Sensor element and gas sensor

Abstract

To enhance fineness and adhesion of a portion of a conductive part that contribute to preventing gas from reaching an inner electrode.SOLUTION: A sensor element 101 is provided, comprising an element body 102 having an oxygen ion-conductive solid-state electrolyte layer, a measurement electrode 44 provided inside the element body 102, and a conductive part 74 having an inner lead part 77, a side lead part 78, and a connector electrode 75a, all connected to the measurement electrode 44 in the described order. The side lead part 78 and / or at least a portion of the inner lead part 77 constitute(s) a fine part finely made of a material containing a precious metal, alumina, and silica.SELECTED DRAWING: Figure 5

Description

Technical Field

[0001] The present invention relates to a sensor element and a gas sensor.

Background Art

[0002] Conventionally, a gas sensor including a sensor element for detecting the concentration of a specific gas such as NOx in a gas to be measured such as exhaust gas of an internal combustion engine is known (see, for example, Patent Document 1). The sensor element of Patent Document 1 includes an element body, an inner electrode, a terminal portion, and a lead portion. The element body has an oxygen ion-conductive solid electrolyte layer, and is columnar extending along the longitudinal direction, and has a front end and a rear end which are both ends along the longitudinal direction, and a side surface which is a surface along the longitudinal direction. The front end side of the element body is exposed to the gas to be measured. The inner electrode is disposed inside the element body. The terminal portion is disposed on the rear end side of the side surface of the element body. The lead portion is arranged to connect the inner electrode and the terminal portion, and has an inner portion disposed inside the element body and a side surface conduction portion exposed from the element body on the side surface. The terminal portion and the side surface conduction portion are each formed using a conductive material obtained by mixing a noble metal powder and an alumina powder. Thereby, the adhesion strength between the terminal portion and the side surface conduction portion and the solid electrolyte body can be increased.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] By the way, in such a sensor element, some gas outside the sensor element (element body) may move inside the element body along the inner part of the lead part or along the space between the lead part and the element body and reach the inner electrode. When such gas reaches the inner electrode, it may cause a decrease in the detection accuracy of the concentration of the specific gas. On the other hand, for example, it is conceivable to suppress such gas from reaching the inner electrode by covering the inner part of the lead part with a dense side conduction part. However, in the sensor element described in Patent Document 1, although the side conduction part contains alumina, the adhesion strength between the side conduction part and the element body can be increased. On the other hand, the porosity of the side conduction part tends to increase, and it is difficult to configure the side conduction part densely.

[0005] The present invention has been made to solve such problems, and the main object is to improve both the airtightness and the adhesion of the part of the conduction part that contributes to suppressing the gas from reaching the inner electrode.

Means for Solving the Problems

[0006] The present invention has adopted the following means to achieve the above-mentioned main object.

[0007] [1] The sensor element of the present invention is a sensor element for detecting the concentration of a specific gas in a gas to be measured, having an oxygen ion conductive solid electrolyte layer and being columnar extending along the longitudinal direction, having a front end and a rear end which are both ends along the longitudinal direction and a side surface which is the surface along the longitudinal direction, and the front end side being exposed to the gas to be measured, an element body, an inner electrode disposed inside the element body, an inner conduction part disposed inside the element body and conducting with the inner electrode, a connector electrode disposed on the rear end side of the side surface, and having a part disposed on the side surface and / or a part exposed to the outside of the sensor element on the side surface and conducting with the inner conduction part, an outer conduction part, and is provided with A portion of the outer conduction part that covers the inner conduction part, and / or at least a part of the inner conduction part, is a dense part that is densely configured including a noble metal, alumina, and silica. is such.

[0008] In this sensor element, regarding the portion of the outer conduction part that covers the inner conduction part side and / or the dense part that is at least a part of the inner conduction part, the adhesion can be enhanced by including not only a noble metal but also alumina and silica, and the density can be enhanced by including silica. Therefore, regarding the portion of the outer conduction part that covers the inner conduction part side and / or at least a part of the inner conduction part, that is, the dense part that contributes to suppressing the gas from reaching the inner electrode of the conduction part, both the density and the adhesion can be made high. The inventors confirmed this through experiments, analysis, etc.

[0009] [2] In the sensor element of the present invention (the sensor element described in [1] above), when the volumes of the contained alumina and silica in the dense part are Va [vol%] and Vs [vol%] respectively, Va / Vs ≤ 1.5 may be satisfied. By doing so, the density of the dense part can be further enhanced.

[0010] [3] In the sensor element of the present invention (the sensor element described in [1] or [2] above), when the volumes of the contained noble metal, alumina, and silica in the dense part are Vp [vol%], Va [vol%], and Vs [vol%] respectively, 1.5 ≤ Vp / (Va + Vs) may be satisfied. By doing so, the effect of enhancing the density of the dense part can be obtained more surely.

[0011] [4] In the sensor element of the present invention (the sensor element described in [3] above), the dense part may satisfy 2.6 ≤ Vp / (Va + Vs). By doing so, the density of the dense part can be further enhanced.

[0012] [5] In the sensor element of the present invention (the sensor element described in any one of [1] to [4] above), the total mass ratio of the contained alkali metal and alkaline earth metal in terms of oxide may satisfy 0.1 wt% or less. By doing so, it is possible to suppress the movement of the alkali metal and / or alkaline earth metal in the dense portion due to electromigration and the decrease in the conductivity of the dense portion, and it is possible to suppress the decrease in the detection accuracy of the concentration of the specific gas due to the decrease in conductivity.

[0013] [6] In the sensor element of the present invention (the sensor element described in any one of [1] to [5] above), the inner electrode may be a measurement electrode used for detecting the concentration of the specific gas.

[0014] [7] In the sensor element of the present invention (the sensor element described in any one of [1] to [6] above), the element body is a laminate in which a plurality of layers including the solid electrolyte layer are laminated in a lamination direction orthogonal to the longitudinal direction. The laminate has, as the side surfaces, a first surface and a second surface which are both end surfaces in the lamination direction, and a third surface and a fourth surface which are both end surfaces in a direction orthogonal to the longitudinal direction and the lamination direction. The inner conduction portion has an inner lead portion drawn out to the third surface or the fourth surface, and the portion of the outer conduction portion that covers the inner conduction portion may be a side surface lead portion disposed on the third surface or the fourth surface and covering the inner lead portion.

[0015] [8] In the sensor element of the present invention (the sensor element described in any one of [1] to [6] above), the element body is a laminate in which a plurality of layers including the solid electrolyte layer are laminated in a lamination direction orthogonal to the longitudinal direction. The laminate has, as the side surfaces, a first surface and a second surface which are both end surfaces in the lamination direction. The element body includes an opening that opens at the first surface or the second surface on the rear end side of the side surface, and has a through hole that penetrates one or more of the plurality of layers in the lamination direction. The outer conduction portion is the connector electrode, and the inner conduction portion may have a through hole conductor disposed in the through hole and covered by the connector electrode.

[0016] [9] In the sensor element of the present invention (the sensor element according to any one of [1] to [6] above), the element body is a laminate in which a plurality of layers including the solid electrolyte layer are laminated in a stacking direction orthogonal to the longitudinal direction, and the laminate has, as the side surfaces, a first surface and a second surface which are both end surfaces in the stacking direction. The element body includes an opening that opens at the first surface or the second surface on the rear end side of the side surface, and has a through hole that penetrates one or more of the plurality of layers in the stacking direction. The inner conduction portion has a through hole conductor disposed in the through hole and exposed to the outside at the opening, and the dense portion may include at least a part of the through hole conductor.

[0017]

[10] The sensor element of the present invention (the sensor element according to any one of [1] to [9] above) may be used for a gas sensor including the sensor element, a case that is cylindrical and extends along the longitudinal direction of the sensor element and has a second front end and a second rear end that are both ends along the longitudinal direction, and in which the sensor element is disposed inside, and a sealing member that seals the second rear end side of the case.

[0018]

[11] The gas sensor of the present invention includes the sensor element according to any one of [1] to [9] above, a case that is cylindrical and extends along the longitudinal direction of the sensor element and has a second front end and a second rear end that are both ends along the longitudinal direction, and in which the sensor element is disposed inside, and a sealing member that seals the second rear end side of the case.

[0019] Since this gas sensor includes the above-described sensor element, the same effects as those exhibited by the above-described sensor element can be obtained. For example, with respect to the portion of the conduction portion that contributes to suppressing the gas from reaching the inner electrode, an effect of achieving both high tightness and high adhesion can be obtained.

Brief Description of the Drawings

[0020]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

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Figure 8

Figure 9

Figure 10

Embodiments for Carrying Out the Invention

[0021] Next, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a longitudinal sectional view of a gas sensor 100 according to an embodiment of the present invention. FIG. 2 is a schematic cross-sectional view schematically showing an example of the configuration of a sensor element 101 included in the gas sensor 100. FIG. 3 is a block diagram showing the electrical connection relationship between the control device 95, each cell of the sensor element 101, and the heater 71a. FIG. 4 is a perspective view of the periphery of the rear end portion of the element body 102 of the sensor element 101. FIG. 5 is a partial cross-sectional view showing an enlarged view of the measurement electrode 44 of the sensor element 101 and the periphery of the conduction portion 74 corresponding thereto. The sensor element 101 has an element body 102 having a long rectangular parallelepiped shape. The longitudinal direction (the left-right direction in FIG. 2) of the element body 102 is defined as the front-rear direction, the thickness direction (the up-down direction in FIG. 2) of the element body 102 is defined as the up-down direction, and the width direction (the direction perpendicular to the front-rear direction and the up-down direction) of the element body 102 is defined as the left-right direction. Since the element body 102 is a rectangular parallelepiped, as shown in FIGS. 2, 4, and 5, as the outer surface of the solid electrolyte layer of the element body 102, there are six surfaces: a first surface 102a (upper surface), a second surface 102b (lower surface), a third surface 102c (left side surface), a fourth surface 102d (right side surface), a fifth surface 102e (front end surface), and a sixth surface 102f (rear end surface).

[0022] As shown in FIG. 1, the gas sensor 100 includes a sensor element 101 having an element body 102, a protective cover 130 that protects the front end side of the element body 102, and a sensor assembly 140 having a connector 150 that conducts with the sensor element 101. This gas sensor 100 is attached to a pipe 190 such as an exhaust gas pipe of an internal combustion engine (such as a diesel engine or a gasoline engine) of a vehicle, for example, as shown in the figure, and the exhaust gas of the internal combustion engine is used as the gas to be measured, and NOx, O 2 , ammonia, and other specific gases are used to measure (detect) the concentration of a specific gas, which is the concentration of a specific gas. In this embodiment, the gas sensor 100 is configured to measure the NOx concentration as the specific gas concentration.

[0023] The protective cover 130 includes a bottomed cylindrical inner protective cover 131 that covers the front end portion of the element body 102, and a bottomed cylindrical outer protective cover 132 that covers the inner protective cover 131. A plurality of holes are formed in the inner protective cover 131 and the outer protective cover 132 for allowing the gas to be measured to flow into the protective cover 130. A sensor element chamber 133 is formed as a space surrounded by the inner protective cover 131, and the front end portion of the element body 102 is disposed in the sensor element chamber 133.

[0024] The sensor assembly 140 includes an element sealing body 141 that encapsulates and fixes the sensor element 101, bolts 147 and an outer cylinder 148 attached to the element sealing body 141, and a connector 150 that contacts and is electrically connected to the connector electrodes 75 formed on the surfaces (upper and lower surfaces) of the rear end portion of the element body 102 of the sensor element 101.

