Sensor element and gas sensor

Abstract

A sensor element (101) is provided with an element main body (layers (1-6)), a reference electrode (42), an outer pump electrode (23), a conductive section (77) having a connector electrode (71) and a reference electrode lead section (78), and a lead insulating layer (79). An internal lead (78b) of the reference electrode lead section (78) causes a reference gas, which is a reference for detecting a specific gas concentration in the gas to be measured, to flow to the reference electrode (42). Among the entire surface area of the lead insulating layer (79) in plan view, a region that is 50% or more from the end on the reference electrode (42) side is configured as a dense region having a porosity of 5% or less.

Description

Technical Field The present invention relates to a sensor element and a gas sensor. Background Art In the past, there are known gas sensors that detect the concentration of specific gases such as NOx in the measured gas such as automobile exhaust gas. Patent document 1 describes a gas sensor having the following sensor element, which has: an element body having a solid electrolyte layer with oxygen ion conductivity, and a measured gas flow portion provided inside for introducing and flowing the measured gas; a reference electrode disposed inside the element body; and a porous reference gas introduction layer for introducing and flowing a reference gas that serves as a detection reference for the specific gas concentration in the measured gas to the reference electrode. Prior art literature Patent Literature Patent Document 1: Japanese Patent Application Publication No. 2018-173320 Summary of the invention In the sensor element of Patent Document 1, the porous reference gas introduction layer may adsorb external water (water vapor) when the sensor element is not driven. In this case, when the sensor element is subsequently driven, the water is gradually discharged to the outside of the element body. However, before the water is discharged to the outside, the oxygen concentration around the reference electrode may decrease. If the time required for the water to be discharged to the outside (drainage time) is long, there is a tendency that the time required from the start of driving the sensor element to the stabilization of the potential of the reference electrode (stabilization time) becomes longer. A main object of the sensor element and the gas sensor of the present invention is to shorten the stabilization time. The sensor element and the gas sensor of the present invention adopt the following means to achieve the above-mentioned main object.

[0001] The sensor element of the present invention is intended to: It is a sensor element used to detect the concentration of a specific gas in the gas being measured. It has: an element body having a solid electrolyte layer with oxygen ion conductivity and provided with a gas flow portion for introducing and flowing the gas to be measured; A reference electrode disposed inside the element body; a measured gas side electrode disposed at a portion of the element body that is exposed to the measured gas; a first conductive portion including a reference electrode terminal disposed outside the element body and a reference electrode lead portion for electrically connecting the reference electrode terminal and the reference electrode; and a lead insulating layer covering a portion of the reference electrode lead portion located inside the element body, The reference electrode lead portion allows a reference gas, which is a detection reference for a specific gas concentration in the measured gas, to flow to the reference electrode. A region that accounts for 50% or more of the entire area of ​​the lead insulating layer in a planar view, calculated from the end portion on the reference electrode side, is a dense region having a porosity of 5% or less. In the sensor element of the present invention, the reference electrode leads allow a reference gas, which is a detection reference for a specific gas concentration in a measured gas, to flow to the reference electrode, and a region of the lead insulating layer that is 50% or more of the entire area in a plan view, calculated from the end portion on the reference electrode side, is configured as a dense region with a porosity of 5% or less. Therefore, since a region of the lead insulating layer that is 50% or more of the entire area in a plan view is configured as a dense region, the amount of water (water vapor) adsorbed by the lead insulating layer during a period when the sensor element is not driven can be suppressed compared to a case where a region of less than 50% is configured as a dense region. In addition, since a region of the lead insulating layer that includes at least the end portion on the reference electrode side is configured as a dense region, even if water is adsorbed by the lead insulating layer during a period when the sensor element is not driven, the water can be easily discharged to the outside of the element body after the sensor element is driven, compared to a case where a porous region with a porosity of more than 5% is provided at a position on the reference electrode side of the lead insulating layer relative to the dense region. This can shorten the time required from the start of driving the sensor element to the time when water is discharged to the outside of the element body (drainage time), and shorten the time required from the start of driving the sensor element to the time when the potential of the reference electrode stabilizes (stabilization time). The inventors of the present invention have confirmed this through experiments and the like.

[0002] In the sensor element (the sensor element described in [1]), a region that accounts for 80% or more of the total area in the plan view from the end on the reference electrode side can be configured as the dense region. This can further reduce the amount of water (water vapor) adsorbed by the lead insulating layer when the sensor element is not driven.

[0003] In the above-mentioned sensor element (the sensor element described in [1] or [2] above), the limiting current when oxygen is absorbed from the vicinity of the reference electrode to the vicinity of the measured gas side electrode can be 0.3μA or more. Here, the limiting current has a positive correlation with the inverse of the diffusion resistance of the lead portion for the reference electrode, which means that: the larger the limiting current, the smaller the diffusion resistance of the lead portion for the reference electrode. Therefore, a limiting current of 0.3μA or more means that: the diffusion resistance of the lead portion for the reference electrode is below a specified value (not too large). According to this configuration, compared with a case where the diffusion resistance of the lead portion for the reference electrode is too large, it is easier to maintain the oxygen concentration around the reference electrode at an appropriate value, and the detection accuracy of a specific gas concentration can be suppressed from decreasing. The inventors of the present invention have confirmed the above situation through experiments, etc. In this case, the limiting current can be 10μA or less.

[0004] In this case (the sensor element described in [3] above), the limiting current may be equal to or greater than 0.5 μA. This can further suppress a decrease in the detection accuracy of the specific gas concentration.

[0005] In the sensor element (the sensor element described in any one of [1] to [4] above), the porosity of the portion of the reference electrode lead portion located inside the element body may be greater than 5% and less than 40%. When the porosity exceeds 5%, the reference gas can be easily circulated to the reference electrode. In addition, when the porosity is less than 40%, the portion of the reference electrode lead portion located inside the element body can be prevented from breaking.

[0006] In the sensor element (the sensor element described in any one of [1] to [5]), the cross-sectional area and / or porosity of the portion of the reference electrode lead portion located inside the element body may be larger on the reference electrode side than on the reference electrode terminal side. As a result, oxygen is easily accumulated around the reference electrode, and the oxygen concentration around the reference electrode can be more easily maintained at an appropriate value.

[0007] The above-mentioned sensor element (the sensor element described in any one of [1] to [6] above) may include: a heater, which is arranged inside the element body and heats the element body; a second conductive portion, which has a heater terminal arranged outside the element body and a heater lead portion that conducts the heater terminal and the heater; and a heater insulating layer, which covers the heater and the heater lead portion located inside the element body, wherein the lead insulating layer and the heater insulating layer are independently arranged in a manner not connected to each other.

[0008] The sensor element (the sensor element described in any one of [1] to [6]) may include: a heater disposed inside the element body and heating the element body; a second conductive portion having a heater terminal disposed outside the element body and a heater lead portion connecting the heater terminal and the heater; and a heater insulating layer covering the heater and the heater lead portion located inside the element body, the element body having a connecting hole connecting the heater insulating layer and the lead insulating layer, and the porosity of the heater insulating layer being 5% or less. Thus, the amount of water adsorbed by the heater insulating layer via the lead insulating layer and the connecting hole during the period when the sensor element is not driven can be suppressed.

