Sensor element and gas detection method using the sensor element

Abstract

To provide a sensor element with which it is possible to suppress a decrease in the detection accuracy of a gas sensor due to use and accurately measure the gas to be measured that includes a measurement object gas in a wide range of concentrations, and a method for detecting the measurement object gas using the sensor element.SOLUTION: Provided is a sensor element 101 for detecting a measurement object gas in the gas to be measured, including a long tabular base part 102 that includes a plurality of oxygen ion conductive laminated solid electrolyte layers 1, 2, 3, 4, 5, 6, a measured gas circulation part 15 for introducing and circulating the gas to be measured, a main pump cell 21 for adjusting oxygen concentration, a measurement object gas decomposition pump cell 50, a pump cell 41 for residual oxygen measurement, and a reference electrode 42 disposed so as to be in contact with reference gas, with a metal material included in a measurement object gas decomposition electrode 51 in the measurement object gas decomposition pump cell 50 having catalytic activity to decompose the measurement object gas. Also provided is a method for detecting, using the sensor element 101, the measurement object gas in the gas to be measured.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a sensor element using an oxygen ion conductive solid electrolyte, and also to a gas detection method for detecting a target gas in a measurement gas using the sensor element. [Background technology]

[0002] Gas sensors are used to detect and measure the concentration of target gas components (oxygen O2, nitrogen oxides NOx, ammonia NH3, hydrocarbons HC, carbon dioxide CO2, etc.) in gases to be measured, such as automobile exhaust gases. For example, the concentration of target gas components in automobile exhaust gases is measured, and the exhaust gas purification system installed in the automobile is optimally controlled based on the measured value.

[0003] Known examples of such gas sensors include a gas sensor element that uses an oxygen ion conductive solid electrolyte such as zirconia (ZrO2).The gas sensor uses the oxygen ion conductivity of the solid electrolyte to detect an electromotive force or a current value corresponding to the concentration of a target gas component in a measured gas, thereby detecting the gas component and measuring its concentration.

[0004] For example, Japanese Patent No. 3050781 discloses a gas sensor that uses a first electrochemical pump cell and a second electrochemical pump cell to control the oxygen partial pressure to a low level that does not substantially affect the measurement of the amount of a gas component to be measured, and detects a current value corresponding to the oxygen generated by reduction or decomposition of the gas component to be measured. In other words, oxygen is removed in advance by the first electrochemical pump cell and the second electrochemical pump cell, and oxygen derived from the target gas component (e.g., nitrogen oxide NOx) is detected.

[0005] Furthermore, Japanese Patent No. 3050781 discloses that there is a linear relationship between the concentration of nitrogen oxides (NOx) and the detected current value (FIG. 5).

[0006] Japanese Patent Application Laid-Open Nos. 2014-209128 and 2014-190940 disclose NOx sensors that have a main pump cell and an auxiliary pump cell for adjusting the oxygen concentration, and disclose that the main pump cell has an inner pump electrode that is, for example, a cermet electrode made of Pt containing 1% Au and zirconia.

[0007] Japanese Patent No. 6292735 discloses a NOx sensor. The NOx sensor has a pump cell for discharging oxygen ions from a measurement gas chamber, and the pump cell is made of a Pt-Au alloy for its pump electrode. Furthermore, the document discloses that an Au adsorption layer is formed on the pump electrode to adsorb Au atoms evaporated from the pump electrode. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Patent No. 3050781 [Patent Document 2] Japanese Patent Application Laid-Open No. 2014-209128 [Patent Document 3] Japanese Patent Application Laid-Open No. 2014-190940 [Patent Document 4] Patent No. 6292735 Summary of the Invention [Problem to be solved by the invention]

[0009] In a conventional gas sensor, as disclosed in, for example, Japanese Patent Application Laid-Open No. 2014-209128, the oxygen generated by decomposition of the measurement target gas is detected as a current value in the measurement pump cell under a condition in which the oxygen partial pressure in the measurement target gas is controlled by a main pump cell and an auxiliary pump cell to be low enough to have no substantial effect on the measurement of the measurement target gas. That is, the oxygen in the measurement target gas is separated from the measurement target gas, and then the oxygen generated from the measurement target gas is detected.

[0010] In such gas sensors, it is required that the main pump cell does not decompose the gas to be measured. Therefore, the pump electrode constituting the main pump cell is formed of a material that does not decompose the gas to be measured (e.g., NOx). A metal material in which Au is added to Pt is used as a material that does not decompose NOx (Japanese Patent Laid-Open No. 2014-209128, Japanese Patent Laid-Open No. 2014-190940, Japanese Patent No. 6292735).

[0011] However, conventional gas sensors have been known to experience a decrease in NOx output after extended use. The inventors have conducted extensive research and found that when a gas sensor is used for extended periods under harsh conditions at high temperatures, Au contained in the pump electrode constituting the main pump cell may evaporate and adhere to the measuring electrode constituting the measuring pump cell, potentially reducing the NOx detection sensitivity.

[0012] In addition, in conventional gas sensors, the detectable concentration range of a measurement target gas (e.g., NOx) is considered to be determined by the amount of the measurement target gas that reaches the measurement electrode, or in other words, by the diffusion resistance from the gas inlet to the measurement electrode in the sensor element included in the gas sensor.

[0013] In conventional gas sensors, the diffusion resistance of the sensor element can be increased to limit the amount of measurement gas that reaches the measurement electrode in order to accurately measure high concentrations of NOx. However, this method has the problem of reduced accuracy in terms of the S / N ratio for measurement gases containing low concentrations of NOx.

[0014] As described above, it has been difficult for conventional gas sensors to accurately measure both a measurement gas containing a high concentration of NOx and a measurement gas containing a low concentration of NOx using a single gas sensor.

[0015] Furthermore, when increasing the diffusion resistance of a sensor element to accurately measure high concentrations of NOx, it is difficult to precisely control the diffusion resistance from the gas inlet of the sensor element to the measurement electrode within a predetermined range during manufacturing. For example, when adjusting the gas inflow rate using a slit-shaped gap, the opening area of ​​the slit must be reduced to increase the diffusion resistance. The smaller the opening area, the greater the change in the opening area will be due to a change in the diffusion resistance. Therefore, to fabricate a sensor element with a diffusion resistance within the desired range, the width and thickness of the slit must be controlled very precisely. Such precise control is difficult during manufacturing. Therefore, in conventional gas sensors, accurately controlling the diffusion resistance has been difficult due to manufacturing and cost considerations.

[0016] Therefore, an object of the present invention is to provide a sensor element that can suppress the deterioration of detection accuracy of a gas sensor due to use and can accurately measure a measurement target gas containing a measurement target gas over a wide concentration range, and a detection method for a measurement target gas using the sensor element. [Means for solving the problem]

[0017] The inventors have discovered that by decomposing the target gas in a target gas decomposition pump cell, removing a certain amount of all oxygen in the target gas including the oxygen produced by decomposition, and then detecting the residual oxygen in a residual oxygen measurement pump cell, it is possible to suppress the deterioration of the detection accuracy of the gas sensor due to use and to accurately measure the target gas over a wide concentration range.

[0018] The present invention includes the following inventions. (1) a long, plate-shaped substrate including a plurality of stacked oxygen ion conductive solid electrolyte layers; a measurement gas flow section for introducing a measurement gas from one longitudinal end of the base section and allowing the measurement gas to flow therethrough; a main pump cell including an inner main pump electrode disposed on an inner surface of the measurement target gas flow portion and an outer pump electrode disposed on the base portion at a position different from the measurement target gas flow portion and corresponding to the inner main pump electrode; a measurement target gas decomposition pump cell including: a measurement target gas decomposition pump electrode disposed on an inner surface of the measurement target gas flow portion at a position farther from the one longitudinal end of the base portion than the inner main pump electrode; and an outer pump electrode disposed at a position different from the measurement target gas flow portion of the base portion, corresponding to the measurement target gas decomposition pump electrode; a residual oxygen measurement pump cell including: a residual oxygen measurement electrode disposed on an inner surface of the measurement target gas flow portion at a position farther from the one longitudinal end of the base portion than the inner main pump electrode; and an outer pump electrode disposed at a position different from the measurement target gas flow portion of the base portion, corresponding to the residual oxygen measurement electrode; a reference electrode disposed inside the base portion so as to be in contact with a reference gas; Including, A sensor element for detecting a measurement object gas in a measurement gas, wherein the metal material contained in the measurement object gas decomposition pump electrode has catalytic activity for decomposing the measurement object gas.

[0019] (2) The sensor element according to (1) above, wherein the metal material contained in the residual oxygen measuring electrode does not have catalytic activity that decomposes the gas to be measured.

[0020] (3) A sensor element as described in (1) or (2) above, in which the measurement target gas decomposition pump electrode and the residual oxygen measuring electrode are arranged in series in this order in the longitudinal direction of the base portion at a position on the inner surface of the measurement target gas flow portion farther from the one end of the base portion in the longitudinal direction than the inner main pump electrode.

[0021] (4) A sensor element as described in (1) or (2) above, wherein the measurement target gas decomposition pump electrode and the residual oxygen measuring electrode are arranged in parallel in the longitudinal direction of the base portion at a position on the inner surface of the measurement target gas flow portion farther from the one end of the base portion in the longitudinal direction than the inner main pump electrode.

[0022] (5) the measurement target gas decomposition pump electrode and an oxygen detection electrode are disposed on the inner surface of the measurement target gas flow portion at a position farther from the one end of the base portion in the longitudinal direction than the inner main pump electrode, The sensor element according to any one of (1) to (4) above, further comprising an oxygen partial pressure detection cell including the oxygen detection electrode and the reference electrode.

[0023] (6) The sensor element according to any one of (1) to (5) above, wherein the gas to be measured is NOx.

[0024] (7) A sensor element described in any one of (1) to (6) above, wherein the metal material contained in the measurement target gas decomposition pump electrode includes at least one metal selected from the group consisting of platinum and rhodium as a metal having catalytic activity for decomposing the measurement target gas.

[0025] (8) A sensor element described in any one of (1) to (7) above, wherein the metal material contained in the measurement target gas decomposition pump electrode does not contain gold or contains gold to an extent that does not inhibit the catalytic activity of decomposing the measurement target gas.

[0026] (9) A sensor element according to any one of (1) to (8) above, wherein the metal material contained in the residual oxygen measuring electrode contains platinum and at least one metal selected from the group consisting of gold and silver as a metal that reduces catalytic activity in decomposing the gas to be measured.

[0027] (10) The metal material contained in the residual oxygen measuring electrode includes gold, The sensor element according to any one of (1) to (9) above, wherein the metal material contains 0.3% by weight or more of gold.

[0028] (11) A sensor element described in any one of (1) to (10) above, wherein at least two of the outer pump electrode corresponding to the inner main pump electrode, the outer pump electrode corresponding to the measurement target gas decomposition pump electrode, and the outer pump electrode corresponding to the residual oxygen measurement electrode are formed as an integrated electrode.

[0029] The present invention further includes a method for detecting a gas to be measured in a gas to be measured, using the sensor element according to any one of (1) to (11) above.

[0030] (12) A long plate-shaped substrate including a plurality of stacked oxygen ion conductive solid electrolyte layers; a measurement gas flow section for introducing a measurement gas from one longitudinal end of the base section and allowing the measurement gas to flow therethrough; a main pump cell including an inner main pump electrode disposed on an inner surface of the measurement target gas flow portion and an outer pump electrode disposed on the base portion at a position different from the measurement target gas flow portion and corresponding to the inner main pump electrode; a measurement target gas decomposition pump cell including: a measurement target gas decomposition pump electrode disposed on an inner surface of the measurement target gas flow portion at a position farther from the one longitudinal end of the base portion than the inner main pump electrode; and an outer pump electrode disposed at a position different from the measurement target gas flow portion of the base portion, corresponding to the measurement target gas decomposition pump electrode; a residual oxygen measurement pump cell including: a residual oxygen measurement electrode disposed on an inner surface of the measurement target gas flow portion at a position farther from the one longitudinal end of the base portion than the inner main pump electrode; and an outer pump electrode disposed at a position different from the measurement target gas flow portion of the base portion, corresponding to the residual oxygen measurement electrode; a reference electrode disposed inside the base portion so as to be in contact with a reference gas; Including, a sensor element in which the metal material contained in the measurement object gas decomposition pump electrode has catalytic activity for decomposing the measurement object gas; an oxygen concentration adjusting step of adjusting the oxygen concentration in the measurement gas flowing into the measurement gas flow section to a predetermined concentration by the main pump cell, thereby obtaining the measurement gas whose oxygen concentration has been adjusted to the predetermined concentration; a current value control step of decomposing the measurement target gas in the measurement target gas at the measurement target gas decomposition pump electrode by the measurement target gas decomposition pump cell, and discharging a predetermined amount of all oxygen in the measurement target gas, including oxygen generated by decomposition of the measurement target gas, from the measurement target gas flow portion so that the current value flowing through the measurement target gas decomposition pump cell is kept constant at a preset value; a detecting step of obtaining a detection current value corresponding to the concentration of residual oxygen present in the measurement target gas flow portion by the residual oxygen measurement pump cell; a concentration calculation step of calculating a concentration of the measurement target gas based on the detected current value; A method for detecting a gas to be measured in a gas to be measured, comprising:

[0031] (13) The detection method according to (12) above, wherein in the current value control step, the set value of the current value is determined by the total amount of the measurement target gas that reaches the measurement target gas decomposition pump electrode of the sensor element.