[0025] The element sealing body 141 includes a cylindrical main fitting 142, a cylindrical inner cylinder 143 welded and fixed coaxially with the main fitting 142, and ceramic supporters 144a to 144c, pressure powders 145a and 145b, and a metal ring 146 encapsulated in the through holes inside the main fitting 142 and the inner cylinder 143. The sensor element 101 is located on the central axis of the element sealing body 141 and penetrates the element sealing body 141 in the front-rear direction. The inner cylinder 143 is formed with a reduced-diameter portion 143a for pressing the pressure powder 145b in the central axis direction of the inner cylinder 143, and a reduced-diameter portion 143b for pressing the ceramic supporters 144a to 144c and the pressure powders 145a and 145b forward via the metal ring 146. Due to the pressing forces from the reduced-diameter portions 143a and 143b, the pressure powders 145a and 145b are compressed between the main fitting 142, the inner cylinder 143 and the sensor element 101, thereby sealing the space between the sensor element chamber 133 in the protective cover 130 and the space 149 in the outer cylinder 148 and fixing the sensor element 101.

[0026] The bolt 147 is fixed coaxially with the main body fitting 142, and has a male screw portion formed on its outer peripheral surface. The male screw portion of the bolt 147 is inserted into a fixing member 191 that is welded to the pipe 190 and has a female screw portion provided on its inner peripheral surface. Thereby, the gas sensor 100 is fixed to the pipe 190 in a state where the front end portion of the element main body 102 of the sensor element 101 and the protection cover 130 of the gas sensor 100 protrude into the pipe 190.

[0027] The outer cylinder 148 covers the inner cylinder 143, the sensor element 101, and the connector 150, and a plurality of lead wires 155 connected to the connector 150 are drawn out to the outside from the rear end. These lead wires 155 are electrically connected to the respective electrodes (described later) of the sensor element 101 via the connector 150. At the rear end side of the outer cylinder 148, the gap between the outer cylinder 148 and the lead wires 155 is sealed by a rubber stopper 157. The outer cylinder 148 is formed with caulking portions 148a and 148b for restricting the relative movement of the rubber stopper 157 in the front-rear direction with respect to the outer cylinder 148. The space 149 inside the outer cylinder 148 is filled with a reference gas. The rear end portion of the element main body 102 of the sensor element 101 is disposed in the space 149.

[0028] As shown in FIG. 2, the sensor element 101 includes an element main body 102, cells 21, 41, 50, 80 to 83, and a heater portion 70. The element main body 102 is a laminate in which six layers, namely, 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 made of an oxygen ion conductive solid electrolyte layer such as zirconia (ZrO 2 ) etc., are laminated in this order from the lower side in the drawing view. Further, the solid electrolytes forming these six layers are dense and airtight. The element main body 102 is manufactured, for example, by performing predetermined processing and printing of circuit patterns on ceramic green sheets corresponding to the respective layers, then laminating them and further firing them to integrate them.

[0029] On the front end side of the element body 102, between the lower surface of the second solid electrolyte layer 6 and the upper surface of the first solid electrolyte layer 4, there are a gas inlet 10, a first diffusion rate-limiting part 11, a buffer space 12, a second diffusion rate-limiting part 13, a first internal cavity (oxygen concentration adjustment chamber) 20, a third diffusion rate-limiting part 30, a second internal cavity (oxygen concentration adjustment chamber) 40, a fourth diffusion rate-limiting part 60, and a third internal cavity (measurement chamber) 61, which are adjacent and formed in a communicating manner in this order.

[0030] The gas inlet 10, the buffer space 12, the first internal cavity 20, the second internal cavity 40, and the third internal cavity 61 are spaces inside the sensor element 101 that are provided in a manner of punching out the spacer layer 5, with the upper part defined by the lower surface of the second solid electrolyte layer 6, the lower part defined by the upper surface of the first solid electrolyte layer 4, and the side part defined by the side surface of the spacer layer 5.

[0031] The first diffusion rate-limiting part 11, the second diffusion rate-limiting part 13, and the third diffusion rate-limiting part 30 are all provided as two horizontally long slits (the opening has a longitudinal direction in a direction perpendicular to the drawing). Also, the fourth diffusion rate-limiting part 60 is provided as one horizontally long slit formed as a gap with the lower surface of the second solid electrolyte layer 6 (the opening has a longitudinal direction in a direction perpendicular to the drawing). Note that the part from the gas inlet 10 to the third internal cavity 61 is also referred to as the measured gas flow-through part.

[0032] The element body 102 is provided with a reference gas introduction part 49 for flowing a reference gas for measuring the NOx concentration to the reference electrode 42 from the outside of the element body 102. The reference gas introduction part 49 has a reference gas introduction space 43 and a reference gas introduction layer 48. The reference gas introduction space 43 is a space provided so as to extend from the sixth surface 102f (rear end surface) of the element body 102 toward the fifth surface 102e (front end surface) side. The reference gas introduction space 43 is provided between the upper surface of the third substrate layer 3 and the lower surface of the spacer layer 5 and at a position where the side part is partitioned by the side surface of the first solid electrolyte layer 4. The reference gas introduction space 43 opens to the rear end surface of the element body 102, and this opening functions as an inlet part 49a of the reference gas introduction part 49. The reference gas is introduced into the reference gas introduction space 43 from this inlet part 49a. The reference gas introduction part 49 introduces the reference gas introduced from the inlet part 49a to the reference electrode 42 while imparting a predetermined diffusion resistance to the reference gas. The reference gas is air in this embodiment.

[0033] The reference gas introduction layer 48 is provided between the upper surface of the third substrate layer 3 and the lower surface of the first solid electrolyte layer 4. The reference gas introduction layer 48 is a porous body made of ceramics such as alumina, for example. A part of the upper surface of the reference gas introduction layer 48 is exposed in the reference gas introduction space 43. The reference gas introduction layer 48 is formed so as to cover the reference electrode 42. The reference gas introduction layer 48 allows the reference gas to flow from the reference gas introduction space 43 to the reference electrode 42.

[0034] The reference electrode 42 is an electrode formed in a manner sandwiched between the upper surface of the third substrate layer 3 and the first solid electrolyte layer 4. As described above, a reference gas introduction layer 48 connected to the reference gas introduction space 43 is provided around the reference electrode 42. Further, as will be described later, it is possible 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 using the reference electrode 42.

[0035] In the measured gas flow path, the gas inlet 10 is a part that opens to the external space, and the measured gas is taken into the sensor element 101 from the external space through the gas inlet 10. The first diffusion rate-limiting part 11 is a part that imparts a predetermined diffusion resistance to the measured gas taken in from the gas inlet 10. The buffer space 12 is a space provided to guide the measured gas introduced from the first diffusion rate-limiting part 11 to the second diffusion rate-limiting part 13. The second diffusion rate-limiting part 13 is a part that imparts a predetermined diffusion resistance to the measured gas introduced from the buffer space 12 into the first internal cavity 20. When the measured gas is introduced from outside the sensor element 101 into the first internal cavity 20, the measured gas suddenly taken into the sensor element 101 from the gas inlet 10 due to the pressure fluctuation of the measured gas in the external space (when the measured gas is the exhaust gas of an internal combustion engine, the pulsation of the exhaust pressure) is not directly introduced into the first internal cavity 20, but after the pressure fluctuation of the measured gas is canceled through the first diffusion rate-limiting part 11, the buffer space 12, and the second diffusion rate-limiting part 13, it is introduced into the first internal cavity 20. As a result, the pressure fluctuation of the measured gas introduced into the first internal cavity 20 becomes negligible. The first internal cavity 20 is provided as a space for adjusting the oxygen partial pressure in the measured gas introduced through the second diffusion rate-limiting part 13. This oxygen partial pressure is adjusted by the operation of the main pump cell 21.

[0036] The main pump cell 21 includes an inner pump electrode 22 having a ceiling electrode part 22a provided on substantially the entire lower surface of the second solid electrolyte layer 6 facing the first internal cavity 20, an outer pump electrode 23 provided in a manner exposed to the outside of the element body 102 in a region corresponding to the ceiling electrode part 22a on the upper surface of the second solid electrolyte layer 6, and the second solid electrolyte layer 6, the spacer layer 5, and the first solid electrolyte layer 4 that form a current path between these electrodes, and is an electrochemical pump cell constituted thereby.

[0037] The inner pump electrode 22 is formed across the upper and lower solid electrolyte layers (the second solid electrolyte layer 6 and the first solid electrolyte layer 4) that partition the first internal cavity 20, and the spacer layer 5 that provides side walls. Specifically, a ceiling electrode portion 22a is formed on the lower surface of the second solid electrolyte layer 6 that provides the ceiling surface of the first internal cavity 20, and a bottom electrode portion 22b is formed on the upper surface of the first solid electrolyte layer 4 that provides the bottom surface of the first internal cavity 20. Then, a side electrode portion (not shown) is formed on the side wall surface (inner surface) of the spacer layer 5 that constitutes both side wall portions of the first internal cavity 20 so as to connect the ceiling electrode portion 22a and the bottom electrode portion 22b, and is disposed in a structure having a tunnel form at the disposed position of the side electrode portion.

[0038] In the main pump cell 21, a desired voltage Vp0 is applied between the inner pump electrode 22 and the outer pump electrode 23, and a pump current Ip0 is caused to flow in the positive or negative direction between the inner pump electrode 22 and the outer pump electrode 23, whereby oxygen in the first internal cavity 20 can be pumped out to the external space, or oxygen in the external space can be pumped into the first internal cavity 20.

[0039] In addition, in order to detect the oxygen concentration (oxygen partial pressure) in the atmosphere in the first internal cavity 20, an electrochemical sensor cell, that is, a main pump control oxygen partial pressure detection sensor cell 80 is constituted by the inner pump electrode 22, the second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, the third substrate layer 3, and the reference electrode 42.

[0040] By measuring the electromotive force (voltage V0) in the main pump control oxygen partial pressure detection sensor cell 80, the oxygen concentration (oxygen partial pressure) in the first internal cavity 20 can be known. Further, by feedback controlling the voltage Vp0 of the variable power supply 24 so that the voltage V0 becomes a target value, the pump current Ip0 is controlled. Thereby, the oxygen concentration in the first internal cavity 20 can be maintained at a predetermined constant value.

[0041] The third diffusion rate-limiting part 30 is a part that imparts a predetermined diffusion resistance to the gas to be measured whose oxygen concentration (oxygen partial pressure) is controlled by the operation of the main pump cell 21 in the first internal cavity 20, and guides the gas to be measured to the second internal cavity 40.

[0042] The second internal cavity 40 is provided as a space for further adjusting the oxygen partial pressure of the gas to be measured introduced through the third diffusion rate-limiting part 30 with respect to the gas to be measured whose oxygen concentration (oxygen partial pressure) has been adjusted in the first internal cavity 20 in advance by the auxiliary pump cell 50. Thereby, since the oxygen concentration in the second internal cavity 40 can be kept constant with high precision, highly accurate NOx concentration measurement can be performed in the gas sensor 100.

[0043] The auxiliary pump cell 50 includes an auxiliary pump electrode 51 having a ceiling electrode part 51a provided substantially over the entire lower surface of the second solid electrolyte layer 6 facing the second internal cavity 40, an outer pump electrode 23 (not limited to the outer pump electrode 23, and any appropriate electrode disposed on the outer peripheral surface of the element body 102 is sufficient), and an auxiliary electrochemical pump cell composed of the second solid electrolyte layer 6, the spacer layer 5, and the first solid electrolyte layer 4.

[0044] The auxiliary pump electrode 51 is disposed in the second internal cavity 40 in a structure having a tunnel form similar to that of the inner pump electrode 22 provided in the first internal cavity 20. That is, a ceiling electrode part 51a is formed on the second solid electrolyte layer 6 that provides the ceiling surface of the second internal cavity 40, a bottom electrode part 51b is formed on the first solid electrolyte layer 4 that provides the bottom surface of the second internal cavity 40, and side electrode parts (not shown) that connect the ceiling electrode part 51a and the bottom electrode part 51b are formed in a tunnel form on both wall surfaces of the spacer layer 5 that provides the side walls of the second internal cavity 40.

