JP2004271515A - Gas sensor element, method of controlling and manufacturing method of same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a gas element which accurately detects the concentration of a specific gas component, such as NOx over a long term by maintaining the NOx decomposition activity of a sensor cell electrode. <P>SOLUTION: The gas sensor element 1 has an oxygen pumping cell 2, which controls the oxygen concentration in an internal space 7 to which a gas to be measured is introduced, and a sensor cell 4, which detects the NOx concentration in the gas to be measured. The gas sensor element 1 is also provided with a control means which maintains the oxygen concentration of the internal space 7 at a low level, when measuring the NOx concentration, and which introduces oxygen into the internal space 7 to allow an electrode 4a, which contains Rh and faces the internal space 7 of the sensor cell 4, to be exposed to an oxidative atmosphere, during specific times, other than when the NOx concentration is measured. By exposing to the oxidative atmosphere, NOx decomposition activity by the electrode 4a is recovered, and degradation in detection accuracy is prevented, when it is used for a long term. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

  The present invention relates to a gas sensor element used for an exhaust system of an internal combustion engine for a vehicle and used for detecting a nitrogen oxide (NOx) concentration in a gas to be measured, a control method of the gas sensor element, and a manufacturing method.

Air pollution caused by exhaust gas and the like discharged from internal combustion engines for automobiles has become a major problem in modern society, and regulations on purification standards for harmful substances in exhaust gas are becoming stricter year by year. As means for reducing harmful components in exhaust gas, there are, for example, a system for suppressing generation of harmful components by controlling combustion of an engine, and a system for purifying harmful components using a catalytic converter. It is considered that if a certain nitrogen oxide (NOx) concentration is detected and the detection result is fed back to these systems, exhaust gas purification can be performed more efficiently. From such a background, a gas sensor element capable of accurately detecting NOx concentration in exhaust gas has been demanded, and various structures have been proposed so far (for example, Patent Document 1).
Patent No. 2885336

  Meanwhile, as a conventionally well-known gas sensor element, there is a stacked gas sensor element using a solid electrolyte having oxygen ion conductivity. As shown in FIG. 12, the gas sensor element 1 has an internal space 7 formed between a solid electrolyte body 51 and a solid electrolyte body 52, and the internal space 7 is provided with a porous protective layer 12 and a pinhole 11. Exhaust gas, which is the gas to be measured, is introduced. The internal space 7 is partitioned into a first internal space 7a and a second internal space 7b. On the first internal space 7a side, an oxygen pump cell 2 including a solid electrolyte body 52 and electrodes 2b and 2a on upper and lower surfaces thereof is provided. Are located. By applying a voltage to the oxygen pump cell 2, oxygen in the first internal space 7a can be pumped to the outside of the device or oxygen in the first internal space 7a can be pumped to the outside of the device.

  On the first internal space 7a side, there is provided an oxygen monitor cell 3 comprising a solid electrolyte body 51 and electrodes 3a and 3b on the upper and lower surfaces thereof and capable of detecting the oxygen concentration in the first internal space 7a. The oxygen pump cell 2 is feedback-controlled so that the oxygen concentration in the first internal space 7a detected by the oxygen monitor cell 3 becomes constant. On the other hand, on the second internal space 7b side, a sensor cell 4 including a solid electrolyte body 51 and electrodes 4a and 4b on the upper and lower surfaces thereof is provided, and is configured to measure oxygen ions generated by decomposition of NOx. ing.

  As described above, since the oxygen concentration in the first internal space 7a is controlled to be constant, the oxygen concentration in the second internal space 7b is also constant. Therefore, the amount of oxygen ions moving in the sensor cell 4, that is, the magnitude of the oxygen ion current in the sensor cell 4 corresponds to the NOx concentration. Thereby, it is possible to accurately measure the concentration of NOx regardless of the increase or decrease in the oxygen concentration in the exhaust gas. In the drawings, 61 and 62 are spacers, 81 is a reference gas space, and 9 is a heater.

  Here, in the gas sensor element 1 having the above-described configuration, rhodium (Rh) as a metal component is applied to the electrode 4a on the side of the second internal space 7b of the sensor cell 4 for detecting the NOx concentration in order to promote reduction and decomposition of NOx. A contained cermet electrode is preferably used.

  However, when the present inventors conducted a durability test of the gas sensor element 1 having the above configuration, it was found that the NOx decomposition activity of the electrode 4a of the sensor cell 4 gradually decreased during long-term operation. Therefore, the oxygen ion current in the sensor cell 4 does not accurately reflect the NOx concentration, and there is a problem that the detection accuracy is deteriorated.

  The present invention has been made in view of such a conventional problem, and a gas sensor element capable of maintaining the NOx decomposition activity of an electrode 4a of a sensor cell 4 and accurately detecting the concentration of a specific gas component such as NOx over a long period of time. It is intended to provide.

The gas sensor element according to claim 1 of the present invention has an internal space into which a gas to be measured is introduced under a predetermined diffusion resistance;
On the surface of the oxygen ion-conductive solid electrolyte body, there is a pair of electrodes provided so that one electrode faces the internal space, and oxygen is introduced into the internal space by energizing the pair of electrodes. An oxygen pump cell for discharging and adjusting the oxygen concentration in the internal space,
On the surface of the oxygen ion conductive solid electrolyte body, there is provided a pair of electrodes provided such that one electrode faces the internal space, and a sensor cell for detecting the concentration of a specific gas component in the gas to be measured. . Also,
During the specific gas component concentration measurement, the oxygen concentration in the internal space is maintained at a low concentration, oxygen is introduced into the internal space at a specific timing except during the specific gas component concentration measurement, and the oxygen is introduced into the internal space of the sensor cell. Control means is provided for controlling the recovery of the activity of the electrode facing the internal space of the sensor cell by exposing the facing electrode to an oxidizing atmosphere.

  It is considered that the decrease in the activity of the sensor cell electrode is due to a decrease in surface area due to aggregation of metal components contained in the electrode. The control means of the present invention controls the exposure of the sensor cell electrode to an oxidizing atmosphere to oxidize a part of the metal component. It is considered that the volume expansion due to the oxidation eliminates the aggregation of the metal components and increases the surface area, so that the activity of the sensor cell electrode is restored. Therefore, high detection accuracy can be maintained over a long period.

  According to the second aspect of the present invention, the electrode facing the internal space of the sensor cell is a cermet electrode containing rhodium.

  The decrease in the activity of the electrode facing the internal space of the sensor cell is particularly likely to occur in an electrode containing rhodium used for detecting NOx or the like. Therefore, it is effective to provide the control means of the present invention to the gas sensor element in which the sensor cell electrode is a cermet electrode containing rhodium, and a gas sensor element having high performance and high durability can be realized.

  In the invention of claim 3, the control means introduces oxygen into the internal space by stopping operation of at least one of the oxygen pump cell and the sensor cell when the gas to be measured is in a high oxygen concentration atmosphere. And controlling the electrode of the sensor cell facing the internal space to be exposed to an oxidizing atmosphere.

  If the operation of the oxygen pump cell and the sensor cell is stopped when the gas to be measured has a large amount of oxygen, oxygen is introduced into the internal space, and the sensor cell electrode can be exposed to an oxidizing atmosphere. At this time, the above-described effect of resolving the activity by eliminating aggregation of the metal component in the sensor cell electrode is obtained.

