (referred to as "target value V1*"), thereby controlling the auxiliary pumping cell 50. The target value V1* is set as a value that brings the oxygen concentration in the second internal cavity 40 to a predetermined low concentration that has substantially no effect on NOx measurement.

[0065] The controller 96 is configured to perform a main pumping control operation in which the main pumping cell 21 is controlled so that the pumping current Ip1, which is generated when the auxiliary pumping cell 50 adjusts the oxygen concentration in the second internal cavity 40 in the auxiliary pumping control operation, reaches a target current (referred to as "target value Ip1*"). Specifically, based on the pumping current Ip1, the controller 96 sets (i.e., performs feedback control) with the target value for the voltage V0 (referred to as "target value V0*") such that the pumping current Ip1 flowing at the voltage Vp1 is set to a fixed target value Ip1*. Furthermore, the controller 96 performs feedback control of the voltage Vp0 of the variable current source 24 such that the voltage V0 is set to the target value V0* (i.e., so that the oxygen concentration in the first internal cavity 20 is set to a target concentration).Through the main pumping control operation, the gradient of the oxygen partial pressure in the measurement object gas introduced from the third diffusion adjustment section 30 into the second internal cavity 40 is set constant. The target value V0* is set to a value at which the oxygen concentration in the first internal cavity 20 is higher than 0% and is a low concentration. The pumping current Ip0 flowing during the main pumping control operation changes with the oxygen concentration in the measurement object gas flowing from the gas inlet 10 into the measurement object gas flow section (ie, the measurement object gas around the sensor element 101). Therefore, the controller 96 can also detect the oxygen concentration in the measurement object gas based on the pumping current Ip0.

[0066] The main pumping control process and the auxiliary pumping control process mentioned above are collectively referred to as "adjustment pumping control process." Furthermore, the first internal cavity 20 and the second internal cavity 40 are collectively referred to as "oxygen concentration adjustment chamber." The main pumping cell 21 and the auxiliary pumping cell 50 are collectively referred to as "adjustment pumping cell." In the adjustment pumping control process performed by the controller 96, the adjustment pumping cell adjusts the oxygen concentration in the oxygen concentration adjustment chamber.

[0067] The controller 96 is configured to perform a metering pump control operation, in which the metering pump cell 41 is controlled so that the voltage V2 reaches a fixed value (referred to as "target value V2*") (i.e., so that the oxygen concentration in the third internal cavity 61 reaches a predetermined low concentration). Specifically, the controller 96 performs feedback control of the voltage Vp2 of the variable current source 46 so that the voltage V2 is set to the target value V2*, thereby controlling the metering pump cell 41. Through the metering pump control operation, oxygen is pumped out from the third internal cavity 61.

[0068] Through the measurement pumping control operation, oxygen generated as a result of NOx being reduced in the measurement target gas in the third internal cavity 61 is pumped out from the third internal cavity 61 so that the oxygen becomes substantially zero. Then, the controller 96 detects the pumping current Ip2 as a value corresponding to the oxygen generated in the third internal cavity 61 from the specific gas (in this case, NOx), and calculates the NOx concentration in the measurement target gas based on the pumping current Ip2.

[0069] The storage unit 98 stores, for example, a relationship expression (e.g., a linear function or a quadratic function expression) or a map representing the correspondence between the pumping current Ip2 and the NOx concentration. Such a relationship expression or map can be obtained in advance through experiments.

[0070] The controller 96 is configured to perform a heater control operation in which a control signal is output to the heater power source 72 such that the temperature of the heater 71a is adjusted to a target temperature (e.g., 800°C). Here, the temperature of the heater 71a can be expressed as a linear function of the resistance value of the heater 71a. In the heater control operation, the controller 96 calculates the resistance value of the heater 71a as a value equivalent to the temperature of the heater 71a (a value that can be converted into a temperature), and controls the heater power source 72 such that the thus calculated resistance value reaches a target resistance value (a resistance value corresponding to the target temperature).For example, the controller 96 may detect the voltage of the heater 71a and the current flowing through the heater 71a, and calculate the resistance value of the heater 71a based on the thus detected voltage and current. Alternatively, the controller 96 may calculate the resistance value of the heater 71a using, for example, a three-terminal method or a four-terminal method. When the heater 71a is energized, the heater power source 72 changes the voltage value to be applied to the heater 71a based on the control signal from the controller 96, for example, thereby adjusting the electric current to be supplied to the heater 71a.

[0071] The control device 95, including the variable power sources 24, 46 and 52, the heater power source 72, etc., which are in the Fig. 3, is connected to the electrodes 22, 23, 42, 44 and 51 and the heater 71a via respective conductive portions 74. The plurality of conductive portions 74 each includes a connector electrode 75 and a lead 76. The lead 76 allows the connector electrode 75 and a corresponding one of the electrodes to be electrically continuous with each other. Fig. 4, all of the plurality of connector electrodes 75 are shown, whereas only one of the leads 76 provided for the measuring electrode 44 is shown.

[0072] The plurality of connector electrodes 75 each acts as a terminal that allows the sensor element 101 and an external element to be electrically continuous with each other. As shown in the Fig. As shown in Fig. 4, the plurality of connector electrodes 75 are each arranged at a rear end part of the first surface 102a (upper surface) or the second surface 102b (lower surface) of the element body 102 of the sensor element 101. Specifically, the plurality of connector electrodes 75 are: connector electrodes 75a to 75d arranged at the rear end part of the first surface 102a of the element body 102 in this order from the left side, and connector electrodes 75e to 75h arranged at the rear end part of the second surface 102b of the element body 102 in this order from the left side. The connector electrodes 75a to 75d and 75h are connected to the measuring electrode 44, the outer pumping electrode 23, the auxiliary pumping electrode 51, the inner pumping electrode 22 and the reference electrode 42 via the respective lines 76 (thus electrically continuous).The connector electrodes 75e to 75g are connected to the heater 71a via the respective leads 76.

[0073] Hereinafter, details of the measuring electrode 44 and the corresponding conductive portion 74 (the connector electrode 75a and the lead 76) will be described with reference to Fig. 4 and Fig. 5. Between each two of the layers 1 to 6 of the element body 102, which are adjacent in the layering direction (top-bottom direction), a hermetic connecting layer is arranged, which in the Fig. 2 is not shown. Accordingly, each adjacent two layers are connected to each other by the corresponding connecting layer. The connecting layers, like layers 1 to 6, preferably have oxygen ion conductivity. In the present embodiment, the connecting layers are each made of a ceramic which, like layers 1 to 6, is predominantly composed of zirconium oxide. As shown in the Fig. As shown in FIG. 5, a connecting layer 7, among the connecting layers, connects the first solid electrolyte layer 4 and the spacer layer 5 to each other. The connecting layer 7 covers substantially the entire upper surface of the first solid electrolyte layer 4, excluding the measurement object gas flow portion formed of the buffer space 12, the first internal cavity 20, the second internal cavity 40, etc.