[0045] In the auxiliary pump cell 50, by applying a desired voltage Vp1 between the auxiliary pump electrode 51 and the outer pump electrode 23, oxygen in the atmosphere in the second internal cavity 40 can be pumped out to the external space or pumped into the second internal cavity 40 from the external space.

[0046] Also, in order to control the oxygen partial pressure in the atmosphere in the second internal cavity 40, an electrochemical sensor cell, that is, an oxygen partial pressure detection sensor cell 81 for auxiliary pump control is constituted by the auxiliary pump electrode 51, the reference electrode 42, the second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, and the third substrate layer 3.

[0047] The auxiliary pump cell 50 pumps based on the electromotive force (voltage V1) detected by this oxygen partial pressure detection sensor cell 81 for auxiliary pump control by a variable power supply 52 whose voltage is controlled. Thereby, the oxygen partial pressure in the atmosphere in the second internal cavity 40 is controlled to a low partial pressure that has substantially no influence on the measurement of NOx.

[0048] At the same time, the pump current Ip1 is used to control the electromotive force of the oxygen partial pressure detection sensor cell 80 for main pump control. Specifically, the pump current Ip1 is input as a control signal to the oxygen partial pressure detection sensor cell 80 for main pump control, and by controlling the above-described target value of the voltage V0, the gradient of the oxygen partial pressure in the measurement gas introduced from the third diffusion rate limiting section 30 into the second internal cavity 40 is always kept constant. When used as a NOx sensor, due to the functions of the main pump cell 21 and the auxiliary pump cell 50, the oxygen concentration in the second internal cavity 40 is maintained at a constant value of about 0.001 ppm.

[0049] The fourth diffusion rate limiting section 60 is a part that imparts a predetermined diffusion resistance to the measurement gas whose oxygen concentration (oxygen partial pressure) is controlled by the operation of the auxiliary pump cell 50 in the second internal cavity 40 and guides the measurement gas to the third internal cavity 61. The fourth diffusion rate limiting section 60 plays a role of limiting the amount of NOx flowing into the third internal cavity 61.

[0050] The third internal space 61 is provided as a space for performing a process related to the measurement of the nitrogen oxide (NOx) concentration in the gas to be measured, which is introduced through the fourth diffusion rate-limiting section 60, after the oxygen concentration (oxygen partial pressure) has been adjusted in the second internal space 40 in advance. The measurement of the NOx concentration is mainly performed in the third internal space 61 by the operation of the measurement pump cell 41.

[0051] The measurement pump cell 41 measures the NOx concentration in the gas to be measured within the third internal space 61. The measurement pump cell 41 is an electrochemical pump cell composed of a measurement electrode 44 provided on the upper surface of the first solid electrolyte layer 4 facing the third internal space 61, an outer pump electrode 23, a second solid electrolyte layer 6, a spacer layer 5, and the first solid electrolyte layer 4. The measurement electrode 44 also functions as a NOx reduction catalyst that reduces NOx present in the atmosphere within the third internal space 61.

[0052] In the measurement pump cell 41, oxygen generated by the decomposition of nitrogen oxide (NOx) in the atmosphere around the measurement electrode 44 can be pumped out, and the generated amount can be detected as a pump current Ip2.

[0053] Also, in order to detect the oxygen partial pressure around the measurement electrode 44, an electrochemical sensor cell, that is, an oxygen partial pressure detection sensor cell 82 for controlling the measurement pump, is constituted by the first solid electrolyte layer 4, the third substrate layer 3, the measurement electrode 44, and the reference electrode 42. The variable power supply 46 is controlled based on the electromotive force (voltage V2) detected by the oxygen partial pressure detection sensor cell 82 for controlling the measurement pump.

[0054] The gas to be measured introduced into the second internal space 40 reaches the measurement electrode 44 within the third internal space 61 through the fourth diffusion rate-limiting section 60 under the condition where the oxygen partial pressure is controlled. Nitrogen oxide (NOx) in the gas to be measured around the measurement electrode 44 is reduced (2NO → N 2 +O 2) Oxygen is generated. Then, the generated oxygen is pumped by the measurement pump cell 41. At this time, the voltage Vp2 of the variable power supply 46 is controlled so that the voltage V2 detected by the oxygen partial pressure detection sensor cell 82 for controlling the measurement pump becomes constant (target value). Since the amount of oxygen generated around the measurement electrode 44 is proportional to the concentration of nitrogen oxides in the gas to be measured, the concentration of nitrogen oxides in the gas to be measured is calculated using the pump current Ip2 in the measurement pump cell 41.

[0055] Also, if the measurement electrode 44, the first solid electrolyte layer 4, the third substrate layer 3, and the reference electrode 42 are combined to form oxygen partial pressure detection means as an electrochemical sensor cell, the electromotive force corresponding to the difference between the amount of oxygen generated by the reduction of the NOx component in the atmosphere around the measurement electrode 44 and the amount of oxygen contained in the reference atmosphere can be detected, and thereby the concentration of the NOx component in the gas to be measured can also be obtained.

[0056] Furthermore, an electrochemical sensor cell 83 is constituted by the second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, the third substrate layer 3, the outer pump electrode 23, and the reference electrode 42. The electromotive force (voltage Vref) obtained by this sensor cell 83 enables the detection of the oxygen partial pressure in the gas to be measured outside the element body 102 of the sensor element 101, specifically, around the outer pump electrode 23.

[0057] In the gas sensor 100 having such a configuration, by operating the main pump cell 21 and the auxiliary pump cell 50, the gas to be measured in which the oxygen partial pressure is always maintained at a constant low value (a value that has substantially no influence on the measurement of NOx) is supplied to the measurement pump cell 41. Therefore, based on the pump current Ip2 that flows when oxygen generated by the reduction of NOx is pumped out from the measurement pump cell 41 in substantially proportion to the concentration of NOx in the gas to be measured, the concentration of NOx in the gas to be measured can be known.

[0058] Here, each of the electrodes 22, 23, 42, 44, 51 will be described. The inner pump electrode 22, the auxiliary pump electrode 51, and the measurement electrode 44 each contain a first noble metal having catalytic activity. Examples of the first noble metal include at least any one of Pt, Rh, Ir, Ru, and Pd. The outer pump electrode 23 and the reference electrode 42 also contain the first noble metal. The inner pump electrode 22 and the auxiliary pump electrode 51 also contain a second noble metal that suppresses the catalytic activity with respect to a specific gas (NOx) by the first noble metal. Thereby, the reduction ability of the inner pump electrode 22 and the auxiliary pump electrode 51 with respect to the NOx component in the gas to be measured is weakened. Examples of the second noble metal include Au. The measurement electrode 44 does not contain the second noble metal. Thereby, the reduction ability with respect to the NOx component in the gas to be measured is enhanced compared to the inner pump electrode 22 and the auxiliary pump electrode 51. The measurement electrode 44 preferably contains at least one of Pt and Rh among the first noble metals, and may contain both Pt and Rh. It is also preferable that the outer pump electrode 23 and the reference electrode 42 do not contain the second noble metal. Each of the electrodes 22, 23, 42, 44, 51 is preferably a cermet containing a noble metal and an oxide having oxygen ion conductivity (e.g., ZrO 2 ). Each of the electrodes 22, 23, 42, 44, 51 is preferably a porous body. In the present embodiment, the inner pump electrode 22 and the auxiliary pump electrode 51 are porous cermet electrodes of Pt containing 1% Au and ZrO 2 . Also, both the outer pump electrode 23 and the reference electrode 42 are porous cermet electrodes of Pt and ZrO 2 . The measurement electrode 44 is a porous cermet electrode of Pt, Rh, and ZrO 2 .

[0059] The heater unit 70 plays a role of temperature adjustment for heating and keeping warm the sensor element 101 in order to enhance the oxygen ion conductivity of the solid electrolyte of the element body 102. The heater unit 70 includes a heater 71a, a heater insulating layer 71b, and a pressure dissipation hole 71c.

[0060] The heater 71a is an electric resistor formed in a manner sandwiched from above and below by the second substrate layer 2 and the third substrate layer 3. When power is supplied from the heater power supply 72, the heater 71a generates heat to heat and keep warm the solid electrolyte of the element body 102 of the sensor element 101. Further, the heater 71a is embedded across the entire region from the first internal cavity 20 to the third internal cavity 61, making it possible to adjust the temperature of the entire sensor element 101 to a temperature at which the solid electrolyte is activated.

[0061] The heater insulating layer 71b is an insulating layer formed of an insulator such as alumina on the upper and lower surfaces of the heater 71a. The heater insulating layer 71b is formed for the purpose of obtaining electrical insulation between the second substrate layer 2 and the heater 71a, and electrical insulation between the third substrate layer 3 and the heater 71a.

[0062] The pressure dissipation hole 71c is a portion provided so as to penetrate the third substrate layer 3 and the reference gas introduction layer 48 and communicate with the reference gas introduction space 43, and is formed for the purpose of alleviating the increase in internal pressure accompanying the rise in temperature within the heater insulating layer 71b.

[0063] The gas sensor 100 further includes a control device 95. As shown in FIG. 3, the control device 95 includes the variable power supplies 24, 46, 52 described above, the heater power supply 72 described above, and a control unit 96. The control unit 96 is a microprocessor having a CPU 97 and a storage unit 98. The storage unit 98 is a non-volatile memory capable of rewriting information, and can store various programs and various data, for example. The control unit 96 inputs the voltage V0 of the oxygen partial pressure detection sensor cell 80 for main pump control, the voltage V1 of the oxygen partial pressure detection sensor cell 81 for auxiliary pump control, the voltage V2 of the oxygen partial pressure detection sensor cell 82 for measurement pump control, the voltage Vref of the sensor cell 83, the pump current Ip0 flowing through the main pump cell 21, the pump current Ip1 flowing through the auxiliary pump cell 50, and the pump current Ip2 flowing through the measurement pump cell 41. Further, the control unit 96 controls the voltages Vp0, Vp1, Vp2 output by the variable power supplies 24, 46, 52 by outputting a control signal to the variable power supplies 24, 46, 52, thereby controlling the main pump cell 21, the measurement pump cell 41, and the auxiliary pump cell 50. The control unit 96 controls the power supplied by the heater power supply 72 to the heater 71a by outputting a control signal to the heater power supply 72. The storage unit 98 also stores target values V0*, V1*, V2*, etc. to be described later. The CPU 97 of the control unit 96 controls each pump cell 21, 41, 50 with reference to these target values V0*, V1*, V2*.

[0064] The control unit 96 performs an auxiliary pump control process for controlling the auxiliary pump cell 50 so that the oxygen concentration in the second internal cavity 40 becomes the target concentration. Specifically, the control unit 96 controls the auxiliary pump cell 50 by feedback controlling the voltage Vp1 of the variable power supply 52 so that the voltage V1 becomes a constant value (referred to as the target value V1*). The target value V1* is determined as a value such that the oxygen concentration in the second internal cavity 40 becomes a predetermined low concentration that has substantially no influence on the measurement of NOx.

[0065] The control unit 96 performs main pump control processing to control the main pump cell 21 so that the pump current Ip1 flowing when the auxiliary pump cell 50 adjusts the oxygen concentration in the second internal cavity 40 becomes the target current (referred to as the target value Ip1*). Specifically, the control unit 96 sets (feedback control) the target value of the voltage V0 (referred to as the target value V0*) based on the pump current Ip1 so that the pump current Ip1 flowing due to the voltage Vp1 becomes a constant target value Ip1*. Then, the control unit 96 performs feedback control on the voltage Vp0 of the variable power supply 24 so that the voltage V0 becomes the target value V0* (that is, so that the oxygen concentration in the first internal cavity 20 becomes the target concentration). By this main pump control processing, the gradient of the oxygen partial pressure in the measurement gas introduced from the third diffusion rate limiting section 30 into the second internal cavity 40 always remains constant. The target value V0* is set to a value such that the oxygen concentration in the first internal cavity 20 is higher than 0% and is a low concentration. Also, the pump current Ip0 flowing during this main pump control processing changes according to the oxygen concentration of the measurement gas flowing into the measurement gas flow section from the gas inlet 10 (that is, the measurement gas around the sensor element 101). Therefore, the control unit 96 can also detect the oxygen concentration in the measurement gas based on the pump current Ip0.

