Gas sensor element and gas sensor unit

A gas sensor device is equipped with a diffusion controlling portion, a pump cell, and a sensor cell. The diffusion controlling portion is formed to face a major surface of a solid electrolyte body and works to control a rate of diffusion of a measurement gas entering a measurement gas chamber. The pump cell has a pump electrode which contains gold and is formed on the major surface. The pump electrode is located downstream of the diffusion controlling portion in a gas flow direction. The pump cell works to regulate a concentration of oxygen in the measurement gas upon application of voltage to the pump electrode. The sensor cell has a sensor electrode formed on the major surface downstream of the diffusion controlling portion in the gas flow direction. The sensor cell works to measure a concentration of nitrogen oxide contained in the measurement gas upon application of voltage to the sensor electrode. The pump electrode is disposed upstream of the sensor electrode at a distance of 0.2 mm or more downstream away from the diffusion controlling portion in the gas flow direction. This enhances the accuracy in measuring the concentration of NOx.

CROSS REFERENCE TO RELATED DOCUMENT

The present application claims the benefit of priority of Japanese Patent Application No. 2016-248149 filed on Dec. 21, 2016, the disclosure of which is incorporated herein by reference.

BACKGROUND

Technical Field

This disclosure generally relates to a gas sensor device and a gas sensor unit including the same.

Background Art

Gas sensor devices in which a measurement gas chamber in which a measurement gas flows in a gas flow direction is defined by a solid electrolyte body are known as gas sensors constituting gas sensor units together with a control circuit for applied voltage control.

For instance, Japanese Patent First Publication No. 2015-200642 discloses a gas sensor device which is equipped with a pump cell, a sensor cell, and a diffusion-controlling portion. The pump cell of the sensor device works to regulate the concentration of oxygen in the measurement gas upon application of voltage to a pump electrode, while the sensor cell works to measure the concentration of nitrogen oxides (NOx) contained in the measurement gas upon application of voltage to a sensor electrode. The sensor device includes a solid electrolyte body. The solid electrolyte body has a first surface and a second surface opposed to each other. The first and second surfaces define a measurement gas chamber. The diffusion-controlling portion is formed between the pump electrode and the sensor electrode and faces the second surface to which the first surface on which the sensor electrode is formed is opposed. The diffusion-controlling portion works to control the diffusion of the measurement gas, thereby minimizing a reduction in sensitivity of an upstream portion of the sensor electrode in the gas flow direction.

The above gas sensor device has a second diffusion-controlling portion formed upstream of the pump electrode in the gas flow direction. The second diffusion-controlling portion faces the first surface which is opposed to the second surface to define the gas measurement chamber and on which the sensor electrode and the pump electrode are formed. This ensures the stability in initially controlling the concentration of oxygen whose diffusion has been controlled before reaching the sensor cell to improve the accuracy in measuring the concentration of NOx in the sensor cell.

The inventors, however, did hard research and have found that in the above gas sensor device, the pump electrode contains gold (Au), which leads to a reduction in accuracy in measuring the concentration of NOx. Specifically, the measurement gas which has passed the second diffusion-controlling portion facing the surface on which the pump electrode is formed hits an upstream portion of the pump electrode in the gas flow direction, thereby resulting in evaporation of Au. Adhesion of the evaporated Au to the sensor electrode will result in a decrease in ability thereof for decomposition activity against NOx, which leads to a reduction in accuracy in measuring the concentration of NOx using the sensor cell. It is thus required to eliminate such a drawback.

SUMMARY

It is an object of this disclosure to provide a gas sensor device and a gas sensor unit which improve the accuracy in measuring the concentration of NOx.

Technical means of the invention for achieving the object will be described below. The reference symbols noted in brackets recited in claims disclosing the technical means of the invention and this disclosure represent correspondence relations to specific means described in embodiments, as will be discussed later, and do not limit the technical field of the invention.

