SENSOR ELEMENT AND GASS SENSOR

Abstract

A sensor element 101 comprises an element body (layers 1 to 6), a reference electrode 42, an outer pump electrode 23, a conductive section 77 having a connector electrode 71 and a reference electrode lead 78, and a conductor insulating layer 79. An inner lead 78b of the reference electrode lead 78 is designed to supply a reference gas, which serves as a reference for detecting the concentration of a specific gas in a gas of the object being measured, to the reference electrode 42. A region of 50% or more, extending from one end on the side of the reference electrode 42, of the total area of ​​the conductor insulating layer 79 in a top view, is configured as a high-density region with a porosity of 5% or less.

Description

Title of the invention: SENSOR ELEMENT AND GAS SENSORTechnical 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 present field is designed to detect a concentration of a specific gas, such as NOx, in a measurement-target gas such as exhaust gas from an automobile. PTL 1 discloses a gas sensor including a sensor element comprising: an element body having an oxygen ion-conductive solid electrolyte layer and a measurement-target gas flow portion located inside the element body and introducing and flowing the measurement-target gas; a reference electrode disposed inside the element body; and a porous reference gas introduction layer that introduces the reference gas used as a reference for detecting the concentration of a specific gas in the measurement-target gas and flows the reference gas to the reference electrode. Document listPatent documents

[0003] PTL 1: JP 2018-173320 A Summary of the inventionTechnical problem

[0004] In the sensor element described above in PTL 1, the porous reference gas introduction layer may adsorb external water (water vapor) during the non-operational period of the sensor element. In such cases, after the sensor element starts operating, the water gradually leaks from the element body. However, until the water is sufficiently leaked, the oxygen concentration around the reference electrode may decrease. If the time required for water leakage (water discharge time) is long, the time from the start of operation of the sensor element until the potential of the reference electrode stabilizes (stabilization time) tends to be longer.

[0005] The main object of the sensor element and the gas sensor according to the present invention is to shorten the stabilization time. Solution to the problem

[0006] To achieve the above 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 having an oxygen ion conductive solid electrolyte layer and a measurement object gas flow portion disposed inside the element body, through which the measurement object gas is supplied and in which the measurement object gas can flow; a reference electrode disposed inside the element body; a measurement object gas side electrode disposed on a portion of the element body exposed to the measurement object gas; a first conductive portion including a reference electrode terminal disposed on an outer side of the element body, and a reference electrode line electrically continuous between the reference electrode terminal and the reference electrode;and a lead insulating layer covering a portion of the reference electrode lead located within the element body, wherein the reference electrode lead is configured to supply a reference gas serving as a reference for determining the concentration of a specific gas in the measurement object gas to the reference electrode, and wherein a region of 50% or more, from an end on the reference electrode side, of the total area of ​​the lead insulating layer in a plan view is formed as a high-density region having a porosity of 5% or less.

[0008] In the sensor element according to the present invention, the reference electrode lead is configured to supply the reference gas, which serves as the reference for detecting the concentration of a specific gas in the measurement target gas, to the reference electrode. The area of ​​50% or more of the total area of ​​the lead insulating layer in plan view, from one end on the reference electrode side, is formed as the high-density region with the porosity of 5% or less. Consequently, since the area of ​​50% or more of the total area of ​​the lead insulating layer in plan view is formed as the high-density region, the amount of water (water vapor) adsorbed by the lead insulating layer during the non-operational period of the sensor element is reduced compared to a case where less than 50% of the total area is formed as the high-density region.In addition, since the region including at least the end on the reference electrode side of the lead insulating layer is formed as the high-density region, even if the lead insulating layer adsorbs water during the non-operational period of the sensor element, the water more easily leaks from the element body after the sensor element starts operating, compared to a case where a porous region with a porosity of more than 5% is provided on the reference electrode side relative to the high-density region in the lead insulating layer. Consequently, the time from the start of operation of the sensor element until water leaks from the element body (water discharge time) is shortened, which in turn shortens the time from the start of operation of the sensor element until the potential of the reference electrode stabilizes (stabilization time).The inventors have confirmed this through experiments and other methods.

[0009] [2] In the above sensor element (the sensor element according to [1] above), the area of ​​80% or more of the total area in plan view from the end on the reference electrode side can be formed as the high-density region. Such a structure further reduces the amount of water (water vapor) adsorbed by the lead insulation layer during the non-operational period of the sensor element.

[0010] [3] In the above sensor element (the sensor element according to [1] or [2] above), the limiting current when oxygen is pumped from the vicinity of the reference electrode to the vicinity of the measured object gas-side electrode can be 0.3 µA or more. The limiting current has a positive correlation with the inverse of the diffusion resistance of the reference electrode line. A higher limiting current indicates a lower diffusion resistance of the reference electrode line. Therefore, a limiting current of 0.3 µA or more indicates that the diffusion resistance of the reference electrode line is at or below a predetermined value (that is, not excessively high).Such a structure makes it easier to maintain the oxygen concentration around the reference electrode at a suitable level compared to a case where the diffusion resistance of the reference electrode lead is excessively high, thereby mitigating the deterioration in the detection accuracy of the concentration of a specific gas. The inventors have confirmed this through experiments and other methods. In this case, the limiting current can also be 10 µA or less.

[0011] [4] In the above sensor element (the sensor element according to [3] above), the limiting current can be 0.5 µA or more. Such a structure further reduces the deterioration of the detection accuracy of the concentration of a specific gas.

[0012] [5] In the above sensor element (the sensor element according to any one of the above [1] to [4]), the porosity of a portion of the

[0013] Reference electrode lead porosity within the element body is greater than 5% and 40% or less. Because the porosity is greater than 5%, the reference gas is more likely to flow to the reference electrode. Additionally, because the porosity is 40% or less, the portion of the reference electrode lead located within the element body is less likely to be severed.

[0014] [6] In the above sensor element (the sensor element according to any one of [1] to [5] above), a cross-sectional area and / or porosity of a portion of the reference electrode lead located inside the element body may be larger on the reference electrode side than on the reference electrode terminal side. Such a structure increases the probability of oxygen accumulating around the reference electrode, thereby facilitating the maintenance of the oxygen concentration around the reference electrode at the appropriate level.

[0015] [7] In the above sensor element (the sensor element according to any one of the above [1] to [6]), the sensor element may further comprise: a heater disposed inside the element body and heating the element body; a second conductive portion including a heater terminal disposed on the outside of the element body and a heater lead electrically continuous between the heater terminal and the heater; and a heater insulating layer covering both the heater and a portion of the heater lead located inside the element body. The lead insulating layer and the heater insulating layer may be provided independently so as not to be connected to each other.

[0016] [8] In the above sensor element (the sensor element according to any one of the above [1] to [6]), the sensor element may further comprise: a heater disposed inside the element body and heating the element body; a second conductive portion including a heater terminal disposed on the outside of the element body and a heater lead electrically continuous between the heater terminal and the heater; and a heater insulating layer covering both the heater and a portion of the heater lead located inside the element body. The element body may include a connection hole connecting the heater insulating layer and the lead insulating layer, and a porosity of the heater insulating layer may be 5% or less.Such a structure reduces the amount of water (water vapor) adsorbed by the heater insulating layer via the lead insulating layer and the connection hole during the non-operational period of the sensor element.

[0017] [9] A gas sensor according to the present invention comprises the sensor element according to any one of the above [1] to [7]. Therefore, the gas sensor provides the same advantageous effects as those achieved by the sensor element according to the present invention, such as an advantageous effect of shortening the stabilization time. Short description of the drawings Fig. 1 is a vertical cross-sectional view of a gas sensor 100. Fig. 2 is a schematic cross-sectional view 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 partially enlarged cross-sectional view of an area around a reference electrode 42 and others. Fig. Figure 5 is a cross-sectional view along AA in the Fig. 4. Fig. 6 is a perspective view of a rear end of an element body 102. Fig. 7 is a cross-sectional view of a part of a sensor element 201 according to a modification. Fig. 8 is a cross-sectional view of a part of a sensor element 301 according to another modification. Fig. 9 is a cross-sectional view of a part of a sensor element 401 according to another modification. Fig. 10 is a cross-sectional view of a part of a sensor element 501 according to another modification. Fig. 11 is a cross-sectional view of a part of a sensor element 601 according to another modification. Fig. 12 is a cross-sectional view along BB in the Fig. 11. Fig. 13 is a schematic cross-sectional view of a sensor element 701 according to another modification. Description of embodiments

[0018] Embodiments of the present invention will be described below with reference to the drawings. Fig. 1 is a vertical cross-sectional view of a gas sensor 100 according to an embodiment of the present invention. Fig. Fig. 2 is a schematic cross-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, including cells. Fig. Figure 4 is a partially enlarged cross-sectional view of an area around a reference electrode 42, a reference electrode lead 78, and a lead 76 of the sensor element 101. The Fig. Figure 5 is a cross-sectional view along AA in the Fig. 4. The Fig. Fig. 6 is a perspective view of a rear end of an element body 102. The sensor element 101 has an elongated rectangular parallelepiped shape. The longitudinal direction of the sensor element 101 (the left-right direction in the Fig. 2) is defined as the front-back direction. The thickness direction of the sensor element 101 (the top-bottom direction in the Fig.2) is defined as the up-down direction. The width direction of the sensor element 101 (the direction perpendicular to the front-back direction and the up-down direction) is defined as the left-right direction.

[0019] As it is in the Fig.1, the gas sensor 100 includes the sensor element 101, a protective cover 130 that protects a part of the sensor element 101 located at the front end, and a sensor assembly 140 that includes a connector 150 that is 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 automobile. The gas sensor 100 is used to measure the concentration of a specific gas, such as NOx or O2, contained in an exhaust gas as a measurement object gas. In the present embodiment, the gas sensor 100 is configured to measure the NOx concentration as the concentration of a specific gas.