[0032] (14) The current value control step includes a plurality of set values ​​of the current value, The detection method according to (12) or (13) above, wherein the current value control step further includes a set value determination step of determining which set value to use from among the plurality of set values.

[0033] (15) The detection method according to (14), wherein the step of determining the set value determines which of the plurality of set values ​​to use based on a predicted concentration of the target gas in the measurement gas.

[0034] (16) The metal material contained in the measurement target gas decomposition pump electrode includes gold, The detection method described in any one of (12) to (15) above, wherein the current value control step includes adjusting the electromotive force between the measurement target gas decomposition pump electrode and the reference electrode to within a range of 350 mV to 500 mV. [Effects of the Invention]

[0035] According to the present invention, the electrode for obtaining the detection value, i.e., the residual oxygen measurement electrode, only needs to have catalytic activity toward oxygen, and does not need to have catalytic activity toward the measurement target gas. Even if a metal (e.g., Au) contained in the pump electrode that reduces the catalytic activity toward the measurement target gas evaporates and the evaporated Au adheres to the residual oxygen measurement electrode, the residual oxygen measurement electrode maintains its catalytic activity toward oxygen, and the detection accuracy of the gas sensor does not decrease.

[0036] In this way, it is possible to suppress a decrease in the detection accuracy of the gas sensor due to use. That is, according to the present invention, it is possible to suppress a change over time in the detection value of the gas to be measured, thereby improving durability.

[0037] Furthermore, according to the present invention, the measurement target gas is not directly detected, but is decomposed in a measurement target gas decomposition pump cell, and a certain amount of all oxygen in the measurement target gas, including the oxygen produced by decomposition, is removed. The residual oxygen in the measurement target gas is then detected using a residual oxygen measurement pump cell. The amount of oxygen removed in the measurement target gas decomposition pump cell correlates with the value of the pump current flowing in the measurement target gas decomposition pump cell. Therefore, the range of residual oxygen concentrations reaching the residual oxygen measurement electrode can be adjusted by the value of the pump current in the measurement target gas decomposition pump cell. As a result, it is possible to accommodate large changes in the concentration of the measurement target gas in the measurement target gas. Thus, according to the present invention, measurement target gases containing a wide concentration range of measurement target gases can be accurately measured. [Brief explanation of the drawings]

[0038] [Figure 1]1 is a schematic vertical cross-sectional view of a sensor element 101 in the longitudinal direction, showing an example of the schematic configuration of a gas sensor 100. FIG. [Figure 2] This is a schematic cross-sectional view taken along line II-II in Figure 1. It is a schematic view showing an example of the general planar arrangement of the inner main pump electrode 22, the measurement target gas decomposition pump electrode 51, and the residual oxygen measuring electrode 44, which are disposed on the lower surface of the second solid electrolyte layer 6. [Figure 3] FIG. 1 is a schematic diagram showing another example (sensor element 201) of the general planar arrangement of an inner main pump electrode 22, a measurement target gas decomposition pump electrode 51, and a residual oxygen measuring electrode 44, which are arranged on the lower surface of a second solid electrolyte layer 6. [Figure 4] 1 is a schematic vertical cross-sectional view of a sensor element 301 in the longitudinal direction, showing another example of the gas sensor 100. FIG. [Figure 5] 1 is a schematic vertical cross-sectional view of a sensor element 401 in the longitudinal direction, showing another example of the gas sensor 100. FIG. [Figure 6] FIG. 9 is a schematic vertical cross-sectional view of a sensor element 901 in the longitudinal direction, showing an example of the schematic configuration of a gas sensor 900 of a comparative embodiment. [Figure 7] 1 is a graph showing the results of durability tests for Examples 1 to 4 and Comparative Example 1. The vertical axis of the graph represents the rate of change in NOx sensitivity (%), and the horizontal axis represents the durability test time (hours: H). DETAILED DESCRIPTION OF THE INVENTION

[0039] The sensor element of the present invention comprises: a long plate-shaped substrate including a plurality of stacked oxygen ion conductive solid electrolyte layers; a measurement gas flow section for introducing a measurement gas from one longitudinal end of the base section and allowing the measurement gas to flow therethrough; a main pump cell including an inner main pump electrode disposed on an inner surface of the measurement target gas flow portion and an outer pump electrode disposed on the base portion at a position different from the measurement target gas flow portion and corresponding to the inner main pump electrode; a measurement target gas decomposition pump cell including: a measurement target gas decomposition pump electrode disposed on an inner surface of the measurement target gas flow portion at a position farther from the one longitudinal end of the base portion than the inner main pump electrode; and an outer pump electrode disposed at a position different from the measurement target gas flow portion of the base portion, corresponding to the measurement target gas decomposition pump electrode; a residual oxygen measurement pump cell including: a residual oxygen measurement electrode disposed on an inner surface of the measurement target gas flow portion at a position farther from the one longitudinal end of the base portion than the inner main pump electrode; and an outer pump electrode disposed at a position different from the measurement target gas flow portion of the base portion, corresponding to the residual oxygen measurement electrode; a reference electrode disposed inside the base portion so as to be in contact with a reference gas; Including, The metal material contained in the measurement object gas decomposition pump electrode has catalytic activity for decomposing the measurement object gas.

[0040] The gas to be measured is, for example, a gas component contained in the exhaust gas of an internal combustion engine, specifically, nitrogen oxides NOx, ammonia NH3, etc.

[0041] [Outline of gas sensor configuration] The sensor element of the present invention will be described below with reference to the drawings. Fig. 1 is a schematic vertical cross-sectional view in the longitudinal direction showing an example of the general configuration of a gas sensor 100 including a sensor element 101. In the following, with Fig. 1 as the reference, the upper side of Fig. 1 will be referred to as the top, the lower side as the bottom, the left side of Fig. 1 as the leading end side, and the right side as the rear end side.

[0042] In the embodiment of FIG. 1, the gas sensor 100 is an example of a limiting current type NOx sensor that detects NOx in a measurement gas by a sensor element 101 and measures its concentration.

[0043] The sensor element 101 is a long, plate-like element including a base portion 102 having a structure in which multiple oxygen-ion conductive solid electrolyte layers are stacked. The long, plate-like shape is also referred to as a long plate shape or a strip shape. The base portion 102 has a structure in which six layers are stacked in this order from bottom to top as viewed in the drawing: a first substrate layer 1, a second substrate layer 2, a third substrate layer 3, a first solid electrolyte layer 4, a spacer layer 5, and a second solid electrolyte layer 6, each of which is made of an oxygen-ion conductive solid electrolyte layer such as zirconia (ZrO). The solid electrolyte forming these six layers is dense and airtight. The six layers may all have the same thickness or may have different thicknesses. The layers are bonded together via adhesive layers made of solid electrolyte, and the base portion 102 includes the adhesive layers. While FIG. 1 illustrates a layer structure consisting of six layers, the layer structure of the present invention is not limited to this and any number and layer structure may be used.

[0044] The sensor element 101 is manufactured by, for example, laminating ceramic green sheets corresponding to each layer after performing predetermined processing and printing a circuit pattern on them, and then firing the sheets to integrate them.

[0045] A gas inlet 10 is formed at one longitudinal end (hereinafter referred to as the tip) of the sensor element 101, between the lower surface of the second solid electrolyte layer 6 and the upper surface of the first solid electrolyte layer 4. The measurement gas flow section 15 is formed in a manner that a first diffusion-controlling section 11, a buffer space 12, a second diffusion-controlling section 13, and an internal space 14 are connected in this order from the measurement gas inlet 10 in the longitudinal direction of the sensor element 101.

[0046] The measurement gas inlet 10, the buffer space 12, and the internal cavity 14 are provided by hollowing out the spacer layer 5, and are spaces within the sensor element 101 that are partitioned at the top by the underside of the second solid electrolyte layer 6, at the bottom by the upper surface of the first solid electrolyte layer 4, and at the sides by the side surfaces of the spacer layer 5.

[0047] Both the first diffusion rate-controlling section 11 and the second diffusion rate-controlling section 13 are provided as two horizontally elongated slits (the openings have their longitudinal direction perpendicular to the plane of the drawing in FIG. 1). The first diffusion rate-controlling section 11 and the second diffusion rate-controlling section 13 may have any shape that provides a desired diffusion resistance, and the shape is not limited to the slits.

[0048] Furthermore, a reference gas introduction space 43 is provided at a position farther from the tip side than the measurement gas flow section 15, between the upper surface of the third substrate layer 3 and the lower surface of the spacer layer 5, and at a position defined at its side by the side surface of the first solid electrolyte layer 4. The reference gas introduction space 43 has an opening at the other end (hereinafter referred to as the rear end) of the sensor element 101. For example, air is introduced into the reference gas introduction space 43 as a reference gas when measuring the NOx concentration.

[0049] The air introduction layer 48 is a layer made of porous alumina, and a reference gas is introduced into the air introduction layer 48 through the reference gas introduction space 43. The air introduction layer 48 is also formed so as to cover the reference electrode 42.

[0050] Reference electrode 42 is an electrode formed in a manner sandwiched between the upper surface of third substrate layer 3 and first solid electrolyte layer 4, and as described above, is surrounded by air introduction layer 48 that connects to reference gas introduction space 43. That is, reference electrode 42 is disposed so as to come into contact with the reference gas via porous air introduction layer 48 and reference gas introduction space 43. Furthermore, as will be described later, reference electrode 42 can be used to measure the oxygen concentration (oxygen partial pressure) in internal space 14.

[0051] In the measurement gas flow section 15, the gas inlet 10 is a portion that opens to the outside space, and the measurement gas is introduced into the sensor element 101 from the outside space through the gas inlet 10.

[0052] In this embodiment, the measurement gas flow section 15 is configured such that the measurement gas is introduced through the gas inlet 10 opening at the tip end surface of the sensor element 101, but the present invention is not limited to this configuration. For example, the measurement gas flow section 15 does not need to have a recess for the gas inlet 10. In this case, the first diffusion rate-controlling section 11 essentially serves as the gas inlet. Furthermore, for example, the measurement gas flow part 15 may have an opening in a side surface along the longitudinal direction of the base part 102, the opening communicating with the buffer space 12 or a position in the internal space 14 near the buffer space 12. In this case, the measurement gas is introduced from the side surface along the longitudinal direction of the base part 102 through the opening. Furthermore, for example, the measurement gas flow portion 15 may be configured so that the measurement gas is introduced through a porous body.

[0053] The first diffusion rate-controlling part 11 is a part that applies a predetermined diffusion resistance to the measurement gas taken in through the gas inlet 10.

[0054] The buffer space 12 is a space provided for guiding the measurement gas introduced from the first diffusion rate-controlling part 11 to the second diffusion rate-controlling part 13 .

[0055] The second diffusion rate-controlling portion 13 is a portion that applies a predetermined diffusion resistance to the measurement gas introduced from the buffer space 12 into the internal space 14 .

[0056] It is sufficient that the amount of the measurement gas introduced into the internal space 14 is within a predetermined range. That is, it is sufficient that a predetermined diffusion resistance is imparted to the entire area from the tip of the sensor element 101 to the second diffusion-controlling section 13. For example, it is also possible that the first diffusion-controlling section 11 directly communicates with the internal space 14, that is, the buffer space 12 and the second diffusion-controlling section 13 do not exist.

[0057] The buffer space 12 is a space provided to mitigate the influence of pressure fluctuations on the detected value when the pressure of the gas to be measured fluctuates.

[0058] When the measurement gas is introduced from the outside of sensor element 101 into internal space 14, the measurement gas is suddenly taken into sensor element 101 from gas inlet 10 due to pressure fluctuations of the measurement gas in the external space (exhaust pressure pulsations if the measurement gas is automobile exhaust gas), but is not introduced directly into internal space 14, but is introduced into internal space 14 after the pressure fluctuations of the measurement gas are canceled out through first diffusion rate-controlling section 11, buffer space 12, and second diffusion rate-controlling section 13. As a result, the pressure fluctuations of the measurement gas introduced into internal space 14 become almost negligible.

[0059] The internal space 14 is provided as a space for detecting the gas to be measured contained in the measurement gas introduced through the second diffusion rate-controlling section 13 .