[0074] The measuring electrode 44 is arranged on the upper surface of the first solid electrolyte layer 4 in a region facing the third inner cavity 61. The lead 76 provided for the measuring electrode 44 includes an inner lead 77 and a peripheral lead 78. The inner lead 77 is arranged inside the element body 102 and is connected to the measuring electrode 44 and the peripheral lead 78 (electrically continuous therewith). Specifically, the inner lead 77 includes first to fourth segments 77a to 77d. The first segment 77a is arranged on the upper surface of the first solid electrolyte layer 4 in a region facing the third inner cavity 61. The right end of the first segment 77a is connected to the measuring electrode 44. The first segment 77a extends linearly in the left-right direction to the left end of the third inner cavity 61.The right end of the second segment 77b is connected to the left end of the first segment 77a. The second segment 77b extends linearly in the left-right direction, but does not reach the left end of the element body 102. The front end of the third segment 77c is connected to the left end of the second segment 77b. The third segment 77c extends linearly in the front-back direction to a position near the rear end of the element body 102. The right end of the fourth segment 77d is connected to the rear end of the third segment 77c. The fourth segment 77d extends linearly in the left-right direction to the left end of the element body 102. The left end of the fourth segment 77d reaches (is guided) to the third surface 102c (the left end surface) of the element body 102 and is covered by and connected to the circumferential surface line 78.The inner lead 77 is a conductor composed predominantly of a noble metal, such as platinum (Pt), or a high-melting-point metal, such as tungsten (W) or molybdenum (Mo). The inner lead 77 is preferably a cermet conductor containing a noble metal or a high-melting-point metal and zirconium oxide, which is the same as the main component of the first solid electrolyte layer 4.

[0075] In the inner lead 77, the entirety of the second and third segments 77b and 77c and the outer periphery of a part of the fourth segment 77d, excluding a part of the left end thereof, are surrounded by a lead insulating layer 79. The lead insulating layer 79 insulates the portion of the inner lead 77 whose outer periphery is surrounded by the lead insulating layer 79 from the first solid electrolyte layer 4 and the spacer layer 5. It should be noted that the outer periphery of the left end part of the fourth segment 77d is not surrounded by the lead insulating layer 79. That is, during the manufacturing of the sensor element 101, the outer periphery of the left end part of the fourth segment 77d is prevented from being surrounded by the lead insulating layer 79. Therefore, the connection (electrical continuity) between the left end of the fourth segment 77d and the circumferential surface line 78 is less likely to be obstructed.The wire insulating layer 79 is an insulator composed of a ceramic such as alumina.

[0076] The circumferential surface lead 78 has a substantially rectangular parallelepiped shape and is arranged near the rear end of the third surface 102c of the element body 102 and is connected to the inner lead 77 and the connector electrode 75a. Specifically, the circumferential surface lead 78 is arranged on the third surface 102c so as to cover the entirety of the left end of the fourth segment 77d of the inner lead 77, thereby preventing the left end of the fourth segment 77d from being exposed to the outside of the sensor element 101. A central part of the right-end surface of the circumferential surface lead 78 is connected to the left end of the fourth segment 77d (therewith electrically continuous). A part of the upper end of the right-end surface of the circumferential surface lead 78 is connected to the left-end surface of the front end portion of the connector electrode 75a.

[0077] The peripheral surface line 78 is a conductor composed predominantly of a noble metal, such as platinum (Pt) or the like. The peripheral surface line 78 is preferably a conductor comprising noble metal and aluminum oxide (Al 2 O 3 ). The circumferential surface line 78 is further preferably a conductor comprising noble metal, aluminum oxide and silicon oxide (SiO 2). For example, in a case where the circumferential surface line 78 is the conductor containing noble metal, alumina, and silicon oxide, when the volumes of noble metal, alumina, and silicon oxide are Vp, Va, and Vs [vol%], respectively, the circumferential surface line 78 may be as follows. The circumferential surface line 78 may satisfy Va / Vs ≤ 1.5. The circumferential surface line 78 may also satisfy Vp / (Va + Vs) ≥ 1.5. In this case, the circumferential surface line 78 may satisfy Vp / (Va + Vs) ≥ 2.6. The circumferential surface line 78 may satisfy Vp / (Va + Vs) ≤ 19.0. The circumferential surface line 78 may satisfy 1.0 ≤ Va / Vs ≤ 1.5. The ratio of the total mass of alkali metal and alkaline earth metal contained in the circumferential surface line 78 on an oxide basis may satisfy 0.1 wt% or less.

[0078] When a porosity of the circumferential line 78 is Rp [%] and a thickness of the circumferential line 78 is Dc [mm], the circumferential line 78 is formed to satisfy Rp / Dc ≤ 145% / mm. When a gas outside the sensor element 101 (element body 102) penetrates into the interior of the element body 102 through the fourth segment 77d of the inner line 77 or through a gap between the fourth segment 77d and the element body 102, the gas can move along the inner line 77 or the gap between the inner line 77 and the element body 102 (a gap between the inner line 77 and the line insulating layer 79 or a gap between the line insulating layer 79 and the element body 102) to the measuring electrode 44 and reach the measuring electrode 44.Examples of a gas outside the sensor element 101 include a volatile organic gas generated from the rubber plug 157 when, for example, the sensor element 101 is exposed to a high-temperature environment. When such a gas reaches the measuring electrode 44, the oxygen concentration around the measuring electrode 44 may decrease. This decrease may cause the voltage V2 to be detected by the oxygen partial pressure detection sensor cell for the measuring pumping control 82 to increase. Consequently, less oxygen may be pumped out from the third internal cavity 61 during the measuring pumping control operation, resulting in a decrease in the pumping current Ip2. This decrease in the pumping current Ip2 may cause a deterioration in the detection accuracy of the concentration of the specific gas (NOx).In view of the above, the circumferential line 78 according to the present embodiment is configured to satisfy Rp / Dc ≤ 145% / mm. Such a configuration causes the porosity Rp of the circumferential line 78 to become small and / or the thickness Dc of the circumferential line 78 to become large, thereby reducing the passage of a gas outside the element body 102 through the circumferential line 78. Consequently, the penetration of such a gas into the interior of the element body 102 through the fourth segment 77d of the inner line 77 or through a gap between the fourth segment 77d and the element body 102 is reduced, which reduces the possibility of such a gas reaching the measuring electrode 44. As a result, the possibility of a decrease in the pumping current Ip2 is reduced, which in turn reduces the possibility of deterioration in the detection accuracy of the concentration of the specific gas (NOx).The present inventors have demonstrated these facts through experiments, analyses, and the like. Furthermore, reducing the probability of such gas reaching the measuring electrode 44 reduces the deterioration of the measuring electrode 44. The circumferential surface line 78 is preferably formed to satisfy Rp / Dc ≤ 120% / mm.