[0020] The protective cover 130 includes a bottomed cylindrical inner protective cover 131 covering the front end of the sensor element 101, 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 for allowing 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 of the sensor element 101 is disposed in the sensor element chamber 133.

[0021] The sensor assembly 140 includes an element sealing unit 141 that seals and attaches 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 the connector electrodes 71 arranged on surfaces (upper and lower surfaces) of the sensor element 101 at a rear end.

[0022] The element sealing unit 141 includes a cylindrical main metal mold 142 and a cylindrical inner cylinder 143 coaxially welded to the main metal mold 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 mold 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 pressing bodies 145a and 145b between the sensor element 101 and the combination of the main metal molding 142 and the inner cylinder 143. Consequently, the pressing bodies 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 fix the sensor element 101.

[0023] The screw 147 is coaxially attached to the main metal molding 142 and has an external threaded portion around its outer peripheral surface. The external threaded portion of the screw 147 is screwed into a mounting member 191 welded to the conduit 190 and having an internal threaded portion around its inner peripheral surface. Thus, the gas sensor 100 is attached to the conduit 190, with a portion of the gas sensor 100, including the front end of the sensor element 101 and the protective cover 130, projecting into the conduit 190.

[0024] 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 extend 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. The gap between the outer cylinder 148 and the lead wires 155 is sealed by a rubber plug 157. The space 149 in the outer cylinder 148 is filled with a reference gas. The rear end of the sensor element 101 is disposed in the space 149.

[0025] As it is in the Fig.As shown in Figure 2, the sensor element 101 is an element comprising a laminated body obtained by stacking six layers in the following order from the bottom side 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 composed of zirconium oxide (ZrO2) or the like. The solid electrolyte formed from each of these six layers has a high density and is hermetic.The sensor element 101 is manufactured, for example, by performing predetermined processing and printing circuit patterns on ceramic green sheets to become the individual layers, then stacking the layers, and then combining the layers by performing calcination.

[0026] At one end (the left side in the Fig. 2) of the sensor element 101 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 inner cavity 20, a third diffusion adjusting portion 30, a second inner cavity 40, a fourth diffusion adjusting portion 60, and a third inner cavity 61 are provided, which are adjacent to each other and connected to each other in this order.

[0027] 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 an upper side defined by the lower surface of the second solid electrolyte layer 6, a lower side defined by the upper surface of the first solid electrolyte layer 4, and side sides defined by the side surfaces of the spacer layer 5.

[0028] 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 elongated 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 elongated in the direction perpendicular to the plane of the drawing) that provides a gap with respect to the bottom 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.

[0029] The reference electrode 42 is arranged between the upper surface of the third substrate layer 3 and the lower surface of the first solid electrolyte layer 4. This reference electrode 42 is arranged directly on the upper surface of the third substrate layer 3, and the upper, front, rear, left, and right surfaces of the reference electrode 42 are covered by the reference electrode insulating layer 43. In addition, the reference electrode 42 is formed as a porous cermet electrode (for example, a cermet electrode made of Pt and ZrO2). As described below, the reference electrode 42 receives a reference gas used as a reference for detecting the NOx concentration, which is supplied from outside the sensor element 101 (here, the space 149 in the Fig. 1) flows through the reference electrode line 78 (see the Fig. 4 and Fig.5) connected to the reference electrode 42. The reference gas is a gas having a predetermined oxygen concentration, and in the present embodiment, it is either air or a gas having the same oxygen concentration as air (for example, a gas containing oxygen with nitrogen as the base gas). Furthermore, as described below, the reference electrode 42 is used to measure the oxygen concentration (oxygen partial pressure) in the first internal cavity 20, the second internal cavity 40, and the third internal cavity 61. The reference electrode insulating layer 43 is provided to maintain electrical insulation between the first solid electrolyte layer 4 and the reference electrode 42. Details regarding the reference electrode insulating layer 43 will be described below.

[0030] In the measurement object gas flow section, the gas inlet 10 is open to an outside space so that the measurement object gas can be 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 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 rapidly introduced 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 of an automobile) 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 flowing through the first diffusion adjusting portion 11, the buffer space 12, and the second diffusion adjusting portion 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 therein via the second diffusion adjusting section 13. The oxygen partial pressure is adjusted by operating a main pump cell 21.

[0031] The main pumping cell 21 is an electrochemical pumping cell consisting of an inner pumping electrode 22 comprising 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 internal 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 face the outside space (the sensor element chamber 133 in the Fig.1) is exposed; and the second solid electrolyte layer 6, which is arranged between the electrodes 22 and 23.

[0032] 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 directly 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 the respective side walls (inner surfaces) of the spacer layer 5, which form opposite side walls 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.

[0033] The inner pumping electrode 22 and the outer pumping electrode 23 are each formed as a porous cermet electrode (for example, a cermet electrode made of Pt and ZrO2 with an Au content of 1 percent). The inner pumping electrode 22, which contacts a measurement target gas, is formed using a material whose reducing ability for NOx components in the measurement target gas is reduced.

[0034] In the main pumping cell 21, a desired pumping 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.

[0035] 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 detecting sensor cell for the main pumping controller 80.

[0036] The oxygen concentration (oxygen partial pressure) in the first internal cavity 20 is determined by measuring an electromotive force (voltage V0) in the oxygen partial pressure detection sensor cell for the main pump controller 80. Furthermore, the pump voltage Vp0 of a variable current source 25 is controlled such that the voltage V0 reaches a fixed value (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.

[0037] 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 means of the operation of the main pump cell 21, and then guiding the measurement object gas to the second inner cavity 40.

[0038] 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 having 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 set level with high accuracy, whereby the gas sensor 100 can achieve highly accurate NOx concentration measurement.

[0039] 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 an outer side of the sensor element 101, if appropriate); and the second solid electrolyte layer 6.

[0040] The auxiliary pumping electrode 51 is arranged in the second internal cavity 40 and has a tunnel-like structure corresponding to the internal pumping electrode 22 arranged 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 upper surface of the first solid electrolyte layer 4, which provides the lower surface for the second internal cavity 40, is directly 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.It should be noted that the auxiliary pumping electrode 51 is also formed using a material whose reducing ability for the NOx components in the measurement object gas is reduced, as is the case with the inner pumping electrode 22.

[0041] In the auxiliary pumping cell 50, a desired voltage Vp1 is applied between the auxiliary pumping electrode 51 and the outer pumping electrode 23 so 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.

[0042] Further, for adjusting 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.

[0043] A variable current source 52, whose voltage is adjusted 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 adjusted to a low partial pressure that has substantially no effect on NOx measurement.

[0044] Furthermore, a pumping current Ip1 generated by the auxiliary pumping cell 50 is used in adjusting 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 adjusted, thereby setting the gradient of the oxygen partial pressure in the measurement target gas to be introduced into the second internal cavity 40 from the third diffusion adjusting section 30 to be 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 approximately 0.001 ppm by the cooperation of the main pumping cell 21 and the auxiliary pumping cell 50.

[0045] The fourth diffusion adjustment 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 adjustment section 60 has a role of restricting the amount of NOx flowing into the third internal cavity 61.

[0046] The third internal cavity 61 is provided as a space in which the process of measuring the concentration of nitrogen oxide (NOx) in the measurement object gas is performed using the measurement object gas whose oxygen concentration (oxygen partial pressure) has been adjusted in advance in the second internal cavity 40 and introduced into the third internal cavity 61 via the fourth diffusion adjustment section 60. The NOx concentration is predominantly measured in the third internal cavity 61 by operating a measuring pump cell 41.

[0047] The measuring pump cell 41 is configured to measure the NOx concentration in the measurement target gas in the third internal cavity 61. The measuring pump cell 41 is an electrochemical pump cell composed of the measuring electrode 44, which is disposed directly on the upper surface of the first solid electrolyte layer 4 in a region facing the third internal cavity 61; and the outer pump electrode 23, the second solid electrolyte layer 6, the spacer layer 5, and the first solid electrolyte layer 4. The measuring electrode 44 is a porous cermet electrode (for example, a cermet electrode made of Pt and ZrO2) formed using a material with a higher reducing ability for NOx components in the measurement target gas than that of the inner pump electrode 22. The measuring electrode 44 also acts as a NOx reduction catalyst that reduces the NOx present in the atmosphere in the third inner cavity 61.

[0048] In the measuring pump cell 41, oxygen resulting from the decomposition of nitrogen oxide in the atmosphere around the measuring electrode 44 is pumped out, and the amount of oxygen thus generated is recorded as pump current Ip2.

[0049] Further, for detecting 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 detecting 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 detecting sensor cell for measuring pump control 82.

[0050] The measurement object gas introduced into the second internal cavity 40 reaches the measuring electrode 44 in the third internal cavity 61 via the fourth diffusion adjustment section 60 under a condition where the oxygen partial pressure is adjusted. The nitrogen oxide in the measurement object gas surrounding the measuring electrode 44 is reduced (2NO → N2 + O2), so that oxygen is generated. Then, the thus generated oxygen is subjected to pumping by the measuring pumping cell 41. During oxygen pumping, 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 set value (target value).Since the amount of oxygen generated around the measuring electrode 44 is proportional to the concentration of nitrogen oxide in the measuring object gas, the nitrogen oxide concentration in the measuring object gas is calculated using the pumping current Ip2 of the measuring pumping cell 41.

[0051] 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 sensor element 101 can be detected.

[0052] 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 reference gas adjusting pumping cell 90. The reference gas adjusting pumping cell 90 performs oxygen pumping by allowing the pumping current Ip3 to flow due to the control voltage Vp3 applied by the power source circuit 92 connected between the outer pumping electrode 23 and the reference electrode 42. Consequently, the reference gas adjusting pumping cell 90 can either pump the oxygen from the space surrounding the outer pumping electrode 23 (the sensor element chamber 133 in the Fig. 1), to the surroundings of the reference electrode 42, or pump the oxygen from the surroundings of the reference electrode 42 to the space surrounding the outer pumping electrode 23.