  Specifically, as in the invention of claim 4, when the gas to be measured is exhaust gas of an internal combustion engine, the control means controls the oxygen pump cell when the exhaust gas is in a lean state or when the internal combustion engine is stopped. By stopping the operation of at least one of the sensor cells, oxygen can be introduced into the internal space, and the electrode of the sensor cell facing the internal space can be exposed to an oxidizing atmosphere.

  In the invention according to claim 5, the control means activates at least one of the oxygen pump cell and the sensor cell so as to introduce oxygen into the internal space, thereby causing the electrode of the sensor cell facing the internal space to operate. Control of exposure to an oxidizing atmosphere is performed.

  By controlling the direction of voltage application to the oxygen pump cell or the sensor cell, oxygen can be introduced into the internal space. Therefore, a similar effect of recovering the activity by exposing the sensor cell electrode to an oxidizing atmosphere can be obtained.

  In the invention according to claim 6, on the surface of the oxygen ion conductive solid electrolyte member, one electrode is provided with a pair of electrodes provided so as to face the internal space, and the oxygen concentration in the internal space is detected. An oxygen monitor cell is provided. At this time, the control means introduces oxygen into the internal space by stopping the operation of at least one of the oxygen pump cell, the sensor cell, and the oxygen monitor cell when the measured gas contains a large amount of oxygen, Control of exposing the electrode facing the internal space of the sensor cell to an oxidizing atmosphere is performed.

  When the gas sensor element has an oxygen monitor cell, the voltage of the oxygen pump cell can be controlled by the signal of the oxygen monitor cell, and the oxygen concentration in the internal space can be controlled to be constant. In the configuration having the oxygen monitor cell, by stopping the operation of the oxygen pump cell, the sensor cell, or the oxygen monitor cell, oxygen is introduced into the internal space, and the sensor cell electrode can be exposed to an oxidizing atmosphere. As a result, the same effect of resolving the activity can be obtained by eliminating the aggregation of the metal components in the sensor cell electrode.

  In the invention according to claim 7, on the surface of the oxygen ion conductive solid electrolyte member, one electrode is provided with a pair of electrodes provided so as to face the internal space, and the oxygen concentration in the internal space is detected. An oxygen monitor cell is provided, and the gas to be measured is used as the exhaust gas of the internal combustion engine. At this time, when the exhaust gas is in a lean state or when the internal combustion engine is stopped, the control means stops the operation of at least one of the oxygen pump cell, the sensor cell, and the oxygen monitor cell to introduce oxygen into the internal space. be able to. Therefore, a similar effect of recovering the activity by exposing the sensor cell electrode to an oxidizing atmosphere can be obtained.

  In the invention of claim 8, on the surface of the oxygen ion conductive solid electrolyte body, there is provided a pair of electrodes provided so that one electrode faces the internal space, and detects the oxygen concentration in the internal space. An oxygen monitor cell is provided. At this time, the control means operates at least one of the oxygen pump cell, the sensor cell, and the oxygen monitor cell so as to introduce oxygen into the internal space, thereby oxidizing the electrode of the sensor cell facing the internal space. Control to expose to atmosphere.

  In the configuration having the oxygen monitor cell, oxygen can be introduced into the internal space by using one of the oxygen pump cell, the sensor cell, and the oxygen monitor cell, and the activity is restored by exposing the sensor cell electrode to an oxidizing atmosphere. Similar effects can be obtained.

  According to the ninth aspect of the present invention, the control means performs control of exposing the electrode facing the internal space of the sensor cell to an oxidizing atmosphere while the temperature of the gas sensor element is 400 ° C. or higher. If the process of restoring the activity of the sensor cell electrode is performed in a state where the temperature of the gas sensor element is high, the oxidation of the metal component is promoted, which is more effective.

  In the invention according to claim 10, the specific gas component is nitrogen oxide. INDUSTRIAL APPLICABILITY The present invention is suitably applied to a gas sensor element for detecting a nitrogen oxide concentration. In particular, when the electrode facing the internal space of the sensor cell is an electrode containing rhodium, by providing the control means, the concentration of nitrogen oxides in the gas to be measured such as exhaust gas can be accurately measured for a long time. Can be detected.

Claim 11 of the present invention is an invention of a control method of a gas sensor element, wherein an internal space into which a gas to be measured is introduced under a predetermined diffusion resistance,
On the surface of the oxygen ion-conductive solid electrolyte body, there is a pair of electrodes provided so that one electrode faces the internal space, and oxygen is introduced into the internal space by energizing the pair of electrodes. An oxygen pump cell for discharging and adjusting the oxygen concentration in the internal space,
On the surface of the oxygen ion conductive solid electrolyte body, there is provided a pair of electrodes provided such that one electrode faces the internal space, and a sensor cell for detecting the concentration of a specific gas component in the gas to be measured. Using a gas sensor element,
During the specific gas component concentration measurement, the oxygen concentration in the internal space is maintained at a low concentration, oxygen is introduced into the internal space at a specific timing except during the specific gas component concentration measurement, and the oxygen is introduced into the internal space of the sensor cell. By exposing the facing electrode to an oxidizing atmosphere, control is performed to restore the activity of the electrode facing the internal space of the sensor cell.

  It is considered that the decrease in the activity of the sensor cell electrode is due to a decrease in surface area due to aggregation of metal components contained in the electrode. Therefore, in the gas sensor element having such a configuration, the sensor cell electrode is exposed to an oxidizing atmosphere to oxidize a part of the metal component. It is considered that the activity is recovered because the aggregation of the metal component is eliminated by volume expansion due to oxidation, and the surface area is increased. With this control method, high detection accuracy can be maintained for a long time.

  In the twelfth aspect, the electrode facing the internal space of the sensor cell is a cermet electrode containing rhodium.

  The decrease in the activity of the electrode facing the internal space of the sensor cell is particularly likely to occur in an electrode containing rhodium used for detecting NOx or the like. Therefore, when the control method of the present invention is applied to a gas sensor element in which the sensor cell electrode is a cermet electrode containing rhodium, performance can be effectively improved and durability can be improved.

  In the control method according to the thirteenth aspect, when the gas to be measured is in a high oxygen concentration atmosphere, by stopping the operation of at least one of the oxygen pump cell and the sensor cell, oxygen is introduced into the internal space, Control of exposing the electrode facing the internal space to an oxidizing atmosphere is performed.

  If the operation of the oxygen pump cell and the sensor cell is stopped when the gas to be measured has a large amount of oxygen, oxygen is introduced into the internal space, and the sensor cell electrode can be exposed to an oxidizing atmosphere. At this time, the above-described effect of resolving the activity by eliminating aggregation of the metal component in the sensor cell electrode is obtained.

  Specifically, as in the control method of the fourteenth aspect, the gas to be measured is an exhaust gas of an internal combustion engine, and when the exhaust gas is in a lean state or when the internal combustion engine is stopped, the measured gas is one of the oxygen pump cell and the sensor cell. By stopping at least one operation, oxygen can be introduced into the internal space, and the electrode of the sensor cell facing the internal space can be exposed to an oxidizing atmosphere.