[0066] The above-described main pump control processing and auxiliary pump control processing are collectively also referred to as adjustment pump control processing. Also, the first internal cavity 20 and the second internal cavity 40 are collectively also referred to as the oxygen concentration adjustment chamber. The main pump cell 21 and the auxiliary pump cell 50 are collectively also referred to as the adjustment pump cell. By the control unit 96 performing the adjustment pump control processing, the adjustment pump cell adjusts the oxygen concentration in the oxygen concentration adjustment chamber.

[0067] The control unit 96 performs a measurement pump control process for controlling the measurement pump cell 41 such that the voltage V2 becomes a constant value (referred to as the target value V2*), that is, such that the oxygen concentration in the third internal cavity 61 becomes a predetermined low concentration. Specifically, the control unit 96 controls the measurement pump cell 41 by feedback-controlling the voltage Vp2 of the variable power supply 46 such that the voltage V2 becomes the target value V2*. By this measurement pump control process, oxygen is pumped out from within the third internal cavity 61.

[0068] By performing the measurement pump control process, oxygen is pumped out from within the third internal cavity 61 such that the oxygen generated by the reduction of NOx in the gas to be measured in the third internal cavity 61 becomes substantially zero. Then, the control unit 96 acquires the pump current Ip2 as a detection value corresponding to the oxygen generated in the third internal cavity 61 from a specific gas (here, NOx), and calculates the NOx concentration in the gas to be measured based on this pump current Ip2.

[0069] The storage unit 98 stores a relational expression (for example, a linear function or a quadratic function expression) or a map or the like as the correspondence relationship between the pump current Ip2 and the NOx concentration. Such a relational expression or map can be obtained in advance by experiments.

[0070] The control unit 96 performs a heater control process of outputting a control signal to the heater power supply 72 to control the temperature of the heater 71a to reach a target temperature (for example, 800 °C). Here, the temperature of the heater 71a can be expressed by a linear function formula of the resistance value of the heater 71a. In the heater control process, the control unit 96 calculates the resistance value of the heater 71a as a value that can be regarded as the temperature of the heater 71a (a value convertible to temperature), and performs feedback control on the heater power supply 72 so that the calculated resistance value becomes the target resistance value (the resistance value corresponding to the target temperature). The control unit 96 can obtain, for example, the voltage of the heater 71a and the current flowing through the heater 71a, and calculate the resistance value of the heater 71a based on the obtained voltage and current. The control unit 96 may calculate the resistance value of the heater 71a by, for example, the three-terminal method or the four-terminal method. When energizing the heater 71a, the heater power supply 72 adjusts the power supplied to the heater 71a by changing the value of the voltage applied to the heater 71a based on, for example, a control signal from the control unit 96.

[0071] Note that including the variable power supplies 24, 46, 52 and the heater power supply 72 shown in FIG. 3, the control device 95 is connected to each of the electrodes 22, 23, 42, 44, 51 and the heater 71a via corresponding conduction parts 74. Each of the plurality of conduction parts 74 has a connector electrode 75 and a lead part 76 for electrically connecting the corresponding electrode and the connector electrode 75 to each other. Although all of the plurality of connector electrodes 75 are shown in FIG. 4, only the lead part 76 corresponding to the measurement electrode 44 is shown for the lead part 76.

[0072] The plurality of connector electrodes 75 each function as a terminal for electrically connecting the sensor element 101 to the outside. As shown in FIG. 4, the plurality of connector electrodes 75 are disposed on the rear end side of the first surface 102a (upper surface) or the second surface 102b (lower surface) of the element body 102 of the sensor element 101. Specifically, the plurality of connector electrodes 75 include connector electrodes 75a to 75d disposed in order from the left on the rear end side of the first surface 102a of the element body 102, and connector electrodes 75e to 75h disposed in order from the left on the rear end side of the second surface 102b of the element body 102. The connector electrodes 75a to 75d, 75h are each connected (electrically conduct) to the measurement electrode 44, the outer pump electrode 23, the auxiliary pump electrode 51, the inner pump electrode 22, and the reference electrode 42 via the corresponding lead portion 76. The connector electrodes 75e to 75g are each connected to the heater 71a via the corresponding lead portion 76.

[0073] Here, the details of the measurement electrode 44 and the conduction portion 74 (connector electrode 75a and lead portion 76) corresponding thereto will be described with reference to FIGS. 4 and 5. Although not shown in FIG. 2, an airtight adhesive layer is provided between two adjacent layers in the stacking direction (vertical direction) among the layers 1 to 6 of the element body 102. Therefore, two adjacent layers are adhered by the corresponding adhesive layer. Each adhesive layer preferably has oxygen ion conductivity, similar to the layers 1 to 6. In the present embodiment, each adhesive layer is made of a ceramic mainly composed of the same zirconia as the layers 1 to 6. As shown in FIG. 5, among the adhesive layers, the adhesive layer 7 adheres the first solid electrolyte layer 4 and the spacer layer 5. The adhesive layer 7 covers most of the upper surface of the first solid electrolyte layer 4 except for the portions where the measured gas flow portions such as the buffer space 12, the first internal cavity 20, and the second internal cavity 40 are located.

[0074] The measurement electrode 44 is provided on the upper surface of the first solid electrolyte layer 4 facing the third internal cavity 61. The lead portion 76 corresponding to the measurement electrode 44 has an inner lead portion 77 and a side lead portion 78. The inner lead portion 77 is disposed inside the element main body 102 and is connected to (electrically conductive with) the measurement electrode 44 and the side lead portion 78. Specifically, the inner lead portion 77 has first to fourth portions 77a to 77d. The first portion 77a is disposed on the upper surface of the first solid electrolyte layer 4 facing the third internal cavity 61. The right end of the first portion 77a is connected to the measurement electrode 44, and the first portion 77a extends linearly to the left side surface of the third internal cavity 61 along the left-right direction. The right end of the second portion 77b is connected to the left end of the first portion 77a, and the second portion 77b extends linearly to a position in front of the left end of the element main body 102 along the left-right direction. The front end of the third portion 77c is connected to the left end of the second portion 77b, and the third portion 77c extends linearly to a position near the rear end of the element main body 102 along the front-rear direction. The right end of the fourth portion 77d is connected to the rear end of the third portion 77c, and the fourth portion 77d extends linearly to the left end of the element main body 102 along the left-right direction. The left end of the fourth portion 77d reaches (is drawn out to) the third surface 102c (left end surface) of the element main body 102 and is covered by and connected to the side lead portion 78. The inner lead portion 77 is a conductor mainly composed of a noble metal such as platinum (Pt) or a high melting point metal such as tungsten (W) or molybdenum (Mo). The inner lead portion 77 is preferably a cermet conductor containing a noble metal or a high melting point metal and zirconia that is the same as the main component of the first solid electrolyte layer 4.

[0075] Of the inner lead portion 77, the entire second and third portions 77b and 77c and the portion of the fourth portion 77d other than the left end portion are surrounded by the lead insulating layer 79. The lead insulating layer 79 insulates the portion of the inner lead portion 77 that surrounds the outer periphery from the first solid electrolyte layer 4 and the spacer layer 5. Note that the outer periphery of the left end portion of the fourth portion 77d is not surrounded by the lead insulating layer 79. This can suppress the outer periphery of the left end portion of the fourth portion 77d from being surrounded by the lead insulating layer 79 during the manufacture of the sensor element 101, and can suppress the connection (electrical conduction) between the left end of the fourth portion 77d and the side lead portion 78 from being inhibited. The lead insulating layer 79 is an insulator of ceramics such as alumina.

[0076] The side lead portion 78 is disposed on the rear end side of the third surface 102c of the element body 102 and is connected to the inner lead portion 77 and the connector electrode 75a. Specifically, the side lead portion 78 is disposed on the third surface 102c so as to cover the entire left end of the fourth portion 77d of the inner lead portion 77 so that the left end is not exposed to the outside of the sensor element 101. The side lead portion 78 is disposed so as to contact the solid electrolyte layer. The side lead portion 78 is disposed so as to span the solid electrolyte layer and the adhesive layer of the element body 102. The central portion of the right end surface of the side lead portion 78 is connected to the left end of the fourth portion 77d (electrically conductive). The upper end portion of the right end surface of the side lead portion 78 is connected to the left end surface of the front end portion of the connector electrode 75a.

[0077] The side lead portion 78 is made of a noble metal, alumina (Al 2 O 3 ) and silica (SiO 2) and is a conductor densely configured. Specific examples of the noble metal include platinum (Pt) or a platinum alloy. By including not only the noble metal but also alumina and silica in the side lead portion 78 in this way, the adhesion of the side lead portion 78 can be enhanced. Also, by including silica in the side lead portion 78, the density of the side lead portion 78 can also be enhanced. Therefore, for the side lead portion 78, both the density and the adhesion can be made high. The inventors confirmed this through experiments, analysis, etc. "Densely configured" means that when the porosity of the side lead portion 78 is Rp [%], Rp is less than 3.0%. It is preferable that the side lead portion 78 has a porosity Rp of less than 0.5%.

[0078] Here, if some gas outside the sensor element 101 (element main body 102) enters the inside of the element main body 102 from the fourth portion 77d of the inner lead portion 77 or the gap between it and the element main body 102, it will move along the inner lead portion 77 and the gap between it and the element main body 102 (the gap between the inner lead portion 77 and the lead insulating layer 79, and the gap between the lead insulating layer 79 and the element main body 102) toward the measurement electrode 44 side and may reach the measurement electrode 44. Examples of some gas outside the sensor element 101 include, for example, volatile organic gas generated from the rubber stopper 157 when the sensor element 101 is exposed to a high-temperature environment, the gas to be measured that slightly enters from the sensor element chamber 133 into the space 149, the gas that enters the inside of the outer cylinder 148 from outside the gas sensor 100 through the gap between the rubber stopper 157 and the outer cylinder 148, air (for example, the reference gas present in the space 149), water vapor, and the like. When such gas reaches the measurement electrode 44, the oxygen concentration around the measurement electrode 44 changes, the voltage V2 detected by the oxygen partial pressure detection sensor cell 82 for measurement pump control changes, and the amount of oxygen pumped out from the third internal cavity 61 by the measurement pump control process also changes, and the pump current Ip2 may change. For example, when the volatile organic gas generated from the rubber stopper 157 reaches the measurement electrode 44, the oxygen concentration around the measurement electrode 44 decreases, the voltage V2 increases, and the pump current Ip2 may decrease. When some gas reaches the measurement electrode 44 and the pump current Ip2 changes in this way, it causes a decrease in the detection accuracy of the concentration of the specific gas (NOx). In contrast, in the present embodiment, the side lead portion 78, which is the portion covering the inner lead portion 77 and is the portion of the conduction portion 74 that contributes to suppressing the gas from reaching the measurement electrode 44, has high tightness. Therefore, it is possible to suppress some gas outside the element main body 102 from passing through the side lead portion 78, suppress such gas from entering the inside of the element main body 102 from the fourth portion 77d of the inner lead portion 77 or the gap between it and the element main body 102, and suppress such gas from reaching the measurement electrode 44. As a result, it is possible to suppress the change in the pump current Ip2 caused by such gas and suppress the decrease in the detection accuracy of the concentration of the specific gas (NOx). Also, by suppressing such gas from reaching the measurement electrode 44, it is possible to suppress the deterioration of the measurement electrode 44.

[0079] In addition, since the side lead portion 78 has high adhesiveness, it is possible to suppress the side lead portion 78 from peeling off from the third surface 102c of the element body 102 during firing shrinkage in the manufacture of the sensor element 101. Further, it is possible to suppress the side lead portion 78 from peeling off from the third surface 102c of the element body 102 due to thermal expansion and contraction of the sensor element 101.