A gas sensor device (100,200,300) in the first mode, as disclosed in order to solve the above problem, which is designed to produce a flow of a measurement gas in a gas flow direction within a measurement gas chamber (10) defined by a major surface (71) of a solid electrolyte body (70). The gas sensor comprises a diffusion controlling portion (50,10b) which is formed to face the major surface and works to control a rate of diffusion of the measurement gas entering the measurement gas chamber, a pump cell (40) which has a pump electrode (41) which contains gold, is formed on the major surface, and located downstream of the diffusion controlling portion in the gas flow direction, the pump cell working to regulate a concentration of oxygen in the measurement gas upon application of voltage to the pump electrode, and a sensor cell (20) which has a sensor electrode (21) formed on the major surface downstream of the diffusion controlling portion in the gas flow direction, the sensor cell working to measure a concentration of nitrogen oxide contained in the measurement gas upon application of voltage to the sensor electrode. The pump electrode is disposed at a place (Sp) which is located upstream of the sensor electrode and at a distance of 0.2 mm or more downstream away from the diffusion controlling portion in the gas flow direction.

A gas sensor unit in the second invention, as disclosed in order to solve the above problem, comprises a gas sensor device (100,200,300) of the first invention, a sensor housing (101) which has the gas sensor device retained therein, a device cover (103) which is secured to the sensor housing to cover an upstream device end portion (100a) of the gas sensor device in the gas flow direction, the device cover directing the measurement gas to the upstream device end portion, and a sensor control circuit (106) which controls application of voltage to the pump electrode and the sensor electrode.

According to the first and second mode, the measurement gas which has passed through the diffusion controlling portion facing the major surface of the solid electrolyte body on which the pump electrode is formed diffuses within the measurement gas chamber before reaching the place which is 0.2 mm or more away from the diffusion controlling portion downstream in the gas flow direction. This decreases a probability that after passing through the diffusion controlling portion, the measurement gas locally hits an upstream portion of the pump electrode. This minimizes the evaporation of Au upstream of the sensor electrode in the gas flow direction, thereby reducing adhesion of evaporated Au to the sensor electrode on the major surface on which the pump electrode is also formed which usually lead to a deterioration in decomposition activity against NOx. This improves the accuracy of the sensor cell in measuring the concentration of NOx.

EMBODIMENT FOR CARRYING OUT THE INVENTION

A plurality of embodiments will be described below with reference to the drawings. The same reference numbers will be used for the same parts in the several embodiments, and the same explanation thereof in detail may be omitted. When only part of the components in each embodiment are described, the explanation of the components already made in another embodiment may be applied thereto. The components described in the several embodiments may alternatively be partially combined in another way without being specified.

First Embodiment

The gas sensor unit1in the first embodiment is, as illustrated inFIG. 1, disposed in the exhaust path2of an internal combustion engine of a vehicle. Exhaust gas flowing through the exhaust path2enters the gas sensor unit1as a measurement gas. The gas sensor unit1measures the concentration of NOx in the measurement gas using the gas sensor device100installed therein. Specifically, the gas sensor unit1includes the sensor housing101, the porcelain insulator102, the covers103and104, the sensor harnesses105, and the sensor control circuit106in addition to the gas sensor device100. “F” inFIG. 1represents a direction in which the measurement gas flows in the gas sensor device100.

The sensor housing101retains therein the gas sensor device100through the porcelain insulator102. The covers103and104are secured to the sensor housing101. The device cover103covers an outer periphery of the upstream device end portion100aof the gas sensor device100in the gas flow direction F. The device cover103has gas inlet holes103athrough which the measurement gas is admitted from the exhaust path2to the upstream device end portion100a. The cover104covers an outer periphery of the downstream device end portion100bof the gas sensor device100in the gas flow direction F. The cover104has the air inlet holes104athrough which atmospheric air is admitted to the downstream device end portion100b. The sensor harnesses105extend from outside to inside the cover104. The sensor control circuit106is disposed outside the sensor housing101and the covers103and104and connected to the gas sensor device100through the sensor harnesses105.

Gas Sensor Device

The structure of the gas sensor device100will be described below in detail.

The gas sensor device100is, as illustrated inFIG. 2, produced by placing the insulating layer90on a first end portion of the solid electrolyte body70through the first spacer91and stacking the heating structural body60through the second spacer92on a second end portion of the solid electrolyte body70which is opposed to the first end portion.