[0079] The circumferential line 78 is preferably formed to satisfy Rp / Dc ≥ 1.5% / mm. Such a structure causes the porosity Rp of the circumferential line 78 to become large and / or the thickness Dc of the circumferential line 78 to become small, thereby increasing the adhesive strength between the circumferential line 78 and the third surface 102c of the element body 102. Consequently, the probability of the circumferential line 78 peeling off from the third surface 102c of the element body 102 when the sensor element 101 undergoes contraction during the calcination process in its manufacture is reduced. Furthermore, the probability of the circumferential line 78 peeling off from the third surface 102c of the element body 102 due to thermal expansion and contraction of the sensor element 101 is also reduced. The present inventors have demonstrated these facts through experiments, analyses and the like.The circumferential surface line 78 is further preferably designed to satisfy Rp / Dc ≥ 4.5% / mm.

[0080] The porosity Rp of the circumferential surface line 78 may be 0.01% or higher. The porosity Rp of the circumferential surface line 78 may be 9.0% or less. The thickness Dc of the circumferential surface line 78 may be 0.001 mm or more. The thickness Dc of the circumferential surface line 78 may be 0.080 mm or less.

[0081] The porosity, such as the porosity Rp of the circumferential surface line 78, is set as a value derived using an image (SEM image) obtained by examination through a scanning electron microscope (SEM) as follows. First, a measurement object is cut to have a cross section (when the measurement object is the circumferential surface line 78, a cross section in the thickness direction of the circumferential surface line 78). The cross section, which is regarded as an examination surface, is embedded with resin and polished, thereby obtaining a test sample.Subsequently, the test surface of the test sample is photographed by SEM photography (using a secondary electron image, an accelerating voltage of 15 kV, and 1000x magnification, but with a magnification of more than 1000 and less than or equal to 5000 if 1000x magnification is insufficient), thereby obtaining an SEM image of the measurement object. Subsequently, the obtained image is analyzed so that a threshold value is determined from a brightness distribution of brightness data of the pixels in the image using the discriminant analysis method (Otsu binarization method). Then, each pixel in the image is binarized into an object portion and a pore portion based on the determined threshold value, and the area of ​​the object portion and the area of ​​the pore portion are calculated. Furthermore, the percentage of the area of ​​the pore portion with respect to the total area (ie,, the total area of ​​the object section and the pore section) as the porosity (in %).

[0082] An exemplary method for manufacturing the sensor element 101 incorporated in the gas sensor 100 will be described below. First, six uncalcined ceramic green sheets, each containing an oxygen ion-conductive solid electrolyte such as zirconia as a ceramic component, are prepared. In each of these green sheets, a plurality of sheet holes to be used for positioning during printing or stacking, as well as necessary through-holes and the like, are provided in advance. Further, the green sheet to become the spacer layer 5 is subjected to a punching process or the like in advance, in which a space to become the measurement-object gas flow portion is provided. Similarly, the green sheet to become the first solid electrolyte layer 4 is subjected to a process for providing a space to become the reference gas introduction space 43.Then, a pattern printing process and a drying process are performed to form various patterns in the ceramic green sheets 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. Specific structures to be formed include, for example, structures serving as electrodes such as the above-described measuring electrode 44, internal leads such as the internal leads 77 connected to the electrodes, lead insulating layers such as the lead insulating layers 79, the connector electrodes 75, the reference gas introducing layer 48, and the heater portion 70.A pattern printing process is performed in which a pattern-forming paste, prepared according to the properties required for an object to be formed, is applied to a green sheet using a known screen printing technique. The drying process is performed using a known drying technique. When the pattern printing process and the drying process are completed, a printing process and a drying process are performed in which a bonding paste from which the bonding layers (including the above bonding layer 7) for stacking and bonding together the green sheets corresponding to the respective layers are to be formed is printed and dried.Subsequently, the green sheets coated with the bonding paste are stacked in a predetermined order while positioned at the sheet holes, and then subjected to a pressure-bonding process in which the green sheets are placed under predetermined temperature and pressure conditions so that they are press-bonded into a single laminated body. The thus-obtained laminated body contains a plurality of sensor elements 101 therein. The laminated body is cut into pieces each having the size of the sensor element 101. Subsequently, a pattern to become the circumferential surface lead 78 is formed by screen printing on a portion obtained by cutting the laminated body, that is, on a part serving as the third surface 102c of the element body 102 of the sensor element 101. Further, the pattern is dried.Then, the laminated body is calcined at a predetermined calcination temperature, thereby obtaining a sensor element 101.

[0083] To form structures that will become the inner lead 77 and the lead insulating layer 79 on the green sheet that will become the first solid electrolyte layer 4, for example, the following steps may alternatively be performed. First, a structure that will become a part of the lead insulating layer 79 covering the bottom surface of the inner lead 77 is formed on the green sheet. Subsequently, a structure that will become the inner lead 77 is formed. Then, parts of the lead insulating layer 79 covering the lateral sides and top surface of the inner lead 77 are formed.

[0084] The pattern-forming paste for forming the circumferential line 78 is a paste containing noble metal such as platinum or the like. The pattern-forming paste for forming the circumferential line 78 is preferably a paste containing noble metal and alumina. The pattern-forming paste for forming the circumferential line 78 is further preferably a paste containing noble metal, alumina, and silicon oxide. The porosity Rp of the circumferential line 78 can be adjusted, for example, by adjusting the particle diameters of particles contained in the pattern-forming paste for forming the circumferential line 78, adjusting a particle diameter and content ratio of a porosity material, or adjusting a calcination temperature and calcination time when the laminated body is calcined.The thickness Dc of the circumferential surface line 78 is adjustable, for example, by adjusting the viscosity of the pattern-forming paste for forming the circumferential surface line 78 or changing the number of printing operations to be performed in the pattern-forming process.

[0085] Next, a gas sensor 100 incorporating the sensor element 101 is manufactured. For example, an element sealing unit 141 is attached to the sensor element 101 to seal and fix the sensor element 101, and a protective cover 130 is attached to a part of the element sealing unit 141 located near the front end of the element body 102 of the sensor element 101. Furthermore, a connector 150 and lead wires 155 are attached to the rear end of the element body 102 of the sensor element 101 so as to be electrically continuous with the connector electrodes 75. Further, an outer cylinder 148 is attached to a part of the element sealing unit 141 located near the rear end of the element body 102, the lead wires 155 are led outward from the outer cylinder 148, and the outer cylinder 148 is welded to the main metal member 142.Next, the lead wires 155 are passed through the through holes provided in the rubber plug 157, the rubber plug 157 is fitted into the outer cylinder 148, and the outer cylinder 148 is swaged to reduce its diameter, thereby forming swaged portions 148a and 148b. Thus, the rubber plug 157 and the outer cylinder 148 are attached to each other. Then, a control device 95 and the sensor element 101 are connected to each other via the lead wires 155. Thus, a gas sensor 100 is obtained.