[0080] When the volumes of the noble metal, alumina, and silica contained in the side lead portion 78 are Vp [Vol%], Va [Vol%], and Vs [Vol%], respectively, the following may be adopted. The side lead portion 78 preferably satisfies Va / Vs ≤ 1.5. Thereby, the denseness of the side lead portion 78 can be further enhanced. The side lead portion 78 may satisfy 0.1 ≤ Va / Vs. The side lead portion 78 preferably satisfies 1.5 ≤ Vp / (Va + Vs). Thereby, the effect of enhancing the denseness of the side lead portion 78 can be more surely obtained. In this case, it is more preferable to satisfy 2.6 ≤ Vp / (Va + Vs). By doing so, the denseness of the side lead portion 78 can be further enhanced. The side lead portion 78 may satisfy Vp / (Va + Vs) ≤ 19.0. The side lead portion 78 more preferably satisfies Va / Vs ≤ 1.5 and 1.5 ≤ Vp / (Va + Vs). The side lead portion 78 still more preferably satisfies Va / Vs ≤ 1.5 and 2.6 ≤ Vp / (Va + Vs). The volume Vp may be 60 Vol% or more. The volume Vp may be 95 Vol% or less. The volume Va may be 1 Vol% or more. The volume Va may be 15 Vol% or less. The volume Vs may be 2 Vol% or more, or may be 4 Vol% or more. The volume Vs may be 25 Vol% or less.

[0081] The side lead portion 78 preferably satisfies that the ratio of the total mass in terms of oxides of the contained alkali metal and alkaline earth metal is 0.1 wt% or less. Here, instead of the side lead portion 78 containing a noble metal, alumina, and silica, it is also conceivable to contain a noble metal and a glass component to improve the airtightness and density. However, in this case, the side lead portion 78 contains, as a glass component, for example, oxides of alkali metals such as sodium oxide (Na 2 O), calcium oxide (CaO), magnesium oxide (MgO), etc. and / or oxides of alkaline earth metals in a large amount. In this case, for example, due to the voltage Vp2 applied during the use of the sensor element 101, the alkali metal and / or alkaline earth metal moves by electromigration (for example, from the inside of the side lead portion 78 to the inner lead portion 77), and the conductivity of the side lead portion 78 decreases (the resistance value increases). As a result, the pump current Ip2 decreases, leading to a decrease in the detection accuracy of the concentration of a specific gas (NOx). On the other hand, the side lead portion 78 of the present embodiment contains alumina and silica, thereby increasing the adhesion and density, and the ratio of the total mass in terms of oxides of the alkali metal and alkaline earth metal is 0.1 wt% or less, so that a decrease in the detection accuracy of the concentration of a specific gas caused by such electromigration can also be suppressed.

[0082] When the thickness of the side lead portion 78 is Dc [mm], the side lead portion 78 may be configured such that the porosity Rp and the thickness Dc satisfy Rp / Dc ≦ 145% / mm. The side lead portion 78 may be configured such that Rp / Dc ≦ 120% / mm. The side lead portion 78 may be configured such that Rp / Dc ≧ 1.5% / mm. The side lead portion 78 may be configured such that Rp / Dc ≧ 4.5% / mm. The porosity Rp of the side lead portion 78 may be 0.01% or more. The thickness Dc of the side lead portion 78 may be 0.001 mm or more. The thickness Dc of the side lead portion 78 may be 0.080 mm or less.

[0083] Note that each porosity such as the porosity Rp of the side lead portion 78 is a value derived as follows using an image (SEM image) obtained by observing using a scanning electron microscope (SEM). First, the measurement target is cut so that the cross-section of the measurement target (when the side lead portion 78 is the measurement target, the cross-section along the thickness direction of the side lead portion 78) is the observation surface, and the cut surface is resin-embedded and polished to obtain an observation sample. Subsequently, an SEM photograph (secondary electron image, acceleration voltage 15 kV, magnification 1000 times, but if 1000 times magnification is inappropriate, a magnification greater than 1000 times and less than or equal to 5000 times) is used to photograph the observation surface of the observation sample to obtain an SEM image of the measurement target. Next, by performing image analysis on the obtained image, a threshold value is determined by the discriminant analysis method (Otsu's binarization) from the luminance distribution of the luminance data of the pixels in the image. Thereafter, each pixel in the image is binarized into an object portion and a pore portion based on the determined threshold value, and the area of the object portion and the area of the pore portion are calculated. Then, the ratio of the area of the pore portion to the total area (the total area of the object portion and the pore portion) is derived as the porosity (unit: %).

[0084] The volumes Vp [Vol%], Va [Vol%], and Vs [Vol%] of the noble metal, alumina, and silica in the side lead portion 78 are values derived as follows using SEM-EDX (EDX: energy dispersive X-ray spectroscopy). First, similar to the measurement of the porosity Rp described above, the measurement target is cut so that the cross-section of the measurement target is the observation surface to obtain an observation sample. Next, elemental mapping is performed at a plurality of locations (for example, three locations) on the observation surface of the observation sample by SEM-EDX. From the results of the elemental mapping, the average value of the elemental concentration is calculated for each of the elements of the noble metal, aluminum, and silicon. Then, the volume Vp [Vol%] is calculated based on the obtained average value of the elemental concentration of the noble metal. Also, the average value of the elemental concentration of aluminum is converted to an oxide to calculate the volume Va [Vol%], and the average value of the elemental concentration of silicon is converted to an oxide to calculate the volume Vs [Vol%].

[0085] The ratio of the total mass in terms of oxides of alkali metals and alkaline earth metals in the side lead portion 78 shall be the value derived as follows using ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometry). First, the measurement target is peeled off from the sensor element 101 and a predetermined amount is weighed to obtain a measurement sample. Next, the measurement sample is dissolved using sulfuric acid to obtain a dissolved solution. Using this dissolved solution, ICP-AES is performed to conduct a qualitative analysis of various elements contained in the measurement sample. Using the test concentration value of Na obtained by this qualitative analysis and the calibration curve of Na obtained in advance, the actual concentration value of Na is determined. The obtained actual concentration value of Na is converted into oxides to calculate the mass ratio of sodium oxide in the measurement target. The mass ratios in terms of oxides of alkali metals and alkaline earth metals other than Na are also calculated by the same procedure. Using these calculation results, the ratio of the total mass in terms of oxides of alkali metals and alkaline earth metals in the measurement target is calculated.

[0086] Next, an example of a method for manufacturing the sensor element 101 of such a gas sensor 100 will be described below. First, six unfired ceramic green sheets containing an oxygen ion conductive solid electrolyte such as zirconia as a ceramic component are prepared. A plurality of sheet holes and necessary through holes for positioning during printing or lamination are formed in advance in this green sheet. Further, in the green sheet that will become the spacer layer 5, a space that will become the measured gas flow path is provided in advance by punching or the like. Similarly, a space that will become the reference gas introduction space 43 is provided in the green sheet that will become the first solid electrolyte layer 4. Then, for each of 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, a pattern printing process and a drying process for forming various patterns on each ceramic green sheet are performed. Specifically, the patterns to be formed are, for example, each electrode such as the measurement electrode 44 described above, inner lead portions such as the inner lead portion 77 connected to each electrode, lead insulating layers such as the lead insulating layer 79, connector electrodes 75, reference gas introduction layers 48, heater portions 70, and the like. Pattern printing is performed by applying a pattern forming paste prepared according to the characteristics required for each formation target to the green sheet using a known screen printing technique. The drying process is also performed using known drying means. After the pattern printing process and the drying process are completed, a printing process and a drying process of an adhesive paste that will become an adhesive layer (including the adhesive layer 7 described above) for laminating and bonding the green sheets corresponding to each layer are performed. Then, the green sheets on which the adhesive paste is formed are laminated in a predetermined order while being positioned by the sheet holes, and a crimping process is performed by applying predetermined temperature and pressure conditions to form a single laminate. The laminate thus obtained includes a plurality of sensor elements 101. The laminate is cut into the size of the sensor element 101. Then, a pattern that will become the side lead portion 78 is formed by screen printing on the portion that will become the third surface 102c of the element body 102 of the sensor element 101, which is the cut surface of the laminate, and the drying process of this pattern is performed. Then, the laminate is fired at a predetermined firing temperature to obtain the sensor element 101.

[0087] When forming the patterns for the inner lead portion 77 and the lead insulating layer 79 on the green sheet that becomes the first solid electrolyte layer 4, for example, the following method may be used. First, form a pattern on the green sheet that covers the lower side of the inner lead portion 77 of the lead insulating layer 79. Subsequently, form the pattern for the inner lead portion 77. Then, form the portions that cover the sides and the upper side of the inner lead portion 77 of the lead insulating layer 79.

[0088] Also, for the paste for forming the pattern of the side lead portion 78, use a paste containing a noble metal such as platinum, alumina, silica, a binder, and a solvent. The pattern-forming paste may further contain at least one of a plasticizer and a dispersion aid. Instead of alumina and silica, materials that become alumina and silica by firing may be mixed in the pattern-forming paste. For example, one or more of aluminum hydroxide, silicic acid, mullite, and kaolinite may be mixed. The adjustment of the porosity Rp of the side lead portion 78 can be achieved, for example, by adjusting the ratios of the noble metal, alumina, and silica contained in the paste for forming the pattern of the side lead portion 78 to adjust the above-mentioned Va / Vs and / or Vp / (Va + Vs) of the side lead portion 78 after firing. The adjustment of the thickness Dc of the side lead portion 78 can be achieved, for example, by adjusting the viscosity of the paste for forming the pattern of the side lead portion 78 or by changing the number of printing times during pattern formation.

[0089] Note that the particles contained in the paste for forming the pattern of the side lead portion 78, particularly the particles of alumina and silica, may contain alkali metals and / or alkaline earth metals as impurities. For these particles, by using high-purity particles or performing a washing process and then mixing with other raw materials to prepare the pattern-forming paste, the proportion of the total mass of the alkali metals and alkaline earth metals in the side lead portion 78 in terms of oxide conversion can be made 0.1 wt% or less.

[0090] Thereafter, a gas sensor 100 incorporating the sensor element 101 is manufactured. For example, an element sealing body 141 is attached to the sensor element 101 and sealed and fixed, and a protective cover 130 is attached to the tip side of the element main body 102 of the sensor element 101 in the element sealing body 141. Also, a connector 150 and a lead wire 155 are attached to the rear end side of the element main body 102 of the sensor element 101 so as to be electrically connected to the connector electrode 75. Further, an outer cylinder 148 is attached to the rear end side of the element main body 102 in the element sealing body 141, the lead wire 155 is drawn out from the outer cylinder 148 to the outside, and the outer cylinder 148 is welded and fixed to the main fitting 142. Subsequently, the rubber plug 57 is inserted into the outer cylinder 148 through the through hole of the rubber plug 157 with the lead wire 155 passed therethrough, the outer cylinder 148 is reduced in diameter by caulking to form caulked portions 148a, 148b, and the rubber plug 57 and the outer cylinder 148 are fixed. Then, the control device 95 and the sensor element 101 are connected via the lead wire 155. Thus, the gas sensor 100 is obtained.

[0091] Next, an example of using the gas sensor 100 will be described. First, the CPU 97 of the control unit 96 controls the heater power supply 72 to supply power to the heater 71a so that the temperature of the heater 71a becomes a target temperature (for example, 800°C or the like). The CPU 97 acquires, for example, a value convertible to the temperature of the heater 71a (for example, the resistance value or current value of the heater 71a), and controls the heater power supply 72 based on this value by feedback control to control the temperature of the heater 71a. When the temperature of the heater 71a reaches the target temperature (or near the target temperature), the CPU 97 starts the control of the above-described pump cells 21, 41, 50 (the adjustment pump control process and the measurement pump control process) and the acquisition of the voltages V0, V1, V2, Vref from the above-described sensor cells 80 to 83. In this state, when the gas to be measured is introduced from the gas inlet 10, the gas to be measured passes through the first diffusion rate-limiting section 11, the buffer space 12, the second diffusion rate-limiting section 13, and reaches the first internal cavity 20. Next, the oxygen concentration of the gas to be measured is adjusted by the main pump cell 21 and the auxiliary pump cell 50 in the first internal cavity 20 and the second internal cavity 40, and the adjusted gas to be measured reaches the third internal cavity 61. Then, the CPU 97 detects the concentration of NOx in the gas to be measured based on the acquired pump current Ip2 and the correspondence relationship stored in the storage unit 98.