The solid electrolyte body70is made of a solid electrolyte material, such as yttria-stabilized zirconia (YSZ), in the shape of a rectangular plate. The solid electrolyte body70exhibits oxygen-iron conductivity when it is placed at high temperature (e.g., about 600° C. or more in case of YSZ). When heated by the heater62which will be described later, the solid electrolyte body70exhibits the oxygen-ion conductivity. The solid electrolyte body70is formed to have the first major surface71and the second major surface72which are opposed to each other and flat.

The insulating layer90and the first spacer91are made of an electrically insulating material such as alumina (Al2O3). The insulating layer90formed in the shape of a rectangular plate has the second surface93which faces the first major surface71that is the first surface of the solid electrolyte body70through the first spacer91. The first spacer91has an opening in the form of a C-shaped plate in a planar view. The measurement gas chamber10is defined between the first major surface71of the solid electrolyte body70and the second surface93of the insulating layer90. The measurement gas chamber10is formed in a box-shape with an opening which is not surrounded by the inner peripheral surface94of the first spacer91. In other words, the measurement gas chamber10is defined by the first major surface71, the second surface93, and the inner peripheral surface94and has an opening portion as the inlet hole10a.

The exhaust gas that is the measurement gas is introduced into the inlet hole10aof the measurement gas chamber10through the gas inlet holes103aof the device cover103. The end of the gas sensor device100to which the inlet hole10aopens is the upstream device end portion100a. The measurement gas entering the inlet hole10aflows away from the inlet hole10aalong the first major surface71in the gas flow direction F.

The heating structural body60is made by retaining the heater62between the heating base plate61and the insulating base plate63. The base plates61and63are made of an electrical insulating material such as Al2O3in the shape of a rectangular plate. The heater62is made of an electrical conducive material such as platinum (Pt) in the shape of a rectangular thin film. The heater62is connected to the sensor control circuit106through the sensor harness105. The heater62is supplied with electrical power from the sensor control circuit106to produce heat.

The second spacer92is made of an electrical insulating material such as alumina (Al2O3). The heating base plate61has the heat dissipating surface64which is opposed to the second major surface72of the solid electrolyte body70through the second spacer92. The second spacer92has an opening in the form of a C-shaped plate in a planar view. The reference gas chamber11is, therefore, formed between the second major surface72of the solid electrolyte body70and the heat dissipating surface64of the heating base plate61in a rectangular box shape with an opening not surrounded by the inner peripheral surface95of the second spacer92. In other words, the reference gas chamber11is defined by the second major surface72, the heat dissipating surface64, and the inner peripheral surface95and an opening portion as a reference inlet (not shown).

The air is introduced as a reference gas into the reference inlet of the reference gas chamber11through the air inlet holes104aof the cover104. The end of the gas sensor device100which is opposed to the inlet hole10aand to which the reference inlet opens is the downstream device end portion100b.

With the above described stacked structure, the gas sensor device100constructs the diffusion controlling portion50, the sensor cell20, and the pump cell40and is also equipped with the heater62.

The diffusion controlling portion50is made of a porous material such as alumina (Al2O3) in the shape of a rectangular thin film having a thickness of, for example, 15 μm. The diffusion controlling portion50is disposed to face the most upstream portion71aof the first major surface71of the solid electrolyte body70. The first major surface71is exposed to the measurement gas chamber10in a rectangular shape. The most upstream portion71ais a portion of the first major surface71which is most upstream in the gas flow portion F and surrounds the inlet hole10a. In the first embodiment, the diffusion controlling portion50is in contact with the most upstream portion71a. With these arrangements, the diffusion controlling portion50permits the measurement gas which contains NOx and oxygen and has entered the inlet hole10ato pass therethrough. The diffusion controlling portion50also provides a flow resistance to the measurement gas to control the rate of diffusion of the measurement gas to the measurement gas chamber10. A portion of the inlet hole10awhich is unoccupied by the diffusion controlling portion50is occupied by the closing wall96that is a portion of the insulating layer90protruding from the second surface93in the shape of a rectangular upright plate, thereby blocking the passage of the measurement gas. The diffusion controlling portion50, therefore, works as a diffusion controlling layer between the closing wall96and the first major surface71to achieve the diffusion control.