[0086] An exemplary use of the gas sensor 100 will be described below. The CPU 97 in the controller 96 first controls the heater power source 72 to supply electric power to the heater 71a so that the temperature of the heater 71a reaches a target temperature (e.g., 800°C). The CPU 97 controls the temperature of the heater 71a by, for example, detecting a value (e.g., the resistance value or the current value of the heater 71a) that can be converted into the temperature of the heater 71a, and performing feedback control of the heater power source 72 based on the thus detected value.When the temperature of the heater 71a reaches the target temperature (or approximately the target temperature), the CPU 97 starts controlling the aforementioned pump cells 21, 41, and 50 (the adjustment pump control process and the measurement pump control process) and detecting the voltages V0, V1, V2, and Vref from the aforementioned sensor cells 80 to 83. In this state, when a measurement object gas is introduced through the gas inlet 10, the measurement object gas moves through the first diffusion adjustment section 11, the buffer space 12, and the second diffusion adjustment section 13 and reaches the first inner cavity 20. Then, the oxygen concentration in the measurement object gas in the first inner cavity 20 and the second inner cavity 40 is adjusted by the main pump cell 21 and the auxiliary pump cell 50, and the thus adjusted measurement object gas reaches the third inner cavity 61.Subsequently, the CPU 97 detects the NOx concentration in the measurement object gas based on the detected pumping current Ip2 and the correspondence relationship stored in the storage unit 98.

[0087] The following explains the correspondence between the elements according to the present embodiment and the elements according to the present invention. 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 according to the present embodiment each correspond to the solid electrolyte layer according to the present invention. The element body 102 corresponds to the element body. The first to fourth surfaces 102a to 102d each correspond to the peripheral surface. The sensing electrode 44 corresponds to the inner electrode. The conductive portion 74 corresponds to the conductive portion. The inner lead 77 corresponds to the inner conductive portion. The connector electrode 75a and the peripheral surface lead 78 each correspond to the outer conductive portion. The peripheral surface lead 78 corresponds to the cover portion.A combination of the protective cover 130 and the sensor assembly 140 corresponds to the housing. The rubber plug 157 corresponds to the sealing element.

[0088] In the sensor element 101 described in detail above, which is incorporated in the gas sensor 100 according to the present embodiment, the circumferential line 78 covers the left end of the fourth segment 77d of the inner line 77. When the porosity of the circumferential line 78 is Rp [%] and the thickness of the circumferential line 78 is Dc [mm], the circumferential line 78 is formed to satisfy Rp / Dc ≤ 145% / mm. Such a structure reduces the passage of a gas outside the sensor element 101 (element body 102), such as the volatile organic gas generated from the rubber plug 157, through the circumferential surface conduit 78. Consequently, the penetration of such a gas into the interior of the element body 102 through the fourth segment 77d of the inner conduit 77 or through a gap between the fourth segment 77d and the element body 102 is reduced, which reduces the probability of such a gas reaching the measuring electrode 44.As a result, the probability of a reduction in the pumping current Ip2 is reduced, which in turn reduces the probability of a deterioration in the accuracy of detecting the concentration of the specific gas.

[0089] In the sensor element 101, the circumferential surface line 78 is configured to satisfy Rp / Dc ≤ 120% / mm. Such a configuration reduces the passage of a gas outside the element body 102 through the circumferential surface line 78.

[0090] In the sensor element 101, the circumferential surface line 78 is formed to satisfy Rp / Dc ≥ 1.5% / mm. Such a structure increases the adhesion between the circumferential surface line 78 and the third surface 102c of the element body 102. Consequently, the probability of the circumferential surface line 78 peeling off from the third surface 102c of the element body 102 when the sensor element 101 undergoes contraction during the calcination process in its manufacture is reduced. Furthermore, the probability of the circumferential surface line 78 peeling off from the third surface 102c of the element body 102 due to thermal expansion and contraction of the sensor element 101 is also reduced.

[0091] In the sensor element 101, the circumferential surface line 78 is formed to satisfy Rp / Dc ≥ 4.5% / mm. Such a structure further improves the adhesion between the circumferential surface line 78 and the third surface 102c of the element body 102.

[0092] The present invention is by no means limited to the above embodiment, and various embodiments are conceivable as long as they are included within the technical scope of the present invention.

[0093] For example, while the circumferential surface line 78 according to the above embodiment is formed of a single layer, such an embodiment is not limitative. For example, as in a sensor element 201 according to a modification disclosed in Fig. 6, or a sensor element 301 according to a further modification shown in Fig. 7, the circumferential surface line 78 may be replaced with a circumferential surface line 278 or 378. The circumferential surface lines 278 and 378 are each formed of two layers. The circumferential surface lines 278 and 378 may each be formed of three or more layers instead of being formed of two layers.

[0094] As it is in the Fig. 6, the circumferential surface line 278 generally has a substantially rectangular parallelepiped shape and includes a first circumferential surface line 278a and a second circumferential surface line 278b. The first circumferential surface line 278a has a substantially rectangular parallelepiped shape and is arranged on the third surface 102c (left surface) of the element body 102 so as to cover the entirety of the left end of the fourth segment 77d of the inner line 77, thereby preventing the left end of the fourth segment 77d from being exposed to the outside of the sensor element 101. The second circumferential surface line 278b has a substantially rectangular parallelepiped shape and is arranged on the left end surface of the first circumferential surface line 278a. In this case, the first circumferential surface line 278a corresponds to the cover portion according to the present invention.Therefore, the first circumferential surface line 278a can be formed corresponding to the circumferential surface line 78 of the sensor element 101, for example, to satisfy Rp / Dc ≤ 145% / mm.

[0095] In the sensor element 301, which is in the Fig. 7, the circumferential surface line 378 generally has a substantially rectangular parallelepiped shape and includes a first circumferential surface line 378a and a second circumferential surface line 378b. The first circumferential surface line 378a has a substantially rectangular parallelepiped shape and is arranged on the third surface 102c (left surface) of the element body 102 such that the entirety of the left end of the fourth segment 77d of the inner line 77 is covered, thereby preventing the left end of the fourth segment 77d from being exposed to the outside of the sensor element 301.The second circumferential surface line 378b covers five surfaces, excluding the right end surface, of the first circumferential surface line 378a and is in contact with the third surface 102c of the element body 102, thereby preventing the fourth segment 77d of the inner line 77 and the first circumferential surface line 378a from being exposed to the outside of the sensor element 301. In this case, the first circumferential surface line 378a may correspond to the cover portion according to the present invention, the second circumferential surface line 378b may correspond to the cover portion according to the present invention, or the entirety of the circumferential surface line 378 may correspond to the cover portion according to the present invention.Therefore, any of the first circumferential line 378a, the second circumferential line 378b, and the entirety of the circumferential line 378 corresponding to the cover portion according to the present invention can be formed, for example, corresponding to the circumferential line 78 of the sensor element 101, which is formed to satisfy Rp / Dc≤145% / mm. In a case where the entirety of the circumferential line 378 corresponds to the cover portion according to the present invention, and a cross section in the thickness direction of the circumferential line 378 (a cross section including the first circumferential line 378a and the second circumferential line 378b) is regarded as the inspection surface, the porosity Rp can be derived using the SEM image obtained by inspection by the SEM.