[0092] Here, the correspondence between the components of this embodiment and the components of the present invention will be clarified. Each of 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 of this embodiment corresponds to the solid electrolyte layer of the present invention, the element body 102 corresponds to the element body, the first to fourth surfaces 102a to 102d correspond to the side surfaces, the measurement electrode 44 corresponds to the inner electrode, the conduction part 74 corresponds to the conduction part, the inner lead part 77 corresponds to the inner conduction part, the connector electrode 75a and the side lead part 78 correspond to the outer conduction part, and the side lead part 78 corresponds to the dense part. Further, the protective cover 130 and the sensor assembly 140 correspond to the case, and the rubber plug 157 corresponds to the sealing member.

[0093] In the sensor element 101 included in the gas sensor 100 of the present embodiment described in detail above, the side lead portion 78 covers the left end of the fourth portion 77d of the inner lead portion 77 and is densely configured to include a noble metal, alumina, and silica. As a result, for the side lead portion 78, which is a part of the conduction portion 74 that contributes to suppressing the reach of gas to the measurement electrode 44, both the airtightness and the adhesion can be made high. And because the side lead portion 78 is airtight, any gas outside the sensor element 101 (element main body 102), such as volatile organic gas generated from the rubber stopper 157, is suppressed from passing through the side lead portion 78, and such gas is suppressed from entering the inside of the element main body 102 from the fourth portion 77d of the inner lead portion 77 or the gap between it and the element main body 102, and such gas is suppressed from reaching the measurement electrode 44. As a result, it is possible to suppress the change in the pump current Ip2 due to such gas and suppress the deterioration of the detection accuracy of the concentration of a specific gas (NOx). Also, because the adhesion of the side lead portion 78 is high, it is possible to suppress the side lead portion 78 from peeling off from the third surface 102c of the element main body 102 during firing shrinkage during the manufacture of the sensor element 101. Also, it is possible to suppress the side lead portion 78 from peeling off from the third surface 102c due to the thermal expansion and contraction of the sensor element 101. In addition, it is possible to suppress the side lead portion 78 from separating from the third surface 102c due to stress (such as contact with a jig) applied from the outside when the fired sensor element 101 flows within the manufacturing process.

[0094] Also, in the sensor element 101, the side lead portion 78 is configured to satisfy Va / Vs ≦ 1.5. Thereby, the airtightness of the side lead portion 78 can be further enhanced, and any gas outside the element main body 102 can be further suppressed from passing through the side lead portion 78.

[0095] Furthermore, in the sensor element 101, the side lead portion 78 is configured to satisfy 1.5 ≤ Vp / (Va + Vs). Thereby, the effect of enhancing the tightness of the side lead portion 78 can be obtained more reliably. In addition, the side lead portion 78 is configured to satisfy 2.6 ≤ Vp / (Va + Vs). Thereby, the tightness of the side lead portion 78 can be further enhanced.

[0096] And, in the sensor element 101, the side lead portion 78 is configured such that the ratio of the total mass in terms of oxides of the contained alkali metal and alkaline earth metal satisfies 0.1 wt% or less. Thereby, it is possible to suppress the movement of the alkali metal and / or alkaline earth metal in the side lead portion 78 due to electromigration and the decrease in the conductivity of the side lead portion 78, and it is possible to suppress the decrease in the detection accuracy of the concentration of the specific gas due to the decrease in conductivity.

[0097] Note that the present invention is not limited to the above-described embodiments at all, and it goes without saying that the present invention can be implemented in various modes as long as it belongs to the technical scope of the present invention.

[0098] For example, in the above-described embodiment, the side lead portion 78 is configured as a single layer, but it is not limited thereto. For example, as shown in the sensor element 201 of the modification example in FIG. 6 and the sensor element 301 of the modification example in FIG. 7, the side lead portion 78 may be replaced with the side lead portions 278 and 378. The side lead portions 278 and 378 are each configured as two layers. Note that the side lead portions 278 and 378 may be configured as three or more layers instead of two layers.

[0099] In the sensor element 201 of FIG. 6, the side lead portion 278 is generally in the shape of a substantially rectangular parallelepiped and has a first side lead portion 278a and a second side lead portion 278b. The first side lead portion 278a is in the shape of a substantially rectangular parallelepiped and is disposed on the third surface 102c (left side surface) of the element body 102 so as to cover the entire left end of the fourth portion 77d of the inner lead portion 77 and prevent the left end thereof from being exposed outside the sensor element 101. The second side lead portion 278b is in the shape of a substantially rectangular parallelepiped and is disposed on the left end surface of the first side lead portion 278a. In this case, the first side lead portion 278a is a portion of the conduction portion 74 that contributes to suppressing the gas from reaching the measurement electrode 44. Therefore, the first side lead portion 278a may be configured as a dense portion of the present invention in the same manner as the side lead portion 78 of the sensor element 101.

[0100] In the sensor element 301 of FIG. 7, the side lead portion 378 is generally in the shape of a substantially rectangular parallelepiped and has a first side lead portion 378a and a second side lead portion 378b. The first side lead portion 378a is in the shape of a substantially rectangular parallelepiped and is disposed on the third surface 102c (left side surface) of the element body 102 so as to cover the entire left end of the fourth portion 77d of the inner lead portion 77 and prevent the left end thereof from being exposed outside the sensor element 101. The second side lead portion 378b covers five surfaces other than the right end surface of the first side lead portion 378a and is in contact with the third surface 102c of the element body 102, so that the fourth portion 77d of the inner lead portion 77 and the first side lead portion 378a are not exposed outside the sensor element 301. In this case, both the first side lead portion 378a and the second side lead portion 378b are portions of the conduction portion 74 that contribute to suppressing the gas from reaching the measurement electrode 44. Therefore, the first side lead portion 378a may be configured as a dense portion of the present invention, the second side lead portion 378b may be configured as a dense portion of the present invention, or the entire side lead portion 378 may be configured as a dense portion of the present invention. Therefore, for those of the first side lead portion 378a, the second side lead portion 378b, and the entire side lead portion 378 that are configured as the dense portion of the present invention, they may be configured in the same manner as the side lead portion 78 of the sensor element 101.

[0101] In the above-described embodiment, the connector electrode 75a is connected to the measurement electrode 44 via the inner lead portion 77 and the side lead portion 78, but the present invention is not limited thereto. For example, as shown in the sensor element 401 of the modification in FIG. 8, the element body 102 may be replaced with the element body 402, and the inner lead portion 77 and the side lead portion 78 may be replaced with the inner lead portion 477 and the through-hole conductor 478. The element body 402 of the sensor element 401 has a through-hole 402h. The through-hole 402h includes an opening that opens on the first surface 102a of the element body 402 and penetrates the spacer layer 5 and the second solid electrolyte layer 6 in the stacking direction (vertical direction). The inner lead portion 477 is disposed inside the element body 402, specifically, between the first solid electrolyte layer 4 and the spacer layer 5, similar to the inner lead portion 77 of the sensor element 101. Note that at least a part of the outer periphery of the inner lead portion 477 is surrounded by a lead insulating layer (not shown), similar to the inner lead portion 77. The through-hole conductor 478 is disposed in the through-hole 402h via an insulating layer 479, the lower end is connected to the inner lead portion 477, and the upper end is covered and connected to the connector electrode 75a so as not to be exposed outside the sensor element 401. In this case, the connector electrode 75a corresponds to the outer conduction portion of the present invention. Further, the connector electrode 75a is a portion of the conduction portion 74 that contributes to suppressing the gas from reaching the measurement electrode 44. Therefore, the connector electrode 75a may be configured as a dense portion of the present invention, similar to the side lead portion 78 of the sensor element 101.

[0102] In the sensor element 401, the connector electrode 75a is configured as a single layer, but it may be configured as two or more layers. In this case, the same consideration can be made as for the side lead portions 278 and 378 of the sensor elements 201 and 301 of the modifications in FIGS. 6 and 7.

[0103] In the above-described sensor elements 101, 201, 301, and 401, the portion of the outer conduction part that covers the inner conduction part is configured as the dense part of the present invention. However, the dense part of the present invention may include the portion of the outer conduction part that covers the inner conduction part and / or at least a part of the inner conduction part. For example, in the sensor elements 101, 201, and 301, at least a part of the inner conduction part, that is, the inner lead part 77, may be configured as the dense part of the present invention in the same manner as the side lead part 78 of the sensor element 101. In the sensor element 401, at least a part of the inner conduction part, that is, the inner lead part 477 and the through-hole conductor 478, may be configured as the dense part of the present invention in the same manner as the side lead part 78 of the sensor element 101. If at least a part of the inner conduction part is dense, it is possible to suppress any gas outside the sensor element 101 (element body 102) from moving along the inside of the inner conduction part to the measurement electrode 44 side. Further, if the adhesion of the dense part of the inner conduction part is high, it is also possible to suppress the gas from moving along the gap between that part and the element body 102 to the measurement electrode 44 side. Thus, even when at least a part of the inner conduction part is configured as the dense part of the present invention, it is possible to suppress any gas outside the sensor element 101 (element body 102) from reaching the measurement electrode 44.

[0104] In the sensor element 401, the upper end of the through-hole conductor 478 is covered by the connector electrode 75, but it is not limited to this. For example, as shown in the sensor element 501 of the modification example in FIG. 9, the connector electrode 75a may be replaced with the connector electrode 575a, and the through-hole conductor 478 may be replaced with the through-hole conductor 578. Similar to the through-hole conductor 478, the through-hole conductor 578 is disposed in the through-hole 402h via the insulating layer 579, and the lower end is connected to the inner lead portion 477. The connector electrode 575a has a circular hole 575h, and the outer peripheral portion of the hole 575h on the lower end surface of the connector electrode 575a is connected to the outer peripheral portion of the upper end of the through-hole conductor 578. Therefore, the portion of the upper end of the through-hole conductor 578 excluding the outer peripheral portion is exposed to the outside of the sensor element 501. In this case, at least a part of the inner conduction portion, that is, the inner lead portion 477 and the through-hole conductor 578, may be configured as the dense portion of the present invention in the same manner as the side lead portion 78 of the sensor element 101. For example, the portion including the portion of the through-hole conductor 578 exposed to the outside of the sensor element 501 may be configured as the dense portion of the present invention, or the entire through-hole conductor 578 may be configured as the dense portion of the present invention. In addition to these, the connector electrode 575a may also be configured as the dense portion of the present invention in the same manner as the side lead portion 78. By doing so, it is possible to suppress the gas outside the sensor element 501 (element body 102) from passing through the connector electrode 575a and to suppress the intrusion into the periphery of the through-hole conductor 578, specifically, the gap between the through-hole conductor 578 and the insulating layer 579 and the gap between the insulating layer 579 and the through-hole 402h.

[0105] In the above-described embodiment, the conduction part 74 corresponding to the measurement electrode 44 has been described. However, the conduction part 74 corresponding to any of the inner pump electrode 22, the auxiliary pump electrode 51, and the reference electrode 42 may be configured in the same manner. For example, just as the conduction part 74 corresponding to the measurement electrode 44 includes the connector electrode 75a and the lead part 76 having the inner lead part 77 and the side lead part 78, when the conduction part 74 corresponding to the inner pump electrode 22 includes the connector electrode 75h and the lead part having the inner lead part and the side lead part, this side lead part may be configured as a dense part of the present invention in the same manner as the side lead part 78. Further, in addition to or instead of this side lead part, at least a part of the inner lead part may be configured as a dense part of the present invention.