[0053] In the gas sensor 100 having the above structure, the measuring pumping cell 41 receives the measurement target 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 target gas can be determined based on the pumping current Ip2 generated as a result of the oxygen generated by NOx reduction being pumped out by the measuring pumping cell 41 in substantially proportional proportion to the NOx concentration in the measurement target gas.

[0054] To further increase the oxygen ion conductivity of the solid electrolyte, the sensor element 101 includes the heater section 70, which serves to adjust the temperature to keep the sensor element 101 warm by heating it. The heater section 70 includes a heater 72, a heater insulation layer 74, and a lead 76.

[0055] The heater 72 is an electrical resistor arranged between the second substrate layer 2 and the third substrate layer 3 from above and below. The heater 72 generates heat when electricity is supplied to it from the outside, thereby heating the solid electrolyte constituting the sensor element 101 and maintaining its temperature. Furthermore, the heater 72 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.

[0056] The heater insulating layer 74 is provided to maintain electrical insulation between the second substrate layer 2 and both the heater 72 and the lead 76, and electrical insulation between the third substrate layer 3 and both the heater 72 and the lead 76. Details regarding the heater insulating layer 74 will be described below.

[0057] The lead 76 is formed in a manner similar to the heater 72 and is arranged between the second substrate layer 2 and the third substrate layer 3 from above and below. The lead 76 is arranged in a shape extending from the rear end of the heater 72 to the rear side and is located slightly forward of the rear end of the sensor element 101. The rear end portion of the lead 76 is connected to the connector electrode 71 via a through hole 73.

[0058] As it is in the Fig.3, the control device 95 includes the above-described variable current sources 25, 46, and 52, the above-described current source circuit 92, and a controller 96. The controller 96 is a microprocessor including a CPU 97, a RAM (not shown), a storage unit 98, etc. The storage unit 98 is a non-volatile memory such as a ROM that serves as a device for storing various types of data. The controller 96 is configured to obtain the voltage V0 to V2 and Vref from each of the sensor cells 80 to 83. The controller 96 is also configured to obtain the pumping current Ip0 to Ip2 and Ip3 flowing through each of the pumping cells 21, 50, 41, and 90.The controller 96 is configured to output control signals to the variable current sources 25, 46, and 52 and the current source circuit 92, and thus control the voltages Vp0 to Vp3 to be output by the variable current sources 25, 46, and 52 and the current source circuit 92, thereby controlling each of the pump cells 21, 41, 50, and 90. The controller 96 is configured to output a control signal to the heater power source (not shown) and thus control the electric current to be supplied from the power source to the heater 72, thereby controlling the temperature of the sensor element 101. The storage unit 98 stores target values ​​V0*, V1*, V2*, Ip1*, and the like, which will be described below.

[0059] The controller 96 performs feedback control with the pump voltage Vp0 of the variable current source 25 such that the voltage V0 reaches the target value V0* (ie, such that the oxygen concentration in the first inner cavity 20 reaches a target concentration).

[0060] The controller 96 performs feedback control with the voltage Vp1 of the variable current source 52 such that the voltage V1 reaches a predetermined value (referred to as "target value V1*") (i.e., such that the oxygen concentration in the second internal cavity 40 reaches a predetermined low oxygen concentration that has substantially no effect on NOx measurement). In connection with this, the controller 96 sets (i.e., performs feedback control) the target value V0* for the voltage V0 based on the pumping current Ip1 such that the pumping current Ip1 flowing at the voltage Vp1 reaches a predetermined value (referred to as "target value Ip1*"). Consequently, the gradient of the oxygen partial pressure in the measurement object gas to be introduced from the third diffusion adjusting section 30 to the second internal cavity 40 is set to be constant.Additionally, the oxygen partial pressure in the atmosphere of the second inner cavity 40 is set to a low level that has no significant effect on the NOx measurement. The target value V0* is set to a value at which the oxygen concentration in the first inner cavity 20 is higher than 0% and is a low concentration.

[0061] The controller 96 performs feedback control with the voltage Vp2 of the variable current source 46 such that the voltage V2 reaches a set value (referred to as the "target value V2*") (i.e., the oxygen concentration in the third internal cavity 61 reaches a predetermined low concentration). Consequently, oxygen is pumped out of the third internal cavity 61 such that the oxygen generated by the reduction of the specific gas (in this case, NOx) in the third internal cavity 61 is effectively reduced to almost zero. Then, the controller 96 detects the pumping current Ip2 as a value corresponding to the oxygen generated from NOx in the third internal cavity 61 and calculates the NOx concentration in the measurement object gas based on the pumping current Ip2. The target value V2* is set to be a value such that the pumping current Ip2 flowing at the controlled voltage Vp2 reaches a limit current.The storage unit 98 stores, for example, a relationship expression (e.g., a linear 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. The controller 96 detects the NOx concentration in the measurement target gas based on the detected pumping current Ip2 and the correspondence relationship stored in the storage unit 98.

[0062] The controller 96 controls the power source circuit 92 to apply the voltage Vp3 to the reference gas adjustment pumping cell 90, thereby allowing the pumping current Ip3 to flow. Consequently, the controller 96 can pump oxygen from the vicinity of the outer pumping electrode 23 to the vicinity of the reference electrode 42 and pump the oxygen out from the vicinity of the reference electrode 42 to the vicinity of the outer pumping electrode 23. In the present embodiment, the voltage Vp3 is set to a DC voltage such that the pumping current Ip3 reaches a predetermined value (a constant DC current). Consequently, the reference gas adjustment pumping cell 90 pumps a constant amount of oxygen from the vicinity of the outer pumping electrode 23 to the vicinity of the reference electrode 42 with the flow of the pumping current Ip3.

[0063] The connector electrodes 71 are each arranged at a portion of the rear end of the sensor element 101. As shown in the Fig. As shown in Figure 6, the connector electrodes 71 include connector electrodes 71a to 71d located at the rear end of the lower surface of the sensor element 101, and connector electrodes 71e to 71h located at the rear end of the upper surface of the sensor element 101. The connector electrodes 71 each function as a terminal that allows the sensor element 101 and the outside to be electrically continuous with each other. The connector electrodes 71e to 71f are connected one-to-one to the inner pumping electrode 22, the outer pumping electrode 23, the measuring electrode 44, and the auxiliary pumping electrode 51 of the sensor element 101. The connector electrode 71a is connected to the reference electrode 42 via the reference electrode line 78, which will be described below. The connector electrode 71b is connected to the heater 72 via the through hole 73 and the lead 76 (see the Fig.2). The connector electrodes 71c and 71d are also connected to the heater 72 through through holes and leads not shown. The variable current sources 25, 46 and 52 and the current source circuit 92 shown in the Fig. 2 are in practice connected to each electrode (the inner pumping electrode 22, the outer pumping electrode 23, the reference electrode 42, the measuring electrode 44 and the auxiliary pumping electrode 51) via the connector electrodes 71 and the connector 150 and the lead 155 shown in the Fig. 1. The power supply for the heater section 70 from the external power source is also conducted via the connector electrodes 71 and the connector 150 and the line 155.

[0064] The structure around the reference electrode 42, the reference electrode line 78 and the line 76 is connected to the Fig.2, 4 to 6. The reference electrode 42 is connected to a conductive portion 77. The conductive portion 77 includes the above-described connector electrode 71a and the reference electrode line 78, which is electrically continuous between the connector electrode 71a and the reference electrode 42. The reference electrode line 78 includes a peripheral surface line 78a and an inner line 78b.

[0065] The peripheral surface lead 78a is arranged on the right surface of the rear end portion of the sensor element 101 and connects the connector electrode 71a and the inner lead 78b (conductive). The inner lead 78b is arranged inside the sensor element 101 and connects the reference electrode 42 and the peripheral surface lead 78a. Specifically, the inner lead 78b is arranged between the third substrate layer 3 and the first solid electrolyte layer 4, extends rearward from the reference electrode 42, is bent to the right at the rear end portion of the sensor element 101, and continues to the right surface of the rear end portion thereof (see FIG. Fig. 5 and Fig. 6). The end portion of the inner lead 78b opposite the reference electrode 42 is connected to the peripheral surface lead 78a.

[0066] The conductive portion 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 conductive portion 77 is preferably a cermet conductor containing a noble metal or a high-melting-point metal and zirconium oxide, which is identical to the main component of the third substrate layer 3. The reference electrode lead 78 (peripheral lead 78a and inner lead 78b) has pores with a porosity of more than 5%. Consequently, the reference electrode lead 78, as described above, allows the reference gas from outside the sensor element 101 (the space 149 formed in the Fig.1) to flow to the reference electrode 42. The porosity of the reference electrode line 78 is preferably 10% or more. In addition, the porosity of the reference electrode line 78, particularly the inner line 78b, is preferably 40% or less. Further, both the peripheral surface line 78a and the inner line 78b should be porous, and the porosity of the peripheral surface line 78a and the porosity of the inner line 78b may be the same or different. The reference electrode line 78 is designed such that the limiting current when oxygen is pumped from the vicinity of the reference electrode 42 to the vicinity of the outer pumping electrode 23 is 0.3 µA or more. The limiting current is preferably 0.5 µA or more. The limiting current has a positive correlation with the inverse of the diffusion resistance of the reference electrode line 78, particularly the inner line 78b.A higher limiting current indicates a lower diffusion resistance of the reference electrode line 78. Therefore, a limiting current of 0.3 µA or more indicates that the diffusion resistance of the reference electrode line 78 is at or below a predetermined value (i.e., not excessively high). The diffusion resistance of the inner line 78b may be 3060 × 10. 3 cm -1 or less and is preferably 2000 × 10 3 cm -1 or less. The diffusion resistance of the inner line 78b is calculated by dividing the length in the direction of extension of the inner line 78b by the product of its width, thickness, and porosity. The limiting current may also be 10 µA or less, indicating that the diffusion resistance of the inner line 78b is above a second predetermined value that is smaller than the first predetermined value (i.e., not excessively low).