  In the control method according to the fifteenth aspect, at least one of the oxygen pump cell and the sensor cell is operated to introduce oxygen into the internal space, thereby exposing an electrode facing the internal space of the sensor cell to an oxidizing atmosphere. Perform control.

  By controlling the direction of voltage application to the oxygen pump cell or the sensor cell, oxygen can be introduced into the internal space. Therefore, a similar effect of recovering the activity by exposing the sensor cell electrode to an oxidizing atmosphere can be obtained.

  In the control method according to the sixteenth aspect, the gas sensor element includes a pair of electrodes provided on a surface of an oxygen ion conductive solid electrolyte body such that one electrode faces the internal space, It is provided with an oxygen monitor cell for detecting the oxygen concentration of the sample.

  When the gas sensor element has an oxygen monitor cell, the voltage of the oxygen pump cell can be controlled by the signal of the oxygen monitor cell, and the oxygen concentration in the internal space can be controlled to be constant. Also in the configuration having this oxygen monitor cell, oxygen is introduced into the internal space, and a control method of exposing the sensor cell electrode to an oxidizing atmosphere is applied, thereby eliminating the aggregation of the metal component in the sensor cell electrode and increasing the activity. A similar effect of recovery is obtained.

Claim 17 is an invention of a method of manufacturing a gas sensor element, wherein an internal space into which a gas to be measured is introduced under a predetermined diffusion resistance,
On the surface of the oxygen ion-conductive solid electrolyte body, there is a pair of electrodes provided so that one electrode faces the internal space, and oxygen is introduced into the internal space by energizing the pair of electrodes. An oxygen pump cell for discharging and adjusting the oxygen concentration in the internal space,
A sensor cell that has a pair of electrodes provided on the surface of the oxygen ion conductive solid electrolyte so that one electrode faces the internal space, and detects the concentration of a specific gas component in the gas to be measured; And
An electrode facing the internal space of the sensor cell is a cermet electrode containing rhodium in a gas sensor element manufacturing process,
After the firing step of the gas sensor element, the electrode facing the internal space of the sensor cell is exposed to an oxidizing atmosphere at 400 to 1000 ° C.

  By the processing in the manufacturing process of the gas sensor element, the activity of the electrode of the sensor cell can be maintained for a long time. Rhodium (Rh), which is an electrode material, becomes Rh2 O3 in a high-temperature oxidizing atmosphere. Therefore, by exposing the internal space side electrode of the sensor cell to an oxidizing atmosphere of 400 to 1000 DEG C. in advance, the internal Rh is reduced. Rh2O3 can be gradually deposited on the surface to increase the Rh concentration on the surface. According to this method, the same effect can be easily obtained without performing control for restoring the activity.

  In the manufacturing method of the eighteenth aspect, after the baking step of the gas sensor element, the gas sensor element is manufactured by alternately exposing the electrode facing the internal space of the sensor cell to an oxidizing atmosphere and a reducing atmosphere at 400 to 1000 ° C.

  As described above, by exposing the internal space side electrode of the sensor cell to the oxidizing atmosphere, the internal Rh is gradually deposited on the surface as Rh2 O3, and the Rh concentration on the surface can be increased. However, as the surface is covered with Rh 2 O 3, the deposition rate of Rh decreases. Therefore, in this manufacturing method, the surface is once exposed to a reducing atmosphere to reduce the Rh 2 O 3 on the surface to metal Rh, and then to an oxidizing atmosphere again. Expose. Thereby, precipitation of Rh can be promoted.

  Hereinafter, a first embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a schematic cross-sectional view of the tip of the gas sensor element 1 of the present invention, and FIG. 2 is a schematic exploded development view of the tip of the gas sensor element 1 of the present invention. In FIG. 1, the upper half shows a sectional view in the axial direction of the element, and the lower half shows a sectional view taken along line AA. The gas sensor element 1 according to the present invention is disposed in a measured gas existing space and is used for detecting a specific gas component in a measured gas, here, a nitrogen oxide (NOx) concentration in an exhaust gas. At the time of measurement, the entire element is housed in a cylindrical case (not shown), fixed to the exhaust pipe wall of the internal combustion engine, which serves as a measured gas existence space, and the front end shown in the drawing is protected by a cover. To be placed through. The rear end of the gas sensor element 1 is protected by a cover and placed in the atmosphere, which becomes a reference oxygen concentration gas.

  1 and 2, the gas sensor element 1 includes a sheet-shaped solid electrolyte member 51 for forming the oxygen pump cell 2, a solid electrolyte member 52 for forming the oxygen monitor cell 3 and the sensor cell 4, and an internal space 7. And a sheet-shaped spacer 62, 63, 64 for forming the reference gas spaces 81, 82, and a heater 9 for heating them. 62, a solid electrolyte member 51, a spacer 61, a solid electrolyte member 52, and spacers 63 and 64 are laminated in this order.

  The internal space 7 is a chamber into which the gas to be measured is introduced from the space in which the gas to be measured is present, in which the tip of the gas sensor element 1 is disposed. As shown in FIG. It is formed by holes 61a and 61b provided in 61. The hole 61a and the hole 61b are connected by a throttle 61c. The internal space 7 is separated from the tip side (left side in FIGS. 1 and 2) of the gas sensor element 1 with the throttle 61c as a boundary. A first internal space 7a composed of a hole 61a and a second internal space 7b composed of a hole 62b are defined.

  The first internal space 7 a communicates with the measured gas existing space via the pinhole 11 penetrating the tip of the solid electrolyte body 52. The pinhole 11 functions as a diffusion resistance means, and the size of the pinhole 11 is determined by the diffusion speed of the gas to be measured introduced into the first internal space 7a and the second internal space 7b through the pinhole 11. The speed is appropriately set so as to achieve the speed.

  Further, a porous protective layer 12 made of porous alumina or the like is formed on the solid electrolyte body 52 so as to cover the pinhole 11 from the side of the space where the gas to be measured exists. , And clogging of the pinhole 11 is prevented.

  The reference gas spaces 81 and 82 are chambers into which air as a reference oxygen concentration gas having a constant oxygen concentration is introduced. The reference gas space 81 is provided in a hole 62a (FIG. 2) provided in the spacer 62 laminated below the solid electrolyte body 51, and the reference gas space 82 is provided in the spacer 63 laminated above the solid electrolyte body 52. The hole 63a is formed. The holes 62a, 63a communicate with the external space in which the atmosphere exists through groove-shaped passages 62b, 63b extending in the longitudinal direction of the gas sensor element 1, respectively. The reference gas space 81, Atmosphere is introduced into 82.

  The spacers 61, 62, 63, and 64 that constitute the internal space 7 and the reference gas spaces 81 and 82 are made of an insulating material such as alumina. The solid electrolyte members 51 and 52 for forming the oxygen pump cell 2, the oxygen monitor cell 3, and the sensor cell 4 are made of a solid electrolyte having oxygen ion conductivity such as zirconia or ceria.

  As shown in FIG. 1, the oxygen pump cell 2 includes a solid electrolyte body 51 and a pair of electrodes 2 a and 2 b opposed to each other so as to sandwich the solid electrolyte body 51. Oxygen is discharged from the internal space 7 to adjust the oxygen concentration in the internal space 7. One of the pair of electrodes 2a, 2b is provided in contact with the upper surface of the solid electrolyte body 51 so as to face the first internal space 7a located on the upstream side of the gas flow in the internal space 7, The other electrode 2 b is provided in contact with the lower surface of the solid electrolyte member 51 so as to face the reference gas space 81.