[0060] On the underside of the second solid electrolyte layer 6 facing the internal cavity 14, an inner main pump electrode 22, a measurement target gas decomposition pump electrode 51, and a residual oxygen measuring electrode 44 are arranged in series in this order in the longitudinal direction of the sensor element 101, starting from a position close to the tip of the sensor element 101. FIG. 2 shows a schematic planar arrangement of the inner main pump electrode 22, the measurement target gas decomposition pump electrode 51, and the residual oxygen measuring electrode 44 arranged on the underside of the second solid electrolyte layer 6. Electrode leads are respectively arranged from each of the electrodes toward the rear end of the sensor element 101, allowing electrical connection to the outside. These electrode leads are not shown in FIG. 2.

[0061] The main pump cell 21 is an electrochemical pump cell including an inner main pump electrode 22 disposed on the inner surface of the measurement gas flow section 15, and an outer pump electrode 23 disposed at a position of the base section 102 different from the measurement gas flow section 15 (on the outer surface of the base section 102 in FIG. 1 ) and corresponding to the inner main pump electrode 22. The phrase "corresponding to the inner main pump electrode 22" means that the outer pump electrode 23 is provided on the inner main pump electrode 22 with a second solid electrolyte layer 6 interposed therebetween.

[0062] That is, the main pump cell 21 is an electrochemical pump cell comprising an inner main pump electrode 22 provided on the lower surface of the second solid electrolyte layer 6 facing the internal cavity 14 at a position close to the tip of the sensor element 101, an outer pump electrode 23 provided on the upper surface of the second solid electrolyte layer 6 in a region corresponding to the inner main pump electrode 22 so as to be exposed to the external space, and the second solid electrolyte layer 6 sandwiched between these electrodes.

[0063] The inner main pump electrode 22 and the outer pump electrode 23 are formed as porous cermet electrodes (electrodes in which a metal component and a ceramic component are mixed). The ceramic component is not particularly limited, but, like the base portion 102, it is preferable to use an oxygen ion conductive solid electrolyte. For example, ZrO2 can be used as the ceramic component. The metal component and ceramic component in the porous cermet electrode can be appropriately determined by those skilled in the art, but, for example, the ceramic component can be approximately 30 wt% to 50 wt% of the total of the metal component and the ceramic component. For example, when Pt is used as the metal component and ZrO2 is used as the ceramic component, the weight ratio may be approximately Pt:ZrO2 = 7.0:3.0 to 5.0:5.0.

[0064] The main pump cell 21 is configured to adjust the oxygen concentration in the measurement gas that has flowed into the measurement gas flow section 15 to a predetermined concentration. Therefore, it is preferable that the inner main pump electrode 22 that comes into contact with the measurement gas decomposes only oxygen without reducing (decomposing) the NOx component in the measurement gas. Specific constituent materials will be described later.

[0065] In the main pump cell 21, by applying a desired pump voltage Vp0 between the inner main pump electrode 22 and the outer pump electrode 23 using a variable power supply 24 and flowing a pump current Ip0 in a positive or negative direction between the inner main pump electrode 22 and the outer pump electrode 23, it is possible to pump oxygen near the inner main pump electrode 22 in the internal space 14 out to the external space, or to pump oxygen from the external space into the internal space 14.

[0066] In addition, in order to detect the oxygen concentration (oxygen partial pressure) in the atmosphere near the inner main pump electrode 22 in the internal space 14, the inner main pump electrode 22, the second solid electrolyte 6, the spacer layer 5, the first solid electrolyte 4, the third substrate layer 3, and the reference electrode 42 constitute an electrochemical sensor cell, i.e., an oxygen partial pressure detection sensor cell 80 for controlling the main pump.

[0067] By measuring the electromotive force V0 in the oxygen partial pressure detection sensor cell 80 for controlling the main pump, the oxygen concentration (oxygen partial pressure) near the inner main pump electrode 22 in the internal space 14 can be determined. Furthermore, the pump current Ip0 is controlled by feedback controlling the pump voltage Vp0 so that the electromotive force V0 is constant. This allows the oxygen concentration near the inner main pump electrode 22 in the internal space 14 to be maintained at a predetermined constant value.

[0068] The measurement target gas decomposition pump cell 50 is an electrochemical pump cell including a measurement target gas decomposition pump electrode 51 disposed on the inner surface of the measurement target gas flow section 15 at a position farther from the longitudinal tip of the base section 102 than the inner main pump electrode 22, and an outer pump electrode 23 disposed on the base section 102 at a position different from the measurement target gas flow section 15 (on the outer surface of the base section 102 in FIG. 1 ) and corresponding to the measurement target gas decomposition pump electrode 51. The phrase "corresponding to the measurement target gas decomposition pump electrode 51" means that the outer pump electrode 23 is provided on the measurement target gas decomposition pump electrode 51 with a second solid electrolyte layer 6 interposed therebetween.

[0069] That is, the measurement target gas decomposition pump cell 50 is an electrochemical pump cell comprising a measurement target gas decomposition pump electrode 51 disposed on the underside of the second solid electrolyte layer 6 facing the internal cavity 14, closer to the rear end of the sensor element 101 than the inner main pump electrode 22, an outer pump electrode 23 (not limited to the outer pump electrode 23, but any suitable electrode located at a position different from the measurement target gas flow section 15, such as outside the sensor element 101, will suffice), and the second solid electrolyte layer 6.

[0070] The measurement object gas decomposition pump cell 50 is configured so that the measurement object gas decomposition pump electrode 51 decomposes the measurement object gas in the measurement object gas, the oxygen concentration of which is maintained at a predetermined constant value in the main pump cell 21, and at least a portion of the total oxygen in the measurement object gas, including oxygen produced by the decomposition of the measurement object gas, can be discharged from the measurement object gas flow section 15. Therefore, the measurement object gas decomposition pump electrode 51 is required to have catalytic activity that reduces (decomposes) the NOx component in the measurement object gas. Specific constituent materials will be described later.

[0071] In the measurement object gas decomposition pump cell 50, by applying a desired voltage Vp1 between the measurement object gas decomposition pump electrode 51 and the outer pump electrode 23 using a variable power supply 52, NOx in the measurement object gas that has reached the measurement object gas decomposition pump electrode 51 can be decomposed, and the oxygen in the measurement object gas, including the oxygen produced by the decomposition of NOx, can be pumped out to the external space.

[0072] In addition, in order to control the oxygen partial pressure in the atmosphere near the measurement target gas decomposition pump electrode 51 within the internal space 14, the measurement target gas decomposition pump electrode 51, the reference electrode 42, the second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, and the third substrate layer 3 constitute an electrochemical sensor cell, i.e., an oxygen partial pressure detection sensor cell 81 for controlling the measurement target gas decomposition pump.

[0073] The measurement object gas decomposition pump cell 50 performs pumping using a variable power supply 52 whose voltage is controlled based on the electromotive force V1 detected by the measurement object gas decomposition pump control oxygen partial pressure detection sensor cell 81. As a result, the oxygen partial pressure in the atmosphere near the measurement object gas decomposition pump electrode 51 in the internal space 14 is controlled to be lower than the oxygen partial pressure near the inner main pump electrode 22 in the internal space 14.

[0074] The pump current Ip1 is also used to control the electromotive force of the main pump control oxygen partial pressure detection sensor cell 80. Specifically, the pump current Ip1 is input as a control signal to the main pump control oxygen partial pressure detection sensor cell 80, and by controlling the electromotive force V0, the gradient of the oxygen partial pressure in the measurement target gas that reaches the measurement target gas decomposition pump electrode 51 in the internal space 14 is controlled to be always constant.

[0075] The residual oxygen measurement pump cell 41 is an electrochemical pump cell including a residual oxygen measurement electrode 44 disposed on the inner surface of the measurement gas flow section 15 at a position farther from the longitudinal tip of the base section 102 than the measurement target gas decomposition pump electrode 51, and an outer pump electrode 23 disposed at a position on the base section 102 different from the measurement target gas flow section 15 (on the outer surface of the base section 102 in FIG. 1 ) corresponding to the residual oxygen measurement electrode 44. "Corresponding to the residual oxygen measurement electrode 44" means that the outer pump electrode 23 is provided on the residual oxygen measurement electrode 44 with a second solid electrolyte layer 6 interposed therebetween.

[0076] That is, the residual oxygen measurement pump cell 41 is an electrochemical pump cell composed of a residual oxygen measurement electrode 44 arranged on the underside of the second solid electrolyte layer 6 facing the internal cavity 14, at a position farther from the longitudinal tip of the sensor element 101 than the measurement target gas decomposition pump electrode 51, an outer pump electrode 23 (not limited to the outer pump electrode 23, but any suitable electrode located at a position different from the measurement target gas flow section 15, such as outside the sensor element 101, will suffice), and the second solid electrolyte layer 6.

[0077] The residual oxygen measuring pump cell 41 is configured to obtain a detection current value corresponding to the concentration of residual oxygen present in the measurement gas flow section 15. NOx in the measurement gas is decomposed in the measurement gas decomposition pump cell 50, and a portion of the total oxygen, including oxygen produced by the decomposition, is discharged and then reaches the residual oxygen measuring electrode 44. The residual oxygen measuring electrode 44 may be configured in any way so as to be able to detect residual oxygen in the measurement gas. Specific constituent materials will be described later.

[0078] Furthermore, in order to detect the oxygen partial pressure around the residual oxygen measuring electrode 44, an electrochemical sensor cell, i.e., an oxygen partial pressure detection sensor cell 82 for controlling a residual oxygen measuring pump, is configured by the second solid electrolyte layer 6, spacer layer 5, first solid electrolyte layer 4, third substrate layer 3, residual oxygen measuring electrode 44, and reference electrode 42. The variable power supply 46 is controlled based on the electromotive force V2 detected by the oxygen partial pressure detection sensor cell 82 for controlling a residual oxygen measuring pump.

[0079] The measurement gas introduced into the internal space 14 has its oxygen partial pressure controlled by the main pump cell 21, and a portion of the total oxygen, including oxygen produced by decomposition of NOx and oxygen originally present in the measurement gas, is pumped out in the measurement gas decomposition pump cell 50. The remaining oxygen remains in the measurement gas when it reaches the residual oxygen measuring electrode 44. The residual oxygen in the measurement gas is pumped by the residual oxygen measurement pump cell 41, and at this time, the voltage Vp2 of the variable power supply 46 is controlled so that the control voltage V2 detected by the residual oxygen measurement pump control oxygen partial pressure detection sensor cell 82 is kept constant.

[0080] The pump current Ip2 in the residual oxygen measurement pump cell 41 is proportional to the residual oxygen concentration in the measurement gas that reaches the vicinity of the residual oxygen measurement electrode 44. The measurement gas that reaches the vicinity of the residual oxygen measurement electrode 44 is considered to be the same as the atmosphere near the measurement gas decomposition pump electrode 51 of the measurement gas decomposition pump cell 50 described above. As described above, the residual oxygen in the atmosphere near the measurement gas decomposition pump electrode 51, i.e., the residual oxygen concentration in the measurement gas that reaches the residual oxygen measuring electrode 44, is correlated with the NOx concentration in the measurement gas. Therefore, the nitrogen oxide concentration in the measurement gas can be calculated using the pump current Ip2.

[0081] In addition, the second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, the third substrate layer 3, the outer pump electrode 23, and the reference electrode 42 constitute an electrochemical sensor cell 83, and the electromotive force Vref obtained by this sensor cell 83 makes it possible to detect the oxygen partial pressure in the measurement gas outside the sensor.

[0082] In the gas sensor 100 having such a configuration, the main pump cell 21 is operated to supply the measurement gas, whose oxygen partial pressure is constantly maintained at a low value, to the measurement gas decomposition pump cell 50. The measurement gas decomposition pump cell 50 decomposes NOx in the measurement gas and discharges a predetermined amount of the total oxygen in the measurement gas, including the oxygen produced by the decomposition, thereby controlling the pump current to a constant current value. As a result, the residual oxygen concentration in the measurement gas that reaches the residual oxygen measuring electrode 44 corresponds to the NOx concentration in the measurement gas. Therefore, the NOx concentration in the measurement gas can be determined based on the pump current Ip2 that flows when the residual oxygen is pumped out by the residual oxygen measuring pump cell 41. The control method using this sensor element 101 will be described in detail later.

[0083] Furthermore, in order to enhance the oxygen ion conductivity of the solid electrolyte, the sensor element 101 includes a heater section 70 that adjusts the temperature by heating and maintaining the temperature of the sensor element 101. The heater section 70 includes a heater electrode 71, a heater 72, a through-hole 73, a heater insulating layer 74, and a pressure release hole 75.