[0067] The upper, front, rear, left, and right surfaces of the reference electrode 42 are covered by the reference electrode insulating layer 43, as described above. In addition, the outer periphery of the entirety of the inner lead 78b, particularly a portion of the inner lead 78b located inside the sensor element 101 (the portion located between the third substrate layer 3 and the first solid electrolyte layer 4), is covered by the lead insulating layer 79. The lead insulating layer 79 is connected to the rear end portion of the reference electrode insulating layer 43 and is integrally formed with the reference electrode insulating layer 43. Thus, the reference electrode 42 is electrically insulated from the first solid electrolyte layer 4, and the inner lead 78b is electrically insulated from both the third substrate layer 3 and the first solid electrolyte layer 4.The reference electrode insulating layer 43 and the lead insulating layer 79 are insulating layers formed from a ceramic such as alumina. The entirety of the reference electrode insulating layer 43 and the lead insulating layer 79 has a high density with a porosity of 5% or less, that is, they are formed as a high-density region. Such a structure reduces the likelihood of the reference gas reaching the reference electrode 42 from outside the sensor element 101 via the lead insulating layer 79.

[0068] The heater portion 70 described above includes the heater 72, the heater insulating layer 74, and the lead 76. The heater insulating layer 74 covers the vicinity of the heater 72 and the vicinity of the lead 76, excluding the connecting portion between the lead 76 and the through-hole 73. Thus, the heater 72 and the lead 76 are electrically insulated from the second substrate layer 2 and the third substrate layer 3. The heater insulating layer 74 is an insulating layer formed of a ceramic such as alumina. The heater insulating layer 74 may be formed as a high-density layer with a porosity of 5% or less or as a porous layer with a porosity of more than 5%.

[0069] The porosity of the above-described parts, such as the reference electrode lead 78, the lead insulating layer 79, and the heater insulating layer 74, is derived as a value as follows using an image (SEM image) obtained in an examination by a scanning electron microscope (SEM). First, a measurement object is cut to have a cross-section. This cross-section, regarded as an examination surface, is embedded with a resin and is polished, thereby obtaining an examination sample. Then, the examination surface of the examination sample is photographed by SEM photography (using a secondary ion image, an acceleration voltage of 15 kV, and 1000x magnification, but a magnification of more than 1000x and less than or equal to 5000x when 1000x magnification is unsuitable), thereby obtaining an SEM image of the measurement object.Subsequently, the obtained image is analyzed to determine a threshold value from a brightness distribution of brightness data of pixels in the image using the discriminant analysis method (Otsu binarization method). Then, each pixel in the image is binarized into a subject portion and a pore portion based on the determined threshold value, and the area of ​​the subject portion and the area of ​​the pore portion are calculated. Furthermore, the percentage of the pore portion area relative to the total area (i.e., the total area of ​​the subject portion and the pore portion) is determined as the porosity (in %).

[0070] The limit current when oxygen is pumped from the vicinity of the reference electrode 42 to the vicinity of the outer pumping electrode 23 is set as a value measured as follows. First, the sensor element 101 is placed in the air, and the heater 72 is energized so that the sensor element 101 is heated to a predetermined operating temperature (for example, 800°C). The variable current sources 25, 46, and 52 are all maintained in a state where no voltage is applied. In this state, the voltage Vp3 is applied between the outer pumping electrode 23 and the reference electrode 42 by the power source circuit 92 so that oxygen is pumped from the vicinity of the reference electrode 42 to the vicinity of the outer pumping electrode 23. During this operation, the pumping current Ip3 flowing between the two electrodes 23 and 42 is measured. The voltage Vp3 is set to a DC voltage.As the voltage Vp3 gradually increases, the pumping current Ip3 also gradually increases. However, when the voltage Vp3 exceeds a certain level, further increasing the voltage Vp3 does not increase the pumping current Ip3, indicating that the pumping current Ip3 has reached its upper limit. This upper limit of the pumping current Ip3 is measured as the limiting current.

[0071] An exemplary method for manufacturing 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, are prepared as a ceramic component. 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. Furthermore, 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.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 patterns to be formed include, for example, structures serving as electrodes such as the above-described reference electrode 42, leads connected to the electrodes such as the inner lead 78a, the lead insulating layer 79, the connector electrodes 71, and the heater portion 70. The pattern printing process, in which a pattern forming paste prepared according to the properties for an object to be formed is applied to a green sheet using a known screen printing technique, is performed.The pattern for the lead insulating layer 79 can be formed by separating the portion covering the bottom surface of the inner lead 78a from the portion forming the top surface. Similarly, the pattern for the heater insulating layer 74 of the heater section 70 can be formed by separating the portion covering the bottom surface of the heater 72 and the lead 76 from the portion covering the top surface. After performing the pattern printing process, a drying process is performed using a known drying technique. When the pattern printing process and the drying process are completed, a printing process is performed for a bonding paste used to stack and bond the green sheets corresponding to the respective layers.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 that will become the circumferential surface lead 78a is formed by screen printing on a portion obtained by cutting the laminated body, that is, on a part serving as the right surface of the sensor element 101. Further, the pattern is dried.Then, the cut laminated body is calcined at a predetermined calcination temperature, thereby obtaining a sensor element 101. Furthermore, the pattern-forming paste for forming the conductive portion 77 (here, the connector electrode 71a and the reference electrode lead 78), particularly for the inner lead 78b, is a paste containing particles of a noble metal or a high-melting-point metal, as described above. The pattern-forming paste for forming the lead insulating layer 79 and the heater insulating layer 74 is a paste containing particles of a ceramic, such as alumina, as described above.

[0072] Here, the limiting current when oxygen is pumped from the vicinity of the reference electrode 42 to the vicinity of the outer pumping electrode 23, as well as the diffusion resistance of the inner conduit 78b, can be adjusted by adjusting the shape and porosity of the inner conduit 78b. This is because, as described above, the diffusion resistance of the inner conduit 78b is based on the length in the extension direction, the width, the thickness, and the porosity. The porosity of the inner conduit 78b can be adjusted, for example, by adjusting the particle diameters of particles contained in the structure-forming paste for forming the inner conduit 78b, adjusting the particle diameter and content ratio of the pore-forming material, or by adjusting the calcination temperature and calcination time when the laminated body is calcined.The same considerations can be applied to adjusting the porosity of the lead insulating layer 79 and the heater insulating layer 74.

[0073] 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. Furthermore, a connector 150 and lead wires 155 are attached to the rear end of the sensor element 101 so as to be electrically continuous with the connector electrodes 71. Additionally, a protective cover 130 is attached to a portion of the element sealing unit 141 near the front end of the sensor element 101. Furthermore, an outer cylinder 148 is attached to a portion of the element sealing unit 141 near the rear end of the sensor element 101, and the lead wires 155 are led out from the outer cylinder 148. Then, a control device 95 and the sensor element 101 are connected to each other via the lead wires 155. In this way, the gas sensor 100 is obtained.

[0074] The following describes the processing performed by the controller 96 when the gas sensor 100 detects the NOx concentration in the measurement target gas. First, the CPU 97 of the controller 96 starts operating the sensor element 101. Specifically, the CPU 97 sends a control signal to the heater power source 78 to heat the sensor element 101 by the heater 72. During this operation, the CPU 97 heats the sensor element 101 to a predetermined operating temperature (for example, 800°C). Next, the CPU 97 starts controlling the above-described pump cells 21, 41, 50, and 90 and detecting the voltages V0, V1, V2, and Vref from the above-described sensor cells 80 to 83.Under this condition, when the measurement object gas is introduced through the gas inlet 10, the measurement object gas passes 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. Next, the oxygen concentration of the measurement object gas is adjusted by the main pumping cell 21 and the auxiliary pumping cell 50 in the first inner cavity 20 and the second inner cavity 40. The adjusted measurement object gas then reaches the third inner cavity 61. 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.

[0075] In this case, the measuring object gas is introduced into the measuring object gas flow section, such as through the gas inlet 10, of the sensor element 101 from the sensor element chamber 133, which is in the Fig. 1. On the other hand, since the reference electrode line 78 (the peripheral line 78a and the inner line 78b) is porous with a porosity of more than 5%, the reference gas is supplied to the reference electrode 42 from outside the sensor element 101 (the space 149 shown in the Fig.1) is introduced via the reference electrode lead 78. In the present embodiment, the sensor element 101 is designed such that the limiting current when oxygen is pumped from the vicinity of the reference electrode 42 to the vicinity of the outer pumping electrode 23 is 0.3 µA or more. This ensures that the diffusion resistance of the reference electrode lead 78, particularly the inner lead 78b, is at or below a predetermined value (i.e., is not excessively high). Such a configuration makes it easier to maintain the oxygen concentration around the reference electrode 42 at a suitable level compared to a case where the diffusion resistance of the inner lead 78b is excessively high, thereby reducing the decrease in the detection accuracy of the concentration of a specific gas. The limiting current is preferably 0.5 µA or more. The porosity of the inner lead 78b is preferably 10% or more.The porosity of the inner conduit 78b is 40% or less. Such a structure makes it less likely that the inner conduit 78b will be separated.