  The sensor cell 4 includes a solid electrolyte body 52 and a pair of electrodes 4a and 4b opposed to each other so as to sandwich the solid electrolyte body 52, and detects the concentration of a specific gas component in the gas to be measured, for example, the NOx concentration. One of the pair of electrodes 4a and 4b is provided in contact with the lower surface of the solid electrolyte body 52 so as to face the second internal space 7b located on the downstream side of the gas flow in the internal space 7; The other electrode 4 b is provided in contact with the upper surface of the solid electrolyte body 52 so as to face the reference gas space 82.

  The oxygen monitor cell 3 includes a solid electrolyte body 52 and a pair of electrodes 3 a and 3 b opposed to each other with the solid electrolyte body 52 interposed therebetween, and detects the oxygen concentration in the internal space 7. One electrode 3a of the pair of electrodes 3a, 3b is provided in contact with the lower surface of the solid electrolyte body 52 so as to face the second internal space 7b, and the other electrode 3b faces the reference gas space 82. The solid electrolyte body 52 is provided in contact with the upper surface of the solid electrolyte body 52. When the electrodes 3a and 3b of the oxygen monitor cell 3 and the electrodes 4a and 4b of the sensor cell 4 are substantially at the same position with respect to the introduction direction of the gas to be measured, the vicinity of the electrodes 3a and 4a facing the second internal space 7b This is preferable because the oxygen concentration becomes almost equal.

  Here, as one of the electrodes 2a and 3a of the oxygen pump cell 2 and the oxygen monitor cell 3, an electrode having low NOx decomposition activity may be used in order to suppress the decomposition of NOx in the gas to be measured. Specifically, a porous cermet electrode containing platinum (Pt) and gold (Au) as main metal components is preferably used. At this time, the content of Au in the metal component is usually desirably about 1 to 10% by weight. The porous cermet electrode is obtained by forming a paste of a metal alloy powder and ceramics such as zirconia and alumina, and firing the paste.

  Further, in order to decompose NOx in the gas to be measured, an electrode having a high activity of decomposing NOx may be used as the one electrode 4a of the sensor cell 4. Specifically, a porous cermet electrode containing platinum (Pt) and rhodium (Rh) as main metal components is preferably used. At this time, the content of Rh in the metal component is preferably about 10 to 50% by weight. As the other electrodes 2b, 3b, 4b of the oxygen pump cell 2, the oxygen monitor cell 3, and the sensor cell 4, for example, a Pt porous cermet electrode is suitably used.

  As shown in FIG. 2, the electrodes of the oxygen pump cell 2, the oxygen monitor cell 3, and the sensor cell 4 have leads 2c, 2d, 3c for extracting electric signals from these electrodes 2a, 2b, 3a, 3b, 4a, 4b. 3d, 4c and 4d are integrally formed. Here, on the upper and lower surfaces of the solid electrolyte members 51 and 52, at the portions other than the electrode formation portion, particularly at the formation portions of the leads 2c, 2d, 3c, 3d, 4c, 4d, these leads 2c, 2d, 3c, 3d, It is preferable to form an insulating layer (not shown) of alumina or the like between 4c and 4d.

  The heater 9 is formed by patterning a heater electrode 14 that generates heat by energization on the upper surface of a heater sheet 13 made of an insulating material such as alumina. The layer 15 is formed. As the heater electrode 14, a cermet electrode of Pt and a ceramic such as alumina is usually used. The heater 9 causes the heater electrode 14 to generate heat by external power supply, and heats the cells 2, 3, and 4 to an activation temperature.

  As shown in FIG. 2, the heater electrode 14 and the electrodes 2a, 2b, 3a, 3b, 4a, 4b of the cells 2, 3, 4 are respectively connected to the leads 2c, 2d, 3c, 3d, 4c, 4d. And the solid electrolyte members 51, 52, spacers 61, 62, 63, 64, the alumina layer 15 and the through-hole SH formed in the heater sheet 13 to the terminal (pad electrode) P of the sensor base. A lead wire is connected to the terminal P via a connector (not shown) by crimping or brazing, so that signals can be exchanged between the external circuit and each of the cells 2, 3, 4, and the heater 9. .

  A method for manufacturing the gas sensor element 1 having the above configuration will be described. First, a raw sheet made of zirconia or the like for the solid electrolyte members 51 and 52, and an alumina raw sheet to be the spacers 61, 62, 63 and 64, the heater sheet 13 and the alumina layer 15 are prepared. These raw sheets can be formed into a sheet shape by a doctor blade method, an extrusion molding method, or the like. Next, the electrodes 2a, 2b, 3a, 3b, 4a, 4b, the heater electrode 14 are placed at predetermined positions on the raw sheets for the solid electrolyte bodies 51, 52, the raw sheet for the spacer 64, and the raw sheet for the heater sheet 13. The leads 2c, 2d, 3c, 3d, 4c, 4d and the terminals P are formed by screen printing or the like. Then, the sheets are stacked in the order shown in FIG. 1 and fired in the air to integrate the elements.

  Next, the operating principle of the gas sensor element 1 having the above configuration will be described. In FIG. 1, the exhaust gas as the gas to be measured passes through the porous protective layer 12 and the pinhole 11, and is introduced into the first internal space 7a. The amount of gas introduced is determined by the diffusion resistance of the porous protective layer 12 and the pinhole 11. Further, the introduced gas is introduced into the second internal space 7b communicating with the first internal space 7a via the throttle 61c.

  When a voltage is applied to the pair of electrodes 2a and 2b of the oxygen pump cell 2 so that the electrode 2b on the reference gas space 81 side becomes a positive electrode, oxygen in the gas to be measured on the electrode 2a on the first internal space 7a side is reduced. It is reduced to oxygen ions and is discharged to the electrode 2b side by a pumping action. Conversely, when a voltage is applied so that the electrode 2a on the first internal space 7a side becomes a positive electrode, oxygen is reduced on the electrode 2b on the reference gas space 81 side to become oxygen ions, and is pumped to the electrode 2a side. be introduced. By this oxygen pump action, the oxygen concentration in the internal space 7 can be controlled.

  On the other hand, when a predetermined voltage (for example, 0.40 V) is applied to the pair of electrodes 3a and 3b of the oxygen monitor cell 3 so that the electrode 3b on the reference gas space 82 side becomes a positive electrode, the second internal space 7b side Oxygen in the gas to be measured is reduced on the electrode 3a to become oxygen ions, and is discharged to the electrode 3b side by a pumping action. Here, since the electrode 3a is a Pt-Au cermet electrode which is inactive in decomposing NOx which is a specific gas component, the oxygen ion current flowing between the electrodes 3a and 3b flows through the porous protective layer 12, the pinhole 11, and the It depends on the amount of oxygen reaching the electrode 3a in the second inner space 7b through the first internal space 7a and the like, and does not depend on the NOx amount. Therefore, if the voltage applied between the electrodes 2a and 2b of the oxygen pump cell 2 is controlled so that the current value flowing between the electrodes 3a and 3b becomes a predetermined constant value (for example, 0.2 μA), the second internal space 7b Oxygen concentration can be controlled to be constant.