[0009] The gas sensor of the present invention is intended to include the sensor element described in any one of [1] to [8] above, and thus can achieve the same effects as those achieved by the sensor element of the present invention, such as the effect of shortening the stabilization time. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a longitudinal cross-sectional view of the gas sensor 100 . Figure 2 It is a cross-sectional schematic diagram schematically showing an example of the structure of the sensor element 101 . Figure 3 It is a block diagram showing the electrical connection relationship between the control device 95 and each unit. Figure 4 It is a partial cross-sectional view in which the periphery of the reference electrode 42 and the like is enlarged. Figure 5 yes Figure 4 AA cross-section diagram. Figure 6 It is a perspective view of the rear end and the surrounding area of ​​the sensor element 101 . Figure 7 2 is a partial cross-sectional view of a sensor element 201 according to a modified example. Figure 8 2 is a partial cross-sectional view of a sensor element 301 according to a modified example. Fig. 9 4 is a partial cross-sectional view of a sensor element 401 according to a modified example. Fig.10 2 is a partial cross-sectional view of a sensor element 501 according to a modified example. Fig.11 2 is a partial cross-sectional view of a sensor element 601 according to a modified example. Fig.12 yes Fig.11 BB cross-section diagram. Fig.132 is a schematic cross-sectional view of a sensor element 701 according to a modified example. DETAILED DESCRIPTION Next, embodiments of the present invention will be described with reference to the drawings. Figure 1 1 is a longitudinal sectional view of a gas sensor 100 according to one embodiment of the present invention. Figure 2 1 is a schematic cross-sectional view schematically showing an example of the configuration of a sensor element 101 included in the gas sensor 100 . Figure 3 It is a block diagram showing the electrical connection relationship between the control device 95 and each unit. Figure 4 It is a partial cross-sectional view showing an enlarged view of the reference electrode 42 , the reference electrode lead portion 78 , and the lead 76 and the surrounding areas of the sensor element 101 . Figure 5 yes Figure 4 AA cross-section diagram. Figure 6 1 is a perspective view of the rear end periphery of the sensor element 101. The sensor element 101 is in the shape of a long rectangular parallelepiped. Figure 2 The left-right direction in the figure) is set as the front-to-back direction, and the thickness direction of the sensor element 101 ( Figure 2 The up-down direction in FIG. 1 is set as the up-down direction, and the width direction of the sensor element 101 (the direction perpendicular to the front-back direction and the up-down direction) is set as the left-right direction. like Figure 1 As shown in the figure, the gas sensor 100 comprises: a sensor element 101; a protective cover 130 that protects the front end side of the sensor element 101; and a sensor assembly 140 that includes a connector 150 that is conductive with the sensor element 101. As shown in the figure, the gas sensor 100 is installed in a pipe 190 such as an exhaust pipe of a vehicle, and is used to measure NOx and O contained in the exhaust gas as the measured gas. 2 In this embodiment, the gas sensor 100 measures the NOx concentration as the specific gas concentration. The protective cover 130 includes an inner protective cover 131 having a bottomed cylindrical shape, which covers the front end of the sensor element 101, and an outer protective cover 132 having a bottomed cylindrical shape, which covers the inner protective cover 131. A plurality of holes for allowing the measured gas to flow in the protective cover 130 are formed in the inner protective cover 131 and the outer protective cover 132. The sensor element chamber 133 is formed as a space surrounded by the inner protective cover 131, and the front end of the sensor element 101 is arranged in the sensor element chamber 133. The sensor assembly 140 comprises: an element enclosure 141, which seals and fixes the sensor element 101; bolts 147 and an outer cylinder 148, which are mounted on the element enclosure 141; and a connector 150, which contacts the connector electrodes 71 formed on the surface (upper and lower surfaces) of the rear end of the sensor element 101 and is electrically connected to these electrodes. The element enclosure 141 includes: a cylindrical main metal member 142; a cylindrical inner tube 143, which is welded and fixed coaxially with the main metal member 142; and ceramic supports 144a to 144c, pressed powder bodies 145a and 145b, and a metal ring 146, which are sealed in through holes on the inner sides of the main metal member 142 and the inner tube 143. The sensor element 101 is located on the central axis of the element enclosure 141 and penetrates the element enclosure 141 in the front-rear direction. The inner tube 143 is formed with: a reduced diameter portion 143a, which is used to press the pressed powder body 145b in the central axis direction of the inner tube 143; and a reduced diameter portion 143b, which is used to press the ceramic supports 144a to 144c and the pressed powder bodies 145a and 145b forward by means of the metal ring 146. The pressed powder bodies 145a and 145b are compressed between the main metal part 142 and the inner tube 143 and the sensor element 101 by the pressing force from the reduced diameter parts 143a and 143b, thereby sealing the space between the sensor element chamber 133 in the protective cover 130 and the space 149 in the outer tube 148 and fixing the sensor element 101. The bolt 147 is coaxially fixed to the main metal fitting 142 and has an external threaded portion formed on the outer peripheral surface. The external threaded portion of the bolt 147 is inserted into the fixing member 191, which is welded to the pipe 190 and has an internal threaded portion on the inner peripheral surface. Thus, the gas sensor 100 is fixed to the pipe 190 in a state where the front end of the sensor element 101 in the gas sensor 100 and a portion of the protective cover 130 protrude into the pipe 190. 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 led out from the rear end to the outside. The lead wires 155 are connected to the electrodes (described later) of the sensor element 101 via the connector 150. The gap between the outer cylinder 148 and the lead wires 155 is closed by a rubber stopper 157. The space 149 in the outer cylinder 148 is filled with a reference gas. The rear end of the sensor element 101 is arranged in the space 149. like Figure 2As shown, the sensor element 101 is an element having a laminated body, which is obtained by laminating 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 separator 5, and a second solid electrolyte layer 6, which are respectively composed of oxygen ion conductive solid electrolyte layers such as zirconium dioxide (ZrO2) in order from the bottom side in the figure. In addition, the solid electrolyte forming these six layers is a dense gas-tight solid electrolyte. The sensor element 101 is manufactured as follows: for example, the ceramic green sheets corresponding to each layer are subjected to prescribed processing and circuit pattern printing, and then they are laminated and fired to achieve integration. At one end of the sensor element 101 ( Figure 2 The left side in the figure) and between the lower surface of the second solid electrolyte layer 6 and the upper surface of the first solid electrolyte layer 4, a gas inlet port 10, a first diffusion rate control part 11, a buffer space 12, a second diffusion rate control part 13, a first internal cavity 20, a third diffusion rate control part 30, a second internal cavity 40, a fourth diffusion rate control part 60 and a third internal cavity 61 are adjacently formed in a sequentially connected manner. 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 which are arranged by hollowing out the isolation layer 5, wherein their upper parts are separated by the lower surface of the second solid electrolyte layer 6, their lower parts are separated by the upper surface of the first solid electrolyte layer 4, and their sides are separated by the side surfaces of the isolation layer 5. The first diffusion rate control section 11, the second diffusion rate control section 13, and the third diffusion rate control section 30 are all provided as two horizontally long slits (the length direction of the opening is in the direction perpendicular to the drawing). In addition, the fourth diffusion rate control section 60 is provided as a horizontally long slit (the length direction of the opening is in the direction perpendicular to the drawing) formed as a gap with the lower surface of the second solid electrolyte layer 6. In addition, the portion from the gas inlet 10 to the third internal cavity 61 is also referred to as the measured gas flow section. A reference electrode 42 is disposed between the upper surface of the third substrate layer 3 and the lower surface of the first solid electrolyte layer 4. The reference electrode 42 is directly formed on the upper surface of the third substrate layer 3, and the upper surface and the front, rear, left and right sides of the reference electrode 42 are covered by a reference electrode insulating layer 43. In addition, the reference electrode 42 is formed as a porous metal ceramic electrode (e.g., Pt and ZrO 2 As will be described later, the reference gas serving as a detection reference for the NOx concentration is passed through the reference electrode lead portion 78 (see reference electrode 42) connected to the reference electrode 42. Figure 4 , Figure 5 ) and from the outside of the sensor element 101 (here Figure 1 The reference electrode 42 is a gas having a predetermined oxygen concentration. In the present embodiment, the reference gas is the atmosphere or a gas having the same oxygen concentration as the atmosphere (e.g., a gas having nitrogen as the base gas and containing oxygen). In addition, as described later, the reference electrode 42 is used to measure the oxygen concentration (oxygen partial pressure) in the first internal cavity 20, the second internal cavity 40, and the third internal cavity 61. The purpose of forming the reference electrode insulating layer 43 is to obtain electrical insulation between the first fixed electrolyte layer 4 and the reference electrode 42. The reference electrode insulating layer 43 is described in detail below. In the measured gas flow section, the gas inlet 10 is a portion open to the external space, and the measured gas enters the sensor element 101 from the external space through the gas inlet 10. The first diffusion rate control section 11 is a portion that applies a predetermined diffusion resistance to the measured gas entering from the gas inlet 10. The buffer space 12 is a space provided to guide the measured gas introduced from the first diffusion rate control section 11 to the second diffusion rate control section 13. The second diffusion rate control section 13 is a portion that applies a predetermined diffusion resistance to the measured gas introduced from the buffer space 12 to the first internal cavity 20. When the measured gas is introduced from the outside of the sensor element 101 into the first internal cavity 20, the measured gas that rapidly enters the sensor element 101 from the gas inlet port 10 due to the pressure fluctuation of the measured gas in the external space (when the measured gas is the exhaust gas of an automobile, it is the pulsation of the exhaust pressure) is not directly introduced into the first internal cavity 20, but is introduced into the first internal cavity 20 after the pressure fluctuation of the measured gas is eliminated by the first diffusion rate control unit 11, the buffer space 12, and the second diffusion rate control unit 13. As a result, the pressure fluctuation of the measured gas introduced into the first internal cavity 20 is almost 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 control unit 13. Such oxygen partial pressure is adjusted by operating the main pump unit 21. The main pump unit 21 is an electrochemical pump unit consisting of an inner pump electrode 22, an outer pump electrode 23, and a second solid electrolyte layer 6 sandwiched by these electrodes, wherein the inner pump electrode 22 has a top electrode portion 22a arranged on substantially the entire surface of the lower surface of the second solid electrolyte layer 6 facing the first internal cavity 20, and the outer pump electrode 23 is arranged in the external space ( Figure 1 The sensor element chamber 133 in the upper surface of the second solid electrolyte layer 6 is provided in a region corresponding to the top electrode portion 22a in a manner that the sensor element chamber 133 in the upper surface is exposed. The inner pump electrode 22 is arranged across the solid electrolyte layers (the second solid electrolyte layer 6 and the first solid electrolyte layer 4) formed on the upper and lower sides that partition the first inner cavity 20, and the isolation layer 5 constituting the side wall. Specifically, a top electrode portion 22a is formed on the lower surface of the second solid electrolyte layer 6 constituting the top surface of the first inner cavity 20, and a bottom electrode portion 22b is directly formed on the upper surface of the first solid electrolyte layer 4 constituting the bottom surface, and a side electrode portion (not shown) is formed on the side wall surface (inner surface) of the isolation layer 5 constituting the two side wall portions of the first inner cavity 20 in a manner that connects the top electrode portion 22a and the bottom electrode portion 22b, thereby configuring a tunnel-shaped structure at the location where the side electrode portion is arranged. The inner pump electrode 22 and the outer pump electrode 23 are formed as porous metal ceramic electrodes (for example, Pt and ZrO containing 1% Au). 