[0084] In the sensor element 101 of this embodiment, the heater 70 is embedded in the base 102, but this is not a limitation. The heater 72 is provided so as to heat the base 102. That is, the heater 72 is required to heat the sensor element 101 to an extent that the sensor element 101 exhibits oxygen ion conductivity sufficient to operate the main pump cell 21, the measurement target gas decomposition pump cell 50, and the residual oxygen measurement pump cell 41. For example, the heater 72 may be embedded in the base 102 as in this embodiment. Alternatively, the heater 70 may be formed as a heater substrate separate from the base 102 and disposed adjacent to the base 102. Alternatively, the heater 70 may be heated by a high-temperature measurement gas. For accurate measurement, it is preferable that the temperature of the sensor element 101 be constant regardless of the temperature of the measurement gas. In consideration of this, it is preferable that the sensor element 101 includes the heater 70 as in this embodiment.

[0085] The heater electrode 71 is an electrode formed in a manner to contact the lower surface of the first substrate layer 1. By connecting the heater electrode 71 to an external power supply, it is possible to supply power to the heater section 70 from the outside.

[0086] The heater 72 is an electrical resistor sandwiched between the second substrate layer 2 and the third substrate layer 3. The heater 72 is connected to the heater electrode 71 via a through hole 73, and generates heat when power is supplied from the outside through the heater electrode 71, thereby heating and keeping warm the solid electrolyte that forms the sensor element 101.

[0087] The heater 72 is embedded throughout the entire internal space 14, making it possible to adjust the entire sensor element 101 to a temperature at which the solid electrolyte is activated. The temperature needs to be adjusted so that the main pump cell 21, the measurement target gas decomposition pump cell 50, and the residual oxygen measurement pump cell 41 can operate. It is not necessary to adjust these areas to the same temperature throughout, and the element 101 may have a temperature distribution.

[0088] The heater insulating layer 74 is an insulating layer made of an insulator such as alumina and formed on the upper and lower surfaces of the heater 72. The heater insulating layer 74 is formed for the purpose of obtaining electrical insulation between the second substrate layer 2 and the heater 72, and between the third substrate layer 3 and the heater 72.

[0089] The pressure release hole 75 penetrates the third substrate layer 3 and is formed so as to connect the heater insulating layer 74 and the reference gas introduction space 43. The pressure release hole 75 can mitigate an increase in internal pressure that accompanies an increase in temperature within the heater insulating layer 74. Note that the pressure release hole 75 may be omitted.

[0090] (Electrode constituent materials) The inner main pump electrode 22, the measurement target gas decomposition pump electrode 51, and the residual oxygen measuring electrode 44 are each a porous cermet electrode (an electrode in which a metal component and a ceramic component are mixed). The ceramic component is not particularly limited, but, like the base portion 102, an oxygen ion-conducting solid electrolyte is preferably used. For example, ZrO2 can be used as the ceramic component. The metal component and ceramic component in the porous cermet electrode can be appropriately determined by those skilled in the art. For example, the ceramic component can be approximately 30% to 50% by weight of the total of the metal component and the ceramic component. For example, when Pt is used as the metal component and ZrO2 is used as the ceramic component, the weight ratio of Pt:ZrO2 may be approximately 7.0:3.0 to 5.0:5.0.

[0091] The metal materials of the electrodes of the sensor element 101 (the inner main pump electrode 22, the measurement object gas decomposition pump electrode 51, and the residual oxygen measuring electrode 44) will be described in detail below.

[0092] (inner main pump electrode) As described above, the main pump cell 21 is configured to be able to adjust the oxygen concentration in the measurement gas that has flowed into the measurement gas flow section 15 to a predetermined concentration. Therefore, it is preferable that the inner main pump electrode 22 in contact with the measurement gas decomposes only oxygen without reducing or decomposing the gases to be measured (nitrogen oxides NOx, ammonia NH3, etc.) in the measurement gas.

[0093] For example, the metal material of the inner main pump electrode 22 may be a material containing platinum (Pt) as a main component and containing an added metal that reduces the catalytic activity of decomposing the gas to be measured.

[0094] Platinum (Pt) is a material used as a catalyst not only in the field of gas sensors but also in a wide range of general applications. Pt has catalytic activity for oxygen and catalytic activity for decomposing a gas to be measured (e.g., NOx). It is believed that by adding a metal that reduces the catalytic activity for decomposing a gas to be measured to Pt, it is possible to reduce the catalytic activity for decomposing a gas to be measured while maintaining catalytic activity for oxygen.

[0095] Examples of metals that reduce catalytic activity for decomposing NOx include gold (Au) and silver (Ag). These metals are considered to have no catalytic activity for decomposing NOx. Preferably, gold (Au) can be used.

[0096] The amount of metal that reduces the catalytic activity of decomposing NOx may be appropriately set so that the inner main pump electrode 22 does not substantially decompose NOx. For example, when gold (Au) is added to platinum (Pt) as the main component, Au may be added in an amount of 0.3 wt % or more, preferably 0.5 wt % or more, and more preferably 0.8 wt % or more, based on the total amount of the metal material. The amount of Au may also be 3.0 wt % or less, preferably 2.0 wt % or less. It is believed that by setting the amount of Au added within this range, the accuracy of measuring the NOx concentration of the gas sensor 100 can be further improved. When adding other metals that reduce the catalytic activity of decomposing NOx, such as Ag, the above amount of Au can also be used as a reference.

[0097] (Measurement target gas decomposition pump electrode) As described above, the measurement object gas decomposition pump cell 50 is configured so that the measurement object gas decomposition pump electrode 51 decomposes the measurement object gas in the measurement object gas, the oxygen concentration of which is maintained at a predetermined constant value in the main pump cell 21, and at least a portion of the total oxygen in the measurement object gas, including oxygen produced by the decomposition of the measurement object gas, can be discharged from the measurement object gas flow section 15. Therefore, in a NOx sensor, the measurement object gas decomposition pump electrode 51 is required to have catalytic activity that reduces (decomposes) the NOx component in the measurement object gas.

[0098] The measurement object gas decomposition pump electrode 51 is a porous cermet electrode. The measurement object gas decomposition pump electrode 51 constitutes an electrochemical pump cell and also functions as a NOx reduction catalyst that reduces NOx present in the atmosphere within the internal space 14.

[0099] The metal material of the measurement target gas decomposition pump electrode 51 is preferably a noble metal material having catalytic activity for decomposing NOx (reducing NOx). For example, platinum (Pt), rhodium (Rh), etc. may be used. For example, Pt may be used, or an alloy of Pt and Rh may be used. For example, when an alloy of Pt and Rh is used, the amount of Rh may be 10% to 90% by weight of the total amount of Pt and Rh. If a noble metal having catalytic activity for NOx is used as the metal material of the measurement target gas decomposition pump electrode 51, substantially all of the NOx will be decomposed by the measurement target gas decomposition pump electrode 51, which is thought to result in high measurement accuracy.

[0100] The metal material of the measurement object gas decomposition pump electrode 51 may contain no gold or may contain gold to an extent that does not inhibit the catalytic activity for decomposing the measurement object gas. In other words, the metal material of the measurement object gas decomposition pump electrode 51 may contain a small amount of gold (Au) to an extent that catalytic activity for NOx is exhibited.

[0101] The metal material containing Pt and Au may contain, for example, 0.3 wt % or less of Au relative to the total weight of the metal material. Preferably, the metal material may contain 0.2 wt % or less of Au, provided that the amount of Au contained in the measurement target gas decomposition pump electrode 51 is less than the amount of Au contained in the inner main pump electrode 22 described above. The amount of Au contained in the measurement target gas decomposition pump electrode 51 may also be the same as the amount of Au contained in the residual oxygen measuring electrode 44 described below. Preferably, the amount of Au may be less than the amount of Au contained in the residual oxygen measuring electrode 44.

[0102] As mentioned above, Au has virtually no catalytic activity for reducing NOx, but if a small amount of Au is added, it is possible for NOx to be decomposed at the measurement target gas decomposition pump electrode 51. Even if Au evaporates from the inner main pump electrode 22 and adheres to the measurement target gas decomposition pump electrode 51 during use of the gas sensor, the measurement target gas decomposition pump electrode 51 originally contains a trace amount of Au, so it is thought that the change in NOx decomposition activity will be small. Therefore, even after long-term use, changes in NOx sensitivity can be suppressed.

[0103] (residual oxygen measurement electrode) As described above, the residual oxygen measurement pump cell 41 is configured to obtain a detection current value corresponding to the concentration of residual oxygen present in the measurement gas flow portion 15. NOx in the measurement gas is decomposed in the measurement gas decomposition pump cell 50, and a portion of the total oxygen, including oxygen produced by the decomposition, is discharged and then reaches the residual oxygen measurement electrode 44. The residual oxygen measurement electrode 44 may be configured in any way so as to be able to detect residual oxygen in the measurement gas.

[0104] The residual oxygen measuring electrode 44 is a porous cermet electrode. The residual oxygen measuring electrode 44 may be made of any material as long as it has catalytic activity for oxygen (O2). For example, platinum (Pt) may be used.

[0105] When the gas sensor is used for a long time under high oxygen concentration and high temperature conditions, Au in the inner main pump electrode 22 evaporates, and even if the evaporated Au adheres to the residual oxygen measuring electrode 44, the residual oxygen measuring electrode 44 maintains its activity toward oxygen, so the detection accuracy of the gas sensor does not decrease. Therefore, the decrease in detection accuracy of the gas sensor due to use can be suppressed. In other words, the change in NOx sensitivity over time can be suppressed. As a result, durability is improved.

[0106] The measurement target gas is decomposed in the measurement target gas decomposition pump cell 50. The residual oxygen measurement pump cell 41 detects residual oxygen in the measurement target gas. By adjusting the pump current Ip1 in the measurement target gas decomposition pump cell 50, it is possible to adjust the residual oxygen concentration near the measurement target gas decomposition pump electrode 51, i.e., the range of residual oxygen concentration reaching the residual oxygen measurement electrode 44. This makes it possible to accommodate large changes in the concentration of the measurement target gas in the measurement target gas. As a result, it is possible to accurately measure the measurement target gas over a wide concentration range.

[0107] The residual oxygen measuring electrode 44 is preferably formed using a material that has substantially no or weak reducing ability for the NOx component in the measurement gas. In other words, it is preferable that the metal material of the residual oxygen measuring electrode 44 does not have catalytic activity for decomposing NOx. In this case, even if some of the NOx in the measurement gas is not decomposed at the measurement target gas decomposition pump electrode 51 for some reason and gas containing residual NOx reaches the residual oxygen measuring electrode 44, the residual NOx will not be decomposed at the residual oxygen measuring electrode 44. It is believed that higher measurement accuracy can be achieved by always detecting only residual oxygen at the residual oxygen measuring electrode 44.

[0108] Specifically, the metal component of the residual oxygen measuring electrode 44 can be a material that contains a precious metal such as platinum (Pt) as the main component, to which a metal that reduces the catalytic activity of decomposing (reducing) NOx is added. Examples of metals that reduce the catalytic activity of decomposing NOx include gold (Au) and silver (Ag). These metals are considered to have no catalytic activity of decomposing NOx. Preferably, gold (Au) can be used.

[0109] The amount of metal added that reduces the catalytic activity of decomposing NOx may be set appropriately so that the residual oxygen measuring electrode 44 does not substantially decompose NOx. For example, when platinum (Pt) is the main component and gold (Au) is added, 0.3 wt % or more of Au may be added relative to the total amount of metal material. Preferably, 0.5 wt % or more may be added. More preferably, 0.8 wt % or more may be added. Furthermore, the amount of Au added may be 3.0 wt % or less. Preferably, 2.0 wt % or less. It is believed that by setting the amount of Au added within this range, the accuracy of measuring residual oxygen may be further improved.

[0110] [Sensor element manufacturing method] Next, an example of a method for manufacturing the above-mentioned sensor element will be described. After performing predetermined processing and printing of a circuit pattern on a plurality of unfired sheet-shaped molded articles (so-called green sheets) containing an oxygen ion conductive solid electrolyte such as zirconia (ZrO2) as a ceramic component, the plurality of sheets are stacked, the stack is cut, and then fired to produce the sensor element 101.

[0111] In the following, an example will be described in which the sensor element 101 made up of six layers as shown in FIG. 1 is fabricated.

[0112] First, six green sheets containing an oxygen ion conductive solid electrolyte such as zirconia (ZrO2) as a ceramic component are prepared. A known forming method can be used to fabricate the green sheets. All six green sheets may be the same thickness, or the thickness may vary depending on the layer being formed. Holes and other components used for positioning during printing and lamination are formed in each of the six green sheets (blank sheets) in advance using a known method such as punching with a punching device. Through holes such as internal voids 14 are also formed in the blank sheet used for the spacer layer 5 in a similar manner. Necessary through holes are also formed in the other layers in advance.