  A predetermined voltage (for example, 0.40 V) is applied to the pair of electrodes 4a and 4b of the sensor cell 4 so that the electrode 4b on the reference gas space 82 side becomes a positive electrode. Here, since the electrode 4a is a Pt-Rh cermet electrode that is active in decomposing NOx as a specific gas component, oxygen and NOx in the gas to be measured are reduced on the electrode 4a on the second internal space 7b side to reduce oxygen. It becomes ions and is discharged to the electrode 4b side by the pumping action. At this time, as described above, in the present embodiment, the oxygen pump cell 2 is controlled so that the current value between the pair of electrodes 3a and 3b of the oxygen monitor cell 3 becomes a predetermined constant value (for example, 0.2 μA). Therefore, if NOx does not exist in the gas to be measured, the current value between the electrodes 4a and 4b of the sensor cell 4 is also controlled to a constant value (for example, 0.2 μA). On the other hand, if NOx is present in the gas to be measured, the current value increases in accordance with the NOx concentration, so that the NOx concentration in the gas to be measured can be detected.

  FIG. 3A shows a current-voltage characteristic of the sensor cell 4. This is because, under the conditions of NO: 1000 ppm and 500 ppm, O2: 100 ppm, and N2: balance gas, the applied voltage of the oxygen pump cell 2 is set to 0.3 V so that the electrode 2 b becomes a positive electrode, and the oxygen monitor cell 3 is stopped (oxygen The measurement was performed assuming that the current of the monitor cell 3 was 0). As shown in FIG. 3A, an oxygen ion current generated by decomposition of NO flows through the sensor cell 4, and the current-voltage characteristic shows a limit current (a state in which the current value is saturated) according to the NOx concentration. Therefore, the operating voltage of the sensor cell 4 is set to a voltage at which the sensor cell current becomes the limit current (0.4 V in the present embodiment), and the NOx concentration is measured by measuring the sensor cell current value (limit current value) at this time. Can be.

  However, when the gas sensor element 1 was installed in an engine exhaust pipe and a durability test was performed for a running distance of 100,000 km, it was found that the current-voltage characteristics of the sensor cell 4 changed. FIG. 3B shows the current-voltage characteristics of the sensor cell after the endurance. Compared to FIG. 3A before endurance, the resistance value in the current increasing region (region on the lower voltage side than the limit current region) is increased, and it is found that the NOx decomposition activity of the electrode 4a of the sensor cell 4 is reduced. Was. For this reason, the applied voltage that reaches the limit current increases, and the operating voltage of the sensor cell 4 becomes 0.4 V and falls outside the limit current region. As a result, as shown in FIG. 4 showing the relationship between the NOx concentration and the sensor output, the NOx sensitivity after the endurance was smaller than that before the endurance, and the detection accuracy was deteriorated. The measurement in FIG. 4 was performed under the conditions of NO: 0 to 1000 ppm, O2: 5%, and N2: balance gas.

  On the other hand, when the operation of the oxygen pump cell 2, the oxygen monitor cell 3, and the sensor cell 4 is stopped (the current of each cell is 0) and only the heater 9 is energized and the gas sensor element 1 after the endurance is exposed to the air, the sensor cell 4 It was found that the NOx decomposition activity of the electrode 4a was restored. FIG. 3C shows the sensor cell current-voltage characteristics after the gas sensor element 1 after the durability of 100,000 km is exposed to the air for 10 minutes. FIG. 4 shows the sensor output characteristics (after oxidation in air) of the gas sensor element 1 exposed to air. As is apparent from the figure, the current-voltage characteristics in FIG. 3C are almost the same as those in FIG. 3A before the endurance, and the sensor output in FIG. 4 has recovered to the same degree as before the endurance.

  It is considered that the above phenomenon is caused by a decrease in surface area due to agglomeration of a metal component constituting the electrode 4a of the sensor cell 4, and an increase in surface area due to oxidation. That is, during the durability, the inside of the second internal space 7b is in a low oxygen concentration atmosphere, and the metal component of the electrode 4a gradually aggregates. Accordingly, the surface area of the electrode decreases, so that the NOx decomposition activity of the electrode 4a decreases. On the other hand, when the electrode 4a of the sensor cell 4 whose decomposition activity is reduced is exposed to a high oxygen concentration atmosphere (preferably the oxygen concentration becomes 1% or more near the electrode 4a), Rh contained in the electrode 4a of the sensor cell 4 is oxidized. It becomes rhodium (Rh2 O3) and expands in volume. Thereby, it is considered that aggregation of the metal component is eliminated, the surface area increases, and the NOx decomposition activity of the electrode 4a is restored. In order to obtain the above effect, it is usually desirable that at least 1% of all Rh atoms contained in the electrode 4a of the sensor cell 4 be oxidized. However, if the amount of Rh2 O3 increases, the NOx decomposition activity of the electrode 4a decreases. Therefore, it is preferable that the Rh atoms contained in the electrode 4a as Rh2 O3 be 5% or less.

  Therefore, in the present invention, a control means for controlling the operations of the oxygen pump cell 2, the oxygen monitor cell 3, and the sensor cell 4 is provided, and when the NOx concentration is measured, the control for maintaining the oxygen concentration in the internal space 7 at a low concentration as described above. Is performed, and oxygen is introduced into the internal space 7 at a specific timing except when measuring the NOx concentration, and control is performed to restore the NOx decomposition activity of the electrode 4a of the sensor cell 4. Specifically, for example, the recovery process is performed at a timing immediately after the engine is stopped or at a timing that does not hinder the NOx concentration measurement (periodically or every traveling distance), so that the NOx decomposition activity of the electrode 4a is reduced. Can be maintained.

  More specifically, as a first method of restoring the NOx decomposition activity of the electrode 4a of the sensor cell 4, at least one of the oxygen pump cell 2, the oxygen monitor cell 3, and the sensor cell 4 when the exhaust pipe of the internal combustion engine has a large amount of oxygen. Stop one operation. For example, if the above control is performed when the engine is stopped or when the exhaust gas of the internal combustion engine, which is the gas to be measured, is in a lean state, the exhaust gas having a high oxygen concentration is introduced into the internal space 7, so that the electrode 4 a of the sensor cell 4 Can be exposed to an oxidizing atmosphere. Preferably, all of the oxygen pump cell 2, the oxygen monitor cell 3, and the sensor cell 4 are stopped, but even if any of the oxygen pump cell 2, the oxygen monitor cell 3, and the sensor cell 4 is operating, oxygen remains in the internal space 7. The above-described effect can be obtained by controlling so as to be in the state of being in the state. If the element temperature is low, Rh is not sufficiently oxidized. Therefore, it is preferable to perform the above-described recovery control while the heater 9 is energized. Preferably, the element temperature (the temperature of the electrode 4a of the sensor cell 4) should be 400 ° C. or more and 1000 ° C. or less. Preferably, the element temperature is set to 600 ° C. or more and 800 ° C. or less.