2 In addition, the inner pump electrode 22 in contact with the measured gas is formed using a material having a weakened reducing ability for the NOx component in the measured gas. In the main pump unit 21, the desired pump voltage Vp0 is applied between the inner pump electrode 22 and the outer pump electrode 23, so that the pump current Ip0 flows between the inner pump electrode 22 and the outer pump electrode 23 in the positive direction or the negative direction. Thus, the oxygen in the first internal cavity 20 can be absorbed into the external space, or the oxygen in the external space can be absorbed into the first internal cavity 20. In addition, in order to detect the oxygen concentration (oxygen partial pressure) in the atmosphere of the first internal cavity 20, an electrochemical sensor unit, i.e., an oxygen partial pressure detection sensor unit 80 for main pump control, is formed by the inner pump electrode 22, the second solid electrolyte layer 6, the isolation layer 5, the first solid electrolyte layer 4, the third substrate layer 3 and the reference electrode 42. By measuring the electromotive force (voltage V0) of the oxygen partial pressure detection sensor unit 80 for main pump control, the oxygen concentration (oxygen partial pressure) in the first internal cavity 20 can be known. In addition, the pump voltage Vp0 of the variable power supply 25 is feedback-controlled so that the voltage V0 is constant (target value), thereby controlling the pump current Ip0. As a result, the oxygen concentration in the first internal cavity 20 can be maintained at a predetermined constant value. The third diffusion rate control unit 30 is a portion that applies 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 unit 21 in the first internal cavity 20 , and guides the gas to be measured to the second internal cavity 40 . The second internal cavity 40 is provided as a space for performing the following processing: the oxygen concentration (oxygen partial pressure) is adjusted in advance in the first internal cavity 20, and then the oxygen partial pressure of the measured gas introduced through the third diffusion rate control unit 30 is further adjusted by the auxiliary pump unit 50. As a result, the oxygen concentration in the second internal cavity 40 can be kept constant with high accuracy, so that the NOx concentration can be measured with high accuracy in the gas sensor 100. The auxiliary pump unit 50 is an auxiliary electrochemical pump unit composed of an auxiliary pump electrode 51, an outer pump electrode 23 (not limited to the outer pump electrode 23, as long as it is a suitable electrode on the outside of the sensor element 101), and a second solid electrolyte layer 6. The auxiliary pump electrode 51 has a top electrode portion 51a that is roughly entirely arranged on the lower surface of the second solid electrolyte layer 6 facing the second internal cavity 40. The auxiliary pump electrode 51 is disposed in the second internal cavity 40 in a tunnel-shaped structure similar to the inner pump electrode 22 previously disposed in the first internal cavity 20. That is, a top electrode portion 51a is formed relative to the second solid electrolyte layer 6 constituting the top surface of the second internal cavity 40, and a bottom electrode portion 51b is directly formed on the upper surface of the first solid electrolyte layer 4 constituting the bottom surface of the second internal cavity 40, and a side electrode portion (not shown) connecting the top electrode portion 51a and the bottom electrode portion 51b is formed in a tunnel-shaped structure formed on both wall surfaces of the isolation layer 5 constituting the side wall of the second internal cavity 40. In addition, the auxiliary pump electrode 51 is formed of a material having a weakened reducing ability for the NOx component in the measured gas, similar to the inner pump electrode 22. In the auxiliary pump cell 50 , a desired voltage Vp1 is applied between the auxiliary pump electrode 51 and the outer pump electrode 23 , thereby allowing oxygen in the atmosphere in the second internal cavity 40 to be absorbed into the external space or oxygen to be absorbed into the second internal cavity 40 from the external space. In addition, in order to control the oxygen partial pressure in the atmosphere within the second internal cavity 40, an electrochemical sensor unit, namely, an auxiliary pump control oxygen partial pressure detection sensor unit 81, is formed by the auxiliary pump electrode 51, the reference electrode 42, the second solid electrolyte layer 6, the isolation layer 5, the first solid electrolyte layer 4, and the third substrate layer 3. The auxiliary pump unit 50 performs pumping using a variable power supply 52 whose voltage is controlled based on the electromotive force (voltage V1) detected by the auxiliary pump control oxygen partial pressure detection sensor unit 81. Thus, the oxygen partial pressure in the atmosphere in the second internal cavity 40 is controlled to a low partial pressure that has substantially no effect on the measurement of NOx. At the same time, the pump current Ip1 is used to control the electromotive force of the main pump control oxygen partial pressure detection sensor unit 80. Specifically, the pump current Ip1 is input as a control signal to the main pump control oxygen partial pressure detection sensor unit 80, and the above-mentioned target value of the voltage V0 is controlled to control the gradient of the oxygen partial pressure in the measured gas introduced from the third diffusion rate control unit 30 into the second internal cavity 40 to be always constant. When used as a NOx sensor, the oxygen concentration in the second internal cavity 40 is maintained at a constant value of about 0.001 ppm by the action of the main pump unit 21 and the auxiliary pump unit 50. The fourth diffusion rate control unit 60 is a portion that applies a predetermined diffusion resistance to the gas to be measured whose oxygen concentration (oxygen partial pressure) is controlled by the operation of the auxiliary pump unit 50 in the second internal cavity 40, and introduces the gas to be measured into the third internal cavity 61. The fourth diffusion rate control unit 60 has the function of limiting the amount of NOx flowing into the third internal cavity 61. The third internal cavity 61 is provided as a space in which the oxygen concentration (oxygen partial pressure) is adjusted in advance in the second internal cavity 40, and then the gas to be measured introduced through the fourth diffusion rate control unit 60 is processed related to the measurement of the nitrogen oxide (NOx) concentration in the gas to be measured. The NOx concentration is measured mainly in the third internal cavity 61 by the operation of the measurement pump unit 41. The measuring pump cell 41 measures the NOx concentration in the measured gas in the third internal cavity 61. The measuring pump cell 41 is an electrochemical pump cell composed of a measuring electrode 44, an outer pump electrode 23, a second solid electrolyte layer 6, a separator 5, and a first solid electrolyte layer 4. The measuring electrode 44 is directly disposed on the upper surface of the first solid electrolyte layer 4 at a position facing the third internal cavity 61. The measuring electrode 44 is a porous metal ceramic electrode (e.g., Pt and ZrO) made of a material having a higher reduction capability for the NOx component in the measured gas than the inner pump electrode 22. 2 The measuring electrode 44 also functions as a NOx reduction catalyst for reducing NOx present in the atmosphere within the third internal cavity 61. The measuring pump cell 41 can absorb oxygen generated by decomposition of nitrogen oxides in the atmosphere around the measuring electrode 44 , and the generated amount can be detected as the pump current Ip2 . In order to detect the oxygen partial pressure around the measuring electrode 44, the first solid electrolyte layer 4, the third substrate layer 3, the measuring electrode 44, and the reference electrode 42 constitute an electrochemical sensor unit, that is, a measuring pump control oxygen partial pressure detection sensor unit 82. The variable power supply 46 is controlled based on the electromotive force (voltage V2) detected by the measuring pump control oxygen partial pressure detection sensor unit 82. The gas to be measured introduced into the second internal cavity 40 passes through the fourth diffusion rate control unit 60 under the condition of controlled oxygen partial pressure and reaches the measuring electrode 44 in the third internal cavity 61. The nitrogen oxides in the gas to be measured around the measuring electrode 44 are reduced (2NO → N 2 +O 2 ) to generate oxygen. The generated oxygen is pumped by the measurement pump unit 41. At this time, the voltage Vp2 of the variable power supply 46 is controlled so that the voltage V2 detected by the measurement pump control oxygen partial pressure detection sensor unit 82 is constant (target value). The amount of oxygen generated around the measurement electrode 44 is proportional to the concentration of nitrogen oxides in the measured gas. Therefore, the concentration of nitrogen oxides in the measured gas is calculated using the pump current Ip2 in the measurement pump unit 41. In addition, the electrochemical sensor unit 83 composed of the second solid electrolyte layer 6, the isolation layer 5, the first solid electrolyte layer 4, the third substrate layer 3, the outer pump electrode 23, and the reference electrode 42 can detect the oxygen partial pressure in the measured gas outside the sensor by using the electromotive force (voltage Vref) obtained by the sensor unit 83. In addition, the second solid electrolyte layer 6, the isolation layer 5, the first solid electrolyte layer 4, the third substrate layer 3, the outer pump electrode 23, and the reference electrode 42 constitute an electrochemical reference gas adjustment pump unit 90. With respect to the reference gas adjustment pump unit 90, a pump current Ip3 flows due to a control voltage Vp3 applied by a power supply circuit 92 connected between the outer pump electrode 23 and the reference electrode 42, thereby pumping oxygen. Accordingly, the reference gas adjustment pump unit 90 can pump oxygen from the space around the outer pump electrode 23 ( Figure 1 Oxygen is absorbed into the vicinity of the reference electrode 42 through the sensor element chamber 133 in the sensor element chamber, or oxygen is absorbed from the vicinity of the reference electrode 42 into the space around the outer pump electrode 23. In the gas sensor 100 having such a structure, the main pump unit 21 and the auxiliary pump unit 50 are operated to supply the measured gas whose oxygen partial pressure is always kept at a constant low value (a value that has substantially no effect on the measurement of NOx) to the measurement pump unit 41. Therefore, based on the pump current Ip2 that is approximately proportional to the NOx concentration in the measured gas and flows due to the oxygen generated by the reduction of NOx being sucked out from the measurement pump unit 41, the NOx concentration in the measured gas can be known. Furthermore, the sensor element 101 includes a heater unit 70 that performs a temperature adjustment function of heating and keeping the sensor element 101 warm so as to improve the oxygen ion conductivity of the solid electrolyte. The heater unit 70 includes a heater 72 , a heater insulating layer 74 , and a lead wire 76 . The heater 72 is a resistor formed by being sandwiched from the upper and lower sides by the second substrate layer 2 and the third substrate layer 3. The heater 72 generates heat by external power supply, thereby heating and keeping warm the solid electrolyte forming the sensor element 101. In addition, the heater 72 is buried in the entire area from the first internal cavity 20 to the third internal cavity 61, and the entire sensor element 101 can be adjusted to a temperature that activates the above-mentioned solid electrolyte. The heater insulating layer 74 is formed to obtain electrical insulation between the second substrate layer 2 and the heater 72 and the lead wire 76, and between the third substrate layer 3 and the heater 72 and the lead wire 76. The heater insulating layer 74 will be described in detail below. The lead wire 76 is formed in a manner sandwiched from the upper and lower sides by the second substrate layer 2 and the third substrate layer 3, similarly to the heater 72. The lead wire 76 extends backward from the rear end of the heater 72 and is arranged slightly forward of the rear end of the sensor element 101. The rear end of the lead wire 76 is connected to the connector electrode 71 via the through hole 73. like Figure 3 As shown, the control device 95 includes: the above-mentioned variable power supplies 25, 46, 52, the above-mentioned power supply circuit 92, and a control unit 96. The control unit 96 is a microprocessor having a CPU 97, a RAM and a storage unit 98 (not shown). The storage unit 98 is a non-volatile memory such as a ROM, and is a device for storing various data. The control unit 96 is input with the voltages V0 to V2 and the voltage Vref of each sensor unit 80 to 83. The control unit 96 is input with the pump currents Ip0 to Ip2 and the pump current Ip3 flowing through each pump unit 21, 50, 41, 90. The control unit 96 outputs a control signal to the variable power supplies 25, 46, 52 and the power supply circuit 92, thereby controlling the voltages Vp0 to Vp3 output by the variable power supplies 25, 46, 52 and the power supply circuit 92, thereby controlling each pump unit 21, 41, 50, 90. The control unit 96 outputs a control signal to a heater power source (not shown) to control the power supplied to the heater 72, thereby adjusting the temperature of the sensor element 101. The storage unit 98 stores target values ​​V0*, V1*, V2*, Ip1*, etc., which will be described later. The control unit 96 performs feedback control on the pump voltage Vp0 of the variable power supply 25 so that the voltage V0 reaches the target value V0* (that is, so that the oxygen concentration in the first internal cavity 20 reaches the target concentration). The control unit 96 performs feedback control on the voltage Vp1 of the variable power supply 52 so that the voltage V1 reaches a constant value (referred to as a target value V1*) (that is, so that the oxygen concentration in the second internal cavity 40 becomes a predetermined low oxygen concentration that does not substantially affect the measurement of NOx). At the same time, the control unit 96 sets the target value V0* of the voltage V0 based on the pump current Ip1 so that the pump current Ip1 flowing due to the voltage Vp1 reaches a constant value (referred to as a target value Ip1*) (feedback control). As a result, the gradient of the oxygen partial pressure in the gas to be measured introduced from the third diffusion rate control unit 30 into the second internal cavity 40 is always constant. In addition, the oxygen partial pressure in the atmosphere in the second internal cavity 40 is controlled to a low partial pressure that does not substantially affect the measurement of NOx. 