The cells20and40share the solid electrolyte body70and the reference electrode80with each other. The reference electrode80is made of an electrical conductive material containing Pt in the shape of a rectangular thin film having a thickness of, for example, 10 μm. The reference electrode80is disposed on a portion of the second major surface72of the solid electrolyte body70downstream of the diffusion controlling surface72in the gas flow direction F. The second major surface72is exposed to the reference gas chamber11in a rectangular shape. The reference electrode80extends over the cells20and40. The reference electrode80is connected to the sensor control circuit106through the sensor harness105. The reference electrode80is, therefore, exposed to the air entering the reference gas chamber11and develops a reference potential when the concentration of NOx is measured.

The sensor cell20includes the sensor electrode21in addition to the parts70and80shared with the pump cell40. The sensor electrode21is made of an electrical conductive material which contains Pt and rhodium (Rh) to enhance decomposition activity against NOx and shaped in the form of a rectangular thin film. The sensor electrode21, thus, develops catalytic activity against NOx molecules. The sensor electrode21is disposed on a portion of the first major surface71of the solid electrolyte body70which is located downstream of the diffusion controlling portion50in the gas flow direction F. The sensor electrode21is opposed to a portion of the reference electrode80through a portion of the solid electrolyte body70. With these arrangements, the sensor electrode21has lead22extending downstream in the gas flow direction F. The lead22is connected to the sensor control circuit106through the sensor harness105.

With the above arrangements, the sensor cell20functions to measure the concentration of NOx in the measurement gas. Specifically, the sensor control circuit106applies a controlled voltage between the electrodes21and80of the sensor cell20. Upon application of such a voltage, the NOx molecules in the measurement gas entering the measurement gas chamber10are adsorbed by an exposed noble-metal surface of the sensor electrode21and then subjected to a catalytic operation, so that they are decomposed into nitrogen ions and oxygen ions. The oxygen ions are transmitted from inside the sensor electrode21to the solid electrolyte body70and then to the reference electrode80. The oxygen ions are then measured in the form of a sensor current. The magnitude of the measured sensor current is used to determine the concentration of NOx contained in the measurement gas.

The pump cell40is equipped with the pump electrode41in addition to the parts70and80shared with the sensor cell20. The pump electrode41is made of an electrical conductive material which contains Au and Pt to have a decreased degree of decomposition activity against NOx molecules in the form of a rectangular thin film having a thickness of, for example, 10 μm. The pump electrode41, therefore, develops the reduction of oxygen molecules. The pump electrode41is, like the sensor electrode21, disposed on a portion of the first major surface71of the solid electrolyte body70which is located downstream of the diffusion controlling portion50and upstream of the sensor electrode21in the gas flow direction F. The pump electrode41is opposed to a portion of the reference electrode80through a portion of the solid electrolyte body70. With these arrangements, the pump electrode41has the lead42extending in a direction P perpendicular to the gas flow direction F. The lead42is connected to the sensor control circuit106through the sensor harness105.

With the above arrangements, the pump cell40functions to regulate the concentration of oxygen contained in the measurement gas. Specifically, the sensor control circuit106applies a controlled voltage between the electrodes41and80of the pump cell40. Upon application of such a voltage, the oxygen molecules in the measurement gas entering the measurement gas chamber10are adsorbed by an exposed noble-metal surface of the pump electrode41and then subjected to a reduction operation, so that they are decomposed into oxygen ions. The oxygen ions are transmitted from inside the pump electrode41to the solid electrolyte body70, to the reference electrode80, and then discharged into the reference gas chamber11. Such a pumping operation decreases or removes the oxygen molecules from the measurement gas, thereby regulating the concentration of oxygen in the measurement gas.

The sensor cell20works to output to the sensor control circuit106the sensor current whose value is as a function of the concentration of NOx in the measurement gas after the concentration of oxygen thereof is regulated by the pump cell40. The sensor control circuit106is responsive to the output from the sensor cell20to calculate the concentration of NOx.