[0113] Various patterns required for each layer are printed and dried on blank sheets used for the six layers: first substrate layer 1, second substrate layer 2, third substrate layer 3, first solid electrolyte layer 4, spacer layer 5, and second solid electrolyte layer 6. Known screen printing techniques can be used to print the patterns. Known drying means can also be used for the drying process.

[0114] When forming the inner main pump electrode 22, the measurement target gas decomposition pump electrode 51, and the residual oxygen measuring electrode 44, first, electrode pastes having the desired electrode compositions described above are prepared for each.

[0115] Next, the electrode paste to be used for the inner main pump electrode 22 is printed and dried in a desired pattern on the second solid electrolyte layer 6. Also, the electrode paste to be used for the measurement target gas decomposition pump electrode 51 is printed and dried in a desired pattern. Also, the electrode paste to be used for the residual oxygen measuring electrode 44 is printed and dried in a desired pattern. The order of these printing steps can be determined as appropriate.

[0116] This process is repeated until various patterns have been printed and dried on each of the six blank sheets. The six printed blank sheets are then stacked in a predetermined order while being positioned using sheet holes, etc., and then pressed under predetermined temperature and pressure conditions to form a laminate. The pressing process is carried out by applying heat and pressure using a laminator such as a known hydraulic press. The temperature, pressure, and time for heating and pressing depend on the laminator used, but can be determined appropriately to achieve good lamination.

[0117] The obtained laminate contains a plurality of sensor elements 101. The laminate is cut into individual sensor elements 101. The cut laminate is fired at a predetermined firing temperature to obtain the sensor elements 101. The firing temperature may be any temperature at which the solid electrolyte constituting the base portion 102 of the sensor element 101 is sintered to form a dense body and at which the electrodes and the like maintain a desired porosity. For example, firing is performed at a firing temperature of about 1300 to 1500°C.

[0118] The obtained sensor element 101 is incorporated into the gas sensor 100 in such a manner that the front end of the sensor element 101 contacts the gas to be measured and the rear end of the sensor element 101 contacts the reference gas.

[0119] [Detection method] A method for detecting NOx using the above-described sensor element 101 will be described in detail below.

[0120] The method for detecting a gas to be measured in a gas to be measured of the present invention comprises the steps of: a long plate-shaped substrate including a plurality of stacked oxygen ion conductive solid electrolyte layers; a measurement gas flow section for introducing a measurement gas from one longitudinal end of the base section and allowing the measurement gas to flow therethrough; a main pump cell including an inner main pump electrode disposed on an inner surface of the measurement target gas flow portion and an outer pump electrode disposed on the base portion at a position different from the measurement target gas flow portion and corresponding to the inner main pump electrode; a measurement target gas decomposition pump cell including: a measurement target gas decomposition pump electrode disposed on an inner surface of the measurement target gas flow portion at a position farther from the one longitudinal end of the base portion than the inner main pump electrode; and an outer pump electrode disposed at a position different from the measurement target gas flow portion of the base portion, corresponding to the measurement target gas decomposition pump electrode; a residual oxygen measurement pump cell including: a residual oxygen measurement electrode disposed on an inner surface of the measurement target gas flow portion at a position farther from the one longitudinal end of the base portion than the inner main pump electrode; and an outer pump electrode disposed at a position different from the measurement target gas flow portion of the base portion, corresponding to the residual oxygen measurement electrode; a reference electrode disposed inside the base portion so as to be in contact with a reference gas; Including, a sensor element in which the metal material contained in the measurement object gas decomposition pump electrode has catalytic activity for decomposing the measurement object gas; an oxygen concentration adjusting step of adjusting the oxygen concentration in the measurement gas flowing into the measurement gas flow section to a predetermined concentration by the main pump cell, thereby obtaining the measurement gas whose oxygen concentration has been adjusted to the predetermined concentration; a current value control step of decomposing the measurement target gas in the measurement target gas at the measurement target gas decomposition pump electrode by the measurement target gas decomposition pump cell, and discharging a predetermined amount of all oxygen in the measurement target gas, including oxygen generated by decomposition of the measurement target gas, from the measurement target gas flow portion so that the current value flowing through the measurement target gas decomposition pump cell is kept constant at a preset value; a detecting step of obtaining a detection current value corresponding to the concentration of residual oxygen present in the measurement target gas flow portion by the residual oxygen measurement pump cell; a concentration calculation step of calculating a concentration of the measurement target gas based on the detected current value; Includes.

[0121] In the oxygen concentration adjusting step, the oxygen concentration in the measurement gas flowing into the measurement gas flow section 15 is adjusted to a predetermined concentration, thereby obtaining measurement gas whose oxygen concentration has been adjusted to the predetermined concentration. That is, the main pump cell 21 controls the oxygen concentration (oxygen partial pressure) in the measurement gas flowing into the internal space 14 so that it is constant at the predetermined concentration.

[0122] The electromotive force V0 in the oxygen partial pressure detection sensor cell 80 for controlling the main pump is kept at a constant value (set value V0 SET The pump voltage Vp0 of the variable power supply 24 in the main pump cell 21 is feedback-controlled so that the electromotive force Vp0 indicates the oxygen partial pressure near the inner main pump electrode 22. Therefore, maintaining the electromotive force Vp0 constant means maintaining the oxygen partial pressure near the inner main pump electrode 22 constant. As a result, the pump current Ip0 in the main pump cell 21 changes depending on the oxygen concentration in the measurement gas.

[0123] When the oxygen partial pressure in the measurement gas reaches the set value V0 SET If the oxygen partial pressure in the measurement gas is higher than the set value V0, the main pump cell 21 discharges oxygen from the internal space 14. SET When the oxygen partial pressure is lower than the oxygen partial pressure corresponding to (for example, when hydrocarbons HC are contained), the main pump cell 21 pumps oxygen from the space outside the sensor element 101 into the internal space 14. Therefore, the pump current Ip0 can take either a positive or negative value.

[0124] In the current value control step, the measurement target gas decomposition pump cell 50 decomposes the measurement target gas in the measurement target gas, the oxygen concentration of which has been adjusted to a predetermined value, at the measurement target gas decomposition pump electrode 51, and a predetermined constant amount of the total oxygen in the measurement target gas, including oxygen produced by the decomposition of the measurement target gas, is discharged from the measurement target gas flow section 15 so that the current value flowing through the measurement target gas decomposition pump cell 50 is kept constant at a preset value. In other words, the main pump cell 21 decomposes NOx in the measurement target gas, the oxygen partial pressure of which has been adjusted, and pumps a constant amount of the total oxygen, including the oxygen produced by the decomposition and the oxygen in the atmospheric gas, to control the pump current value to a constant value.

[0125] The electromotive force V1 detected by the oxygen partial pressure detection sensor cell 81 for controlling the measurement target gas decomposition pump is kept at a constant value (set value V1 SET The pump voltage Vp1 of the variable power supply 52 in the measurement object gas decomposition pump cell 50 is feedback-controlled so that the set value Vp1 is equal to the oxygen partial pressure in the vicinity of the measurement object gas decomposition pump electrode 51. Since the electromotive force V1 indicates the oxygen partial pressure in the vicinity of the measurement object gas decomposition pump electrode 51, maintaining the electromotive force V1 constant means maintaining the oxygen partial pressure in the vicinity of the measurement object gas decomposition pump electrode 51 constant. SET can be set to a value that provides a low oxygen concentration that allows NOx in the measurement gas to be sufficiently decomposed.

[0126] Set value of electromotive force V1 V1 SET For example, the voltage may be set to 350 mV to 500 mV, and preferably to 380 mV to 430 mV.

[0127] It is presumed that when the measurement object gas decomposition pump electrode 51 contains a small amount of Au, the catalytic activity of the measurement object gas decomposition pump electrode 51 for NOx is somewhat suppressed. However, it is thought that by increasing the electromotive force V1, it is possible to decompose NOx more effectively. In other words, this shows that by controlling the electromotive force V1, it is possible to control the amount of decomposition of the measurement object gas at the measurement object gas decomposition pump electrode 51 within a desired range. When the measurement object gas decomposition pump electrode 51 contains a small amount of Au, the set value V1SET For example, the voltage may be set to 350 mV to 500 mV, and preferably to 400 mV to 480 mV.

[0128] When the electromotive force V1 is increased, the pump voltage Vp1 applied between the measurement object gas decomposition pump electrode 51 and the outer pump electrode 23 based on the electromotive force V1 also increases. In the so-called limiting current region, the pump current Ip1 that flows remains almost constant even when the pump voltage Vp1 increases. However, it is believed that the larger the electromotive force V1 (or the pump voltage Vp1), the more the decomposition of NOx can be promoted at the measurement object gas decomposition pump electrode 51. In other words, it is believed that almost all of the NOx can be decomposed, improving measurement accuracy.

[0129] The measurement gas introduced into the internal cavity 14 has its oxygen partial pressure controlled by the main pump cell 21, and reaches the measurement gas decomposition pump electrode 51. Nitrogen oxides (NOx) in the measurement gas around the measurement gas decomposition pump electrode 51 are reduced (2NO → N2 + O2) to generate oxygen. This generated oxygen, along with the oxygen in the measurement gas whose oxygen partial pressure has been controlled by the main pump cell 21, are pumped by the measurement gas decomposition pump cell 50. At this time, the voltage Vp1 of the variable power supply 52 is controlled so that the control voltage V1 detected by the measurement gas decomposition pump control oxygen partial pressure detection sensor cell 81 remains constant.

[0130] At the same time, the pump current Ip1 in the measurement object gas decomposition pump cell 50 is set to a constant value (set value Ip1 SET The set value V0 of the electromotive force V0 is set based on the pump current Ip1 so that the set value V0 SET As a result, the partial pressure of oxygen in the measurement gas that reaches the measurement object gas decomposition pump electrode 51 becomes constant.

[0131] As described above, the measurement gas reaches the measurement object gas decomposition pump electrode 51 with its oxygen partial pressure controlled in the main pump cell 21. In other words, the oxygen partial pressure P(O2) in the measurement gas when it reaches the measurement object gas decomposition pump electrode 51 is kept constant.

[0132] NOx in the measurement gas is decomposed (2NO→N2+O2) to generate oxygen at the measurement gas decomposition pump electrode 51. The generated oxygen partial pressure is denoted as P(NOx).

[0133] As a result, the oxygen partial pressure in the vicinity of the measurement target gas decomposition pump electrode 51 is The partial pressure of oxygen originally present in the gas being measured [P(O2) = constant amount] The oxygen partial pressure generated by the decomposition of NOx [P(NOx) = amount according to the NOx concentration] The total amount is [P(Total) = P(O2) + P(NOx)].

[0134] That is, the amount of oxygen in the vicinity of the measurement object gas decomposition pump electrode 51 varies depending on the NOx concentration in the measurement object gas.

[0135] The total amount of the measurement gas reaching the measurement target gas decomposition pump electrode 51 is determined by the diffusion resistance from the gas inlet 10 to the measurement target gas decomposition pump electrode 51. The diffusion resistance is approximately equal to the diffusion resistance provided by the first diffusion rate-controlling section 11 and the second diffusion rate-controlling section 13. A constant oxygen partial pressure is synonymous with a constant oxygen amount (number of oxygen molecules).

[0136] As described above, the pump current Ip1 in the measurement object gas decomposition pump cell 50 is set to a constant value (set value Ip1 SET The pump current Ip1 is directly correlated with the amount of oxygen transferred in the measurement object gas decomposition pump cell 50. In other words, the pump current Ip1 is controlled to be a constant value (set value Ip1 SET ) is to always keep a constant amount [P(Ip1 SETAs a result, some oxygen remains near the measurement target gas decomposition pump electrode 51. This residual oxygen partial pressure [denoted as P(R)] is expressed by the following equation (1):

[0137] P(R) = P(O2) + P(NOx) - P(Ip1 SET ) (1)

[0138] Here, as described above, P(O2) and P(Ip1 SET ) is a constant value, and P(NOx) is an amount that corresponds to the NOx concentration in the measurement gas. Therefore, the amount of residual oxygen [P(R)] is considered to be an amount that corresponds to the NOx concentration in the measurement gas.

[0139] Also, from the viewpoint of current values, it can be considered as follows: The pump current Ip1 in the measurement target gas decomposition pump cell 50 is considered to be composed of a pump current Ip(NOx) that pumps oxygen generated by the reduction of NOx, and a pump current Ip(O2) that pumps oxygen in the measurement target gas whose oxygen partial pressure has been controlled by the main pump cell 21, as shown in the following equation (2). Note that for convenience of explanation, Ip(NOx) and Ip(O2) are described separately, but they cannot actually be detected separately as pump currents.