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 becomes a low oxygen concentration. The control unit 96 performs feedback control on the voltage Vp2 of the variable power supply 46 so that the voltage V2 reaches a constant value (referred to as a target value V2*) (that is, so that the oxygen concentration in the third internal cavity 61 becomes a predetermined low concentration). As a result, oxygen is sucked out from the third internal cavity 61 so that the oxygen generated by the reduction of a specific gas (here, NOx) in the measured gas in the third internal cavity 61 is substantially zero. In addition, the control unit 96 obtains the pump current Ip2 as a detection value corresponding to the oxygen generated in the third internal cavity 61 from NOx, and calculates the NOx concentration in the measured gas based on the pump current Ip2. The target value V2* is predetermined to be a value such that the pump current Ip2 flowing due to the feedback-controlled voltage Vp2 becomes a limiting current. In the storage unit 98, a relational expression (for example, a linear function expression), a map, etc. are stored as a correspondence between the pump current Ip2 and the NOx concentration. The above-mentioned relational expression or map can be solved in advance by experiments. Then, the control unit 96 detects the NOx concentration in the gas to be measured based on the acquired pump current Ip2 and the above-mentioned correspondence relationship stored in the storage unit 98 . The control unit 96 controls the power supply circuit 92 in such a manner that the voltage Vp3 is applied to the reference gas adjustment pump unit 90, thereby causing the pump current Ip3 to flow. Thus, the control unit 96 can absorb oxygen from the periphery of the outer pump electrode 23 to the periphery of the reference electrode 42, or absorb oxygen from the periphery of the reference electrode 42 to the periphery of the outer pump electrode 23. In the present embodiment, the voltage Vp3 is set to a DC voltage such that the pump current Ip3 reaches a predetermined value (a DC current of a constant value). Therefore, by causing the pump current Ip3 to flow, the reference gas adjustment pump unit 90 absorbs a constant amount of oxygen from the periphery of the outer pump electrode 23 to the periphery of the reference electrode 42. A connector electrode 71 is provided on the rear end side of the sensor element 101. Figure 6As shown, the connector electrode 71 includes: connector electrodes 71a~71d, which are arranged at the rear end of the lower surface of the sensor element 101; and connector electrodes 71e~71h, which are arranged at the rear end of the upper surface of the sensor element 101. The connector electrode 71 functions as a terminal for electrically connecting the sensor element 101 and the outside. The connector electrodes 71e~71f are electrically connected one-to-one with the inner pump electrode 22, the outer pump electrode 23, the measuring electrode 44 and the auxiliary pump electrode 51 of the sensor element 101. The connector electrode 71a is connected to the reference electrode 42 by means of the reference electrode lead portion 78 described later. The connector electrode 71b is connected to the heater 72 by means of the through hole 73 and the lead 76 (refer to Figure 2 The connector electrodes 71c and 71d are also connected to the heater 72 via through holes and lead wires (not shown). Figure 2 The variable power supplies 25, 46, 52 and the power supply circuit 92 shown in the figure are actually connected by the connector electrodes 71, Figure 1 The connector 150 and the lead wire 155 are connected to each electrode (the inner pump electrode 22, the outer pump electrode 23, the reference electrode 42, the measuring electrode 44 and the auxiliary pump electrode 51). The power supply from the outside to the heater unit 70 is also connected via the connector electrode 71, Figure 1 The connector 150 and the lead 155 are used for this purpose. Here, use Figure 2 , Figures 4 to 6 , the structure of the reference electrode 42, the reference electrode lead portion 78 and the lead 76 and its surroundings will be described in detail. The reference electrode 42 is connected to a conducting portion 77. The conducting portion 77 includes: the above-mentioned connector electrode 71a, and a reference electrode lead portion 78 that conducts the connector electrode 71a and the reference electrode 42. The reference electrode lead portion 78 includes a side lead 78a and an internal lead 78b. The side lead 78a is disposed on the right side of the rear end portion of the sensor element 101, and connects the connector electrode 71a and the internal lead 78b (makes them conductive). The internal lead 78b is disposed inside the sensor element 101, and connects the reference electrode 42 and the side lead 78a. Specifically, the internal lead 78b is disposed between the third substrate layer 3 and the first solid electrolyte layer 4, extends backward from the reference electrode 42, bends to the right side at the rear end portion of the sensor element 101, and is disposed on the right side of the rear end portion of the sensor element 101 (refer to Figure 5 , Figure 6 ). The end of the internal lead 78b on the opposite side to the reference electrode 42 is connected to the side lead 78a. The conductive portion 77 is a conductor whose main component is a noble metal such as platinum (Pt) or a high melting point metal such as tungsten (W) or molybdenum (Mo). The conductive portion 77 is preferably a metal ceramic conductor containing a noble metal or a high melting point metal and zirconium dioxide, which is the same main component as the third substrate layer 3. The reference electrode lead portion 78 (side lead portion 78a and internal lead portion 78b) is porous with a porosity exceeding 5%. Thus, the reference electrode lead portion 78 allows the reference gas to flow in from the outside ( Figure 1 The reference electrode lead 78 preferably has a porosity of 10% or more. The reference electrode lead 78, especially the internal lead 78b, preferably has a porosity of 40% or less. In addition, as long as the side lead 78a and the internal lead 78b are both porous, the porosity of the side lead 78a and the porosity of the internal lead 78b may be the same or different. In addition, the reference electrode lead 78 is designed so that the limiting current when oxygen is sucked out from the periphery of the reference electrode 42 to the periphery of the outer pump electrode 23 is 0.3μA or more. The limiting current is preferably 0.5μA or more. The limiting current has a positive correlation with the inverse of the diffusion resistance of the reference electrode lead 78, especially the internal lead 78b, which means that the larger the limiting current, the smaller the diffusion resistance of the reference electrode lead 78. Therefore, the limiting current of 0.3μA or more means that the diffusion resistance of the internal lead 78b is less than the first specified value (not too large). The diffusion resistance of the inner lead 78b can be 3060×10 3 cm -1 Below, preferably 2000×10 3 cm -1 The diffusion resistance of the internal lead 78b is obtained by dividing the length of the internal lead 78b in the extension direction by the product of the width, thickness and porosity of the internal lead 78b. The limiting current can be 10μA or less. This means that the diffusion resistance of the internal lead 78b is greater than or equal to the second specified value that is less than the first specified value (not too small). As described above, the upper surface and the front, rear, left and right sides of the reference electrode 42 are covered by the reference electrode insulating layer 43. In addition, the entire internal lead 78b, that is, the outer periphery of the portion of the internal lead 78b located inside the sensor element 101 (the portion sandwiched by the third substrate layer 3 and the first solid electrolyte layer 4) is covered by the lead insulating layer 79. The lead insulating layer 79 is connected to the rear end of the reference electrode insulating layer 43 and is formed integrally with the reference electrode insulating layer 43. Thus, the reference electrode 42 is electrically insulated from the first solid electrolyte layer 4, and the internal lead 78b is electrically insulated from the third substrate layer 3 and the first solid electrolyte layer 4. The reference electrode insulating layer 43 and the lead insulating layer 79 are insulating layers formed of ceramics such as alumina. The entire reference electrode insulating layer 43 and the lead insulating layer 79 are dense with a porosity of less than 5%, that is, they are configured as dense regions. As a result, the reference gas is suppressed from reaching the reference electrode 42 from the outside of the sensor element 101 through the lead insulating layer 79. As described above, the heater unit 70 includes a heater 72, a heater insulating layer 74, and a lead 76. The heater insulating layer 74 covers the periphery of the heater 72 and the periphery of the lead 76, except for the connection portion between the lead 76 and the through hole 73. Thus, the heater 72 and the lead 76 are electrically insulated from the second substrate layer 2 and the third substrate layer 3. The heater insulating layer 74 is an insulating layer formed of ceramics such as alumina. The heater insulating layer 74 may be dense with a porosity of 5% or less, or may be porous with a porosity of more than 5%. In addition, the porosity of the reference electrode lead portion 78, the lead insulation layer 79, the heater insulation layer 74, etc. is set to the value derived as follows using an image (SEM image) obtained by observation using a scanning electron microscope (SEM). First, the object to be measured is cut in such a way that the cross section of the object to be measured is set as the observation surface, and the cross-section surface is resin-embedded and polished to prepare an observation sample. Next, the observation surface of the observation sample is photographed with a SEM photograph (secondary electron image, acceleration voltage 15kV, magnification 1000 times, but when the magnification 1000 times is not appropriate, a magnification greater than 1000 times and less than 5000 times is used), thereby obtaining a SEM image of the object to be measured. Next, the obtained image is analyzed, and the threshold is determined using a discriminant analysis method (Otsu's binarization) based on the brightness distribution of the brightness data of the pixels in the image. Thereafter, based on the determined threshold, each pixel in the image is binarized into an object portion and a pore portion, 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 entire area (the total area of ​​the object portion and the pore portion) was derived as the porosity (unit: %). In addition, the limiting current when oxygen is sucked out from the vicinity of the reference electrode 42 to the vicinity of the outer pump electrode 23 is set to a value measured as follows. First, the sensor element 101 is arranged in the atmosphere, and the heater 72 is energized to heat the sensor element 101 to a predetermined driving temperature (e.g., 800° C.). The variable power supplies 25, 46, and 52 are all set to a state where no voltage is applied. In this state, a voltage Vp3 is applied between the outer pump electrode 23 and the reference electrode 42 through the power supply circuit 92 in such a manner that oxygen is sucked out from the vicinity of the reference electrode 42 to the vicinity of the outer pump electrode 23. The pump current Ip3 flowing between the two electrodes 23 and 42 at this time is measured. The voltage Vp3 is set to a DC voltage. Thereafter, when the voltage Vp3 is gradually increased, the pump current Ip3 also gradually increases. However, when a voltage Vp3 of a certain value or more is applied, the pump current Ip3 does not increase even if the voltage Vp3 is increased, and reaches an upper limit. The upper limit of the pump current Ip3 is measured as the limiting current. Next, an example of a method for manufacturing the gas sensor 100 is described below. First, six unfired ceramic green sheets containing an oxygen ion conductive solid electrolyte such as zirconium dioxide as a ceramic component are prepared. A plurality of sheet holes, necessary through holes, etc. for positioning during printing and lamination are formed in advance on the green sheets. In addition, a space that becomes a gas flow portion to be measured is provided on the green sheet constituting the isolation layer 5 by punching treatment or the like in advance. Then, a pattern printing process is performed to form various patterns on each ceramic green sheet corresponding to the first substrate layer 1, the second substrate layer 2, the third substrate layer 3, the first solid electrolyte layer 4, the isolation layer 5, and the second solid electrolyte layer 6. Specifically, the pattern formed is a pattern of each electrode such as the reference electrode 42, the lead wires connected to each electrode such as the internal lead wire 78a, the lead wire insulation layer 79, the connector electrode 71, the heater portion 70, etc. The pattern printing is performed in the following manner: a pattern forming paste prepared according to the characteristics required for each formation object is applied to the green sheet using a known screen printing technique. In addition, as for the pattern of the lead insulating layer 79, it is sufficient to form a portion that covers the lower side of the internal lead 78a and a portion that covers the upper side separately. Similarly, as for the pattern of the heater insulating layer 74 that becomes the heater portion 70, it is sufficient to form a portion that covers the lower side of the heater 72 and the lead 76 and a portion that covers the upper side separately. When the pattern printing process is performed, the drying process is performed using a known drying means. When the pattern printing and drying are completed, the printing and drying process of the adhesive paste for stacking and bonding the raw sheets corresponding to each layer to each other is performed. Then, the following compression bonding process is performed: the raw sheets formed with the adhesive paste are positioned using the sheet holes, and they are stacked in a prescribed order and compressed under prescribed temperature and pressure conditions, thereby forming them into a stack. The stack obtained in this way contains a plurality of sensor elements 101. The stack is cut and divided into the size of the sensor element 101. After that, a pattern of the side lead 78a is formed on the cut surface of the laminate, that is, the portion that becomes the right surface of the sensor element 101, by screen printing, and the pattern is dried. Then, the cut laminate is fired at a predetermined firing temperature to obtain the sensor element 101. In addition, the pattern forming paste of the conductive portion 77 (here, the connector electrode 71a and the reference electrode lead portion 78), especially the internal lead 78b, uses a paste containing particles of the above-mentioned noble metal or high melting point metal, for example. The pattern forming paste of the lead insulating layer 79 and the heater insulating layer 74 uses a paste containing particles of ceramics such as the above-mentioned alumina, for example. Here, the limiting current when oxygen is sucked out from the periphery of the reference electrode 42 to the periphery of the outer pump electrode 23 and the diffusion resistance of the internal lead 78b can be adjusted by adjusting the shape and porosity of the internal lead 78b. This is because: as described above, the diffusion resistance of the internal lead 78b depends on the length, width, thickness, and porosity in the extension direction of the internal lead 78b. The porosity of the internal lead 78b can be adjusted by, for example, adjusting the particle size of the particles contained in the pattern forming paste of the internal lead 78b, or by adjusting the particle size and content ratio of the pore-forming material, or by adjusting the firing temperature and firing time when the laminate is fired. The adjustment of the porosity of the lead insulating layer 79 and the heater insulating layer 74 can also be considered in the same way. After that, the gas sensor 100 in which the sensor element 101 is embedded is manufactured. For example, the element enclosure 141 is installed on the sensor element 101 to be sealed and fixed, and the connector 150 and the lead 155 are installed on the rear end side of the sensor element 101 in a manner that is conductive with the connector electrode 71. The protective cover 130 is installed on the front end side of the sensor element 101 in the element enclosure 141. Furthermore, the outer cylinder 148 is installed on the rear end side of the sensor element 101 in the element enclosure 141, and the lead 155 is led out from the outer cylinder 148 to the outside. Then, the control device 95 and the sensor element 101 are connected by the lead 155. In this way, the gas sensor 100 is obtained. Next, the processing performed by the control unit 96 when the gas sensor 100 detects