The heater62is laid to overlap the cells20and40in a planar view (not shown). This causes heat, as produced by the heater62, to be transmitted to the heating base plate61and then emitted from the heat dissipating surface64. The heat emitted from the heat dissipating surface64is transmitted to the parts80,70,21, and41of the cells20and40through the reference gas chamber11, thereby heating the cells20and40. In the first embodiment, the heating center of the heater62at which the temperature of the heater62becomes highest is located closer to the pump electrode41than to the sensor electrode21in the gas flow direction F, thereby causing the pump electrode41to be kept by the heater62at a temperature (e.g., 800° C.) higher than that (e.g., 600° C.) at which the sensor electrode21is kept.

Positional Relation Among Components

The positional relation among the electrodes21and14of the cells20and40and the diffusion controlling portion50will be described below as positional relation among the parts of the gas sensor device100.

The pump electrode41is, as can be seen inFIG. 2, disposed at a place Sp which is located upstream of the sensor electrode21and downstream of the diffusion controlling portion50in the gas flow direction F within the measurement gas chamber10. Specifically, the place Sp is, as illustrated inFIG. 3, an area which is located at a given distance Dpu away from the diffusion controlling portion50to the downstream side in the gas flow direction F and in which the pump electrode41is formed. The distance Dpu is selected as a minimum distance between the downstream end surface50aof the diffusion controlling portion50and the upstream end surface41aof the pump electrode41in the gas flow direction F. The upstream end surface41aof the pump electrode41arranged at the place Sp is located at an interval of 0.2 mm or more (i.e., the distance Dpu) away from the downstream end surface50aof the diffusion controlling portion50. The reasons why the distance Dpu is selected to be 0.2 mm or more will be described below.

The inventors of this application searched a surface composition of the sensor electrode21and an output error of the sensor electrode21for difference values of the distance Dpu, as demonstrated inFIG. 4, in order to find adverse effects of the distance Dpu on the evaporation of Au from the pump electrode41and adhesion of Au to the sensor electrode21.FIG. 4represents the surface composition percentage after the electrodes41and21are exposed to air at 800° C. for 200 hours and the output error when the electrodes41and21are exposed to a test gas in which the concentration of NOx is 100 ppm at 800° C. after the above air-exposure. The surface composition percentage inFIG. 4is expressed by an average of percentages by weight of Au composition (i.e., wt % of Au) in a plurality of areas, as each defined by a circle having a diameter of 0.4 mm on the surface of the sensor electrode21. The percentages by weight are derived using X-ray photoelectron spectroscopy (XPS). The output error inFIG. 4is expressed by a difference between the concentration of NOx that is a function of the sensor current outputted from the sensor electrode21and the concentration of NOx (i.e., 100 ppm) in the test gas. “δC” indicates an allowable range of the output error.

The results inFIG. 4show that when the distance Dpu is less than 0.2 mm, the Au composition percentage of the sensor electrode21increases, in other words, the amount of Au adhered to the sensor electrode21increases. This results in an increase in output error of the sensor electrode21to increase (inFIG. 4, the output error increases to the minus side smaller than the concentration of NOx in the test gas). This is thought of as being because it is easy for the test gas after passing through the diffusion controlling portion50, as denoted by arrows representing flows of gas inFIG. 5, to reach the upstream portion41bof the pump electrode41which is located at a distance of less than 0.2 mm downstream away from the diffusion controlling portion50in the gas flow direction F. It becomes easy for Au to evaporate from the upstream portion41bthe test gas reaches and then adheres to the sensor electrode21, which may lead to an increase in the output error.

FIG. 4shows that when the distance Dpu is 0.2 mm or more, the Au composition percentage of the sensor electrode21greatly decreases, that is, the amount of Au adhered to the sensor electrode21greater decreases. This causes the output error of the sensor electrode21to decrease to within the allowable range δC. This is thought of as being because the test gas after passing through the diffusion controlling portion50, as denoted inFIG. 6by arrows representing flows of gas in the first embodiment, hardly locally reaches the upstream portion41bof the pump electrode41which is located at a distance of 0.2 mm or more downstream away from the diffusion controlling portion50in the gas flow direction F. It is becomes difficult for Au to evaporate from the upstream portion41bof the test gas hardly reaches. This results in a decrease in evaporated Au reaching and adhering to the sensor electrode21, which leads to a decrease in the output error.