[0140] Ip1=Ip(NOx)+Ip(O2)=constant (2)

[0141] As described above, since the pump current Ip1 is controlled to a constant value, according to the relationship in equation (2), oxygen corresponding to the magnitude of Ip(NOx) remains in the measured gas as residual oxygen near the measured gas decomposition pump electrode 51.

[0142] When the NOx concentration in the measurement gas is high, the pump current Ip(NOx) that pumps the oxygen generated by the reduction of NOx becomes large, while the pump current Ip(O2) that pumps a portion of the oxygen in the measurement gas whose oxygen partial pressure is controlled by the main pump cell 21 becomes small. As a result, the residual oxygen concentration in the atmosphere near the measurement gas decomposition pump electrode 51 becomes high.

[0143] Conversely, when the NOx concentration in the measurement gas is low, the pump current Ip(NOx) that pumps the oxygen generated by the reduction of NOx becomes small, while the pump current Ip(O2) that pumps a portion of the oxygen in the measurement gas whose oxygen partial pressure is controlled by the main pump cell 21 becomes large. As a result, the residual oxygen concentration in the atmosphere near the measurement gas decomposition pump electrode 51 becomes low.

[0144] Thus, the higher the NOx concentration in the measurement gas, the higher the residual oxygen concentration in the atmosphere near the measurement gas decomposition pump electrode 51. In other words, the residual oxygen concentration in the atmosphere near the measurement gas decomposition pump electrode 51 has a correlation with the NOx concentration in the measurement gas.

[0145] Furthermore, as described above, the pump current Ip1 is controlled to a constant value. Therefore, even if Au adheres to the measurement object gas decomposition pump electrode 51 and its catalytic activity for the measurement object gas decreases slightly, the measurement object gas can be decomposed at the measurement object gas decomposition pump electrode 51 by the pump voltage Vp1 applied to keep the pump current Ip1 constant. This prevents a decrease in the detection accuracy of the gas sensor due to the use of the detection electrode. In other words, it prevents changes over time in the detection value of the measurement object gas. As a result, durability is improved.

[0146] Pump current Ip1 setting value Ip1 SETcan be set appropriately so as to pump out oxygen derived from the gas to be measured (NOx in this embodiment). For example, it may be set to 1 μA to 15 μA. Preferably, it may be set to 3 μA to 10 μA. For example, it can be set to 7 μA.

[0147] Setting value Ip1 SET It is preferable that the current value be equal to or greater than Ip(NOx), which should flow according to the amount of NOx in the gas being measured. If the set value is set in this range, it is believed that the residual oxygen concentration will not become saturated. If the set value is set in this range, it is believed that the NOx concentration can be accurately detected, especially when the NOx concentration is high.

[0148] Setting value Ip1 SET It is preferable that the Ip(NOx) is not too large compared with the Ip(NOx) that should flow according to the amount of NOx in the measurement gas. With such a set value, it is possible to keep the S / N ratio within an appropriate range in the detection step described below, even when the NOx concentration is particularly low, and it is believed that the NOx concentration can be accurately detected.

[0149] For example, the setting value Ip1 SET may be set based on the total amount of the measurement gas that reaches the measurement gas decomposition pump electrode 51. The total amount of the measurement gas that reaches the measurement gas decomposition pump electrode 51 is determined by the diffusion resistance from the measurement gas inlet 10 of the sensor element 101 to the measurement gas decomposition pump electrode 51.

[0150] For example, the diffusion resistor and the optimum set value Ip1 SET The diffusion resistance from the measurement object gas inlet 10 to the measurement object gas decomposition pump electrode 51 is measured for the sensor element 101, and the set value Ip1 is set according to the value of the diffusion resistance. SET can be determined.

[0151] Alternatively, it may be determined using the value of the diffusion resistance from the measurement gas inlet 10 to the inner main pump electrode 22. The value of the diffusion resistance from the measurement gas inlet 10 to the inner main pump electrode 22 is considered to correspond to the amount of measurement gas flowing into the measurement gas flow section 15.

[0152] In this way, the setting value Ip1 SET By determining this, the detected current value will not be saturated in the detection step described later, and the S / N ratio can be kept within an appropriate range, so that a highly accurate detected current value can be obtained.

[0153] The current value constant control step may further include a set value determination step. SET may be variable. That is, a plurality of set values ​​Ip1 SET may be determined in advance and appropriately changed during use of the gas sensor 100 (during measurement of the gas to be measured). In this case, in the setting value determination step, the current value control step may determine a plurality of the setting values ​​Ip1 SET Which of the setting values ​​Ip1 SET Determine whether to use the setting value Ip1 SET is a set of multiple values ​​Ip1 SET may be switched stepwise or may be changed continuously.

[0154] For example, when the NOx concentration in the measurement gas is high, the set value Ip1 SET On the other hand, when the NOx concentration in the measurement gas is low, it is possible to increase the set value Ip1 to prevent the detection value from saturating in the detection step described later. SET It is conceivable to reduce the S / N ratio by doing so.

[0155] Set value Ip1 according to the NOx concentration in the measured gas SET When changing the NOx concentration in the measurement gas, the set value Ip1 is calculated based on the calculated NOx concentration. SETThat is, in the setting value determination step, it may be determined which of the plurality of setting values ​​to use based on the predicted concentration of the measurement target gas in the measurement gas.

[0156] When the gas to be measured is exhaust gas from an automobile, it is possible to estimate the amount of NOx generated based on conditions such as engine speed, exhaust gas temperature, and hydrocarbon (HC) concentration. For example, a relationship (map) between conditions such as engine speed, exhaust gas temperature, and hydrocarbon (HC) concentration and the NOx concentration in exhaust gas is created. During measurement, information such as engine speed, exhaust gas temperature, and hydrocarbon (HC) concentration is obtained, and it is possible to estimate whether the NOx concentration in the gas to be measured is low or high from the map. In the set value determination step, when the NOx concentration is estimated to be low, the set value Ip1 for low concentration is set. SET (Low) is applied, and when the NOx concentration is estimated to be high, the set value for high concentration Ip1 is applied. SET (High) may be applied.

[0157] In the detection step, the residual oxygen measurement pump cell 41 obtains a detection current value corresponding to the concentration of residual oxygen present in the measurement target gas flow portion 15. That is, the residual oxygen in the measurement target gas is detected as the pump current Ip2.

[0158] The electromotive force V2 detected by the oxygen partial pressure detection sensor cell 82 for controlling the residual oxygen measurement pump is kept at a constant value (set value V2 SET The pump voltage Vp2 of the variable power supply 46 in the residual oxygen measurement pump cell 41 is feedback-controlled so that the set value Vp2 is equal to the set value Vp2 (referred to as "set value Vp2"). Since the electromotive force V2 indicates the oxygen partial pressure in the vicinity of the residual oxygen measurement electrode 44, maintaining the electromotive force V2 constant means maintaining the oxygen partial pressure in the vicinity of the residual oxygen measurement electrode 44 constant. SET The set value V2 can be set as a value at which the amount of oxygen present in the vicinity of the residual oxygen measuring electrode 44 becomes substantially zero. SET For example, the voltage may be set to 350 mV to 500 mV, and preferably to 380 mV to 450 mV.

[0159] The residual oxygen measurement pump cell 41 pumps out the residual oxygen in the measurement gas so that the oxygen partial pressure in the vicinity of the residual oxygen measurement electrode 44 becomes substantially zero. At this time, the pump current Ip2 in the residual oxygen measurement pump cell 41 is detected. The pump current Ip2 is referred to as the detected current value.

[0160] The temperature in the vicinity of the residual oxygen measuring electrode 44 may be kept lower than the temperature in the vicinity of the inner main pump electrode 22. For example, it is preferably 750° C. or lower.

[0161] When the temperature near the residual oxygen measuring electrode 44 is low, the Au contained in the residual oxygen measuring electrode 44 is less likely to evaporate, and the recrystallization of Pt after it oxidizes and evaporates is also suppressed. Therefore, even after long-term use, the decomposition of NOx at the residual oxygen measuring electrode 44 can be suppressed. Therefore, even after long-term use, the residual oxygen measuring electrode 44 can measure only residual oxygen with high accuracy. As a result, durability is further improved.

[0162] In the concentration calculation step, the NOx concentration in the measurement gas is calculated from the detected current value.

[0163] The detected value of the pump current Ip2 (detected current value) corresponds to the amount of residual oxygen in the measurement gas. As described above, the amount of residual oxygen in the measurement gas corresponds to the NOx concentration in the measurement gas. As a result, there is a correlation between the pump current Ip2 and the NOx concentration in the measurement gas. Based on this correlation, the NOx concentration in the measurement gas is calculated from the pump current Ip2 (detected current value).

[0164] The correlation between the pump current Ip2 and the NOx concentration in the measurement gas is determined by the set value Ip1 in the constant current control step described above. SET Therefore, the set value Ip1 SET When a plurality of set values ​​Ip1 are used, SET The correlation between each of these is examined in advance, and the respective set values ​​Ip1SET Based on the correlation according to the above, the NOx concentration in the measurement gas is calculated from the pump current Ip2 (detected current value).

[0165] In the above-described embodiment, the case of detecting NOx has been described. For example, when ammonia (NH3) is used as the measurement target gas, the ammonia (NH3) in the measurement target gas introduced into the internal space 14 may be first oxidized to NO in the main pumping cell 21. Thereafter, by detecting NO as described in the above-described embodiment, ammonia (NH3) can be detected.

[0166] In the above-described embodiment, the inner main pump electrode 22, the measurement target gas decomposition pump electrode 51, and the residual oxygen measuring electrode 44 are arranged in series on the underside of the second solid electrolyte layer 6 facing one internal space 14, from the front end to the rear end of the sensor element 101 in the longitudinal direction of the sensor element 101, but the embodiments of the present invention are not limited to this.

[0167] [Modification 1 of the present invention] The sensor element 201 of Modification 1 has an inner main pump electrode 22 disposed on the underside of the second solid electrolyte layer 6 near the tip of the sensor element 201, facing one internal cavity 14. A measurement target gas decomposition pump electrode 51 and a residual oxygen measuring electrode 44 are disposed in parallel along the longitudinal direction of the sensor element 201 at positions farther from the tip of the sensor element 201 than the inner main pump electrode 22. FIG. 3 shows a schematic planar arrangement of the inner main pump electrode 22, the measurement target gas decomposition pump electrode 51, and the residual oxygen measuring electrode 44 disposed on the underside of the second solid electrolyte layer 6 of the sensor element 201 of Modification 1. Electrode leads are disposed from each of the electrodes toward the rear end of the element, enabling external connection. As in FIG. 2, these electrode leads are not shown in FIG. 3.

[0168] In the sensor element 201 of variant 1, the oxygen concentration near the inner main pump electrode 22 in the internal cavity 14 in the main pump cell 21 is maintained at a predetermined constant value, and the measurement gas with oxygen adjusted to a predetermined concentration simultaneously reaches both the measurement target gas decomposition pump electrode 51 and the residual oxygen measurement electrode 44, which are arranged in parallel.

[0169] In the sensor element 201 of the first modification, the residual oxygen measuring electrode 44 is continuously exposed to gas containing NOx. In such a case, it is conceivable that some of the NOx will be decomposed by the residual oxygen measuring electrode 44. Considering this point, it is considered that a series electrode arrangement, as in the sensor element 101 of the above embodiment, is more preferable.

[0170] [Modification 2 of the present invention] Fig. 4 is a schematic vertical cross-sectional view of the sensor element 301 in the longitudinal direction of the sensor element 301 of Modification 2. In Fig. 4, the same components as those in Fig. 1 are denoted by the same reference numerals, and therefore description thereof will be omitted.

[0171] In the sensor element 301 of the second modification, the inner main pump electrode 22 faces the first internal space 20, the measurement object gas decomposition pump electrode 51 faces the second internal space 40, and the residual oxygen measuring electrode 44 faces the third internal space 61. In other words, one electrode is disposed in each of the separate internal spaces that communicate with each other via a diffusion-controlling section.

[0172] In the sensor element 301 of variant 2, the measured gas flow section 16 is formed by adjacently forming, in the longitudinal direction of the sensor element 301 from the measured gas inlet 10, a first diffusion-controlling section 11, a buffer space 12, a second diffusion-controlling section 13, a first internal cavity 20, a third diffusion-controlling section 30, a second internal cavity 40, a fourth diffusion-controlling section 60, and a third internal cavity 61 in this order in a manner that they are connected to each other.

[0173] In the sensor element 301 of the second modification, the third diffusion rate-controlling portion 30 is provided as two horizontally elongated slits (with the opening having the longitudinal direction perpendicular to the paper in FIG. 3 ), similar to the first diffusion rate-controlling portion 11 and the second diffusion rate-controlling portion 13. The fourth diffusion rate-controlling portion 60 is provided as a single horizontally elongated slit (with the opening having the longitudinal direction perpendicular to the paper in FIG. 3 ) between the spacer layer 5 and the second solid electrolyte layer 6. The third diffusion rate-controlling portion 30 and the fourth diffusion rate-controlling portion 60 may have any shape that provides a desired diffusion resistance, and the shape is not limited to the slits.