[0106] In the above-described embodiment, the oxygen concentration adjustment chamber had the first internal cavity 20 and the second internal cavity 40, but it is not limited thereto. For example, the oxygen concentration adjustment chamber may further include another internal cavity, or one of the first internal cavity 20 and the second internal cavity 40 may be omitted. Similarly, in the above-described embodiment, the adjustment pump cell had the main pump cell 21 and the auxiliary pump cell 50, but it is not limited thereto. For example, the adjustment pump cell may further include another pump cell, or one of the main pump cell 21 and the auxiliary pump cell 50 may be omitted. For example, when the oxygen concentration of the gas to be measured can be sufficiently lowered by only the main pump cell 21, the auxiliary pump cell 50 may be omitted. When the auxiliary pump cell 50 is omitted, the control unit 96 may perform only the main pump control process as the adjustment pump control process. Further, in the main pump control process, the setting of the target value V0* based on the pump current Ip1 described above may be omitted. Specifically, a predetermined target value V0* is stored in the storage unit 98 in advance, and the control unit 96 may control the main pump cell 21 by feedback-controlling the voltage Vp0 of the variable power supply 24 so that the voltage V0 becomes the target value V0*.

[0107] In the above-described embodiment, the sensor element 101 of the gas sensor 100 is provided with the first internal cavity 20, the second internal cavity 40, and the third internal cavity 61, but is not limited thereto. For example, as shown in the sensor element 601 of the modification of FIG. 10, it may not be provided with the third internal cavity 61. In the sensor element 601 of the modification of FIG. 10, between the lower surface of the second solid electrolyte layer 6 and the upper surface of the first solid electrolyte layer 4, there are a gas inlet 10, a first diffusion rate-limiting portion 11, a buffer space 12, a second diffusion rate-limiting portion 13, a first internal cavity 20, a third diffusion rate-limiting portion 30, and a second internal cavity 40, which are adjacently formed in a communicating manner in this order. Further, the measurement electrode 44 is disposed on the upper surface of the first solid electrolyte layer 4 within the second internal cavity 40. The measurement electrode 44 is covered by a fourth diffusion rate-limiting portion 45. The fourth diffusion rate-limiting portion 45 is a film formed of a ceramic porous body such as alumina (Al 2 O 3 ). The fourth diffusion rate-limiting portion 45, similar to the fourth diffusion rate-limiting portion 60 of the above-described embodiment, plays a role of restricting the amount of NOx flowing into the measurement electrode 44. Further, the fourth diffusion rate-limiting portion 45 also functions as a protective film for the measurement electrode 44. The ceiling electrode portion 51a of the auxiliary pump electrode 51 is formed up to directly above the measurement electrode 44. Even with the sensor element 601 having such a configuration, similar to the above-described embodiment, the NOx concentration can be detected by the measurement pump cell 41. In the sensor element 601 of FIG. 10, the periphery of the measurement electrode 44 functions as a measurement chamber. That is, the periphery of the measurement electrode 44 plays the same role as the third internal cavity 61.

[0108] In the above-described embodiment, the outer pump electrode 23 serves as an electrode paired with the inner pump electrode 22 in the main pump cell 21 (also referred to as an outer main pump electrode), an electrode paired with the auxiliary pump electrode 51 of the auxiliary pump cell 50 (also referred to as an outer auxiliary pump electrode), and an electrode paired with the measurement electrode 44 of the measurement pump cell 41 (also referred to as an outer measurement electrode), but is not limited thereto. Any one or more of the outer main pump electrode, the outer auxiliary pump electrode, and the outer measurement electrode may be provided outside the element body 102 in contact with the gas to be measured separately from the outer pump electrode 23.

[0109] In the above-described embodiment, the sensor element 101 is configured to detect the NOx concentration in the gas to be measured. However, the present invention is not limited thereto as long as it can detect the concentration of a specific gas in the gas to be measured. For example, the concentration of an oxide other than NOx may be used as the specific gas concentration. When the specific gas is an oxide, oxygen is generated when the specific gas itself is reduced in the third internal cavity 61 as in the above-described embodiment. Therefore, the measurement pump cell 41 can detect the specific gas concentration by obtaining a detection value (for example, pump current Ip2) corresponding to this oxygen. Further, the specific gas may be a non-oxide such as ammonia. When the specific gas is a non-oxide, oxygen is generated when the gas after conversion is reduced in the third internal cavity 61 by converting the specific gas into an oxide (for example, converting ammonia into NO). Therefore, the measurement pump cell 41 can detect the specific gas concentration by obtaining a detection value (for example, pump current Ip2) corresponding to this oxygen. For example, the inner pump electrode 22 in the first internal cavity 20 functions as a catalyst, so that ammonia can be converted into NO in the first internal cavity 20.

[0110] In the above-described embodiment, the element body 102 of the sensor element 101 is a laminate having a plurality of solid electrolyte layers (layers 1 to 6), but the present invention is not limited to this. The element body 102 only needs to include at least one solid electrolyte layer having oxygen ion conductivity. For example, in FIG. 2, layers 1 to 5 other than the second solid electrolyte layer 6 may be layers made of a material other than the solid electrolyte layer (for example, a layer made of alumina). In this case, each electrode included in the sensor element 101 may be disposed on the second solid electrolyte layer 6. For example, the measurement electrode 44 in FIG. 2 may be disposed on the lower surface of the second solid electrolyte layer 6. Further, instead of providing the reference gas introduction space 43 in the first solid electrolyte layer 4, it may be provided in the spacer layer 5, and instead of providing the reference gas introduction layer 48 between the first solid electrolyte layer 4 and the third substrate layer 3, it may be provided between the second solid electrolyte layer 6 and the spacer layer 5, and the reference electrode 42 may be provided behind the third internal cavity 61 and on the lower surface of the second solid electrolyte layer 6. When the laminate includes layers other than the solid electrolyte layer, the dense portion of the present invention may be in contact with the solid electrolyte layer in the same manner as the side lead portion 78 in the above-described embodiment, or may be in contact with a layer made of a material other than the solid electrolyte layer (for example, a layer made of alumina).

[0111] In the above-described embodiment, the control unit 96 sets the target value V0* of the voltage V0 based on the pump current Ip1 (feedback control) so that the pump current Ip1 becomes the target value Ip1*, and feedback-controls the pump voltage Vp0 so that the voltage V0 becomes the target value V0*. However, other controls may be performed. For example, the control unit 96 may feedback-control the pump voltage Vp0 based on the pump current Ip1 so that the pump current Ip1 becomes the target value Ip1*. That is, the control unit 96 may omit the acquisition of the voltage V0 from the main pump control oxygen partial pressure detection sensor cell 80 and the setting of the target value V0*, and directly control the pump voltage Vp0 (and thus control the pump current Ip0) based on the pump current Ip1.

[0112] In the above-described embodiment, the form of the gas sensor 100 including the sensor elements 101, 201, etc. has been described. However, it goes without saying that the form of the sensor elements 101, 201, etc. used in the gas sensor 100 may also be the same.

Example

[0113] Hereinafter, an example of specifically manufacturing a sensor element will be described as an example. Note that the present invention is not limited to the following examples.

[0114] [Examples 1 to 8, Comparative Examples 1 to 3] By the above-described manufacturing method, the sensor element 101 shown in FIG. 2 or the gas sensor 100 shown in FIG. 1 was manufactured to obtain Examples 1 to 8. In manufacturing the sensor element 101, the ceramic green sheet was formed by tape casting after mixing zirconia particles added with 4 mol% of yttria as a stabilizer, an organic binder, and an organic solvent. For the paste for forming the pattern of the side lead portion 78, a paste containing a noble metal, alumina, and silica was used. In Examples 1 to 8, the volume Vp [vol%], Va [vol%], and Vs [vol%] of the noble metal, alumina, and silica in the side lead portion 78 were variously changed as shown in Table 1 below. This was done by changing the ratio of the noble metal, alumina, and silica contained in the paste for forming the pattern of the side lead portion 78. Further, the volume Vp [vol%], Va [vol%], and Vs [vol%] of the noble metal, alumina, and silica in the side lead portion 78 were variously changed as shown in Table 1 below so that the side lead portion 78 did not contain either alumina or silica, and sensor elements 101 and gas sensors 100 equipped with the same were manufactured to obtain Comparative Examples 1 to 3.

[0115]

Table 1

[0116] [Evaluation of tightness] For the sensor elements 101 of Examples 1 to 8 and Comparative Examples 1 to 3, the porosity Rp [%] of the side lead portion 78 was measured by the above-described method. When the porosity Rp was less than 0.5%, it was evaluated that the tightness was very high (A). When the porosity Rp was 0.5% or more and less than 3%, it was evaluated that the tightness was high (B). When the porosity Rp was 3.0% or more, it was evaluated that the tightness was low (F).

[0117] [Evaluation of Adhesion] For the sensor elements 101 of Examples 1 to 8 and Comparative Example 3, a test for measuring the adhesion strength of the side lead portion 78 to the third surface 102c of the element body 102 was carried out according to the known Sebastian method (see "Thin Film Handbook", edited by the 131st Committee on Thin Films of the Japan Society for the Promotion of Science [Ohmsha]). Specifically, first, the sensor element 101 was cut into a predetermined size to prepare a test piece in a state where the side lead portion 78 was disposed on the third surface 102c. Next, a stud pin made of copper with a diameter of 1.3 mm was adhered to the side lead portion 78 of this test piece with an epoxy-based adhesive. Then, using an autograph, a force was applied to peel the side lead portion 78 of the test piece from the third surface 102c through the stud pin. When the epoxy-based adhesive was broken and the stud pin and the side lead portion 78 were separated before the side lead portion 78 was peeled from the element body 102, it was evaluated that the adhesion of the side lead portion 78 was high (A). On the other hand, when the side lead portion 78 was peeled from the element body 102 before the stud pin and the side lead portion 78 were separated, it was evaluated that the adhesion of the side lead portion 78 was low (F). For Comparative Examples 1 and 2, since the evaluation of the denseness was "low (F)" as shown in Table 1, the evaluation of the adhesion was omitted.

[0118] The volume Vp [vol%], Va [vol%], Vs [vol%], Va / Vs, Vp / (Va + Vs), the evaluation results of the denseness, and the evaluation results of the adhesion of the side lead portions 78 of Examples 1 to 8 and Comparative Examples 1 to 3 are summarized in Table 1.

[0119] As can be seen from Table 1, both Comparative Examples 1 and 2 without silica were evaluated as having "low (F)" tightness, while Examples 1 to 8 and Comparative Example 3 containing silica were all evaluated as having "very high (A)" or "high (B)" tightness. Therefore, it is considered that the tightness can be increased by including silica. Also, Comparative Example 3 without alumina was evaluated as having "low (F)" adhesion, while Examples 1 to 8 containing alumina and silica were all evaluated as having "high (A)" adhesion. Therefore, it is considered that the adhesion can be increased by including alumina and silica.

[0120] Also, when comparing Examples 3, 6 to 8 where the total volume of alumina and silica is the same for each other (Va + Vs = 20 [vol%]), Example 6 which does not satisfy Va / Vs ≤ 1.5 was evaluated as having "high (B)" tightness, while Examples 3, 7 to 8 which satisfy Va / Vs ≤ 1.5 were all evaluated as having "very high (A)" tightness. Therefore, it is considered that the tightness of the side lead portion 78 can be further increased by satisfying Va / Vs ≤ 1.5.

[0121] Note that all of Examples 1 to 8 satisfy 1.5 ≤ Vp / (Va + Vs), and all were evaluated as having "very high (A)" or "high (B)" tightness. Furthermore, when comparing Examples 1 to 5, 7 to 8 which satisfy Va / Vs ≤ 1.5, Example 4 which does not satisfy 2.6 ≤ Vp / (Va + Vs) was evaluated as having "high (B)" tightness, while Examples 1 to 3, 5, 7 to 8 which satisfy 2.6 ≤ Vp / (Va + Vs) were all evaluated as having "very high (A)" tightness. Therefore, it is considered that the tightness of the side lead portion 78 can be further increased by satisfying 2.6 ≤ Vp / (Va + Vs).