the NOx concentration in the measured gas is described. First, the CPU 97 of the control unit 96 starts driving the sensor element 101. Specifically, the CPU 97 sends a control signal to the heater power supply 78 to heat the sensor element 101 using the heater 72. At this time, the CPU 97 heats the sensor element 101 to a specified driving temperature (for example, 800°C). Next, the CPU 97 starts controlling the above-mentioned pump units 21, 41, 50, 90 and obtains the voltages V0, V1, V2, and Vref from the above-mentioned sensor units 80 to 83. In this state, when the measured gas is introduced from the gas inlet 10, the measured gas passes through the first diffusion rate control unit 11, the buffer space 12, and the second diffusion rate control unit 13 to reach the first internal cavity 20. Next, the oxygen concentration of the measured gas is adjusted by the main pump unit 21 and the auxiliary pump unit 50 in the first internal cavity 20 and the second internal cavity 40, and the adjusted measured gas reaches the third internal cavity 61. Then, the CPU 97 detects the NOx concentration in the measured gas based on the obtained pump current Ip2 and the corresponding relationship stored in the storage unit 98. Here, the measured gas is Figure 1The sensor element chamber 133 is introduced into the gas flow portion such as the gas inlet 10 in the sensor element 101. On the other hand, since the reference electrode lead portion 78 (side lead 78a and internal lead 78b) is porous with a porosity exceeding 5%, the reference gas is introduced from the outside ( Figure 1 The space 149 shown in the figure is introduced into the reference electrode 42 via the reference electrode lead portion 78. In the present embodiment, the sensor element 101 is designed so that the limiting current when oxygen is sucked out from the periphery of the reference electrode 42 to the periphery of the outer pump electrode 23 is 0.3 μA or more, that is, the diffusion resistance of the reference electrode lead portion 78, especially the internal lead 78b, is less than the specified value (not too large). As a result, it is easier to maintain the oxygen concentration around the reference electrode 42 at an appropriate value compared to a case where the diffusion resistance of the internal lead 78b is too large, and the reduction in the detection accuracy of the specific gas concentration can be suppressed. In addition, as described above, the limiting current is preferably 0.5 μA or more, and the porosity of the internal lead 78b is preferably 10% or more. In addition, the porosity of the internal lead 78b is 40% or less. As a result, the disconnection of the internal lead 78b can be suppressed. In addition, during the period when the sensor element 101 is not driven, the inner lead 78b, the lead insulating layer 79, etc. may absorb water outside the sensor element 101, that is, in the space 149. The water in the space 149 may be originally slightly present in the space 149, or may enter the space 149 from the gap between the rubber stopper 157 and the outer cylinder 148. When the control unit 96 starts driving the sensor element 101, the sensor element 101 is heated by the heater 72, so that the water in the inner lead 78b, the lead insulating layer 79, etc. turns into gas and is discharged to the outside of the sensor element 101, that is, in the space 149. However, during the period until the water is completely discharged, the oxygen concentration around the reference electrode 42 may decrease due to the presence of gaseous water. Therefore, the time from the start of driving the sensor element 101 to the stabilization of the potential of the reference electrode 42 (hereinafter referred to as "stabilization time") changes depending on the time required for the water to be discharged from the inner lead 78b, the lead insulating layer 79, etc. to the outside of the sensor element 101 (hereinafter referred to as "drainage time"). In the present embodiment, the entire lead insulating layer 79 is dense (dense region). Thus, compared with a case where at least a portion of the lead insulating layer 79 is porous (porous region) with a porosity exceeding 5%, the amount of water (water vapor) adsorbed by the lead insulating layer 79 during the period when the sensor element 101 is not driven can be suppressed. In addition, in the present embodiment, the internal lead 78b is a conductor mainly composed of a noble metal or a high melting point metal. The noble metal and high melting point metal used in the internal lead 78b have the property of not being easy to adsorb water compared with alumina used in the lead insulating layer 79, etc., and therefore, the internal lead 78b is not easy to adsorb water compared with the lead insulating layer 79. Thus, the amount of water (water vapor) adsorbed by the internal lead 78b during the period when the sensor element 101 is not driven can be suppressed. Accordingly, by suppressing the amount of water (water vapor) adsorbed by the lead insulating layer 79 and the internal lead 78b during the period when the sensor element 101 is not driven, the drainage time can be shortened, thereby shortening the stabilization time. Here, the correspondence between the constituent elements of the present embodiment and the constituent elements of the present invention is clarified. The first substrate layer 1, the second substrate layer 2, the third substrate layer 3, the first solid electrolyte layer 4, the isolation layer 5 and the second solid electrolyte layer 6 of the present embodiment are respectively equivalent to the solid electrolyte layers of the present invention, and they are equivalent to the element body. In addition, the reference electrode 42 is equivalent to the reference electrode, the outer pump electrode 23 is equivalent to the measured gas side electrode, the connector electrode 71a is equivalent to the reference electrode terminal, the reference electrode lead portion 78 is equivalent to the reference electrode lead portion, and the conductive portion 77 is equivalent to the first conductive portion. In addition, the lead insulating layer 79 is equivalent to the lead insulating layer. In the sensor element 101 of the present embodiment described in detail above, the reference electrode lead portion 78 allows the reference gas to flow from the outside of the sensor element 101 to the reference electrode 42, and the limiting current when oxygen is sucked from the periphery of the reference electrode 42 to the periphery of the outer pump electrode 23 is 0.3 μA or more, and the lead insulating layer 79 is configured as a dense (dense region) with a porosity of 5% or less. Since the lead insulating layer 79 is configured as a dense (dense region) as a whole, the amount of water (water vapor) adsorbed by the lead insulating layer 79 during the period when the sensor element 101 is not driven can be suppressed compared to the case where at least a part of the lead insulating layer 79 is configured as porous. As a result, the time from the start of driving the sensor element 101 to the discharge of water to the outside of the sensor element 101 (drainage time) can be shortened, thereby shortening the time from the start of driving the sensor element 101 to the stabilization of the potential of the reference electrode (stabilization time). In addition, by setting the limiting current to be above 0.3 μA, that is, the diffusion resistance of the reference electrode lead portion 78, especially the internal lead 78b, is below the specified value (not too large), it is easier to maintain the oxygen concentration around the reference electrode 42 at an appropriate value compared to the case where the diffusion resistance of the internal lead 78 is too large, thereby suppressing the reduction in the detection accuracy of the specific gas concentration (NOx concentration). The present invention is not limited to the above-described embodiment, and can be implemented in various forms as long as it falls within the technical scope of the present invention. For example, in the above-mentioned embodiment, the entire lead insulating layer 79 is dense (dense region). Figure 7As shown in the sensor element 201 of the modified example, the area of ​​the lead insulating layer 79 in the plan view that is 50% or more and less than 100% of the total area calculated from the end on the reference electrode 42 side is constituted as a dense area 79a having a porosity of 5% or less and being dense, and the remaining area is constituted as a porous porous area 79b. The total area of ​​the lead insulating layer 79 in the plan view is the total area of ​​the lead insulating layer 79 when viewed from above or from above, in other words, the projected area of ​​the lead insulating layer 79 on the upper surface or the lower surface of the sensor element 201. It is preferred that the area of ​​the lead insulating layer 79 in the plan view that is 80% or more of the total area calculated from the end on the reference electrode 42 side is constituted as a dense area. When a part of the entire area of ​​the lead insulating layer 79 in a plan view is configured as a dense region and the rest is configured as a porous region, the region including the end portion on the reference electrode 42 side in the lead insulating layer 79 is configured as a dense region. Compared with a case where a porous region is provided at a position on the reference electrode 42 side relative to the dense region in the lead insulating layer 79, even if water is adsorbed on the lead insulating layer 79 during a period when the sensor element 101 is not driven, the water can be easily discharged to the outside of the sensor element 101 after the driving of the sensor element 101 is started. As a result, the drainage time can be shortened, and the stabilization time can be shortened. In the above-mentioned embodiment, the cross-sectional area and porosity of the internal lead 78b of the reference electrode lead portion 78 are substantially constant as a whole. Figure 8 As shown in the sensor element 301, the cross-sectional area of ​​the internal lead 78b can be made larger on the reference electrode 42 side than on the side lead 78a side (connector electrode 71a side). In addition or instead of this, the porosity of the internal lead 78b can be made larger on the reference electrode 42 side than on the side lead 78a side. By making the cross-sectional area and / or porosity of the internal lead 78 larger on the reference electrode 42 side than on the side lead 78a side, the diffusion resistance of the internal lead 78b is made smaller on the reference electrode 42 side than on the side lead 78a side. As a result, oxygen is easily accumulated around the reference electrode 42, and it is easier to maintain the oxygen concentration around the reference electrode 42 at an appropriate value, which can suppress the reduction in the detection accuracy of the specific gas concentration (NOx concentration). In the above embodiment, the heater unit 70 includes the heater 72, the heater insulating layer 74 and the lead wire 76, and the lead wire insulating layer 79 and the lead wire insulating layer 79 are independently provided in a manner not to communicate with each other inside the sensor element 101. Fig. 9As shown in the sensor element 401, the heater portion 70 further includes a pressure release hole (connecting hole) 75. The pressure release hole 75 is provided to penetrate the third substrate layer 3 and connect the lead insulation layer 79 and the heater insulation layer 74. The purpose of forming the pressure release hole 75 is to alleviate the internal pressure increase accompanying the temperature increase in the heater insulation layer 74. In the case where the heater portion 70 includes the pressure release hole 75, the overall structure of the heater insulation layer 74 is dense with a porosity of less than 5%. Since the heater portion 70 is provided with the pressure release hole 75 and the overall structure of the heater insulation layer 74 is dense (dense area), the amount of water adsorbed by the heater insulation layer 74 through the lead insulation layer 79 and the pressure release hole 75 during the period when the sensor element 101 is not driven can be suppressed. In the above-described embodiment, the reference electrode insulating layer 43 is densely constructed together with the entire lead insulating layer 79. However, the reference electrode insulating layer 43 may be porous. In the above embodiment, the upper surface and the front, rear, left, and right side surfaces of the reference electrode 42 are covered by the reference electrode insulating layer 43. Fig.10 As shown in the sensor element 501, a reference gas chamber 47 as a space for storing a reference gas is provided inside the sensor element 101, and a reference electrode 42 is provided in the reference gas chamber 47. Fig.10 In the sensor element 501, the reference gas chamber 47 is a space inside the sensor element 501 in which the first solid electrolyte layer 4 is hollowed out. Specifically, it is a space surrounded by the lower surface of the isolation layer 5, the front, back, left and right sides of the first solid electrolyte layer 4, and the upper surface of the third substrate layer 3. A dense layer 48 is arranged in the reference gas chamber 47. The dense layer 48 is a dense layer with a porosity of 5% or less, which covers the inner peripheral surface of the reference gas chamber 47. In addition, the dense layer 48 does not cover the upper surface of the reference electrode 44. As a result, the reference electrode 42 is exposed in the space surrounded by the dense layer 48 in the reference gas chamber 47. In addition, the dense layer 48 also covers the portion of the internal lead 78b arranged in the reference gas chamber 43. As a result, the reference gas can be stored in the reference gas chamber 47 in advance, so that the change in oxygen concentration around the reference electrode 42 can be further suppressed. Furthermore, instead of providing the dense layer 48 in the reference gas chamber 47 , the reference gas chamber 47 may be filled with a porous body, or the dense layer 48 and the porous body may not be provided in the reference gas chamber 47 . In the above embodiment, the outer periphery of the portion of the inner lead 78b located inside the sensor element 101 (the portion sandwiched between the third substrate layer 3 and the first solid electrolyte layer 4) is covered with the lead insulating layer 79. Fig.11 , Fig.12As shown in the sensor element 601, a portion of the outer periphery of the portion of the inner lead 78b located inside the sensor element 101 (midway in the extending direction of the inner lead 78b) is exposed in the preliminary chamber 78c. Fig.11 It is a partial cross-sectional view showing an enlarged view of the reference electrode 42 , the reference electrode lead portion 78 , and the lead 76 and its surroundings of the sensor element 601 . Fig.12 yes Fig.11 The preliminary chamber 78c is a space formed in the element body (laminated body) of the sensor element 601. Fig.11 and Fig.12 In the example, the preparatory chamber 78c is a cylindrical space formed in the form of a hole that passes through the first solid electrolyte layer 4 from top to bottom. In addition, the preparatory chamber 78c also passes through the portion of the lead insulating layer 79 that covers the upper side of the internal lead 78a from top to bottom, so that the upper surface of the internal lead 78a is exposed in the preparatory