[0174] In the sensor element 301 of the second modification, the inner main pump electrode 22 is formed across the upper and lower solid electrolyte layers (the second solid electrolyte layer 6 and the first solid electrolyte layer 4) that define the first internal space 20, and the spacer layer 5 that provides the side walls. Specifically, a ceiling electrode portion 22a is formed on almost the entire lower surface of the second solid electrolyte layer 6 that provides the ceiling surface of the first internal space 20, and a bottom electrode portion 22b is formed on almost the entire upper surface of the first solid electrolyte layer 4 that provides the bottom surface. Side electrode portions (not shown) are formed on the side wall surfaces (inner surfaces) of the spacer layer 5 that configure both side wall portions of the first internal space 20 so as to connect the ceiling electrode portion 22a and the bottom electrode portion 22b, and are arranged in a tunnel-like structure at the locations where the side electrode portions are provided.

[0175] In the sensor element 301 of the second modification, the measurement target gas decomposition pump electrode 51 is disposed in the second internal space 40 in a tunnel-shaped structure similar to the inner main pump electrode 22 disposed in the first internal space 20. That is, a ceiling electrode portion 51a is formed on almost the entire lower surface of the second solid electrolyte layer 6, which provides the ceiling surface of the second internal space 40, and a bottom electrode portion 51b is formed on almost the entire upper surface of the first solid electrolyte layer 4, which provides the bottom surface of the second internal space 40. Side electrodes (not shown) connecting the ceiling electrode portion 51a and the bottom electrode portion 51b are formed on both wall surfaces of the spacer layer 5, which provide the side walls of the second internal space 40, respectively, forming a tunnel-shaped structure.

[0176] In the sensor element 301 of the second modification, the residual oxygen measuring electrode 44 is formed on almost the entire upper surface of the first solid electrolyte layer 4 which provides the bottom surface of the third internal space 61 .

[0177] In the sensor element 301 of the second modification, the adjustment of the oxygen concentration by the main pump cell 21, the decomposition of NOx by the measurement target gas decomposition pump cell 50, and the detection of residual oxygen by the residual oxygen measurement pump cell 41 are all performed in separate internal spaces that are connected via a diffusion-controlling section. This is expected to enable more accurate adjustment of the oxygen concentration in the measurement target gas. As a result, further improvement in measurement accuracy is expected.

[0178] [Modification 3 of the present invention] 5 is a vertical cross-sectional view of a sensor element 401 of Modification 3 taken along the longitudinal direction thereof. Similar to the sensor element 301 of Modification 2, the sensor element 401 of Modification 3 has a configuration in which the adjustment of oxygen concentration by the main pump cell 21, the decomposition of NOx by the measurement target gas decomposition pump cell 50, and the detection of residual oxygen by the residual oxygen measurement pump cell 41 are carried out in separate internal spaces that are connected to each other via diffusion-controlling sections. In FIG. 5, the same components as those in FIG. 4 are designated by the same reference numerals, and therefore further description thereof will be omitted.

[0179] In sensor element 401 of modified example 3, a measurement object gas decomposition pump electrode 51 and an oxygen detection electrode 53 are disposed on the inner surface of measurement gas flow section 16 at a position farther from the longitudinal tip of base section 402 (sensor element 401) than inner main pump electrode 22. Sensor element 401 of modified example 3 includes oxygen detection electrode 53 and reference electrode 42, and also includes an oxygen partial pressure detection cell (oxygen partial pressure detection sensor cell 481 for controlling the measurement object gas decomposition pump in sensor element 401 of modified example 3) that detects the oxygen concentration in the vicinity of measurement object gas decomposition pump electrode 51.

[0180] In the sensor element 401 of the third modification, the measurement object gas decomposition pump electrode 51 is formed on almost the entire lower surface of the second solid electrolyte layer 6 which provides the ceiling surface of the second internal space 40 .

[0181] In the sensor element 401 of the third modified example, the oxygen partial pressure detection sensor cell 481 for controlling the measurement target gas decomposition pump is configured as an electrochemical sensor cell by an oxygen detection electrode 53, a reference electrode 42, a first solid electrolyte layer 4, and a third substrate layer 3.

[0182] The oxygen detection electrode 53 is formed on almost the entire upper surface of the first solid electrolyte layer 4, which provides the bottom surface of the second internal space 40. The oxygen detection electrode 53 is formed as an electrode separate from the measurement target gas decomposition pump electrode 51. In other words, there is no side electrode portion between the measurement target gas decomposition pump electrode 51 and the oxygen detection electrode 53, as in the sensor element 301 of the second modification.

[0183] The oxygen detection electrode 53 is configured as an electrode for detecting the oxygen concentration in the second internal space 40. Like the inner main pump electrode 22, the oxygen detection electrode 53 may be configured to decompose only oxygen without reducing or decomposing the measurement target gas (e.g., NOx) in the measurement target gas. Alternatively, like the measurement target gas decomposition pump electrode 51, the oxygen detection electrode 53 may have catalytic activity to reduce (decompose) the measurement target gas components (such as NOx components) in the measurement target gas. When NOx is decomposed by the oxygen detection electrode 53, the oxygen generated by the decomposition is pumped out by the measurement target gas decomposition pump cell 50.

[0184] 5, the measurement object gas decomposition pump electrode 51 is formed on the ceiling surface of the second internal space 40, and the oxygen detection electrode 53 is formed on the bottom surface of the second internal space 40, but this configuration is not limited thereto. In this manner, the oxygen detection electrode 53 only needs to be provided near the measurement object gas decomposition pump electrode 51 so as to be exposed to substantially the same atmosphere as the measurement object gas decomposition pump electrode 51. For example, the measurement object gas decomposition pump electrode 51 may be formed on the bottom surface of the second internal space 40, and the oxygen detection electrode 53 may be formed on the ceiling surface of the second internal space 40. Alternatively, the measurement object gas decomposition pump electrode 51 and the oxygen detection electrode 53 may be formed in parallel in the longitudinal direction of the sensor element 101 on the ceiling surface or bottom surface of the second internal space 40. Furthermore, the measurement object gas decomposition pump electrode 51 and the oxygen detection electrode 53 may be formed in this order on the ceiling surface or bottom surface of the second internal space 40 in the longitudinal direction from the tip side of the sensor element 101.

[0185] The electromotive force V1a detected by the oxygen partial pressure detection sensor cell 481 for controlling the measurement object gas decomposition pump indicates the oxygen partial pressure in the second internal space 40. In other words, the electromotive force V1a indicates the oxygen partial pressure in the second internal space 40 in a state in which the oxygen partial pressure is controlled by the measurement object gas decomposition pump cell 50.

[0186] 4, in sensor element 301, oxygen partial pressure detection sensor cell 81 for controlling the measurement object gas decomposition pump detects electromotive force V1 between measurement object gas decomposition pump electrode 51 and reference electrode 42. Pump current Ip1 flows through measurement object gas decomposition pump cell 50, which includes measurement object gas decomposition pump electrode 51. Sensor element 101 in FIG. 1 has a similar configuration.

[0187] According to the study by the present inventors, in such a case, the electromotive force V1 between the measurement object gas decomposition pump electrode 51 and the reference electrode 42 is (1) The concentration difference electromotive force V(oxygen) generated by the difference in oxygen concentration between the measurement target gas decomposition pump electrode 51 and the reference electrode 42, (2) the thermoelectric power V (thermal) generated by the temperature difference between the measurement object gas decomposition pump electrode 51 and the reference electrode 42, and (3) The potential difference V(IR) caused by the flow of the pump current Ip1 through the measurement object gas decomposition pump electrode 51, i.e., the potential difference V(IR) caused by the pump current Ip1 and the resistance value of the measurement object gas decomposition pump electrode 51. Furthermore, for example, when oxygen is pumped into the reference electrode 42 to control the reference gas atmosphere near the reference electrode 42, a current also flows through the reference electrode 42. In this case, the electromotive force V1 is considered to further include the potential difference V(IR)' generated by the current flowing through the reference electrode 42 and the resistance value of the reference electrode 42, in addition to the above.

[0188] On the other hand, referring to FIG. 5, in the sensor element 401 of the third modification, the electromotive force V1a in the oxygen partial pressure detection sensor cell 481 for controlling the measurement object gas decomposition pump is detected as the electromotive force between the oxygen detection electrode 53, which is separate from the measurement object gas decomposition pump electrode 51, and the reference electrode 42. Since no current flows through the oxygen detection electrode 53, there is no potential difference corresponding to the above-mentioned (3) potential difference V(IR) in the electromotive force V1a. In other words, the value of the pump current Ip1 flowing through the measurement object gas decomposition pump cell 50 does not affect the electromotive force V1a. For example, even if the pump current Ip1 is increased (set value Ip1) when measuring a high concentration measurement object gas, the electromotive force V1a does not change. SET Even if the value of V1 is set to a large value, the electromotive force V1a is not affected. Therefore, the electromotive force V1a can detect the oxygen partial pressure in the second internal space 40 more accurately. Therefore, the oxygen partial pressure in the second internal space 40, i.e., the residual oxygen concentration in the measurement target gas that reaches the residual oxygen measuring electrode 44, can be controlled more accurately. As a result, the NOx concentration can be measured more accurately. In particular, high measurement accuracy can be maintained even when measuring a measurement target gas with a high concentration.

[0189] [Comparative form] 6 is a schematic vertical cross-sectional view of a sensor element 901 in the longitudinal direction, showing an example of the schematic configuration of a gas sensor 900 of a comparative example. The arrangement of the internal spaces 20, 40, and 61 of the measurement gas flow section 16 is the same as that of the sensor element 301 of Modification 2. In FIG. 6, the same components as those in FIG. 4 are denoted by the same reference numerals, and therefore, description thereof will be omitted.

[0190] The comparative sensor element 901 includes a main pump cell 21 that adjusts the oxygen concentration in the gas to be measured, an auxiliary pump cell 950 that further adjusts the oxygen concentration, and a measurement pump cell 941 that detects the gas to be measured.

[0191] The auxiliary pump cell 950 is an auxiliary electrochemical pump cell including an auxiliary pump electrode 951 having a ceiling electrode portion 951a provided over substantially the entire lower surface of the second solid electrolyte layer 6 facing the second internal space 40, the outer pump electrode 23, and the second solid electrolyte layer 6. The auxiliary pump cell 950 is configured to pump oxygen from the atmosphere in the second internal space 40 by applying a desired voltage Vp3 between the auxiliary pump electrode 951 and the outer pump electrode 23 by a variable power supply 952. The auxiliary pump electrode 951, the reference electrode 42, the second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, and the third substrate layer 3 form an auxiliary pump control oxygen partial pressure detection sensor cell 981.

[0192] The auxiliary pump electrode 951 is configured to decompose only oxygen without reducing or decomposing NOx, similar to the inner main pump electrode 22. The auxiliary pump electrode 951 has a tunnel-like structure in which a ceiling electrode portion 951a is formed on the second solid electrolyte layer 6 that provides the ceiling surface of the second internal space 40, a bottom electrode portion 951b is formed on the first solid electrolyte layer 4 that provides the bottom surface of the second internal space 40, and side electrode portions (not shown) that connect the ceiling electrode portion 951a and the bottom electrode portion 951b are formed on both wall surfaces of the spacer layer 5 that provide the side walls of the second internal space 40.

[0193] The measurement pump cell 941 is an electrochemical pump cell configured with a measurement electrode 944 provided on the upper surface of the first solid electrolyte layer 4 facing the third internal space 61, the outer pump electrode 23, the second solid electrolyte layer 6, the spacer layer 5, and the first solid electrolyte layer 4. The measurement electrode 944, the reference electrode 42, the second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, and the third substrate layer 3 configure an oxygen partial pressure detection sensor cell 982 for controlling the measurement pump.

[0194] The measurement electrode 944 is configured to also function as a NOx reduction catalyst that reduces NOx present in the atmosphere within the third internal space 61. The measurement electrode 944 is formed on almost the entire upper surface of the first solid electrolyte layer 4, which provides the bottom surface of the third internal space 61.

[0195] In the comparative gas sensor 900, the pump current Ip0 is controlled by feedback controlling the pump voltage Vp0 so that the electromotive force V0 in the main pump control oxygen partial pressure detection sensor cell 80 is constant. This allows the oxygen concentration in the vicinity of the inner main pump electrode 22 in the first internal space 20 to be maintained at a predetermined constant value. The main pump current Ip0 in the main pump cell 21 changes depending on the oxygen concentration in the measurement gas.