The space Sp, as defined by the distance Dpu, is also determined in the first embodiment, as illustrated inFIG. 3, by the distance Dpd at which the pump electrode41is located upstream away from the sensor electrode21in the gas flow direction F. The distance Dpd is a minimum interval between the upstream end surface21aof the sensor electrode21and the downstream end surface41cof the pump electrode41in the gas flow direction F. The downstream end surface41cof the pump electrode41lying at the place Sp is arranged at an interval of 0.3 mm to 0.7 mm, that is, a distance of 0.3 mm or more to 0.7 mm or less (i.e., the distance Dpd) away from the upstream end surface21aof the sensor electrode21. When the distance Dpd is less than 0.3 mm, it result in a risk that a leakage current occurs between the sensor electrode21and the pump electrode41, which results in a decrease in measurement accuracy of the gas sensor unit1. The distance Dpd is, therefore, selected to be 0.3 mm or more. Alternatively, when the distance Dpd is more than 0.7 mm, it results in a risk that the size of the gas sensor device100, i.e., the gas sensor unit1, is increased, thus resulting in an increase in manufacturing costs thereof. The distance Dpd is, therefore, selected to be 0.7 mm or less.

Operation and Effects

The operation and effects of the first embodiment will be described below.

In the first embodiment, the measurement gas which has passed through the diffusion controlling portion50facing the first major surface71of the solid electrolyte body70on which the pump electrode41is formed diffuses within the measurement gas chamber10before reaching the place Sp which is 0.2 mm or more away from the diffusion controlling portion50downstream in the gas flow direction F. This decreases a probability that after passing through the diffusion controlling portion50, the measurement gas locally hits the upstream portion41bof the pump electrode41at the place Sp. This minimizes the evaporation of Au upstream of the sensor electrode21in the gas flow direction F, thereby reducing adhesion of evaporated Au to the sensor electrode21on the first major surface71on which the pump electrode41is also formed which usually lead to a deterioration in decomposition activity against NOx. This improves the accuracy of the sensor cell20in measuring the concentration of NOx.

In a case where the exhaust gas emitted from an internal combustion engine is the measurement gas in the first embodiment, it is desirable to introduce the measurement gas into the gas sensor unit1from a portion of the exhaust path2near the cylinder in which the exhaust gas is created in order to improve the accuracy in measuring the concentration of NOx. This, however, results in a risk that the high-temperature measurement gas, as introduced from the exhaust path2into the measurement gas chamber10, hits the pump electrode41downstream of the diffusion controlling portion50in the gas flow direction F, thereby facilitating the evaporation of Au. The high-temperature measurement gas, however, will diffuse before reaching the pump electrode41which is located 0.2 mm or more away from the diffusion controlling portion50downstream in the gas flow direction F, thereby minimizing the evaporation of Au. This enhances the measurement accuracy of the gas sensor unit1.

In the first embodiment, when the heater62heats the pump cell40and the sensor cell20, the temperature of the pump electrode41becomes higher than that of the sensor electrode21. When the temperature of the exhaust gas entering the measurement gas chamber10as the measurement gas is excessively increased by some disturbance, it will result in an increased degree of evaporation of Au from the pump electrode41whose temperature is increased by exposure to the measurement gas to be more than that increased by the heater62. However, the pump electrode41whose temperature is increased to be more than that increased by the heater62is located 0.2 mm or more away from the diffusion controlling portion downstream in the gas flow direction F, thereby enhancing the diffusion of the measurement gas before reaching the pump electrode41, thereby reducing the evaporation of Au to improve the measurement accuracy of the gas sensor unit1.

In the first embodiment, there is a low probability that the leakage current occurs between the sensor electrode21and the pump electrode41which is 0.3 to 0.7 mm away from the sensor electrode21upstream in the gas flow direction F, thereby enabling the size of the gas sensor device100and the gas sensor unit1to be reduced to decrease the production costs thereof.

Second Embodiment

The second embodiment is, as can be seen inFIG. 7, a modification of the first embodiment.