[0076] In addition, during the non-operational period of the sensor element 101, the inner lead 78b and the lead insulating layer 79 may adsorb water from outside the sensor element 101, particularly from the space 149. The water in the space 149 may be present in small amounts or may penetrate through the gap between the rubber plug 157 and the outer cylinder 148. When the controller 96 starts the operation of the sensor element 101, the heater 72 heats the sensor element 101, causing the water in the inner lead 78b and the lead insulating layer 79 to evaporate and escape to the outside of the sensor element 101, particularly into the space 149. However, until the water is sufficiently discharged, the presence of evaporated water may reduce the oxygen concentration around the reference electrode 42.Therefore, the time required for water to drain from the inner lead 78b and the lead insulating layer 79 to the outside of the sensor element 101 (hereinafter referred to as "water drainage time") affects the time from the start of operation of the sensor element 101 to the stabilization of the potential of the reference electrode 42 (hereinafter referred to as "stabilization time"). In the present embodiment, the entire lead insulating layer 79 is formed to have a high density (as a high-density region). Such a structure reduces the amount of water (water vapor) adsorbed by the lead insulating layer 79 during the non-operational period of the sensor element 101 compared to a case where at least a portion of the lead insulating layer 79 is formed to be porous with a porosity of more than 5% (as a porous region).In the present embodiment, the inner lead 78b is a conductor composed predominantly of a noble metal or a high-melting-point metal. The noble metal or the high-melting-point metal used for the inner lead 78b generally has properties that make it less likely to adsorb water compared to alumina used for the lead insulating layer 79, so the inner lead 78b is less likely to adsorb water than the lead insulating layer 79. Such a structure reduces the amount of water (water vapor) adsorbed by the inner lead 78b during the non-operational period of the sensor element 101.Since the amount of water adsorbed by both the lead insulating layer 79 and the inner lead 78b is reduced during this non-operational period of the sensor element 101, the water discharge time is shortened, which in turn shortens the stabilization time.

[0077] 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, and these layers correspond to the element body. In addition, the reference electrode 42 corresponds to the reference electrode, the external pumping electrode 23 corresponds to the measurement-object gas-side electrode, the connector electrode 71a corresponds to the reference electrode terminal, the reference electrode lead 78 corresponds to the reference electrode lead, and the conductive portion 77 corresponds to the first conductive portion. Further, the lead insulating layer 79 corresponds to the lead insulating layer.

[0078] In the sensor element 101 incorporated in the gas sensor 100 according to the present embodiment described in detail above, the reference electrode lead 78 allows the reference gas to flow from outside the sensor element 101 to the reference electrode 42. The limit current is 0.3 µA or more when oxygen is pumped out from the vicinity of the reference electrode 42 to the vicinity of the external pumping electrode 23. The entire lead insulating layer 79 is formed to have a high density (as a high-density region) with a porosity of 5% or less.Since the entirety of the wiring insulating layer 79 is formed to have a high density (as a high-density region), the amount of water (water vapor) adsorbed by the wiring insulating layer 79 during the non-operational period of the sensor element 101 is reduced compared to a case where at least a portion of the wiring insulating layer 79 is formed to be porous. Such a structure shortens the time from the start of operation of the sensor element 101 until the water is discharged from the sensor element 101 (water discharge time), thereby shortening the time from the start of operation of the sensor element 101 until the potential of the reference electrode 42 stabilizes (stabilization time).Furthermore, since the limit current is 0.3 µA or more, indicating that the diffusion resistance of the reference electrode line 78, particularly the inner line 78b, is at or below a predetermined value (is not excessively high), maintaining the oxygen concentration around the reference electrode 42 at an appropriate level is facilitated compared with a case where the diffusion resistance of the reference electrode line is excessively high, thereby reducing the decrease in the detection accuracy of the concentration of a specific gas (NOx concentration).

[0079] The present invention is in no way limited to the above embodiment, and various embodiments are conceivable as long as they are included in the technical scope of the present invention.

[0080] For example, in the above-described embodiment, the entirety of the wiring insulation layer 79 is formed to have a high density (as a high-density region). As shown in a sensor element 201 according to a modification in Fig.However, as shown in Fig. 7, the lead insulating layer 79 may be formed such that a region of 50% or more but less than 100%, from the end on the reference electrode 42 side, of the total area of ​​the lead insulating layer 79 in plan view is a high-density region 79a having the porosity of 5% or less, while the remaining region is a porous region 79b. The total area of ​​the lead insulating layer 79 in plan view refers to the total area of ​​the lead insulating layer 79 when viewed from above or below, or in other words, refers to the projected area of ​​the lead insulating layer 79 on the upper or lower surface of the sensor element 201. A structure in which a region of 80% or more, from the end on the reference electrode 42 side, of the total area of ​​the lead insulating layer 79 in plan view is formed as the high-density region is preferable.In a case where a partial area of ​​the entire surface of the wiring insulating layer 79 in plan view is formed as the high-density region while the remaining area is formed as the porous region, since the area including at least the end on the reference electrode 42 side of the wiring insulating layer 79 is formed as the high-density region, even if the wiring insulating layer 79 adsorbs water during the non-operation period of the sensor element 101, the water is more easily discharged after the start of operation of the sensor element 101, compared with a case where the porous region is provided on the reference electrode 42 side relative to the high-density region in the wiring insulating layer 79. Consequently, such a structure shortens the water discharge time, thereby shortening the stabilization time.

[0081] In the embodiment described above, the cross-sectional area and porosity of the inner line 78b of the reference electrode line 78 are approximately constant overall. As in a sensor element 301 in the Fig.However, as shown in Fig. 8, the cross-sectional area of ​​the inner line 78b on the reference electrode 42 side may be larger than that on the circumferential line 78a side (connector electrode 71a side). Additionally or alternatively, the porosity of the inner line 78b on the reference electrode 42 side may be larger than that on the circumferential line 78a side. As the cross-sectional area and / or porosity of the inner line 78b on the reference electrode 42 side becomes larger than that on the circumferential line 78a side, the diffusion resistance of the inner line 78b on the reference electrode 42 side becomes smaller than that on the circumferential line 78a side. Such a structure increases the probability that oxygen accumulates around the reference electrode 42, thereby facilitating the maintenance of the oxygen concentration around the reference electrode at the appropriate level.Consequently, the decrease in the detection accuracy of the concentration of a specific gas (NOx concentration) is reduced.

[0082] In the above-described embodiment, the heater portion 70 includes the heater 72, the heater insulating layer 74, and the lead 76, and the lead insulating layer 79 and the heater insulating layer 79 are provided independently such that they are not connected to each other within the sensor element 101. As in a sensor element 401 in the Fig.However, as shown in FIG. 9, the heater portion 70 may further include a pressure relief hole (connection hole) 75. The pressure relief hole 75 is provided to penetrate the third substrate layer 3 and connect the lead insulating layer 79 and the heater insulating layer 74. This pressure relief hole 75 is provided to reduce the increase in internal pressure resulting from the rise in temperature within the heater insulating layer 74. In a case where the heater portion 70 includes the pressure relief hole 75, the entirety of the heater insulating layer 74 is formed to have a high density, with the porosity being 5% or less.Since the heater portion 70 includes a pressure release hole 75 and the entirety of the heater insulating layer 74 is formed to have a high density (as a high-density region), the amount of water adsorbed by the heater insulating layer 74 via the lead insulating layer 79 and the pressure release hole 75 is reduced during the non-operational period of the sensor element 101.

[0083] In the above-described embodiment, the reference electrode insulating layer 43 is formed to have a high density along the entirety of the line insulating layer 79. However, the reference electrode insulating layer 43 may also be porous.

[0084] In the above-described embodiment, the upper, front, rear, left, and right surfaces of the reference electrode 42 are covered by the reference electrode insulating layer 43. As shown in a sensor element 501 in the Fig. 10, however, a reference gas chamber 47 may be provided within the sensor element 101 as a space for accumulating the reference gas, and the reference electrode 42 may be arranged within this reference gas chamber 47. In the sensor element 501 shown in the Fig.As shown in Figure 10, the reference gas chamber 47 is an internal space provided within the sensor element 501 by hollowing out the first solid electrolyte layer 4. Specifically, it is the space enclosed by the lower surface of the spacer layer 5, with the inner front, rear, left, and right side surfaces facing the first solid electrolyte layer 4, and the upper surface of the third substrate layer 3. A high-density layer 48 is disposed within the reference gas chamber 47. The high-density layer 48 is a high-density layer with a porosity of 5% or less, which covers the inner peripheral surface of the reference gas chamber 47. In addition, the high-density layer 48 does not cover the upper surface of the reference electrode 44. Consequently, the reference electrode 42 is exposed to the space enclosed by the high-density layer 48 within the reference gas chamber 47.Furthermore, the high-density layer 48 also covers the portion of the inner conduit 78b located within the reference gas chamber 43. This further reduces the variation in oxygen concentration around the reference electrode 42 because the reference gas can accumulate in the reference gas chamber 47. Instead of having the high-density layer 48 disposed within the reference gas chamber 47, a porous body may be filled into the reference gas chamber 47, or neither the high-density layer 48 nor the porous body may be provided within the reference gas chamber 47.

[0085] In the above-described embodiment, the outer periphery of the portion of the inner lead 78b located inside the sensor element 101 (the portion located between the third substrate layer 3 and the first solid electrolyte layer 4) is covered by the lead insulating layer 79. As in a sensor element 601 in Fig. 11 and Fig. However, as shown in Fig. 12, a portion of the outer periphery of the portion of the inner conduit 78b located inside the sensor element 101 (a part in the extending direction of the inner conduit 78b) may be exposed within a pre-chamber 78c. Fig. Figure 11 is a partially enlarged cross-sectional view of an area around a reference electrode 42, a reference electrode lead 78, and a lead 76 of the sensor element 601. The Fig. 12 is a cross-sectional view along BB in the Fig.11. The pre-chamber 78c is a space formed in the element body (laminated body) of the sensor element 601. In the examples shown in Fig. 11 and Fig.12, the prechamber 78c is a cylindrical space formed as a hole that penetrates vertically through the first solid electrolyte layer 4. Since the portion of the inner conduit 78b is exposed within the prechamber 78c, in other words, since the prechamber 78c is in contact with the portion of the inner conduit 78b, the reference gas flowing through the inner conduit 78b diffuses into the prechamber 78c, which has a smaller diffusion resistance than the inner conduit 78b. This allows the prechamber 78c to act as a buffer for variations in the oxygen concentration in the reference gas. In addition, the prechamber 78c may have a shape other than cylindrical, and may, for example, have the shape of a rectangular prism. Further, there may be a plurality of prechambers 78c instead of just one. In the examples shown in Fig. 11 and Fig.12, the upper surface of the inner conduit 78c is exposed within the pre-chamber 78c, however, multiple surfaces of the inner conduit 78c may be exposed within the pre-chamber 78c as the inner conduit 78c passes through the pre-chamber 78c.