  According to the first method, the engine is stopped every 10 hours, and after the engine is stopped, only the heater 9 is energized for 5 minutes to control the recovery of the electrode 4a of the sensor cell 4 (oxygen pump cell 2, oxygen monitor cell 3, sensor cell 4). (The operation was stopped.) Then, the engine was restarted, and an endurance test for running a total of 100,000 km was performed. As a result, as shown in FIG. 5, it was found that the NOx sensitivity did not change even after the endurance, and high detection accuracy could be maintained. This is considered to be because the NOx decomposing activity of the electrode 4a is maintained for the above-described reason by energizing only the heater 9 in a state where the gas sensor element 1 is exposed to the high oxygen concentration atmosphere after the engine is stopped. Here, the recovery processing is performed for 5 minutes every 10 hours, but the recovery processing interval and processing time can be set arbitrarily.

  As the control means, for example, a NOx sensor element ECU (not shown) using a microcomputer is used. FIG. 6 shows an example of a control flowchart by this ECU. The ECU first determines in step 101 whether the ignition switch of the vehicle is ON. If the ignition switch is OFF, the ECU proceeds to step 102. In step 102, it is determined whether or not the ignition switch has just changed from ON to OFF. If the determination in step 102 is affirmative, the routine proceeds to step 103, where a timer for 5 minutes is started from that point.

  In step 104, it is determined whether five minutes have elapsed since the timer was started. If not, the routine proceeds to step 105, where the oxygen pump cell 2, the oxygen monitor cell 3 Then, the applied voltage to the sensor cell 4 is cut off. Thereafter, the process returns to step 101 and repeats the following steps. At step 104, when it is confirmed that five minutes have elapsed since the timer was started, the process proceeds to step 106. In step 106, the heater 9 is turned off in addition to the oxygen pump cell 2, the oxygen monitor cell 3, and the sensor cell 4, and the recovery process ends. If the ignition switch of the vehicle is ON in step 101, the process proceeds to step 107, where the heater 9, the oxygen pump cell 2, the oxygen monitor cell 3, and the sensor cell 4 are operated normally to perform an operation of detecting the NOx concentration.

  As a second method of restoring the NOx decomposition activity of the electrode 4a of the sensor cell, at least one of the oxygen pump cell 2, the oxygen monitor cell 3, and the sensor cell 4 may be operated so that oxygen is introduced into the internal space 7. it can. For example, when a voltage is applied to the oxygen pump cell 2 so that the electrode 2a on the first internal space 7a side becomes a positive electrode, oxygen is pumped to the first internal space 7a side, so that the electrode 4a of the sensor cell 4 is exposed to an oxidizing atmosphere. Can be exposed. Also in this case, it is preferable to perform the recovery process while the heater 9 is energized.

  The following test was conducted to confirm the effect of the second method. In the 100,000 km durability test, the gas sensor element 1 was continuously operated, and an electric current was applied to the oxygen pump cell 2 so as to supply oxygen to the first internal space 7a every hour for one minute. (In the present embodiment, a voltage of 0.5 V is applied so that the electrode 2a on the first internal space 7a side becomes a positive electrode. As a result, the same effect as that obtained by the first method is obtained. confirmed.

  FIG. 7 shows a timing chart of the voltage applied to the oxygen pump cell 2 when performing this control. An event timer may be provided in the NOx sensor element ECU serving as the control means, and control may be performed to reverse the applied voltage of the oxygen pump cell 2 for one minute every one hour as shown in the figure.

  As another example for confirming the effect of the second method, the following test was performed. In the 100,000 km endurance test, the gas sensor element 1 was continuously operated, and a current was passed through the sensor cell 4 so that oxygen was supplied to the second internal space 7b every hour for one minute. (In the present embodiment, a voltage of 0.5 V is applied so that the electrode 4a on the second internal space 7b side becomes a positive electrode. Thus, the internal space 7 is formed using the sensor cell 4 instead of the oxygen pump cell 2. It was confirmed that the same effect as in the case of the first method was obtained even if oxygen was introduced into the inside.

  FIG. 8 shows a timing chart of the voltage applied to the sensor cell 4 when performing this control. An event timer may be provided in the NOx sensor element ECU serving as control means, and control may be performed to reverse the applied voltage of the sensor cell 4 for one minute every one hour as shown in the figure.

  FIGS. 9A and 9B are diagrams showing another operation principle, and will be described as a second embodiment of the gas sensor element 1 to which the method of the present invention is applied. The configuration of the gas sensor element 1 is the same as that of the first embodiment. In the first embodiment, the voltage applied between the electrodes 2a and 2b of the oxygen pump cell 2 is controlled so that the current value between the electrodes 3a and 3b of the oxygen monitor cell 3 is constant. From the relationship between the applied voltage of the oxygen pump cell and the current of the oxygen pump cell, by applying a voltage corresponding to the oxygen concentration so that the oxygen pump cell current becomes the limiting current, the oxygen concentration in the first internal space 7a is reduced to a predetermined low oxygen level. Control to concentration.

  However, when the oxygen concentration in the internal space 7 is controlled by this method, the oxygen concentration in the second internal space 7b becomes lower than the control based on the detection value of the oxygen monitor cell 3 as in the first embodiment. If the current flowing between the electrodes 4a and 4b of the sensor cell 4 is used as the sensor signal as it is, the detection accuracy of NOx deteriorates. Therefore, in the present embodiment, a current difference detection circuit is provided, and a difference between a current flowing between the electrodes 3a and 3b of the oxygen monitor cell 3 and a current flowing between the electrodes 4a and 4b of the sensor cell 4 is used as a sensor signal. In this way, the influence of the oxygen concentration fluctuation in the second internal space 7b can be eliminated, a sensor output independent of the oxygen concentration in the gas to be measured can be obtained, and the NOx concentration can be accurately detected.

  Also in the gas sensor element 1 having this configuration, as described above, by performing the process of recovering the NOx decomposition activity of the electrode 4a of the sensor cell 4 at a specific timing, high NOx detection accuracy can be maintained for a long time.

  In each of the above-described embodiments, the oxygen concentration in the second internal space 7b is detected by the value of the current flowing through the oxygen monitor cell 3, but may be detected by the electromotive force generated in the oxygen monitor cell 3. This will be described as a third embodiment with reference to FIG. In the present embodiment, the basic configuration including the oxygen monitor cell 3 and the sensor cell 4 is the same as that of the first embodiment, but the arrangement is different, and a second oxygen pump cell 20 is provided. . Further, the reference gas space 82 is not provided, and only the reference gas space 81 is provided.

  In FIG. 10, the oxygen pump cell 2 has electrodes 2a and 2b provided on the upper and lower surfaces of a solid electrolyte member 52, and one electrode 2a facing the first internal space 7a is provided on the lower surface of the solid electrolyte member 52 and the other electrode 2a. The electrode 2b is provided on the upper surface of the solid electrolyte body 52 so as to face the space where the gas to be measured exists. The oxygen monitor cell 3 is provided such that one electrode 3a is provided on the upper surface of the solid electrolyte body 51 so as to face the first internal space 7a, and the other electrode 3b is provided on the lower surface of the solid electrolyte body 51 and the reference gas space 81. Provided facing. In the sensor cell 4, one electrode 4 a facing the second internal space 7 b is provided on the upper surface of the solid electrolyte body 51, and the other electrode 4 b is provided on the lower surface of the solid electrolyte body 51 facing the reference gas space 81. Note that the electrode 4b is a common electrode with the electrode 3b of the oxygen monitor cell 3.