chamber 78c. The diffusion resistance of the preparatory chamber 78c is smaller than that of the internal lead 78b. By exposing a portion of the internal lead 78b in the preparatory chamber 78c, in other words, by contacting a portion of the internal lead 78b with the preparatory chamber 65, the reference gas flowing through the internal lead 78b diffuses to the preparatory chamber 78c having a smaller diffusion resistance than that of the internal lead 78b. Thus, the preparatory chamber 78c can play a role as a buffer for changes in the oxygen concentration in the reference gas. In addition, the preparatory chamber 78c may also be a shape other than a cylindrical shape, such as a quadrangular prism. In addition, there may be multiple preparatory chambers 78c instead of just one. In Fig.11 and Fig.12 In the example, although the upper surface of the internal lead 78c is exposed in the preliminary chamber 78c, the internal lead 78c may pass through the preliminary chamber 78c so that a plurality of surfaces of the internal lead 78c are exposed in the preliminary chamber 78c. In the above embodiment, the sensor element 101 of the gas sensor 100 includes the first internal cavity 20, the second internal cavity 40, and the third internal cavity 61, but the present invention is not limited thereto. Fig.13 As shown in a sensor element 701 of a modified example, the third internal cavity 61 is not provided. Fig.13 In the sensor element 701 of the modified example, a gas inlet 10, a first diffusion rate control part 11, a buffer space 12, a second diffusion rate control part 13, a first internal cavity 20, a third diffusion rate control part 30, and a second internal cavity 40 are formed adjacent to each other in a sequentially connected manner between the lower surface of the second solid electrolyte layer 6 and the upper surface of the first solid electrolyte layer 4. In addition, a measuring electrode 44 is arranged on the upper surface of the first solid electrolyte layer 4 in the second internal cavity 40. The measuring electrode 44 is covered by a fourth diffusion rate control part 45. The fourth diffusion rate control part 45 is made of aluminum oxide (Al 2 O 3) or other ceramic porous bodies. The fourth diffusion rate control unit 45 plays the role of limiting the amount of NOx flowing into the measuring electrode 44, similarly to the fourth diffusion rate control unit 60 of the above-mentioned embodiment. In addition, the fourth diffusion rate control unit 45 also functions as a protective film for the measuring electrode 44. The top electrode portion 51a of the auxiliary pump electrode 51 is formed to be directly above the measuring electrode 44. Even with the sensor element 701 having such a configuration, the NOx concentration can be detected by using the measuring pump unit 41, similarly to the above-mentioned embodiment. Fig.13 In the sensor element 701 , the periphery of the measuring electrode 44 functions as a measuring chamber. That is, the periphery of the measuring electrode 44 functions similarly to the third internal cavity 61 . In the sensor element 101, the circuit of the reference gas adjustment pump unit 90 may be omitted, or the gas sensor 100 may not include the power supply circuit 92. In addition, the gas sensor 100 may not include the control device 95. For example, the gas sensor 100 may include an external connection connector mounted on the lead wire 155 and used to connect the control device 95 and the lead wire 155, instead of the control device 95. In the above embodiment, the surface of the front side of the sensor element 101 including the outer pump electrode 23 (the portion exposed in the sensor element chamber 133 ) may be covered with a porous protective layer made of ceramics such as alumina. In the above-mentioned embodiment, the CPU 97 performs a process of feedback-controlling the voltage Vp2 of the variable power supply 46 so that the voltage V2 reaches the target value V2*, and detects the NOx concentration in the measured gas based on the detected value (pump current Ip2) at this time, but the present invention is not limited to this. For example, the CPU 97 may control the measurement pump unit 41 (for example, control the voltage Vp2) so that the pump current Ip2 reaches a constant target value Ip2*, and detect the NOx concentration using the detected value (voltage V2) at this time. By controlling the measurement pump unit 41 so that the pump current Ip2 reaches the target value Ip2*, oxygen is sucked out from the third internal cavity 61 at a substantially constant flow rate. Therefore, the oxygen concentration in the third internal cavity 61 changes according to the amount of oxygen generated by the reduction of NOx in the measured gas in the third internal cavity 61, and the voltage V2 changes accordingly. Therefore, the voltage V2 becomes a value corresponding to the NOx concentration in the measured gas. Therefore, the control unit 96 can calculate the NOx concentration based on the voltage V2. In this case, for example, the correspondence relationship between the voltage V2 and the NOx concentration may be stored in advance in the storage unit 98 . In the above embodiment, the sensor element 101 detects the NOx concentration in the measured gas, but it is not limited to the above as long as the concentration of the specific gas in the measured gas is detected. For example, the concentration of oxides other than NOx can also be set as the concentration of the specific gas. In the case where the specific gas is an oxide, as in the above embodiment, the specific gas itself generates oxygen when it is reduced in the third internal cavity 61, so the measurement pump unit 41 can obtain a detection value corresponding to the oxygen (for example, the pump current Ip2) and detect the specific gas concentration. In addition, the specific gas can also be a non-oxide such as ammonia. In the case where the specific gas is a non-oxide, by converting the specific gas into an oxide (for example, if it is ammonia, it is converted into NO), the converted gas generates oxygen when it is reduced in the third internal cavity 61, so the measurement pump unit 41 can obtain a detection value corresponding to the oxygen (for example, the pump current Ip2) and detect the specific gas concentration. For example, the inner pump electrode 22 of the first internal cavity 20 acts as a catalyst, and ammonia can be converted into NO in the first internal cavity 20. In the above embodiment, the element body of the sensor element 101 is set as a stacked body having a plurality of solid electrolyte layers (layers 1 to 6), but it is not limited thereto. The element body of the sensor element 101 only needs to include at least one oxygen ion conductive solid electrolyte layer. For example, Figure 2 In the embodiment, the layers 1 to 5 other than the second solid electrolyte layer 6 may be formed of a material other than the solid electrolyte layer (for example, a layer formed of aluminum oxide). In this case, it is sufficient that each electrode of the sensor element 101 is disposed on the second solid electrolyte layer 6. For example, Figure 2 The measuring electrode 44 in the embodiment may be disposed on the lower surface of the second solid electrolyte layer 6. In addition, the reference electrode 42 may be disposed on the lower surface of the second solid electrolyte layer 6 at a position behind the measured gas flow portion. In the above embodiment, the inner pump electrode 22 is made of Pt and ZrO containing 1% Au. 2 The inner pump electrode 22 only needs to include a noble metal with catalytic activity (for example, at least one of Pt, Rh, Ir, Ru, and Pd), and a noble metal (for example, Au) having the ability to suppress the catalytic activity of the noble metal with catalytic activity for a specific gas. As for the auxiliary pump electrode 51, similarly to the inner pump electrode 22, it only needs to include a noble metal with catalytic activity and a noble metal having the ability to suppress the catalytic activity. The outer pump electrode 23, the reference electrode 42, and the measuring electrode 44 only need to include the above-mentioned noble metal with catalytic activity, respectively. Each electrode 22, 23, 42, 44, and 51 preferably includes a noble metal and an oxide with oxygen ion conductivity (for example, ZrO 2) of metal ceramics, however, at least one of these electrodes may not be metal ceramics. Each electrode 22, 23, 42, 44, 51 is preferably a porous body, however, at least one of these electrodes may not be a porous body. In the above-mentioned embodiment, the outer pump electrode 23 is used as: the outer main pump electrode, which is a part of the main pump unit 21 and is disposed outside the sensor element 101 and exposed to the measured gas; the outer auxiliary pump electrode, which is a part of the auxiliary pump unit 50 and is disposed outside the sensor element 101 and exposed to the measured gas; the outer measurement electrode, which is a part of the measurement pump unit 41 and is disposed outside the sensor element 101 and exposed to the measured gas; and the measured gas side electrode, which is a part of the reference gas adjustment pump unit 90 and is disposed outside the sensor element 101 and exposed to the measured gas, but the present invention is not limited thereto. It is also possible to make one or more of the outer main pump electrode, the outer auxiliary pump electrode, the outer measurement electrode, and the measured gas side electrode be separately disposed outside the sensor element 101 from the outer pump electrode 23. In the above-described embodiment, the control unit 96 sets the target value V0* of the voltage V0 based on the pump current Ip1 so that the pump current Ip1 reaches the target value Ip1* (feedback control), and performs feedback control on the pump voltage Vp0 so that the voltage V0 reaches the target value V0*, but other controls may be performed. For example, the control unit 96 may also perform feedback control on the pump voltage Vp0 based on the pump current Ip1 so that the pump current Ip1 reaches the target value Ip1*. That is, the control unit 96 may omit obtaining the voltage V0 from the main pump control oxygen partial pressure detection sensor unit 80 and setting the target value V0*, and directly control the pump voltage Vp0 based on the pump current Ip1 (and further control the pump current Ip0). In the above-mentioned embodiment, the pump current Ip3 is set to a DC current of a constant value, but it is not limited to this. For example, the pump current Ip3 may also be a pulsed intermittent current. In addition, the pump current Ip3 is a DC current of a constant value in the present embodiment, and is always a current flowing in the direction of absorbing oxygen to the periphery of the reference electrode 42, but it is not limited to this. For example, there may be a period in which the pump current Ip3 flows in the direction of absorbing oxygen from the periphery of the reference electrode 42. Even in this case, the overall oxygen movement direction when observed over a sufficiently long prescribed period is the direction of absorbing oxygen to the periphery of the reference electrode 42. Example Hereinafter, an example of manufacturing a sensor element will be described as an embodiment. In addition, the present invention is not limited to the following embodiment. [Example 1] Using the above manufacturing method, Figure 1 to Figure 6The gas sensor 100 shown is set as Example 1. In addition, when manufacturing the sensor element 101, the ceramic green sheet is obtained by mixing zirconium dioxide particles to which 4 mol% of yttrium trioxide is added as a stabilizer, an organic binder, a dispersant, a plasticizer and an organic solvent and molding them by tape casting. The reference electrode 42 is set as a porous metal ceramic electrode of Pt and zirconium dioxide. The pattern for the reference electrode 42 is formed using a paste made by mixing Pt powder, zirconium dioxide powder, a binder and a pore-forming material. The conductive portion 77 is set as a porous metal ceramic of Pt and zirconium dioxide with Pt as the main component. The pattern of the conductive portion 77 is formed using a paste made by mixing Pt powder, zirconium dioxide powder, a binder and a pore-forming material. As a paste for the lead insulating layer 79, alumina powder and a binder solution are mixed in a weight ratio of 1:2. The porosity of the internal lead 78b in the conductive portion 77 is 15%. The ratio of the dense area calculated from the end on the reference electrode 42 side in the overall area of ​​the lead insulating layer 79 when viewed from above is 100%. The limiting current when oxygen is sucked out from the vicinity of the reference electrode 42 to the vicinity of the outer pump electrode 23 is 1.0 μA. In addition, the porosity is measured by the image analysis using the SEM image described above. The limiting current is measured using the above method. [Example 2] A gas sensor 100 identical to that of Example 1 was produced except that the limiting current was set to 2.0 μA, which was referred to as Example 2. The limiting current was changed by changing the shape of the inner lead 78 b (specifically, the length in the extending direction) compared to Example 1. [Example 3] The same gas sensor 100 as in Example 1 was manufactured except that the porosity of the internal lead 78b was set to 6%, the ratio of the dense region from the end on the reference electrode 42 side in the entire area of ​​the lead insulating layer 79 in a plan view was set to 50%, and the limiting current was set to 0.3 μA. This was referred to as Example 3. In addition, the paste for the porous region in the lead insulating layer 79 was prepared by further mixing a pore-forming material into the paste for the lead insulating layer 79 of Example 1 (the same is true in Examples 4 to 8 and Comparative Example 2 described later). The limiting current was changed by changing the shape (specifically, the length in the extension direction) and porosity of the internal lead 78b with respect to Example 1. [Example 4] A gas sensor 100 identical to that of Example 1 was manufactured except that the ratio of the dense region calculated from the end on the reference electrode 42 side in the overall area of ​​the lead insulating layer 79 in a plan view was set to 80% and the limiting current was set to 0.7 μA, and this was referred to as Example 4. In addition, the limiting current was changed by changing the shape of the internal lead 78b (specifically, the length in the extension direction) with respect to Example 1. [Example 5] A gas sensor 100 identical to that of Example 1 was manufactured except that the porosity of the internal lead 78b was set to 10%, the ratio of the dense region of the entire area of ​​the lead insulating layer 79 in a plan view from the end on the reference electrode 42 side was set to 70%, and the limiting current was set to 0.5 μA, and this was referred to as Example 5. In addition, the limiting current was changed by changing the shape (specifically, the length in the extension direction) and the porosity of the internal lead 78b with respect to Example 1. [Example 6] A gas sensor 100 identical to that of Example 1 was manufactured except that the porosity of the internal lead 78b was set to 10%, the ratio of the dense region of the entire area of ​​the lead insulating layer 79 in a plan view from the end on the reference electrode 42 side was set to 90%, and the limiting current was set