[0096] While the above embodiment relates to a case where the connector electrode 75a is connected to the measuring electrode 44 via the inner lead 77 and the peripheral surface lead 78, such an embodiment is not limitative. For example, as is the case in a sensor element 401 according to a modification disclosed in Fig. 8, the element body 102 may be replaced with an element body 402, and the inner lead 77 and the peripheral surface lead 78 may be replaced with an inner lead 477 and a through-hole conductor 478. The element body 402 of the sensor element 401 has a through-hole 402h. The through-hole 402h has an opening provided on the first surface 102a of the element body 402 and extending through the spacer layer 5 and the second solid electrolyte layer 6 in the lamination direction (top-bottom direction). The inner lead 477 is disposed inside the element body 402, specifically, between the first solid electrolyte layer 4 and the spacer layer 5, as is the case with the inner lead 77 of the sensor element 101. At least a portion of the inner lead 477 is surrounded at the outer periphery thereof by a lead insulating layer (not shown), as is the case with the inner lead 77.The through-hole conductor 478 is disposed in the through-hole 402h with an insulating layer 479 interposed therebetween. The lower end of the through-hole conductor 478 is connected to the inner lead 477. The entire upper end surface of the through-hole conductor 478 is covered and connected to the connector electrode 75a so that it is not exposed to the outside of the sensor element 401. In this case, the connector electrode 75a corresponds to the outer conductive portion and the cover portion according to the present invention. Therefore, the connector electrode 75a can be formed similarly to the peripheral surface lead 78 of the sensor element 101, for example, formed to satisfy Rp / Dc ≤ 145% / mm.

[0097] While the connector electrode 75a of the sensor element 401 is formed of a single layer, the connector electrode 75a may be formed of two layers or three or more layers. In this case, the connector electrode 75a may be formed as one of the peripheral surface leads 278 and 378 of the sensor elements 201 and 301 according to the modifications described in Fig. 6 and Fig. 7 are shown.

[0098] While the sensor element 401 includes the through-hole conductor 478 whose upper end is covered by the connector electrode 75a, such an embodiment is not limiting. For example, as in a sensor element 501 according to a modification disclosed in Fig. 9, the connector electrode 75a may be replaced with a connector electrode 575a, and the through-hole conductor 478 may be replaced with a through-hole conductor 578. Like the through-hole conductor 478, the through-hole conductor 578 is disposed in the through-hole 402h with an insulating layer 579 interposed therebetween, and the lower end of the through-hole conductor 578 is connected to the inner lead 477. The connector electrode 575a has a circular hole 575h. On the lower end surface of the connector electrode 575a, the outer periphery of the hole 575h is connected to the outer periphery of the upper end of the through-hole conductor 578. Therefore, the upper end of the through-hole conductor 578, excluding the outer periphery, is exposed to the outside of the sensor element 501.In this case, the part of the upper end of the through-hole conductor 578 (within a predetermined distance from the upper end in the up-down direction) may correspond to the cover portion according to the present invention, or the entirety of the through-hole conductor 578 may correspond to the cover portion according to the present invention. In a case where the upper end portion of the through-hole conductor 578 corresponds to the cover portion according to the present invention, the upper end portion of the through-hole conductor 578 can be considered to cover the remaining portion thereof. In a case where the entirety of the through-hole conductor 578 corresponds to the cover portion according to the present invention, the entirety of the through-hole conductor 578 can be considered to cover the portion of the inner lead 477 connected thereto.The upper end portion or the entirety of the through-hole conductor 578 corresponding to the cover portion according to the present invention can be formed according to the circumferential surface conductor 78 of the sensor element 101, and, for example, can be formed to satisfy Rp / Dc ≤ 145% / mm. The thickness Dc is defined as the length in the shortest direction of the upper end portion or the entirety of the through-hole conductor 578. Specifically, the thickness Dc is defined as the shorter of: a radial distance between a circumference of the part of the upper end of the through-hole conductor 578 exposed to the outside of the sensor element 501 and a circumference of the entirety of the upper end of the through-hole conductor 578, and a length in the up-down direction of the upper end portion or the entirety of the through-hole conductor 578 of the cover portion according to the present invention.In addition to such parts, the connector electrode 575a may be formed corresponding to the peripheral surface lead 78. Such a structure reduces the passage of a gas outside the sensor element 501 (element body 102) through the connector electrode 575a. Consequently, the ingress of such a gas into the vicinity of the through-hole conductor 578, particularly a gap between the through-hole conductor 578 and the insulating layer 579 or a gap between the insulating layer 579 and the through-hole 402h, is reduced.

[0099] In the above embodiment, the conductive portion 74 provided for the measuring electrode 44 was described. Other conductive portions 74 provided for each of the inner pumping electrode 22, the auxiliary pumping electrode 51, and the reference electrode 42 can be formed as above. For example, as in the case where the conductive portion 74 provided for the measuring electrode 44 includes the connector electrode 75a and the lead 76 including the inner lead 77 and the circumferential lead 78, when the conductive portion 74 provided for the inner pumping electrode 22 includes a connector electrode 75h and a lead including an inner lead and a circumferential lead, the circumferential lead can be formed corresponding to the circumferential lead 78 of the sensor element 101, for example, formed to satisfy Rp / Dc≤145% / mm.

[0100] While the oxygen concentration adjusting chamber according to the above embodiment includes the first internal cavity 20 and the second internal cavity 40, such an embodiment is not limiting. For example, the oxygen concentration adjusting chamber may further include another internal cavity, or one of the first internal cavity 20 and the second internal cavity 40 may be omitted. Accordingly, while the adjusting pumping cell according to the above embodiment includes the main pumping cell 21 and the auxiliary pumping cell 50, such an embodiment is not limiting. For example, the adjusting pumping cell may further include another pumping cell, or one of the main pumping cell 21 and the auxiliary pumping cell 50 may be omitted.For example, if the oxygen concentration in the measurement object gas can be satisfactorily reduced using the main pumping cell 21 alone, the auxiliary pumping cell 50 may be omitted. If the auxiliary pumping cell 50 is omitted, the controller 96 only needs to perform the main pumping control process as the setting pumping control process. Further, in the main pumping control process, setting the target value V0* based on the above-described pumping current Ip1 may be omitted. Specifically, if a predetermined target value V0* is stored in advance in the storage unit 98, the controller 96 only needs to control the main pumping cell 21 by performing feedback control of the voltage Vp0 of the variable current source 24 such that the voltage V0 reaches the target value V0*.