[0086] In the above-described embodiment, the sensor element 101 of the gas sensor 100 includes the first inner cavity 20, the second inner cavity 40, and the third inner cavity 61. However, this is not limiting. For example, as shown in a sensor element 701 according to a further modification in Fig. 13, the sensor element 701 does not include the third inner cavity 61. In the sensor element 701 according to a further modification in the Fig.13, between the lower surface of the second solid electrolyte layer 6 and the upper surface of the first solid electrolyte layer 4, there are provided a gas inlet 10, a first diffusion adjusting portion 11, a buffer space 12, a second diffusion adjusting portion 13, a first internal cavity 20, a third diffusion adjusting portion 30, and a second internal cavity 40, which are adjacent to each other and connected to each other in this order. In addition, the measuring electrode 44 is arranged on the upper surface of the first solid electrolyte layer 4 within the second internal cavity 40. The measuring electrode 44 is covered by the fourth diffusion adjusting portion 45. The fourth diffusion adjusting portion 45 is a membrane made of a porous ceramic body, such as alumina (Al2O3).Like the fourth diffusion adjusting portion 60 in the above-described embodiment, the fourth diffusion adjusting portion 45 has a role of restricting the amount of NOx flowing into the measuring electrode 44. In addition, 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 is formed directly above the measuring electrode 44. Even with such a configuration of the sensor element 701, the NOx concentration can be detected by the measuring pumping cell 41 as in the above-described embodiment. In the sensor element 701 shown in FIG. Fig. As shown in Figure 13, the area surrounding the measuring electrode 44 acts as a measuring chamber. In other words, the area surrounding the measuring electrode 44 serves a role similar to that of the third internal cavity 61.

[0087] In the sensor element 101 described above, the circuit of the reference gas adjustment pump cell 90 may be omitted, or the gas sensor 100 may not include the power source circuit 92. Additionally, the gas sensor 100 may not include the control device 95. For example, the gas sensor 100 may include a connector for external connection attached to the lead wires 155 for connecting the control device 95 and the lead wires 155 instead of including the control device 95.

[0088] In the sensor element 101 described above, the surface of the front side of the sensor element 101, which includes the outer pumping electrode 23 and is exposed to the sensor element chamber 133, may be covered with a porous protective layer made of a ceramic such as alumina.

[0089] In the above-described embodiment, the CPU 97 controls the voltage Vp2 of the variable current source 46 so that the voltage V2 reaches the target value V2*, and detects the NOx concentration in the measurement target gas based on the detected value (pumping current Ip2). However, this is not limiting. For example, the CPU 97 may control the measurement pump cell 41 (e.g., control the voltage Vp2) so that the pumping current Ip2 reaches a constant target value Ip2*, and detect the NOx concentration based on the detected value (the voltage V2). Since the measurement pump cell 41 is controlled so that the pumping current Ip2 reaches the target value Ip2*, oxygen is extracted from the third internal cavity 61 at a nearly constant flow rate.Consequently, the oxygen concentration in the third internal cavity 61 changes according to the amount of oxygen generated by the reduction of NOx in the measurement target gas. This, in turn, leads to variations in the voltage V2. Therefore, the voltage V2 corresponds to the NOx concentration in the measurement target gas. As a result, the controller 96 can calculate the NOx concentration based on this voltage V2. In this case, for example, the relationship between the voltage V2 and the NOx concentration can be stored in advance in the storage unit 98.

[0090] In the above-described embodiment, the sensor element 101 is configured to detect the NOx concentration in the measurement target gas. However, it is not limited to this and can be used to detect the concentration of any specific gas in the measurement target gas. For example, it can be used to detect the concentration of oxides other than concentrations of a specific gas, not only NOx. When the specific gas is an oxide as in the above-described embodiment, oxygen is generated when the specific gas itself is reduced in the third internal cavity 61. Therefore, the measurement pump cell 41 detects the detected value (for example, the pumping current Ip2) corresponding to this oxygen to detect the concentration of the specific gas. The specific gas may also be a non-oxide, such as ammonia.In this case, if the specific gas is a non-oxide, converting the specific gas into an oxide (for example, converting ammonia to NO) generates oxygen when the converted gas is reduced in the third internal cavity 61. Therefore, the measuring pump cell 41 detects the detected value (for example, the pumping current Ip2) corresponding to this oxygen, so that the concentration of the specific gas is detected. For example, the inner pumping electrode 22 of the first internal cavity 20 acts as a catalyst, thereby converting ammonia within the first internal cavity 20 into NO.

[0091] In the above-described embodiment, the element body of the sensor element 101 is formed as a laminated body with multiple solid electrolyte layers (layers 1 to 6). However, this is not limiting. The element body of the sensor element 101 should include at least one oxygen ion-conductive solid electrolyte layer. For example, in the Fig. 2, the layers 1 to 5, which are different from the second solid electrolyte layer 6, consist of layers made of materials different from solid electrolyte layers (for example, layers made of aluminum oxide). In this case, each electrode of the sensor element 101 can be arranged on the second solid electrolyte layer 6. For example, the measuring electrode 44 in the Fig.2 may be arranged on the lower surface of the second solid electrolyte layer 6. In addition, the reference electrode 42 may be provided behind the measurement object gas flow section and on the lower surface of the second solid electrolyte layer 6.

[0092] In the above-described embodiment, the inner pumping electrode 22 is formed as a cermet electrode made of Pt containing 1% Au and ZrO2. However, this is not limiting. The inner pumping electrode 22 should contain a noble metal having catalytic activity (for example, at least one of Pt, Rh, Ir, Ru, or Pd) and a noble metal having the ability to reduce catalytic activity (for example, Au) that reduces the catalytic activity of the noble metal having catalytic activity for the specific gas. The auxiliary pumping electrode 51 should contain a noble metal having catalytic activity and a noble metal having the ability to reduce catalytic activity, as is the case with the inner pumping electrode 22.The outer pumping electrode 23, the reference electrode 42, and the measuring electrode 44 should each contain a noble metal with catalytic activity, as described above. Each of the electrodes 22, 23, 42, 44, and 51 is preferably a cermet containing a noble metal with catalytic activity and an oxide with oxygen ion conductivity (e.g., ZrO2), although one or more of these electrodes may not be cermets. Each of the electrodes 22, 23, 42, 44, and 51 is preferably a porous body, although one or more of these electrodes may not be porous bodies.

[0093] In the above-described embodiment, the outer pumping electrode 23 serves as a plurality of electrodes, including an outer main pumping electrode that is a part of the main pumping cell 21 and is disposed on the portion exposed to the measurement object gas outside the sensor element 101, an outer auxiliary pumping electrode that is a part of the auxiliary pumping cell 50 and is disposed on the portion exposed to the measurement object gas outside the sensor element 101, an outer measuring electrode that is a part of the measuring pumping cell 41 and is disposed on the portion exposed to the measurement gas outside the sensor element 101, and the measurement object gas-side electrode that is a part of the reference gas adjustment pumping cell 90 and is disposed on the portion exposed to the measurement gas outside the sensor element 101. However, this is not limitative.One or more of the outer main pumping electrode, the outer auxiliary pumping electrode, the outer measuring electrode, and the measuring object gas side electrode may be provided separately from the outer pumping electrode 23 on the outside of the sensor element 101.

[0094] In the above-described embodiment, the controller 96 sets the target value V0* for the voltage V0 based on the pumping current Ip1 so that the pumping current Ip1 reaches the target value Ip1* (feedback control), and controls the pumping voltage Vp0 so that the voltage V0 reaches the target value V0*. However, other control methods may be employed. For example, the controller 96 may control the pumping voltage Vp0 based on the pumping current Ip1 so that the pumping current Ip1 reaches the target value Ip1*. In other words, the controller 96 may not perform 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 directly control the pumping voltage Vp0 based on the pumping current Ip1 (and thus control the pumping current Ip0).