to 0.5 μA. The limiting current was changed by changing the length of the internal lead 78b in the extension direction and the porosity with respect to Example 1. [Example 7] A gas sensor 100 identical to that of Example 1 was manufactured except that the porosity of the internal lead 78b was set to 8%, the ratio of the dense region of the entire area of ​​the lead insulating layer 79 in a plan view from the end on the reference electrode 42 side was set to 75%, and the limiting current was set to 0.4 μA. This was referred to as Example 7. In addition, the limiting current was changed by changing the shape (specifically, the length in the extension direction) and the porosity of the internal lead 78b with respect to Example 1. [Example 8] A gas sensor 100 identical to that of Example 1 was manufactured except that the porosity of the internal lead 78b was set to 10%, the ratio of the dense region of the entire area of ​​the lead insulating layer 79 in a plan view from the end on the reference electrode 42 side was set to 80%, and the limiting current was set to 0.7 μA. This was referred to as Example 8. In addition, the limiting current was changed by changing the porosity of the internal lead 78b with respect to Example 1. [Example 9] A gas sensor 100 identical to that of Example 1 was manufactured except that the porosity of the inner lead 78b was set to 40% and the limiting current was set to 2.0 μA, which was referred to as Example 9. The limiting current was changed by changing the shape (specifically, the length in the extension direction) and porosity of the inner lead 78b compared to Example 1. [Example 10] The sensor element 601 having the auxiliary chamber 78c is manufactured (see Fig.11 and Fig.12 ) instead of the sensor element 101 without the preparation chamber (refer to Figure 1 to Figure 6 ) is set as Example 10. Regarding the sensor element 601, a space (hole) that becomes the preliminary chamber 78c is set in advance by punching processing or the like in the green sheet that becomes the first solid electrolyte layer 4, and the shape of the pattern of the portion that becomes the upper side of the inner lead 78a in the lead insulating layer 79 is set to have a shape of the space (hole) that becomes the preliminary chamber 78c. In addition, it is manufactured by the same manufacturing method as in Example 1. In addition, the porosity of the inner lead 78b is set to 15%, the ratio of the dense area calculated from the end of the reference electrode 42 side in the overall area of ​​the lead insulating layer 79 when viewed from above is set to 100%, and the limiting current is set to 2.0μA. Regarding the limiting current, changes are made to Example 1 by providing the preliminary chamber 78c. [Comparative Example 1] A gas sensor 100 identical to that of Example 1 was produced except that the porosity of the inner lead 78b was set to 5% and the limiting current was set to 0.2 μA, which was referred to as Comparative Example 1. The limiting current was changed by changing the shape (specifically, the length in the extension direction) and porosity of the inner lead 78b compared to Example 1. [Comparative Example 2] Comparative Example 2 was a gas sensor 100 similar to that of Example 1 except that the ratio of the dense region from the end on the reference electrode 42 side in the entire area of ​​the lead insulating layer 79 in a plan view was set to 40%. [Evaluation Test 1: Evaluation of Stabilization Time] The gas sensor 100 of Example 1 was stored in a constant temperature and humidity chamber at a temperature of 40°C and a humidity of 85% for one week, so that water was adsorbed on the internal lead 78b and the lead insulation layer 79. Next, the gas sensor 100 of Example 1 was installed in the piping. A model gas in which the base gas was nitrogen, the oxygen concentration was 0%, and the NO concentration was 1500ppm was prepared, and it was circulated in the piping as the measured gas. In this state, the sensor element 101 was driven by the control device 95. Specifically, the control device 95 energized the heater 72 to heat the sensor element 101 and maintain the temperature of the sensor element 101 at 800°C. In addition, it was set that the control device 95 continuously controlled the above-mentioned pump units 21, 41, 50 and obtained the voltages V0, V1, V2, and Vref from the above-mentioned sensor units 80 to 83. The pump unit 90 was adjusted with respect to the reference gas so that it did not operate. The above state is maintained for 60 minutes from the start of driving (heating) of the sensor element 101, and the pump current Ip2 is continuously measured during this period. The value of the pump current Ip2 after 60 minutes from the start of the measurement is taken as the reference value (100%), and the change rate of the value of the pump current Ip2 after 10 minutes from the start of the measurement relative to the reference value is calculated. The change rate of the pump current Ip2 is calculated similarly for the gas sensors 100 of Examples 2 to 10 and Comparative Examples 1 and 2. Here, during the period from the start of the drive of the sensor element 101 to the discharge of water from the internal lead 78b and the lead insulation layer 79, the oxygen concentration around the reference electrode 42 decreases due to the presence of water in the gas, so the potential of the reference electrode 42 is unstable. Therefore, before the potential of the reference electrode 42 stabilizes, even if the NOx concentration of the measured gas is constant, the pump current Ip2 is unstable. In addition, it is considered that: the smaller the change rate of the pump current Ip2, the more fully the water is discharged from the internal lead 78b and the lead insulation layer 79 at the moment when 10 minutes have passed from the start of the measurement, and the more stable the potential of the reference electrode 42. Therefore, the time from the start of the drive of the sensor element 101 to the stabilization of the potential of the reference electrode 42, that is, the length of the stabilization time, can be evaluated according to the size of the change rate. When the calculated change rate is 3% or less, it is determined that the stabilization time is very short ("A"). When the calculated change rate exceeds 3% and is 5% or less, it is determined that the stabilization time is short ("B"). When the calculated change rate exceeds 5%, it is determined that the stabilization time is long ("F"). [Evaluation Test 2: Evaluation of NOx Detection Accuracy] The gas sensor 100 of Example 1 is mounted on the piping. Then, the heater 72 is powered on and its temperature is set to 850°C to heat the sensor element 101. In addition, the reference gas adjustment pump unit 90 is operated. The control voltage Vp3 applied by the power supply circuit 92 of the reference gas adjustment pump unit 90 is set to a pulse voltage (duty ratio 20%) with a period T of 10msec, an on time Ton of 2.0msec, and an off time Toff of 8.0msec. The control voltage Vp3 applied by the power supply circuit 92 is set to a peak value of 10μA for the control current Ip3 flowing when the voltage is on. The effective value Ip3ef of the control current Ip3 is 2μA. In this state, a model gas is prepared in which the base gas is nitrogen, the oxygen concentration is 10%, and the NOx concentration is 500ppm, and it is used as the measured gas and circulated in the piping. This state is maintained for 20 minutes, and the voltage Vref during this period is measured. Examples 2 to 8 and Comparative Examples 1 and 2 are measured in the same manner. There is a tendency that the lower the oxygen concentration around the reference electrode 42 is than the oxygen concentration at the start of the measurement (= oxygen concentration in the atmosphere), the lower the voltage Vref is from the value at the start of the measurement as time passes. In addition, there is a tendency that the lower the voltage Vref is, the larger the pump current Ip2 is than the correct value (the value corresponding to the NOx concentration of 500 ppm). Therefore, when the value of the voltage Vref at the start of the measurement is set to 100%, even if the measured voltage Vref falls within the prescribed range (80% or more) after 20 minutes, it is determined that the change (reduction) in the oxygen concentration around the reference electrode 42 is very small and the detection accuracy of the NOx concentration is very high ("A"). In the case where the measured voltage Vref falls within the prescribed range until 15 minutes have passed, but is lower than the lower limit of the prescribed range before 20 minutes have passed, it is determined that the oxygen around the reference electrode 42 is slightly less, but the detection accuracy of the NOx concentration is high ("B"). In the case where the measured voltage Vref is lower than the lower limit of the prescribed range before 15 minutes have passed, it is determined that the oxygen around the reference electrode 42 is insufficient and the detection accuracy of the NOx concentration is low ("F"). For Examples 1 to 10 and Comparative Examples 1 and 2, the porosity of the internal lead 78b, the proportion of the dense area of ​​the lead insulating layer 79 in the overall area when viewed from above, the limiting current when oxygen is sucked from the vicinity of the reference electrode 42 to the vicinity of the outer pump electrode 23, the evaluation results of the stabilization time and the evaluation results of the NOx detection accuracy are shown in Table 1. Table 1 As shown in Table 1, for Examples 1 to 10 in which the porosity of the internal lead 78b was more than 5% and less than 40%, the ratio of the dense region from the end on the reference electrode 42 side in the overall area of ​​the lead insulating layer 79 in a plan view was more than 50%, and the limiting current when oxygen was sucked from the periphery of the reference electrode 42 to the periphery of the outer pump electrode 23 was more than 0.3 μA, the evaluation of the stabilization time was "A" or "B" and the evaluation of the NOx detection accuracy was "A" or "B". That is, in Examples 1 to 10, the stabilization time was short and the NOx detection accuracy was good. In contrast, in Comparative Example 1, the evaluation of the NOx detection accuracy was "F". It is considered that in Comparative Example 1, the porosity of the internal lead 78b is 5%, that is, the internal lead 78b is dense and the limiting current when oxygen is sucked from the periphery of the reference electrode 42 to the periphery of the outer pump electrode 23 is 0.2μA, and thus the movement of the reference gas between the periphery of the reference electrode 42 and the outside of the sensor element 101 is greatly restricted, so that the oxygen concentration around the reference electrode 42 is not easy to maintain, and the NOx detection accuracy is poor. In addition, in Comparative Example 2, the evaluation of the stabilization time is "F". It is considered that in Comparative Example 2, the proportion of the dense area from the end of the reference electrode 42 side in the overall area of ​​the lead insulating layer 79 in the top view is 40%, that is, the proportion of the dense area is low, and thus the amount of water adsorbed by the lead insulating layer 79 during the period when the sensor element 101 is not driven is large, so it takes time to discharge water from the lead insulating layer 79, and the stabilization time becomes long. Based on the above comparison, it is believed that: in Examples 1 to 10, the porosity of the internal lead 78b is greater than 5% and less than 40%, the proportion of the dense area of ​​the lead insulating layer 79 as viewed from above, which is calculated from the end on the reference electrode 42 side, is greater than 50%, and the limiting current when oxygen is sucked out from the vicinity of the reference electrode 42 to the vicinity of the outer pump electrode 23 is greater than 0.3μA, which can shorten the stabilization time and suppress the reduction in the NOx detection concentration. In addition, in Examples 1 to 10, the proportion of the dense region from the end on the reference electrode 42 side in the overall area of ​​the lead insulating layer 79 in a plan view is 80% or more in Examples 1, 2, 4, 6, 8 to 10, and the evaluation of the stabilization time is "A", and the evaluation of the stabilization time is "B" in Examples 3, 5, and 7, where the proportion is 50% or more and less than 80%. It is therefore considered that the proportion of the dense region from the end on the reference electrode 42 side in the overall area of ​​the lead insulating layer 79 in a plan view is preferably 80% or more. In addition, in Examples 1 to 10, the limiting current when oxygen is sucked from the periphery of the reference electrode 42 to the periphery of the outer pump electrode 23 is 0.5 μA or more in Examples 1, 2, 4 to 6, 8 to 10, and the NOx detection accuracy is evaluated as "A", and the limiting current is 0.3 μA or more and less than 0.5 μA in Examples 3 and 7, and the NOx detection accuracy is evaluated as "B". Therefore, it is considered that the limiting current is preferably 0.5 μA or more. Industrial Applicability The present invention can be used in a gas sensor for detecting the concentration of a specific gas such as NOx in a measured gas such as automobile exhaust gas. Explanation of symbols 1 first substrate layer, 2 second substrate layer, 3 third substrate layer, 4 first solid electrolyte layer, 5 isolation layer, 6 second solid electrolyte layer, 10 gas inlet, 11 first diffusion rate control unit, 12 buffer space, 13 second diffusion rate control unit, 20 first internal cavity, 21 main pump unit, 22 inner pump electrode, 22a top electrode unit, 22b bottom electrode unit, 23 outer pump electrode, 25 variable power supply, 30 third diffusion rate control unit, 40 second internal cavity, 41 measurement pump unit, 42 reference electrode, 43 reference electrode electrode insulating layer, 44 measuring electrode, 45 fourth diffusion rate control unit, 46 variable power supply, 47 reference gas chamber, 50 auxiliary pump unit, 51 auxiliary pump electrode, 51a top electrode portion, 51b bottom electrode portion, 52 variable power supply, 60 fourth diffusion rate control unit, 61 third internal cavity, 70 heater portion, 71, 71a to 71h connector electrode, 72 heater, 73 through hole, 74 heater insulating layer, 75 pressure release hole, 76 lead, 77 conductive portion, 78 internal lead, 78 reference electrode lead portion, 78a side Surface lead, 78a internal lead, 78b internal lead, 78c preliminary chamber, 79 lead insulation layer, 79a dense region, 79b porous region, 80 oxygen partial pressure detection sensor unit for main pump control, 81 oxygen partial pressure detection sensor unit for auxiliary pump control, 82 oxygen partial pressure detection sensor unit for measurement pump control, 83 sensor unit, 90 reference gas adjustment pump unit, 92 power supply circuit, 95 control device, 96 control unit, 97 CPU, 98 storage unit, 100 gas sensor, 101, 201, 301, 50 1, 601, 701 sensor element, 130 protective cover, 131 inner protective cover, 132 outer protective cover, 133 sensor element chamber, 140 sensor assembly, 141 element sealing body, 142 main metal part, 143 inner cylinder, 143a, 143b reduced diameter part, 144a~144c ceramic support, 145a, 145b pressed powder, 146 metal ring, 147 bolt, 148 outer cylinder, 149 space, 150 connector, 155 lead wire, 157 rubber plug, 190 piping, 191 fixing parts.