[0101] While the sensor element 101 of the gas sensor 100 according to the above embodiment includes the first inner cavity 20, the second inner cavity 40, and the third inner cavity 61, such an embodiment is not limiting. For example, the third inner cavity 61 may be omitted, as is the case in a sensor element 601 according to a modification shown in Fig. 10. In the sensor element 601 according to the modification shown in Fig. 10, the gas inlet 10, the first diffusion adjusting portion 11, the buffer space 12, the second diffusion adjusting portion 13, the first internal cavity 20, the third diffusion adjusting portion 30, and the second internal cavity 40 are provided adjacent to each other and communicate with each other in this order between the lower surface of the second solid electrolyte layer 6 and the upper surface of the first solid electrolyte layer 4. Further, the measuring electrode 44 is arranged on the upper surface of the first solid electrolyte layer 4 in the second internal cavity 40. The measuring electrode 44 is covered by a fourth diffusion adjusting portion 45. The fourth diffusion adjusting portion 45 is a film formed of a porous ceramic body made of alumina (Al 2 O 3) or the like. Corresponding to the fourth diffusion adjusting portion 60 according to the above embodiment, the fourth diffusion adjusting portion 45 has a role of restricting the amount of NOx flowing to the measuring electrode 44. Moreover, the fourth diffusion adjusting portion 45 also functions as a protective film for the measuring electrode 44. The upper electrode portion 51a of the auxiliary pumping electrode 51 extends to a position just above the measuring electrode 44. As in the case of the above embodiment, the sensor element 601 with such a structure can detect the NOx concentration using the measuring pumping cell 41. In the sensor element 601 shown in the Fig. 10, the space around the measuring electrode 44 acts as a measuring chamber. In particular, the space around the measuring electrode 44 has a role corresponding to the role of the third inner cavity 61. While the sensor element 101 of the gas sensor 100 according to the above embodiment has the first inner cavity 20, the second inner cavity 40, and the third inner cavity 61, such an embodiment is not limiting. For example, the third inner cavity 61 may be omitted, as is the case in a sensor element 601 according to a modification shown in Fig. 10. In the sensor element 601 according to the modification shown in Fig. 10, the gas inlet 10, the first diffusion adjusting portion 11, the buffer space 12, the second diffusion adjusting portion 13, the first internal cavity 20, the third diffusion adjusting portion 30, and the second internal cavity 40 are provided adjacent to each other and communicate with each other in this order between the lower surface of the second solid electrolyte layer 6 and the upper surface of the first solid electrolyte layer 4. Further, the measuring electrode 44 is arranged on the upper surface of the first solid electrolyte layer 4 in the second internal cavity 40. The measuring electrode 44 is covered by a fourth diffusion adjusting portion 45. The fourth diffusion adjusting portion 45 is a film formed of a porous ceramic body made of alumina (Al 2 O 3) or the like. Corresponding to the fourth diffusion adjusting portion 60 according to the above embodiment, the fourth diffusion adjusting portion 45 has a role of restricting the amount of NOx flowing to the measuring electrode 44. Moreover, the fourth diffusion adjusting portion 45 also functions as a protective film for the measuring electrode 44. The upper electrode portion 51a of the auxiliary pumping electrode 51 extends to a position just above the measuring electrode 44. As in the case of the above embodiment, the sensor element 601 with such a structure can detect the NOx concentration by using the measuring pumping cell 41. In the sensor element 601 shown in the Fig. As shown in Figure 10, the space around the measuring electrode 44 acts as a measuring chamber. In particular, the space around the measuring electrode 44 has a role that corresponds to the role of the third inner cavity 61.

[0102] The outer pumping electrode 23 according to the above embodiment includes a role electrode (also referred to as an "outer main pumping electrode") paired with the inner pumping electrode 22 in the main pumping cell 21, a role electrode (also referred to as an "outer auxiliary pumping electrode") paired with the auxiliary pumping electrode 51 in the auxiliary pumping cell 50, and a role electrode (also referred to as an "outer measuring electrode") paired with the measuring electrode 44 in the measuring pumping cell 41. However, such an embodiment is not limitative. Any one or more of the outer main pumping electrode, the outer auxiliary pumping electrode, and the outer measuring electrode may be arranged separately from the outer pumping electrode 23 and outside the element body 102 so as to come into contact with the measurement object gas.

[0103] While the sensor element 101 according to the above embodiment is configured to detect the concentration of NOx in the measurement target gas, such an embodiment is not limitative as long as the sensor element is configured to detect the concentration of any specific gas in the measurement target gas. For example, the concentration of any oxide other than NOx may be set as the concentration of a specific gas. When the specific gas is an oxide, oxygen is generated when the specific gas itself is reduced in the third internal cavity 61, as is the case with the above embodiment. Therefore, the measurement pumping cell 41 can detect the concentration of a specific gas by detecting a value (for example, the pumping current Ip2) detected corresponding to the oxygen. Alternatively, the specific gas may be a non-oxide such as ammonia.If the specific gas is a non-oxide, the specific gas is converted into an oxide (e.g., NO in the case of ammonia), so that oxygen is generated when the converted gas is reduced in the third inner cavity 61. Consequently, the measuring pumping cell 41 can detect a value (e.g., the pumping current Ip2) corresponding to this oxygen and thus detect the concentration of a specific gas. For example, since the inner pumping electrode 22 in the first inner cavity 20 acts as a catalyst, the ammonia in the first inner cavity 20 is converted into NO.

[0104] While the element body 102 of the sensor element 101 according to the above embodiment is a laminated body comprising a plurality of solid electrolyte layers (ie, layers 1 to 6), such an embodiment is not limiting. The element body 102 only needs to comprise at least one oxygen ion-conductive solid electrolyte layer. For example, layers 1 to 5 formed by the second solid electrolyte layer 6 in the Fig. 2, may be layers (e.g., layers composed of aluminum oxide) composed of a material different from that of solid electrolyte layers. In this case, the electrodes in the sensor element 101 may be arranged on the second solid electrolyte layer 6. For example, the measuring electrode 44 in the Fig. 2 may be disposed on the lower surface of the second solid electrolyte layer 6. Furthermore, the reference gas introduction space 43 may be disposed in the spacer layer 5 instead of being disposed in the first solid electrolyte layer 4, and the reference gas introduction layer 48 may be provided between the second solid electrolyte layer 6 and the spacer layer 5 instead of being provided between the first solid electrolyte layer 4 and the third substrate layer 3, and the reference electrode 42 may be provided rearward of the third inner cavity 61 and on the lower surface of the second solid electrolyte layer 6.When the laminated body includes any layer other than the solid electrolyte layers, the high-density segment according to the present invention may be in contact with the solid electrolyte layers as is the case with the circumferential line 78 according to the above embodiment, or may be in contact with a layer (for example, a layer composed of alumina) composed of a material other than the solid electrolyte layer.

[0105] In the above embodiment, the controller 96 sets (ie, performs feedback control) the target value V0* of the voltage V0 based on the pumping current Ip1 so that the pumping current Ip1 is set to the target value Ip1*, and performs feedback control of the pumping voltage Vp0 so that the voltage V0 is set to the target value V0*, but may perform other types of control. For example, the controller 96 may perform feedback control of the pumping voltage Vp0 based on the pumping current Ip1 so that the pumping current Ip1 is set to the target value Ip1*. Specifically, the controller 96 may omit the detection of the voltage V0 from the oxygen partial pressure detection sensor cell to the main pumping controller 80 and the setting of the target value V0*, and may directly control the pumping voltage Vp0 (and thus the pumping current Ip0) based on the pumping current Ip1.