The gas sensor device200of the second embodiment is equipped with the protruding wall297. Specifically, the protruding wall297is made of an electrical insulating material, such as alumina (Al2O3), in the shape of a rectangular upright plate. The protruding wall297is disposed on the first major surface71of the solid electrolyte body70on which the electrodes21and41are disposed and located downstream of the pump electrode41and upstream of the sensor electrode21in the gas flow direction F. In other words, the protruding wall297is disposed downstream of the diffusion controlling portion50in the gas flow direction F and interposed between the pump electrode41at the place Sp and the sensor electrode21. The protruding wall297extends from the first major surface71of the solid electrolyte body70to a level below the second surface93of the insulating layer90so as to have an air gap210bof, for example, 15 μm between itself and the second surface93. The protruding wall297, thus, works to limit the flow of the measurement gas except for the air gap210bthat is a portion of the measurement gas chamber10and faces the second surface93and also to give a flow resistance to the measurement gas in the air gap210bto control the rate of diffusion of the measurement gas in the measurement gas chamber10.

In the second embodiment, the protruding wall297extending from the first major surface71of the solid electrolyte body70between the pump electrode41and the sensor electrode21in the gas flow direction F is laid to create the air gap210bbetween itself and the second surface93which faces the first major surface71and defines the measurement gas chamber10. The air gap210bworks to control the rate of diffusion of the measurement gas and facilitates the adhesion of Au evaporated from the pump electrode41to the surface of the protruding wall297or the second surface93. This minimizes the adhesion of evaporated Au to the sensor electrode21located downstream of the air gap210bin the gas flow direction F which will lead to a deterioration in decomposition activity against NOx. This enhances the measurement accuracy of the gas sensor device200.

Third Embodiment

The third embodiment is, as can be seen inFIG. 8, a modification of the first embodiment.

The gas sensor device300of the third embodiment is also equipped with the monitor cell330which is laid to overlap the heater62in a planar view (not shown). Specifically, the monitor cell330includes the monitor electrode331in addition to the parts70and80shared with the cells20and40. The monitor electrode331is made of an electrical conductive material which contains Au and Pt to have a decreased degree of decomposition activity against NOx molecules in the form of a rectangular thin film having a thickness of, for example, 10 μm. The monitor electrode331, therefore, develops the reduction of oxygen molecules.

The monitor electrode331is disposed on the first major surface71of the solid electrolyte body70on which the electrodes21and41are arranged. The monitor electrode331is located downstream away from the diffusion controlling portion50and the pump electrode41in the gas flow direction F and also away from the sensor electrode21in the direction P perpendicular to the sensor electrode21. The monitor electrode331is opposed to a portion of the reference electrode80through a portion of the solid electrolyte body70and placed in substantially the same condition of exposure to the measurement gas as in the sensor electrode21arranged in the perpendicular direction P.

The distance at which the downstream end surface41cof the pump electrode41is away upstream from the upstream end surface331aof the monitor electrode331in the gas flow direction F is, as illustrated inFIG. 9, the distance Dpd that is substantially the same as in the sensor electrode21. In other words, the pump electrode41is arranged upstream of the monitor electrode331and the sensor electrode21in the gas flow direction F and located at the place Sp at the distance Dpd from the electrodes331and21. The upstream end surface331aof the monitor electrode331and the upstream end surface21aof the sensor electrode2are, therefore, arranged at the same interval away from the downstream end surface50aof the diffusion controlling portion50as well as away from the downstream end surface41cof the pump electrode41.

With the above arrangements, the monitor electrode331, as illustrated inFIG. 8, has the lead332extending downstream in the gas flow direction F. The lead332is connected to the sensor control circuit106through the sensor harness105.

With the above arrangements, the monitor cell330functions to perform an oxygen concentration measuring operation for correction of the concentration of NOx in the measurement gas. Specifically, the sensor control circuit106applies a controlled voltage between the electrodes331and80of the monitor cell330. Upon application of such a voltage, oxygen molecules in the measurement gas entering the measurement gas chamber10are adsorbed by the exposed noble-metal surface of the monitor electrode331and then reduced, so that they are decomposed into oxygen ions. The oxygen ions are then transmitted from inside the monitor electrode331into the solid electrolyte body70and then to the reference electrode80. The oxygen ions are measured in the form of a monitor current. The concentration of oxygen in the measurement gas is, therefore, determined as a function of the magnitude of the monitor current.