  The second oxygen pump cell 20 includes a solid electrolyte body 52 and a pair of electrodes 20a and 2b provided on the surface thereof. One electrode 20a is provided on the lower surface of the solid electrolyte body 52 so as to face the second internal space 7b, and the other electrode 2b serves as a common electrode with the oxygen pump cell 2. The second oxygen pump cell 20 has a function of discharging the residual oxygen in the measured gas introduced into the second internal space 7b without being exhausted by the oxygen pump cell 2 to the measured gas existing space.

  The operation in this case will be described with reference to FIG. In the above configuration, the electrode 3a of the oxygen monitor cell 3 faces the first internal space 7a, and the electrode 3b faces the reference gas space 81 into which the atmosphere is introduced. An electromotive force based on the Nernst equation is generated between the electrodes 3a and 3b due to a difference in oxygen concentration between the first internal space 7a where both electrodes are in contact and the reference gas space 81. Since the oxygen concentration in the reference gas space 81 is constant, the electromotive force generated between the electrodes 3a and 3b reflects the oxygen concentration in the first internal space 7a. Therefore, if the voltage applied between the electrodes 2a and 2b of the oxygen pump cell 2 is controlled so that the electromotive force generated between the electrodes 3a and 3b becomes a predetermined constant value (for example, 0.20 V), the second internal space The concentration of oxygen flowing into 7b can be controlled to be constant. Further, in the present embodiment, the second oxygen pump cell 20 is formed by the solid electrolyte body 52 and the electrodes 20a and 2b, and the oxygen that cannot be discharged by the oxygen pump cell 2 and flows into the second internal space 7b is Exhaust. As a result, the oxygen concentration in the second internal space 7b becomes substantially zero, and the sensor cell 4 enables highly accurate NOx concentration measurement.

  Also in the gas sensor element 1 having this configuration, by performing the process of recovering the NOx decomposition activity of the electrode 4a of the sensor cell 4 at a specific timing according to the present invention, the NOx concentration can be detected with high accuracy for a long time.

  In the above embodiments, the control method for recovering the gas sensor element 1 manufactured by the normal method when the detection accuracy is reduced by long-term use has been described. However, by changing the manufacturing method, the detection accuracy can be improved. The decrease can be suppressed, and the recovery control can be made unnecessary. This manufacturing method is to form a raw sheet made of zirconia or the like for the solid electrolyte body, a spacer, a heater sheet, and an alumina raw sheet to be an alumina layer, print electrodes, heater electrodes, lead portions, terminals, and then laminate them. The process up to baking in the air is performed in the same manner as described above, and thereafter, an oxidizing process of exposing the integrated device to a high-temperature oxidizing atmosphere is performed. The oxidation treatment temperature is preferably from 400 to 1000C, more preferably from 600 to 900C. At a temperature of 1000 ° C. or higher, the metal Rh is more stable than Rh 2 O 3, so that the above-mentioned precipitation phenomenon cannot be expected. At a temperature of 400 ° C. or lower, the deposition rate becomes slow and no effect is obtained. Generally, the oxidizing atmosphere should preferably have an oxygen concentration of 1% or more and the oxidizing treatment time should be 1 hour or more.

  The operation and effect of this manufacturing method will be described. Rh becomes Rh2 O3 in a high-temperature oxidizing atmosphere. The electrode 4a on the inner space side of the sensor cell 4 is a Pt-Rh electrode, and when exposed to an oxidizing atmosphere, internal Rh is gradually deposited on the surface as Rh2 O3, and the Rh concentration on the surface increases. The electrode surface concentration of Rh, which is a NOx decomposition active component, is increased. Further, since the deposited Rh is not poisoned by the inert substance (Au) scattered from the electrode 2a or the like of the oxygen pump cell 2, the NOx decomposition activity of the sensor cell 4 is significantly improved.

  FIG. 11 shows the results of an endurance test for a running distance of 100,000 km, in which the gas sensor element 1 which had been treated in air at 900 ° C. for 10 hours after the firing step was installed in an engine exhaust pipe. As is apparent from the figure, there is almost no change in sensitivity before and after the endurance. As described above, the gas sensor element 1 manufactured by performing the oxidation treatment after the firing step can maintain a high NOx decomposition activity for a long period of time, and thus does not require recovery control. Therefore, a recovery control device is not required, and a high effect can be obtained with a simple configuration.

  In the above-described manufacturing method, when the Pt-Rh electrode 4a on the inner space side of the sensor cell 4 is exposed to an oxidizing atmosphere, the internal Rh is gradually deposited on the surface as Rh2 O3, and the Rh concentration on the surface increases. However, as the surface is covered with Rh2 O3, the deposition rate of Rh decreases. Then, you may make it expose alternately to an oxidizing atmosphere and a reducing atmosphere. That is, after the oxidation treatment, the substrate is once exposed to a reducing atmosphere to reduce Rh2 O3 on the surface to metal Rh. Thereafter, by exposing to the oxidizing atmosphere again, the precipitation of Rh can be further promoted. In this method, as a specific method of reducing Rh2 O3 to metal Rh, a reducing atmosphere may be formed by introducing a flammable gas such as H2, or an electrode in the inner space side may be formed in the sensor cell 4. A voltage may be applied so that 4a becomes a negative electrode to perform electrical reduction.

  This manufacturing method can be suitably applied to any of the configurations of the gas sensor elements 1 of the above embodiments.

It is a typical sectional view of a gas sensor element in a 1st embodiment of the present invention. FIG. 2 is an exploded development view of the gas sensor element according to the first embodiment. FIG. 3 is a diagram showing current-voltage characteristics of a sensor cell, in which (a) is a characteristic diagram before endurance, (b) is after endurance, and (c) is a characteristic diagram after oxidation treatment in air. FIG. 4 is a diagram illustrating a relationship between NOx concentration and sensor output for explaining the effect of the present invention. FIG. 4 is a diagram illustrating a relationship between NOx concentration and sensor output for explaining the effect of the present invention. FIG. 4 is a view showing a flowchart of recovery control of NOx decomposition activity according to the present invention. FIG. 3 is a diagram showing a time chart of recovery control of NOx decomposition activity according to the present invention. FIG. 3 is a diagram showing a time chart of recovery control of NOx decomposition activity according to the present invention. It is a typical sectional view of a gas sensor element in a 2nd embodiment of the present invention. It is a typical sectional view of a gas sensor element in a 3rd embodiment of the present invention. FIG. 4 is a diagram illustrating a relationship between NOx concentration and sensor output for explaining the effect of the present invention. It is a typical sectional view of the conventional gas sensor element.