[0106] While the above embodiment has been described as an embodiment of the gas sensor 100 including any of the sensor elements 101, 201, etc., the above embodiment can of course be applied to any of the sensor elements 101, 201, etc. incorporated into a gas sensor 100. EXAMPLES

[0107] Specific examples of the sensor element are described below. It should be noted that the present invention is not limited to the following examples.

[0108] Each of the experimental examples was prepared by fabricating the sensor element 101 shown in the Fig. 2, and the gas sensor 100 shown in the Fig. 1, using the manufacturing method described above. In the manufacture of the sensor element 101, ceramic green sheets were each prepared by mixing zirconia particles to which 4 mol% of yttria was added as a stabilizer with an organic binder and an organic solvent, and then forming the mixture by tape molding. The pattern-forming paste for forming the circumferential lead 78 was a paste containing noble metal, alumina, and silicon oxide.Of each of the experimental examples, the porosity Rp of the circumferential surface line 78 was varied within a range of 0.01% or higher and 9.0% or lower by adjusting the particle diameters of the particles contained in the pattern-forming paste for forming the circumferential surface line 78, adjusting the particle diameter and content ratio of the porosity material, or adjusting the calcination temperature and calcination time when the laminated body is calcined. Further, in each of the experimental examples, the thickness Dc of the circumferential surface line 78 was varied within a range of 0.001 mm or more and 0.080 mm or less. The . Fig. 11 is an explanatory diagram showing specifications of the sensor element 101 in each of the experimental examples, particularly a relationship between the thickness Dc [mm], the porosity Rp [%] and Rp / Dc [% / mm] of the circumferential surface line 78. A reference gas sensor was also prepared by manufacturing the gas sensor 100 shown in the Fig. 1 was obtained by the above-described manufacturing method using the rubber plug 157 processed to utilize the generation of the volatile organic gas in the evaluation test 1 described below. In the reference gas sensor, the porosity Rp of the circumferential surface line 78 was set to 1.0%, and the thickness Dc of the circumferential surface line 78 was set to 0.020 mm. [Assessment Test 1]

[0109] The reference gas sensor and the gas sensors 100 according to experimental examples were each subjected to an evaluation test to check the detection accuracy of the pumping current Ip2. First, each of the reference gas sensor and the gas sensor 100 according to experimental examples was attached to a piping such that the front end portion of the element body 102 of the sensor element 101 protruded into the piping. Further, temperature sensors were attached to the rubber plug 157 of each gas sensor. Subsequently, the heater control process was started to heat the heater 71a to 800°C. Further, an air-fuel mixture, which is a mixture of liquefied petroleum gas (LPG) and air, was burned with a burner to obtain a gas used as the measurement object gas and allowed to flow into the piping.In this step, the flow rates of LPG and air were adjusted so that the air-fuel ratio λ was set to 1.05, and the temperature of the measurement object gas was set to 740 °C. It should be noted that the present inventors previously demonstrated through other experiments and analyses that the NOx concentration of the measurement object gas substantially reached 100 ppm under the above conditions. Therefore, while causing the measurement object gas to flow through the piping, the setting pumping control operation and the measuring pumping control operation were performed, in which the pumping current Ip2 was detected for two hours. A value obtained by converting the pumping current Ip2 of the reference gas sensor into the NOx concentration [ppm] was set as the reference value.Furthermore, a value obtained by converting the pumping current Ip2 of each of the gas sensors 100 according to experimental examples into the NOx concentration [ppm] was set as an evaluation item value. For each of the gas sensors 100 according to experimental examples, a concentration difference at each of different time points was calculated by subtracting the evaluation item value from the reference value. When the maximum value of the concentration difference was 7 ppm or less, the gas sensor was evaluated as excellent (A). When the maximum value of the concentration difference was greater than 7 ppm and less than 15 ppm, the gas sensor was evaluated as good (B). When the maximum value of the concentration difference was 15 ppm or more, the gas sensor was evaluated as poor (F).Since the reference gas sensor has been processed to avoid the generation of the volatile organic gas as described above, it is assumed that the reference value of the reference gas sensor is not sufficiently sensitive at any time with respect to Rp / Dc [% / mm]. [Assessment Test 2]

[0110] The gas sensors 100 according to experimental examples were each subjected to a thermal expansion and contraction test to evaluate the durability of the circumferential surface lead 78 against peeling from the third surface 102c of the element body 102. Specifically, each of the gas sensors 100 was placed within a temperature cycling oven and subjected to a process of being kept at -40°C for 15 minutes, followed by 15 minutes at 200°C. This procedure constituted one cycle, and the cycle was repeated 600 times. Next, each of the gas sensors 100 was removed from the temperature cycling oven. The continuity between the measuring electrode 44 and the connector electrode 75a via the circumferential surface lead 78 was examined. If continuity was present, it was further examined whether any peeling of the circumferential surface lead 78 from the third surface 102c of the element body 102 occurred. Fig. 12 and Fig. 13 is a diagram for describing a method of examining whether there is any peeling of a circumferential surface lead 78 from the element body 102. First, the sensor element 101 was cut along the thick dashed line shown in Fig. 12. The thick dashed line in the Fig. 12 passes through the front-rear center of the circumferential surface line 78 to divide the sensor element 101 into a front end side and a rear end side. Fig. 13 shows the cut surface of the rear end side of the sensor element 101. Subsequently, five areas of the sensor element 101, which are shown in the Fig. 13 are shown by the thick solid lines, examined by an SEM with 500x magnification and five areas of the sensor element 101, which are shown in the Fig. 13 were observed by the SEM at 3000x magnification. The five regions represented by the thick solid lines and the five regions represented by the thick dashed lines were each set to include the boundary between the circumferential surface lead 78 and the third surface 102c of the element body 102. If at least one of the ten regions had a part where the distance between the circumferential surface lead 78 and the third surface 102c of the element body 102 was 0.020 mm or more, it was judged that peeling occurred. If none of the ten regions had a part where the distance was 0.020 mm or more, it was judged that no peeling occurred.For each of the gas sensors 100 according to Examples 1 to 6 and Comparative Example 1, if there was no continuity between the sensing electrode 44 and the connector electrode 75a via the circumferential surface lead 78, the gas sensor 100 was rated as poor (F). If there was continuity but some peeling of the circumferential surface lead 78 from the third surface 102c of the element body 102 occurred, the gas sensor was rated as good (B). If there was no peeling of the circumferential surface lead 78 from the third surface 102c of the element body 102 occurred, the gas sensor was rated as excellent (A).