The monitor cell330, therefore, outputs to the sensor control circuit106the monitor current whose value corresponds to the concentration of oxygen remaining in the measurement gas whose concentration of oxygen has been regulated by the pump cell40. Simultaneously, the sensor cell20outputs to the sensor control circuit106the sensor current whose value corresponds to a before-corrected concentration of NOx which is expected to contain the concentration of oxygen which still remains in the measurement gas after regulated by the pump cell40and is higher than zero. The sensor control circuit106is responsive to the above outputs to 17 to calculate the remaining concentration of oxygen and the before-corrected concentration of NOx as results of measurements thereof. The sensor control circuit106performs correction by subtracting the residual concentration of oxygen from the before-corrected concentration of NOx to calculate the concentration of NOx from which an error arising from the residual concentration of oxygen is removed as a result of the measurement.

In the third embodiment, reach and adhesion of Au, as evaporated from the pump electrode41located upstream in the gas flow direction F, to the monitor electrode331which is arranged adjacent the sensor electrode21in the direction P perpendicular to the gas flow direction F and formed on the first major surface71of the solid electrolyte body70is minimized in the same way as the sensor electrode21. Therefore, upon application of voltage to the monitor electrode331of the monitor cell330, the monitor cell330works to measure the concentration of oxygen remaining in the measurement gas after regulated by the pump cell40, thereby enabling the accuracy in correcting the concentration of NOx measured by the sensor cell20to be improved. If the evaporation of Au from the pump electrode41results in a variation in ability of the pump cell40to measure the concentration of oxygen, the monitor cell330is capable of measuring the concentration of oxygen remaining in the measurement gas, thus ensuring the improvement of accuracy in correction of the concentration of NOx.

Other Embodiments

While the some embodiments have been described above, the invention should not be limited thereof, it should be appreciated that the invention can be embodied in various ways without departing from the principle of the invention.

As a modification 1 of the first to third embodiment, an air gap which is defined between the closing wall96and the first major surface71, that is, faces the first major surface71, as illustrated inFIG. 10(i.e., the modification 1 of the first embodiment), may be, like the air gap210bin the second embodiment, serves as the diffusion controlling portion10b. As a modification 2 of the second embodiment, a diffusion controlling layer which is made of a porous material, like the diffusion controlling portion50, may be, as illustrated inFIG. 11, disposed as the diffusion controlling portion250instead of the air gap210bto control the rate of diffusion between the protruding wall297and the second surface93.

As a modification of the first to third embodiment, the exhaust gas may be introduced as the measurement gas into the measurement gas chamber10from an EGR path diverging from the exhaust path2of the internal combustion engine or a portion of an intake path of the internal combustion engine which is located downstream of a joint of the intake path and the EGR path As a modification 4 of the first to third embodiment, the heater62may be designed to heat the pump cell40and the sensor cell20so as to increase the temperature of the pump electrode41to be lower than or substantially identical with that of the sensor electrode21.

As a modification 5 of the first to third embodiment, the distance Dpd between the pump electrode41and the sensor electrode21may be selected to be less than 0.3 mm or more than 0.7 mm. A modification of 6 is implemented by a combination of the second and third embodiment.

A modification 7 of the first to third embodiment, the sensor control circuit106only for use with the gas sensor devices100,200, and300may be omitted. For instance, an ECU for internal combustion engines may alternatively be used to control the application of voltage to the gas the sensor electrode21, the pump electrode, and the monitor electrode331. In the modification 7, it is desirable to use an ECU located as close to the gas sensor device100,200, or300as possible for the voltage application.

As a modification 8 of the first to third embodiment, the reference electrodes80may be prepared one for each of the cells20,40, and330. As a modification 9 of the first to third embodiment, an adsorption layer which works to adsorb Au may be disposed on the first major surface71or the second surface93.