[0122] [Examples 9 to 11] Sensor elements 101 and gas sensors 100 having the same values of Vp [vol%], Va [vol%], and Vs [vol%] as in Examples 2 and 3 were fabricated and used as Examples 9 and 10, respectively. Further, as shown in Table 2 below, the raw material ratio of the paste for forming the pattern of the side lead portion 78 was changed so that the side lead portion 78 contained sodium oxide, and the sensor element 101 and gas sensor 100 of Example 11 were fabricated.

[0123]

Table 2

[0124] [Measurement of Alkali Metals and Alkaline Earth Metals] Regarding the side lead portions 78 of Examples 9 to 11, the ratios of the total mass in terms of oxides of the contained alkali metals and alkaline earth metals were measured by the above-described method. Specifically, first, the side lead portion 78 was peeled off from the sensor element 101, 0.5 g was weighed to obtain a measurement sample, and the measurement sample was placed in a PTFE container. Next, 7.5 mL of a sulfuric acid solution with a water:sulfuric acid ratio of 1:3 was added to the PTFE container, and the PTFE container was further placed in a stainless steel container and covered and sealed. The stainless steel container was placed in a thermostatic bath and maintained at 230°C for 24 hours excluding the heating-up time and cooling-down time to dissolve the measurement sample and obtain a dissolved solution. Water was added to the dissolved solution to make a 50 mL sample. Using this sample, ICP-AES was performed to conduct qualitative analysis of various elements contained in the sample. Using the test concentration value of Na obtained by this qualitative analysis and the calibration curve of Na determined in advance, the actual concentration value of Na was determined. The obtained actual concentration value of Na was converted to oxides to calculate the mass ratio of sodium oxide (Na 2 O) in the side lead portion 78. The mass ratios in terms of oxides of alkali metals and alkaline earth metals other than Na were also calculated by the same procedure. Using these calculation results, the ratio of the total mass in terms of oxides of alkali metals and alkaline earth metals in the side lead portion 78 was calculated.

[0125] [Evaluation of Conductivity] Regarding the gas sensors 100 of Examples 9 to 11, the presence or absence of a decrease in the conductivity of the side lead portion 78 after the durability test was evaluated. First, the offset current of the pump current Ip2 before the durability test was examined as follows. Each of the gas sensors 100 of Examples 9 to 11 was attached to a pipe such that the tip side portion of the element body 102 of the sensor element 101 protruded into the pipe, and a model gas containing no NOx was prepared, specifically, a model gas in which the base gas was nitrogen, the oxygen concentration was 0%, and the NOx concentration was 0 ppm, and this was flowed through the pipe as the gas to be measured. In parallel, the heater control process was started, and when the heater 71a reached 800 °C, the adjustment pump control process and the measurement pump control process were started, and the pump current Ip2 flowing in this state was measured as the offset current (the pump current Ip2 flowing when no NOx is contained in the gas to be measured). Next, a durability test was conducted. Specifically, for each of the gas sensors 100 of Examples 9 to 11, the exhaust gas of a diesel engine was flowed through the pipe as the gas to be measured, and in parallel, the heater control process, the adjustment pump control process, and the measurement pump control process were continuously executed. The temperature of the side lead portion 78 at this time was 280 °C. This state was maintained for 2000 hours. Then, the offset current of the pump current Ip2 after the durability test was measured in the same manner as before the durability test. Thereafter, for each of the gas sensors 100 of Examples 9 to 11, the variation (difference) in the offset current before and after the durability test was calculated, and the value obtained by converting this variation into NOx concentration [ppm] was calculated as the offset variation amount. Here, when the conductivity of the side lead portion 78 decreases (the resistance value increases) due to the durability test, the pump current Ip2 flowing through the side lead portion 78 changes to a different value even when the NOx concentration is the same. Therefore, when the above-described offset variation amount was within 7 ppm, it was evaluated that the effect of suppressing the decrease in the conductivity of the side lead portion 78 was high (A). When the offset variation amount was greater than 7 ppm, it was evaluated that the effect of suppressing the decrease in the conductivity of the side lead portion 78 was low (B).

[0126] Table 2 summarizes the volume Vp [vol%], Va [vol%], Vs [vol%] of the side lead portion 78 in Examples 9 to 11, the volume Vn [vol%] of sodium oxide, the ratio of the total mass in terms of oxides of alkali metals and alkaline earth metals, and the evaluation results of the effect of suppressing the decrease in conductivity.

[0127] As can be seen from Table 2, in Example 11 where the ratio of the total mass in terms of oxides of alkali metals and alkaline earth metals exceeds 0.10 wt%, the evaluation of the effect of suppressing the decrease in conductivity was "low (B)", whereas in Examples 9 and 10 where the ratio of the total mass in terms of oxides of alkali metals and alkaline earth metals satisfies 0.10 wt% or less, the evaluation of the effect of suppressing the decrease in conductivity was "high (A)" for both. In Examples 9 and 10, since the amount of alkali metals and / or alkaline earth metals contained in the side lead portion 78 is small (the ratio of the total mass in terms of oxides is 0.10 wt% or less), it is considered that the decrease in conductivity of the side lead portion 78 due to migration by these electromigrations is suppressed.

Industrial Applicability

[0128] The present invention can be used for a sensor element for detecting the concentration of a specific gas such as NOx in a gas to be measured such as the exhaust gas of an internal combustion engine, and a gas sensor including the same.

Explanation of Signs

[0129] 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, 7 Adhesive layer, 10 Gas inlet, 11 First diffusion rate limiting part, 12 Buffer space, 13 Second diffusion rate limiting part, 20 First internal cavity, 21 Main pump cell, 22 Inner pump electrode, 22a, 51a Ceiling electrode part, 22b, 51b Bottom electrode part, 23 Outer pump electrode, 24 Variable power source, 30 Third diffusion rate limiting part, 40 Second internal cavity, 41 Measurement pump cell, 42 Reference electrode, 43 Reference gas introduction space, 44 Measurement electrode, 45 Fourth diffusion rate limiting part, 46 Variable power source, 48 Reference gas introduction layer, 49 Reference gas introduction part, 49a Inlet part, 50 Auxiliary pump cell, 51 Auxiliary pump electrode, 52 Variable power source, 60 Fourth diffusion rate limiting part, 61 Third internal cavity, 70 Heater part, 71a Heater, 71b Heater insulating layer, 71c Pressure dissipation hole, 72 Heater power source, 74 Conductive part, 75, 75a~75h, 575a Connector electrode, 76 Lead part, 77, 477 Inner lead part, 77a~77d First to fourth parts, 78, 278, 378 Side lead part, 79 Lead insulating layer, 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 measurement pump control, 83 Sensor cell, 95 Control device, 96 Control part, 97 CPU, 98 Memory part, 100 Gas sensor, 101, 201, 301, 401, 501, 601 Sensor element, 102, 402 Element body, 102a First surface, 102b Second surface, 102c Third surface, 102d Fourth surface, 102e Fifth surface, 102f Sixth surface, 130 Protection cover, 131 Inner protection cover, 132 Outer protection cover, 133 Sensor element chamber, 140 Sensor assembly, 141 Element seal, 142 Main body fitting, 143 Inner cylinder, 143a, 143b Reduced diameter part, 144a~144c Ceramic supporter, 145a, 145b Compressed powder body, 146 Metal ring, 147 Bolt, 148 Outer cylinder, 149 Space, 150 Connector, 155 Lead wire, 157 Rubber plug, 190 Pipe, 191 Fixing member, 278a, 378a First side lead part, 278b, 378b Second side lead part, 402h Through hole, 478, 578 Through hole conductor, 479 Insulating layer, 575h Hole.

Claims

1. A sensor element for detecting the concentration of a specific gas in a gas to be measured, having an oxygen ion conductive solid electrolyte layer and being columnar extending along a longitudinal direction, having a front end and a rear end which are both ends along the longitudinal direction and a side surface which is a surface along the longitudinal direction, and the front end side being exposed to the gas to be measured; an element body, an inner electrode disposed inside the element body, a conduction part having an inner conduction part disposed inside the element body and conducting with the inner electrode, a connector electrode disposed on the rear end side of the side surface, and having a part disposed on the side surface and / or a part exposed to the outside of the sensor element on the side surface and conducting with the inner conduction part, comprising: a part of the outer conduction part covering the inner conduction part, and / or at least a part of the inner conduction part is a dense part densely formed including a noble metal, alumina, and silica, a sensor element.

2. When the volume of alumina and silica contained in the dense part is Va [vol%] and Vs [vol%] respectively, Va / Vs≤1.5 is satisfied, The sensor element according to Claim 1.

3. When the volume of the noble metal, alumina, and silica contained in the dense part is Vp [vol%], Va [vol%], and Vs [vol%] respectively, 1.5≤Vp / (Va + Vs) is satisfied, The sensor element according to Claim 1 or 2.

4. The dense part satisfies 2.6≤Vp / (Va + Vs), The sensor element according to Claim 3.

5. The dense part satisfies that the ratio of the total mass in terms of oxides of the contained alkali metal and alkaline earth metal is 0.1 wt% or less, The sensor element according to Claim 1 or 2.

6. The sensor element according to Claim 1 or 2, wherein the inner electrode is a measurement electrode used for detecting the concentration of the specific gas, a sensor element.

7. The sensor element according to Claim 1 or 2, wherein the element body is a laminate in which a plurality of layers including the solid electrolyte layer are laminated in a lamination direction orthogonal to the longitudinal direction, the laminate has, as the side surface, a first surface and a second surface which are both end surfaces in the lamination direction, and a third surface and a fourth surface which are both end surfaces in a direction orthogonal to the longitudinal direction and the lamination direction, the inner conduction part has an inner lead part drawn out to the third surface or the fourth surface, Of the outer conduction portions, the portion covering the inner conduction portion is a side lead portion disposed on the third surface or the fourth surface and covering the inner lead portion. Sensor element.

8. The sensor element according to claim 1 or 2, wherein The element body is a laminate in which a plurality of layers including the solid electrolyte layer are laminated in a stacking direction orthogonal to the longitudinal direction. The laminate has, as the side surfaces, a first surface and a second surface which are both end surfaces in the stacking direction. The element body includes an opening that opens on the first surface or the second surface on the rear end side of the side surface, and has a through hole that penetrates one or more of the plurality of layers in the stacking direction. The outer conduction portion is the connector electrode. The inner conduction portion has a through hole conductor disposed in the through hole and covered by the connector electrode. Sensor element.

9. The sensor element according to claim 1 or 2, wherein The element body is a laminate in which a plurality of layers including the solid electrolyte layer are laminated in a stacking direction orthogonal to the longitudinal direction. The laminate has, as the side surfaces, a first surface and a second surface which are both end surfaces in the stacking direction. The element body includes an opening that opens on the first surface or the second surface on the rear end side of the side surface, and has a through hole that penetrates one or more of the plurality of layers in the stacking direction. The inner conduction portion has a through hole conductor disposed in the through hole and exposed to the outside at the opening. The dense portion includes at least a part of the through hole conductor. Sensor element.

10. The sensor element according to claim 1 or 2, wherein The sensor element; A case that is cylindrical and extends along the longitudinal direction of the sensor element, has a second front end and a second rear end that are both ends along the longitudinal direction, and in which the sensor element is disposed inside; A sealing member that seals the second rear end side of the case; It is used for a gas sensor comprising Sensor element.

11. The sensor element according to claim 1 or 2, and A case that is cylindrical and extends along the longitudinal direction of the sensor element, has a second front end and a second rear end that are both ends along the longitudinal direction, and in which the sensor element is disposed inside; A sealing member that seals the second rear end side of the case; A gas sensor comprising

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