[0111] As can be seen from the results of the evaluation test 1, the experimental examples as described in the Fig. 11, a region up to the upper left of the solid line La was evaluated as poor (F), whereas a region between the solid line La and the dashed line Lb (partially coinciding with the solid line La) was evaluated as good (B), and a region up to the lower right of the dashed line Lb was evaluated as very good (A). That is, as can be seen from the results of Evaluation Test 1, among the experimental examples, a region of Rp / Dc > 145% / mm was evaluated as poor, whereas a region of 120% / mm < Rp / Dc ≤ 145% / mm was evaluated as good, and a region of Rp / Dc ≤ 120% / mm was evaluated as very good.Therefore, the range where Rp / Dc ≤ 145% / mm of the experimental examples further prevents the pumping current Ip2 from decreasing due to the volatile organic gas generated from the rubber plug 157, which in turn reduces the deterioration of the specific gas (NOx) concentration detection accuracy. Furthermore, the range where Rp / Dc ≤ 120% / mm of the experimental examples further prevents the pumping current Ip2 from decreasing due to the volatile organic gas generated from the rubber plug 157, which in turn reduces the deterioration of the specific gas (NOx) concentration detection accuracy.

[0112] As can be seen from the results of the evaluation test 2, the experimental examples as described in the Fig.12, a region to the lower right of the solid line Lc was evaluated as poor (F), whereas a region between the solid line Lc and the dashed line Ld (partially identical to the solid line Lc) was evaluated as good (B), and a region to the upper left of the dashed line Ld was evaluated as very good (A). That is, as can be seen from the results of Evaluation Test 2, among the experimental examples, a region of Rp / Dc < 1.5% / mm was evaluated as poor, whereas a region of 1.5% / mm < Rp / Dc < 4.5% / mm was evaluated as good, and a region of Rp / Dc ≥ 4.5% / mm was evaluated as very good. Therefore, the range of the experimental examples where Rp / Dc ≥ 1.5% / mm reduces the amount of peeling of the circumferential surface lead 78 from the third surface 102c of the element body 102 due to thermal expansion and contraction of the sensor element 101 of the gas sensor 100.Furthermore, the range of the experimental examples where Rp / Dc ≥ 4.5% / mm further reduces the amount of peeling of the circumferential surface lead 78 from the third surface 102c of the element body 102 due to thermal expansion and contraction of the sensor element 101 of the gas sensor 100.

[0113] This application claims priority to Japanese Patent Application No. 2023-198186, filed on November 22, 2023, the entire contents of which are incorporated herein by reference. Commercial applicability

[0114] The present invention is applicable to a sensor element configured to detect the concentration of a specific gas such as NOx in a measurement object gas such as exhaust gas from an internal combustion engine, and to a gas sensor including the same. QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] JP 4628920 B

[0003] JP 2023-198186

[0113]

Claims

[1] A sensor element configured to detect a concentration of a specific gas in a measurement object gas, the sensor element comprising: an element body comprising an oxygen ion-conductive solid electrolyte layer and having a columnar shape extending in a longitudinal direction, the element body having a front end and a rear end that are opposite ends in the longitudinal direction, and further having a peripheral surface that is a surface extending in the longitudinal direction, wherein a part of the element body located at the front end is allowed to be exposed to the measurement object gas; an inner electrode disposed within the element body; and a conductive portion comprising an inner conductive portion and an outer conductive portion, wherein the inner conductive portion is disposed within the element body and is electrically continuous with the inner electrode, wherein the outer conductive portion comprises a connector electrode disposed on a part of the peripheral surface located near the rear end, wherein the outer conductive portion further comprises a segment disposed on the peripheral surface and / or a segment disposed on the peripheral surface and exposed to an outside of the sensor element, wherein the outer conductive portion is electrically continuous with the inner conductive portion, wherein, when a porosity is Rp [%] and a thickness is Dc [mm], a covering portion of the outer conductive portion covering the inner conductive portion satisfies Rp / Dc ≤ 145% / mm. [2] The sensor element according to claim 1, wherein the cover portion satisfies Rp / Dc ≤ 120% / mm. [3] The sensor element according to claim 1 or 2, wherein the cover portion satisfies Rp / Dc ≥ 1.5% / mm. [4] The sensor element according to claim 3, wherein the cover portion satisfies Rp / Dc ≥ 4.5% / mm. [5] The sensor element according to any one of claims 1 to 4, wherein the porosity Rp is 0.01% or more and 9.0% or less. [6] The sensor element according to any one of claims 1 to 5, wherein the thickness Dc is 0.001 mm or more and 0.080 mm or less. [7] The sensor element according to any one of claims 1 to 6, wherein the inner electrode is a measuring electrode used in detecting the concentration of the specific gas. [8] Sensor element according to one of claims 1 to 7, wherein the element body is a laminated body obtained by stacking a plurality of layers including the solid electrolyte layer in a lamination direction orthogonal to the longitudinal direction, wherein the laminated body has a first surface and a second surface which are opposite end surfaces in the lamination direction; and a third surface and a fourth surface which are opposite end surfaces in a direction orthogonal to the longitudinal direction and to the lamination direction, wherein the first to fourth surfaces are each the circumferential surface, wherein the inner conductive portion comprises an inner lead led to the third surface or the fourth surface, and wherein the cover portion is a circumferential surface conduit disposed on the third surface or the fourth surface and covering the inner conduit. [9] Sensor element according to one of claims 1 to 7, wherein the element body is a laminated body obtained by stacking a plurality of layers including the solid electrolyte layer in a lamination direction orthogonal to the longitudinal direction, wherein the laminated body has a first surface and a second surface which are opposite end surfaces in the lamination direction and are each the peripheral surface, wherein the element body has a through-hole with an opening provided in a part of the peripheral surface located near the rear end, the peripheral surface being the first surface or the second surface, the through-hole extending through one or more of the plurality of layers in the lamination direction, wherein the inner conductive portion comprises a through-hole conductor disposed in the through-hole, and wherein the cover portion is the connector electrode and covers the through-hole conductor. [10] Sensor element according to one of claims 1, 2 and 5 to 7, wherein the element body is a laminated body obtained by stacking a plurality of layers including the solid electrolyte layer in a lamination direction orthogonal to the longitudinal direction, wherein the laminated body has a first surface and a second surface which are opposite end surfaces in the lamination direction and are each the peripheral surface, wherein the element body has a through-hole with an opening provided in a part of the peripheral surface located near the rear end, the peripheral surface being the first surface or the second surface, the through-hole extending through one or more of the plurality of layers in the lamination direction, and wherein the cover portion includes a part of a through-hole conductor exposed to the outside at the opening, the through-hole conductor being arranged in the through-hole, including the part exposed to the outside, and connected to the connector electrode. [11] Sensor element according to one of claims 1 to 10, which is a sensor element for insertion into a gas sensor, the gas sensor comprising: the sensor element; a housing having a cylindrical shape extending in the longitudinal direction of the sensor element, the housing having a second front end and a second rear end which are opposite ends in the longitudinal direction, the sensor element being arranged within the housing; and a sealing element that seals the second rear end of the housing. [12] Gas sensor, comprising: the sensor element according to one of claims 1 to 10; a housing having a cylindrical shape extending in the longitudinal direction of the sensor element, the housing having a second front end and a second rear end which are opposite ends in the longitudinal direction, the sensor element being arranged within the housing; and a sealing element that seals the second rear end of the housing.

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