[0095] In the above-described embodiment, the pumping current Ip3 is a constant direct current, but this is not limiting. For example, the pumping current Ip3 may be a pulsed, discontinuous current. In addition, in the above-described embodiment, the pumping current Ip3 is a constant direct current that always flows in the direction of drawing oxygen into the area around the reference electrode 42, but this is not limiting. For example, there may be a period of time during which the pumping current Ip3 flows in the direction of pumping oxygen out of the vicinity of the reference electrode 42. Even in such a case, the overall movement of oxygen, if examined over a sufficiently long predetermined period of time, may be in the direction of pumping oxygen into the vicinity of the reference electrode 42. Examples

[0096] Specific manufacturing examples of the sensor element are described below. It should be noted that the present invention is not limited to the following examples. [Example 1]

[0097] Example 1 was prepared by manufacturing the gas sensor 100 which is Fig.1 to 6 using the manufacturing method described above. In the manufacture of the sensor element 101, ceramic green sheets were each obtained 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 reference electrode 42 was prepared as a porous cermet electrode composed of Pt and zirconia. The structure for the reference electrode 42 was formed using a paste produced by mixing a Pt powder, a zirconia powder, a binder, and a pore-forming agent. The conductive portion 77 was prepared as a porous cermet composed predominantly of Pt together with zirconia.The structure for the conductive portion 77 was formed using a paste prepared by mixing Pt powder, zirconium oxide powder, a binder, and a pore-forming agent. The paste for the wiring insulating layer 79 was prepared by mixing alumina powder and a binder solution in a weight ratio of 1:2. The porosity of the inner wiring 78b in the conductive portion 77 was 15%. The ratio of the high-density region from the end on the reference electrode 42 side to the total area of ​​the wiring insulating layer 79 in a plan view was 100%. The limiting current when oxygen is pumped out from the vicinity of the reference electrode 42 to the vicinity of the outer pumping electrode 23 was 1.0 µA. The porosity measurement was performed using image analysis based on the SEM images described above.The measurement of the limiting current was carried out using the method described above. [Example 2]

[0098] Except for setting the limit current to 2.0 µA, a gas sensor 100 identical to Example 1 was manufactured for Example 2. The limit current was changed by changing the shape of the inner lead 78b (particularly, the length in the extending direction) compared with Example 1. [Example 3]

[0099] Except for setting the porosity of the inner lead 78b to 6%, setting the high-density region from the end on the reference electrode 42 side to the total area of ​​the lead insulating layer 79 in a plan view to 50%, and setting the limiting current to 0.3 µA, a gas sensor 100 identical to Example 1 was manufactured for Example 3. The paste for the porous region of the lead insulating layer 79 was prepared by further mixing a pore-forming agent into the paste used for the lead insulating layer 79 in Example 1 (the same applies to Examples 4 to 8 and Comparative Example 2, which will be described later). The limiting current was changed by changing the shape of the inner lead 78b (particularly, the length in the extending direction) and the porosity compared to Example 1. [Example 4]

[0100] Except for setting the ratio of the high-density region from the end on the reference electrode 42 side to the total area of ​​the lead insulating layer 79 in a plan view to 80% and setting the limit current to 0.7 µA, a gas sensor 100 identical to Example 1 was manufactured for Example 4. The limit current was changed by changing the shape of the inner lead 78b (particularly, the length in the extending direction) compared to Example 1. [Example 5]

[0101] Except for setting the porosity of the inner lead 78b to 10%, setting the ratio of the high-density region from the end on the reference electrode 42 side to the total area of ​​the lead insulating layer 79 in a plan view to 70%, and setting the limiting current to 0.5 µA, a gas sensor 100 identical to Example 1 was manufactured for Example 5. The limiting current was changed by changing the shape of the inner lead 78b (particularly, the length in the extending direction) and the porosity compared to Example 1. [Example 6]

[0102] Except for setting the porosity of the inner lead 78b to 10%, setting the ratio of the high-density area from the end on the reference electrode 42 side to the total area of ​​the lead insulating layer 79 in a plan view to 90%, and setting the limiting current to 0.5 µA, a gas sensor 100 identical to Example 1 was manufactured for Example 6. The limiting current was changed by changing the length in the extension direction and the porosity compared to Example 1. [Example 7]

[0103] Except for setting the porosity of the inner lead 78b to 8%, setting the ratio of the high-density region from the end on the reference electrode 42 side to the total area of ​​the lead insulating layer 79 in a plan view to 75%, and setting the limiting current to 0.4 µA, a gas sensor 100 identical to Example 1 was manufactured for Example 7. The limiting current was changed by changing the shape of the inner lead 78b (particularly, the length in the extending direction) and the porosity compared to Example 1. [Example 8]

[0104] Except for setting the porosity of the inner lead 78b to 10%, setting the ratio of the high-density region from the end on the reference electrode 42 side to the total area of ​​the lead insulating layer 79 in a plan view to 80%, and setting the limiting current to 0.7 µA, a gas sensor 100 identical to Example 1 was manufactured for Example 8. The limiting current was changed by changing the porosity of the inner lead 78b compared to Example 1. [Example 9]

[0105] Except for setting the porosity of the inner line 78b to 40% and setting the limiting current to 2.0 µA, a gas sensor 100 identical to Example 1 was manufactured for Example 9. The limiting current was changed by changing the shape of the inner line 78b (particularly, the length in the extending direction) and the porosity compared to Example 1. [Example 10]

[0106] A gas sensor 100 was manufactured which included a sensor element 601 with a prechamber 78c (cf. Fig. 11 and Fig. 12) instead of a sensor element 101 without a prechamber (cf. Fig.1 to 6), for example, 10. The sensor element 601 was manufactured using a manufacturing method similar to Example 1, except for the following points: a space (hole) for the pre-chamber 78c was pre-punched in the green sheet serving as the first solid electrolyte layer 4, and the shape of the structure covering the top of the inner lead 78b in the lead insulating layer 79 included a space (hole) for the pre-chamber 78c. The porosity of the inner lead 78b was set to 15%, the ratio of the high-density region from the end on the reference electrode 42 side to the total area of ​​the lead insulating layer 79 in a plan view was set to 100%, and the limiting current was set to 2.0 µA. The limiting current was changed compared to Example 1 by including the pre-chamber 78c. [Comparison example 1]

[0107] Except for setting the porosity of the inner line 78b to 5% and setting the limiting current to 2.0 µA, a gas sensor 100 identical to Example 1 was manufactured for Comparative Example 1. The limiting current was changed by changing the shape of the inner line 78b (particularly, the length in the extending direction) and the porosity compared to Example 1. [Comparison example 2]

[0108] Except for setting the ratio of the high-density region from the end on the reference electrode 42 side to the total area of ​​the lead insulating layer 79 in a plan view to 40%, a gas sensor 100 identical to Example 1 was manufactured for Comparative Example 2. [Assessment Test 1: Assessment of Stabilization Time]

[0109] The gas sensor 100 according to Example 1 was stored in a constant temperature and humidity chamber at 40°C and 85% humidity for one week so that water could be adsorbed by the inner pipe 78b and the pipe insulating layer 79. Next, the gas sensor 100 according to Example 1 was attached to a pipe. A model gas was prepared with nitrogen as the base gas, an oxygen concentration of 0%, and a NOx concentration of 1500 ppm, which was then passed through the pipe as the measurement object gas. In this state, the sensor element 101 was driven by the control device 95. Specifically, the control device 95 supplied power to the heater 72 to heat the sensor element 101 while maintaining its temperature at 800°C.In addition, the control device 95 was in the state of continuously controlling each of the above-described pump cells 21, 41, and 50 and detecting the respective voltages V0, V1, V2, and Vref of each of the above-described sensor cells 80 to 83. The operation of the reference gas adjustment pump cell 90 was not permitted. The above condition was maintained for 60 minutes from the start of operation of the sensor element 101 (heating start), during which the pump current Ip2 was continuously measured. The value of the pump current Ip2 after 60 minutes from the start of measurement was set as the reference value (100%), and the change rate of the pump current Ip2 after 10 minutes from the start of measurement with respect to the reference value was calculated. The change rate of the pump current Ip2 was calculated according to the gas sensors 100 according to Examples 2 to 10 and Comparative Examples 1 and 2.

[0110] At this time, during the period from the start of operation of the sensor element 101 until the water in the inner lead 78b and the lead insulating layer 79 has been sufficiently discharged, the presence of evaporated water reduces the oxygen concentration around the reference electrode 42, resulting in an unstable potential at the reference electrode 42. Therefore, until the potential of the reference electrode 42 stabilizes, the pumping current Ip2 will not be stable even if the NOx concentration of the measurement object gas remains constant. A smaller change rate of the pumping current Ip2 indicates that water has been sufficiently discharged from the inner lead 78b and the lead insulating layer 79, and the potential of the reference electrode 42 is considered stable at that point 10 minutes after the start of measurement.Therefore, the duration of the stabilization time, which is the time from the start of operation of the sensor element 101 until the potential of the reference electrode 42 stabilizes, can be evaluated based on the magnitude of the change rate. If the calculated change rate is 3% or less, the stabilization time was evaluated as short (denoted as "A"). If the calculated change rate is more than 3% and 5% or less, the stabilization time was evaluated as short (denoted as "B"). If the calculated change rate exceeds 5%, the stabilization time was evaluated as long (denoted as "F"). [Evaluation Test 2: Evaluation of NOx Detection Accuracy]

[0111] The gas sensor 100 according to Example 1 was attached to a lead. Power was applied to the heater 72 to maintain its temperature at 850°C, and the sensor element 101 was heated. In addition, the reference gas adjusting pump cell 90 was operated. The control voltage Vp3 applied by the power source circuit 92 of the electrochemical reference gas adjusting pump cell 90 was a pulse voltage with a period T of 10 ms, an on-time Ton of 2.0 ms, and an off-time Toff of 8.0 ms (duty ratio 20%). The control voltage Vp3 applied by the power source circuit 92 was set such that the peak value of the control current Ip3 flowing during the power-on time was 10 µA. The effective value Ip3ef of the control current Ip3 was 2 µA.In this condition, a model gas was prepared with nitrogen as the base gas, an oxygen concentration of 10%, and a NOx concentration of 500 ppm, which was then passed through the line as the target gas. This condition was maintained for 20 minutes, during which time the voltage Vref was measured. The same measurements were performed with Examples 2 to 8 and Comparative Examples 1 and 2.

[0112] The lower the oxygen concentration around the reference electrode 42 is compared to the oxygen concentration at the start of measurement (which is identical to the oxygen concentration in the air), the more the voltage Vref tends to decrease over time from the value at the start of measurement. Furthermore, the lower the voltage Vref is, the more the pumping current Ip2 tends to be comparable to the correct value (the value corresponding to a NOx concentration of 500 ppm). Therefore, when the value of the voltage Vref at the start of measurement was set to 100% and the measured voltage Vref remained within the specified range (80% or more) even after 20 minutes, the change (decrease) in the oxygen concentration around the reference electrode 42 was very small, and the detection accuracy of the NOx concentration was evaluated as very high (referred to as "A").If the measured voltage Vref remained within the specified range after 15 minutes but fell below the lower limit of the range before 20 minutes had elapsed, the oxygen around the reference electrode 42 was slightly low, but the detection accuracy of the NOx concentration was rated as high (denoted as "B"). If the measured voltage Vref fell below the lower limit of the range before 15 minutes had elapsed, the oxygen around the reference electrode 42 was insufficient, and the detection accuracy of the NOx concentration was rated as low (denoted as "F").