Explanation of reference numerals

DESCRIPTION OF SYMBOLS 1 Gas sensor element 11 Pinhole 12 Porous protective layer 2 Oxygen pump cell 21, 22 A pair of electrodes 3 Oxygen monitor cell (monitor cell)
4 Sensor Cell 41, 42 Pair of Electrodes 51, 52 Solid Electrolyte Body 61, 62, 63, 64 Spacer 7 Internal Space 7a First Internal Space 7b Second Internal Space 81, 82 Reference Gas Space 9 Heater P Terminal

Claims (18)

An internal space into which the gas to be measured is introduced under a predetermined diffusion resistance,
On the surface of the oxygen ion-conductive solid electrolyte body, there is a pair of electrodes provided so that one electrode faces the internal space, and oxygen is introduced into the internal space by energizing the pair of electrodes. An oxygen pump cell for discharging and adjusting the oxygen concentration in the internal space,
On the surface of the oxygen ion-conductive solid electrolyte body, there is provided a pair of electrodes provided so that one electrode faces the internal space, and a sensor cell for detecting the concentration of a specific gas component in the gas to be measured. A gas sensor element,
During the specific gas component concentration measurement, the oxygen concentration in the internal space is maintained at a low concentration, oxygen is introduced into the internal space at a specific timing except during the specific gas component concentration measurement, and the oxygen is introduced into the internal space of the sensor cell. A gas sensor element comprising: control means for controlling the recovery of the activity of an electrode facing the internal space of the sensor cell by exposing the facing electrode to an oxidizing atmosphere.   The gas sensor element according to claim 1, wherein the electrode facing the internal space of the sensor cell is a cermet electrode containing rhodium.   The control means stops the operation of at least one of the oxygen pump cell and the sensor cell when the gas to be measured is in a high oxygen concentration atmosphere, thereby introducing oxygen into the internal space and the internal space of the sensor cell. 3. The gas sensor element according to claim 1, wherein the electrode facing the substrate is exposed to an oxidizing atmosphere.   The gas to be measured is an exhaust gas of an internal combustion engine, and the control means stops the operation of at least one of the oxygen pump cell and the sensor cell when the exhaust gas is in a lean state or when the internal combustion engine is stopped, 3. The gas sensor element according to claim 1, wherein oxygen is introduced into the internal space, and an electrode of the sensor cell facing the internal space is exposed to an oxidizing atmosphere.   The control means exposes an electrode facing the internal space of the sensor cell to an oxidizing atmosphere by operating at least one of the oxygen pump cell and the sensor cell to introduce oxygen into the internal space. The gas sensor element according to claim 1 or 2, wherein On the surface of the oxygen ion conductive solid electrolyte body, one pair of electrodes has a pair of electrodes provided so as to face the internal space, and has an oxygen monitor cell for detecting the oxygen concentration of the internal space,
The control means, when there is a large amount of oxygen in the gas to be measured, by stopping the operation of at least one of the oxygen pump cell, the sensor cell, and the oxygen monitor cell to introduce oxygen into the internal space, 3. The gas sensor element according to claim 1, wherein the electrode facing the internal space is exposed to an oxidizing atmosphere. On the surface of the oxygen ion conductive solid electrolyte body, one pair of electrodes has a pair of electrodes provided so as to face the internal space, and has an oxygen monitor cell for detecting the oxygen concentration of the internal space,
The gas to be measured is an exhaust gas of an internal combustion engine, and the control means stops the operation of at least one of the oxygen pump cell, the sensor cell, and the oxygen monitor cell when the exhaust gas is in a lean state or when the internal combustion engine is stopped. 3. The gas sensor element according to claim 1, wherein oxygen is introduced into the internal space, and an electrode of the sensor cell facing the internal space is exposed to an oxidizing atmosphere. On the surface of the oxygen ion conductive solid electrolyte body, one pair of electrodes has a pair of electrodes provided so as to face the internal space, and has an oxygen monitor cell for detecting the oxygen concentration of the internal space,
The control means exposes an electrode facing the internal space of the sensor cell to an oxidizing atmosphere by operating at least one of the oxygen pump cell, the sensor cell, and the oxygen monitor cell to introduce oxygen into the internal space. The gas sensor element according to claim 1 or 2, wherein:   The gas sensor element according to any one of claims 1 to 8, wherein the control means controls the exposure of the electrode facing the internal space of the sensor cell to an oxidizing atmosphere while the temperature of the gas sensor element is 400 ° C or higher.   10. The gas sensor element according to claim 1, wherein the specific gas component is a nitrogen oxide. An internal space into which the gas to be measured is introduced under a predetermined diffusion resistance,
On the surface of the oxygen ion-conductive solid electrolyte body, there is a pair of electrodes provided so that one electrode faces the internal space, and oxygen is introduced into the internal space by energizing the pair of electrodes. An oxygen pump cell for discharging and adjusting the oxygen concentration in the internal space,
On the surface of the oxygen ion conductive solid electrolyte body, there is provided a pair of electrodes provided such that one electrode faces the internal space, and a sensor cell for detecting the concentration of a specific gas component in the gas to be measured. Using a gas sensor element,
During the specific gas component concentration measurement, the oxygen concentration in the internal space is maintained at a low concentration, oxygen is introduced into the internal space at a specific timing except during the specific gas component concentration measurement, and the oxygen is introduced into the internal space of the sensor cell. A method for controlling a gas sensor element, comprising recovering the activity of an electrode facing the internal space of the sensor cell by exposing the facing electrode to an oxidizing atmosphere.   The control method of a gas sensor element according to claim 11, wherein the electrode facing the internal space of the sensor cell is a cermet electrode containing rhodium.   When the gas to be measured is in a high oxygen concentration atmosphere, by stopping the operation of at least one of the oxygen pump cell and the sensor cell, oxygen is introduced into the internal space, and an electrode facing the internal space of the sensor cell. 13. The method according to claim 11, wherein the gas sensor element is exposed to an oxidizing atmosphere.   The gas to be measured is an exhaust gas of the internal combustion engine, and when the measurement gas is in a lean state or when the internal combustion engine is stopped, the operation of at least one of the oxygen pump cell and the sensor cell is stopped. The method of controlling a gas sensor element according to claim 11, wherein the method further comprises: introducing an electrode facing the internal space of the sensor cell to an oxidizing atmosphere.   The electrode facing the internal space of the sensor cell is exposed to an oxidizing atmosphere by operating at least one of the oxygen pump cell and the sensor cell to introduce oxygen into the internal space. Or a method for controlling a gas sensor element according to claim 12.   An oxygen monitor cell for detecting the oxygen concentration in the internal space, wherein the gas sensor element has a pair of electrodes provided on a surface of an oxygen ion conductive solid electrolyte body such that one electrode faces the internal space. The method for controlling a gas sensor element according to any one of claims 11 to 15, comprising: An internal space into which the gas to be measured is introduced under a predetermined diffusion resistance,
On the surface of the oxygen ion-conductive solid electrolyte body, there is a pair of electrodes provided so that one electrode faces the internal space, and oxygen is introduced into the internal space by energizing the pair of electrodes. An oxygen pump cell for discharging and adjusting the oxygen concentration in the internal space,
A sensor cell that has a pair of electrodes provided on the surface of the oxygen ion conductive solid electrolyte so that one electrode faces the internal space, and detects the concentration of a specific gas component in the gas to be measured; And
An electrode facing the internal space of the sensor cell is a cermet electrode containing rhodium in a gas sensor element manufacturing process,
A method for manufacturing a gas sensor element, comprising: exposing an electrode facing the internal space of the sensor cell to an oxidizing atmosphere at 400 to 1000 ° C. after a firing step of the gas sensor element. 18. The method according to claim 17, wherein, after the firing step of the gas sensor element, the electrode facing the internal space of the sensor cell is alternately exposed to an oxidizing atmosphere and a reducing atmosphere at 400 to 1000C.

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