Claims

1. A sensor element for detecting the concentration of a specific gas in a measured gas, comprising: an element body having a solid electrolyte layer with oxygen ion conductivity and provided with a gas flow portion for introducing and flowing the gas to be measured; A reference electrode disposed inside the element body; a measured gas side electrode disposed at a portion of the element body that is exposed to the measured gas; a first conductive portion including a reference electrode terminal disposed outside the element body and a reference electrode lead portion for conducting the reference electrode terminal and the reference electrode; as well as a lead insulating layer covering a portion of the reference electrode lead portion located inside the element body, The reference electrode lead portion allows a reference gas, which is a detection reference for a specific gas concentration in the measured gas, to flow to the reference electrode. A region that accounts for 50% or more of the entire area of ​​the lead insulating layer in a planar view, calculated from the end portion on the reference electrode side, is a dense region having a porosity of 5% or less.

2. The sensor element according to claim 1, characterized in that The dense region is constituted by a region that accounts for 80% or more of the entire area in the plan view, calculated from the end portion on the reference electrode side.

3. The sensor element according to claim 1 or 2, characterized in that The limiting current when oxygen is sucked from the vicinity of the reference electrode to the vicinity of the measured gas side electrode is 0.3 μA or more.

4. The sensor element according to claim 3, characterized in that The limiting current is greater than 0.5 μA.

5. The sensor element according to claim 1 or 2, characterized in that: The porosity of a portion of the reference electrode lead portion located inside the element body is greater than 5% and is equal to or less than 40%.

6. The sensor element according to claim 1 or 2, characterized in that The cross-sectional area and / or porosity of a portion of the reference electrode lead portion located inside the element body is larger on the reference electrode side than on the reference electrode terminal side.

7. The sensor element according to claim 1 or 2, characterized in that: have: a heater, which is disposed inside the element body and heats the element body; a second conductive portion including a heater terminal disposed outside the element body and a heater lead portion for electrically connecting the heater terminal and the heater; and a heater insulating layer covering the heater and the heater lead portion located inside the element body, The lead insulating layer and the heater insulating layer are independently provided so as not to communicate with each other.

8. The sensor element according to claim 1 or 2, characterized in that have: a heater, which is disposed inside the element body and heats the element body; a second conductive portion including a heater terminal disposed outside the element body and a heater lead portion for electrically connecting the heater terminal and the heater; and a heater insulating layer covering the heater and the heater lead portion located inside the element body, The element body has a connecting hole connecting the heater insulating layer and the lead insulating layer. The heater insulating layer has a porosity of 5% or less. 9 . A gas sensor comprising the sensor element according to claim 1 or 2.

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