[0113] For each of Examples 1 to 10 and Comparative Examples 1 and 2, the porosity of the inner lead 78b, the ratio of the high-density region from the end on the reference electrode 42 side to the total area of ​​the lead insulating layer 79 in a plan view, the limiting current when oxygen was pumped from the vicinity of the reference electrode 42 to the vicinity of the outer pumping electrode 23, the evaluation results of the stabilization time, and the evaluation results of the NOx detection accuracy are shown in Table 1. [Table 1] level Porosity of the inner pipe [%] Ratio of the high-density area to the total area of ​​the cable insulation layer [%] Presence of the antechamber Limit current [µA] Assessment of the stabilization period Evaluation of NOx detection accuracy Example 1 15 100 No 1,0 A A Example 2 15 100 No 2,0 A A Example 3 6 50 No 0,3 B B Example 4 15 80 No 0,7 A A Example 5 10 70 No 0,5 B A Example 6 10 90 No 0,5 A A Example 7 8 75 No 0,4 B B Example 8 10 80 No 0,7 A A Example 9 40 100 No 2,0 A A Example 10 15 100 Yes 2,0 A A Comparison example 1 5 100 No 0,2 A F Comparison example 2 15 40 No 1,0 F A

[0114] As shown in Table 1, in Examples 1 to 10, where the porosity of the inner lead 78b is more than 5% and 40% or less, the ratio of the high-density area from the end on the reference electrode 42 side to the total area of ​​the lead insulating layer 79 in plan view is 50% or more, and the limiting current when oxygen is pumped from the vicinity of the reference electrode 42 to the vicinity of the outer pumping electrode 23 is 0.3 μA or more, the evaluation of the stabilization time was "A" or "B" and the evaluation of the NOx detection accuracy was also "A" or "B." In other words, Examples 1 to 10 had a short stabilization time and good NOx detection accuracy. In contrast, in Comparative Example 1, the evaluation of the NOx detection accuracy was "F."In Comparative Example 1, the porosity of the inner lead 78b was 5%, indicating that the inner lead 78b had a high density, and the limiting current when oxygen was pumped from the vicinity of the reference electrode 42 to the vicinity of the outer pumping electrode 23 was 0.2 μA. It is assumed that the oxygen concentration around the reference electrode 42 is difficult to maintain, and NOx detection accuracy is poor because the movement of the reference gas between the vicinity of the reference electrode 42 and the exterior of the sensor element 101 is severely restricted. In Comparative Example 2, the evaluation of the stabilization time was "F." In Comparative Example 2, the ratio of the high-density region from the end on the reference electrode 42 side to the total area of ​​the lead insulating layer 79 in the plan view is 40%, indicating that the ratio of the high-density region is low.As a result, the amount of water adsorbed by the lead insulating layer 79 during the non-operational period of the sensor element 101 is large, and it takes time for the water to be discharged from the lead insulating layer 79, resulting in a longer stabilization time. Based on the comparisons, it is considered that in Examples 1 to 10, the porosity of the inner lead 78b is more than 5% and 40% or less, the ratio of the high-density region from the end on the reference electrode 42 side to the total area of ​​the lead insulating layer 79 in plan view is 50% or more, and the limit current when oxygen is pumped from the vicinity of the reference electrode 42 to the vicinity of the outer pumping electrode 23 is 0.3 µA or more. This structure likely contributes to a shorter stabilization time while mitigating the decrease in the NOx detection concentration.

[0115] Furthermore, among Examples 1 to 10, the stabilization time evaluation for Examples 1, 2, 4, 6, and 8 to 10, in which the ratio of the high-density region from the end on the reference electrode 42 side to the total area of ​​the wiring insulating layer 79 in the plan view was 80% or more, was "A." In contrast, the stabilization time evaluation for Examples 3, 5, and 7, in which this ratio was 50% or more but less than 80%, was "B." Therefore, it is considered that the ratio of the high-density region from the end on the reference electrode 42 side to the total area of ​​the wiring insulating layer 79 in the plan view is preferably 80% or more.

[0116] Furthermore, among Examples 1 to 10, for Examples 1, 2, 4 to 6, and 8 to 10, in which the limiting current when oxygen is pumped from the vicinity of the reference electrode 42 to the vicinity of the outer pumping electrode 23 was 0.5 µA or more, the NOx detection accuracy rating was "A." In contrast, for Examples 3 and 7, in which the limiting current was 0.3 µA or more but less than 0.5 µA, the NOx detection accuracy rating was "B." Therefore, it is considered that the limiting current is preferably 0.5 µA or more. Industrial applicability

[0117] The present invention is applicable to a gas sensor that detects the concentration of a specific gas such as NOx in a measurement object gas such as the exhaust gas of an automobile. List of reference symbols

[0118] 1 First substrate layer, 2 Second substrate layer, 3 Third substrate layer, 4 First solid electrolyte layer, 5 Spacer layer, 6 Second solid electrolyte layer, 10 Gas inlet opening, 11 First diffusion adjustment section, 12 Buffer space, 13 Second diffusion adjustment section, 20 First inner cavity, 21 Main pump cell, 22 Inner pump electrode, 22a Upper electrode section, 22b Lower electrode section, 23 Outer pump electrode, 25 Variable power supply, 30 Third diffusion adjustment section, 40 Second inner cavity, 41 Measuring pump cell, 42 Reference electrode, 43 Reference electrode insulating layer, 44 Measuring electrode, 45 Fourth diffusion adjustment section, 46 Variable power supply, 47 Reference gas chamber, 50 Auxiliary pump cell, 51 Auxiliary pump electrode, 51a Upper electrode section, 51b Lower Electrode section, 52 variable power supply, 60 fourth diffusion adjustment section, 61 third inner cavity, 70 heater section, 71, 71a to 71h connector electrode,72 heater, 73 through hole, 74 heater insulating layer, 75 pressure release hole, 76 lead, 77 conductive portion, 78 inner lead, 78 reference electrode lead, 78a circumferential surface lead, 78a inner lead, 78b inner lead, 78c pre-chamber, 79 lead insulating layer, 79a high-density region, 79b porous region, 80 oxygen partial pressure detection sensor cell for main pump control, 81 oxygen partial pressure detection sensor cell for auxiliary pump control, 82 oxygen partial pressure detection sensor cell for measuring pump control, 83 sensor cell, 90 reference gas adjustment pump cell, 92 power source circuit, 95 control device, 96 controller, 97 CPU, 98 storage unit, 100 gas sensor, 101, 201, 301, 501, 601, 701 sensor element, 130 protective cover, 131 inner protective cover, 132 outer protective cover, 133 sensor element chamber, 140 sensor assembly, 141 element sealing unit, 142 main metal molding, 143 inner cylinder, 143a, 143b diameter reducing section,144a to 144c Ceramic carrier, 145a, 145b Press body, 146 Metal ring, 147 Screw, 148 Outer cylinder, 149 Space, 150 Connector, 155 Lead wires, 157 Rubber plug, 190 Lead, 191 Attachment element, 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 2018-173320 A

[0003]

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 having an oxygen ion-conductive solid electrolyte layer and a measurement object gas flow section located within the element body, through which the measurement object gas is supplied and in which the measurement object gas can flow; a reference electrode disposed within the element body; a measurement object gas-side electrode arranged on a portion of the element body exposed to the measurement object gas; a first conductive portion comprising a reference electrode terminal disposed on an outer side of the element body and a reference electrode line electrically continuous between the reference electrode terminal and the reference electrode; and a lead insulating layer covering a portion of the reference electrode lead located within the element body, wherein the reference electrode line is configured to supply a reference gas serving as a reference for determining the concentration of a specific gas in the measurement object gas to the reference electrode, and wherein a region of 50% or more, from an end on the reference electrode side, of the total area of ​​the lead insulating layer in a plan view is formed as a high-density region having a porosity of 5% or less. [2] The sensor element according to claim 1, wherein the region of 80% or more, from the end on the side of the reference electrode, of the total area in the plan view is formed as the high-density region. [3] The sensor element according to claim 1 or 2, wherein the limit current when oxygen is pumped from the vicinity of the reference electrode to the vicinity of the measurement object gas side electrode is 0.3 µA or more. [4] The sensor element according to claim 3, wherein the limit current is 0.5 µA or more. [5] The sensor element according to claim 1 or 2, wherein the porosity of a portion of the reference electrode lead located within the element body is more than 5% and 40% or less. [6] The sensor element according to claim 1 or 2, wherein a cross-sectional area and / or a porosity of a portion of the reference electrode lead located within the element body is larger on the reference electrode side than on the reference electrode terminal side. [7] Sensor element according to claim 1 or 2, further comprising: a heating device arranged within the element body and heating the element body; a second conductive portion comprising a heater terminal disposed on the outside of the element body and a heater line electrically continuous between the heater terminal and the heater; and a heater insulation layer covering both the heater and a portion of the heater line located within the element body, wherein the wire insulating layer and the heater insulating layer are provided independently such that they are not connected to each other. [8] Sensor element according to claim 1 or 2, further comprising: a heating device arranged within the element body and heating the element body; a second conductive portion comprising a heater terminal disposed on the outside of the element body and a heater line electrically continuous between the heater terminal and the heater; and a heater insulation layer covering both the heater and a portion of the heater line located within the element body, wherein the element body includes a connection hole connecting the heater insulating layer and the wire insulating layer, and a porosity of the heater insulation layer is 5% or less. [9] Gas sensor, comprising: the sensor element according to claim 1 or 2.

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