[0196] Furthermore, the pump voltage Vp3 of the variable power supply 952 in the auxiliary pump cell 950 is feedback-controlled so that the electromotive force V3 in the auxiliary pump control oxygen partial pressure detection sensor cell 981 becomes a predetermined value. As a result, the oxygen partial pressure in the atmosphere in the second internal space 40 is controlled to a low level that does not substantially affect the measurement of NOx. At the same time, the set value of the electromotive force V0 in the main pump control oxygen partial pressure detection sensor cell 80 is set based on the pump current Ip3 so that the pump current Ip3 in the auxiliary pump cell 950 becomes a constant value.

[0197] The measurement gas introduced into the second internal space 40 reaches the measurement electrode 944 in the third internal space 61 through the fourth diffusion-controlling part 60 under conditions where the oxygen partial pressure is controlled. Nitrogen oxides in the measurement gas around the measurement electrode 944 are reduced (2NO → N2 + O2) to generate oxygen. This generated oxygen is then pumped by the measurement pump cell 941, and the voltage Vp4 of the variable power supply 946 is controlled so that the control voltage V4 detected by the measurement pump control oxygen partial pressure detection sensor cell 982 remains constant. Because the amount of oxygen generated around the measurement electrode 944 is proportional to the nitrogen oxide concentration in the measurement gas, the pump current Ip4 in the measurement pump cell 941 is used to calculate the nitrogen oxide concentration in the measurement gas. [Example]

[0198] Examples in which sensor elements were specifically fabricated and tested will be described below as examples, but the present invention is not limited to the following examples.

[0199] [Example 1] As Example 1, a sensor element 101 shown in FIGS. 1 and 2 was produced.

[0200] The electrode pastes used for the electrodes 22, 51, and 44 were prepared as follows. Each electrode paste was prepared by adding a solvent, binder, and dispersant to powders blended so that the weight ratio of the metal component to the ceramic component was 6.0:4.0. ZrO2 was used as the ceramic component for each electrode paste. The metal components of each electrode paste were as follows:

[0201] The electrode paste used for the inner main pump electrode 22 contained Pt and Au as metal components. The concentration of Au relative to the total amount of Pt and Au was 0.80% by weight. The electrode paste used for the measurement target gas decomposition pump electrode 51 contained Pt and Rh as metal components. The concentration of Rh relative to the total amount of Pt and Rh was 50% by weight. The electrode paste used for the residual oxygen measuring electrode 44 contained Pt and Au as metal components, similar to the electrode paste used for the inner main pump electrode 22. The concentration of Au relative to the total amount of Pt and Au was 0.80% by weight.

[0202] Using each of the prepared electrode pastes, the sensor element 101 was prepared according to the above-described method for manufacturing the sensor element 101. A gas sensor 100 incorporating the prepared sensor element 101 was fabricated so that a durability test, which will be described later, could be performed.

[0203] [Example 2] As Example 2, a sensor element 201 shown in FIG. 3 was produced.

[0204] The electrode pastes prepared in Example 1 were used. Each electrode paste was printed on the second solid electrolyte layer 6 so that the measurement target gas decomposition pump electrode 51 and the residual oxygen measurement electrode 44 were arranged in parallel. Except for this, the sensor element was prepared in the same manner as the sensor element 101 of Example 1. A gas sensor incorporating the prepared sensor element 201 was fabricated so that a durability test, which will be described later, could be performed.

[0205] [Example 3] As Example 3, a sensor element 301 shown in FIG. 4 was produced.

[0206] The blank sheet used for the spacer layer 5 had through-holes formed in the internal spaces 20, 40, 61, etc. shown in FIG. 4 . The electrode pastes prepared in Example 1 were used. The electrode paste used for the inner main pump electrode 22 was printed at predetermined positions on the second solid electrolyte layer 6, the first solid electrolyte layer 4, and the spacer layer 5. The electrode paste used for the measurement target gas decomposition pump electrode 51 was printed at predetermined positions on the second solid electrolyte layer 6, the first solid electrolyte layer 4, and the spacer layer 5. The electrode paste used for the residual oxygen measuring electrode 44 was printed at a predetermined position on the first solid electrolyte layer 4. Other than that, the sensor element 301 was prepared in the same manner as the sensor element 101 of Example 1. A gas sensor incorporating the prepared sensor element 301 was prepared so that the durability test described below could be performed.

[0207] [Example 4] As Example 4, a sensor element 401 shown in FIG. 5 was produced.

[0208] The electrode pastes prepared in Example 1 were used. The electrode paste used for the oxygen detection electrode 53 was prepared in the same manner as the electrode pastes prepared in Example 1, except that Pt was used as the metal component. The electrode paste used for the measurement target gas decomposition pump electrode 51 was printed at a predetermined position on the second solid electrolyte layer 6. The electrode paste used for the oxygen detection electrode 53 was printed at a predetermined position on the first solid electrolyte layer 4. Other than that, the sensor element was prepared in the same manner as the sensor element 301 of Example 3. A gas sensor incorporating the prepared sensor element 401 was produced so that the durability test described below could be performed.

[0209] [Comparative Example 1] As a comparative example, a sensor element 901 shown in FIG. 6 was fabricated.

[0210] The electrode paste used for the auxiliary pump electrode 951 was the same as that used for the inner main pump electrode 22. The electrode paste used for the measurement electrode 944 was the same as that used for the measurement target gas decomposition pump electrode 51. Except for this, the sensor element 901 was fabricated in the same manner as the sensor element 301 of Example 3. A gas sensor 900 incorporating the fabricated sensor element 901 was fabricated so that a durability test, which will be described later, could be performed.

[0211] [Durability test] A durability test was conducted using a diesel engine to evaluate the degree of deterioration in NOx detection sensitivity. The NOx sensitivity (Ip2 current value) of the gas sensor at an NO concentration of 500 ppm was measured before and after the durability test, and the rate of change in NOx sensitivity before and after the durability test was calculated. The rate of change in NOx sensitivity was used to evaluate and judge the degree of deterioration in NOx detection sensitivity. Specifically, the test was conducted as follows.

[0212] The measurement of NOx sensitivity and the durability test were carried out while the gas sensor was in operation. The gas sensors of Examples 1 to 4 and Comparative Example 1 were operated with the set values ​​shown in Table 1.

[0213] [Table 1]

[0214] First, the gas sensor of Example 1 was measured in a model gas device. The gas sensor of Example 1 was attached to the measurement pipe of the model gas device. The gas sensor of Example 1 was driven. A model gas with NO=500 ppm and O2=0% was flowed through the measurement pipe, and the Ip2 current value (Ip2 fresh ) was measured. Similarly, for each of Examples 2 to 4 and Comparative Example 1, the Ip2 current value (Ip2 fresh ) was measured. The gas components other than NO and O2 in the model gas used for the measurement were H2O (3%) and N2 (balance).

[0215] Next, a durability test was conducted using a diesel engine. The gas sensors of Examples 1 to 4 and Comparative Example 1 were each attached to the exhaust pipe of an automobile. Then, the gas sensors of Examples 1 to 4 and Comparative Example 1 were driven. Under this condition, a 40-minute driving pattern consisting of an engine speed of 1500 to 3500 rpm and a load torque in the range of 0 to 350 N·m was repeated until 1000 hours had elapsed. The gas temperature at that time was 200°C to 600°C, and the NOx concentration was 0 to 1500 ppm.

[0216] The durability test was temporarily stopped when 500 hours had elapsed since the start of the test, and the gas sensors of Examples 1 to 4 and Comparative Example 1 were taken out. For each of the taken-out gas sensors of Examples 1 to 4 and Comparative Example 1, the Ip2 current value (Ip2 aged500H ) were measured respectively.

[0217] The amount of change in NOx detection sensitivity before and after the durability test was calculated for each of the gas sensors of Examples 1 to 4 and Comparative Example 1. That is, the Ip2 current value (Ip2 fresh ) after 500 hours of durability testing (Ip2 aged500H The rate of change in NOx sensitivity was calculated.

[0218] NOx sensitivity change rate (%) = (Ip2 aged500H / Ip2 fresh -1) x 100

[0219] Ip2 current value after 500 hours of durability test (Ip2 aged500H ), the gas sensors of Examples 1 to 4 and Comparative Example 1 were again attached to the exhaust gas pipe. Then, the durability test using the diesel engine was resumed and continued until the cumulative elapsed time reached 1000 hours.

[0220] After 1000 hours of the durability test, the Ip2 current value (Ip2 fresh) after 1000 hours of durability testing (Ip2 aged1000H The rate of change in NOx sensitivity was calculated.

[0221] Table 2 and Fig. 7 show the durability test results for Examples 1 to 4 and Comparative Example 1. In Fig. 7, the vertical axis of the graph represents the rate of change in NOx sensitivity (%), and the horizontal axis represents the durability test time (hours).

[0222] [Table 2]

[0223] As shown in Table 2 and FIG. 7, it was confirmed that the gas sensors of Examples 1 to 4 were all able to suppress the rate of change (%) in NOx sensitivity compared to the gas sensor 900 of Comparative Example 1.

[0224] As described above, according to the present invention, the measurement target gas concentration can be obtained by removing a portion of the oxygen derived from the measurement target gas in the measurement target gas decomposition pump cell 50, and detecting the residual oxygen at a concentration corresponding to the measurement target gas concentration as a current value in the residual oxygen measurement pump cell 41.

[0225] According to the present invention, even if the Au in the inner main pump electrode 22 evaporates and the evaporated Au adheres to the residual oxygen measuring electrode 44 due to long-term use of the gas sensor in a high-oxygen concentration and high-temperature range, the catalytic activity of the residual oxygen measuring electrode 44 toward oxygen is maintained, and the detection accuracy of the gas sensor does not decrease. Therefore, it is possible to suppress a decrease in the detection accuracy of the gas sensor due to use.

[0226] In this way, it is possible to suppress a decrease in the detection accuracy of the gas sensor due to use. That is, according to the present invention, it is possible to suppress a change over time in the detection value of the gas to be measured, thereby improving durability.

[0227] Furthermore, according to the present invention, the measurement target gas is not directly detected, but is decomposed in the measurement target gas decomposition pump cell 50, and a fixed amount of all oxygen in the measurement target gas, including the oxygen produced by decomposition, is removed. The residual oxygen in the measurement target gas is then detected by the residual oxygen measurement pump cell 41. The amount of oxygen removed in the measurement target gas decomposition pump cell 50 correlates with the value of the pump current Ip1 flowing through the measurement target gas decomposition pump cell 50. Therefore, the range of residual oxygen concentrations reaching the residual oxygen measuring electrode 44 can be adjusted by the value of the pump current Ip1 in the measurement target gas decomposition pump cell 50. As a result, it is possible to accommodate large changes in the concentration of the measurement target gas in the measurement target gas. In this way, according to the present invention, measurement target gases containing a wide concentration range of measurement target gases can be accurately measured.

[0228] The present invention also includes the following embodiments.

[0229] a long plate-shaped substrate including a plurality of stacked oxygen ion conductive solid electrolyte layers; a measurement gas flow section for introducing a measurement gas from one longitudinal end of the base section and allowing the measurement gas to flow therethrough; a main pump cell including an inner main pump electrode disposed on an inner surface of the measurement target gas flow portion and an outer pump electrode disposed on the base portion at a position different from the measurement target gas flow portion and corresponding to the inner main pump electrode; a measurement target gas decomposition pump cell including: a measurement target gas decomposition pump electrode disposed on an inner surface of the measurement target gas flow portion at a position farther from the one longitudinal end of the base portion than the inner main pump electrode; and an outer pump electrode disposed at a position different from the measurement target gas flow portion of the base portion, corresponding to the measurement target gas decomposition pump electrode; a residual oxygen measurement pump cell including: a residual oxygen measurement electrode disposed on an inner surface of the measurement target gas flow portion at a position farther from the one longitudinal end of the base portion than the inner main pump electrode; and an outer pump electrode disposed at a position different from the measurement target gas flow portion of the base portion, corresponding to the residual oxygen measurement electrode; a reference electrode disposed inside the base portion so as to be in contact with a reference gas; Including, the metal material contained in the measurement target gas decomposition pump electrode has catalytic activity for decomposing the measurement target gas, the main pump cell has a function of adjusting the oxygen concentration in the measurement gas flowing into the measurement gas flow section to a predetermined concentration, and obtaining the measurement gas whose oxygen concentration has been adjusted to the predetermined concentration; the measurement object gas decomposition pump cell has a function of decomposing the measurement object gas in the measurement object gas at the measurement object gas decomposition pump electrode, and discharging a predetermined constant amount of all oxygen in the measurement object gas, including oxygen generated by decomposition of the measurement object gas, from the measurement object gas flow portion so that a current value flowing through the measurement object gas decomposition pump cell is kept constant at a preset value; The residual oxygen measuring pump cell is a gas sensor that detects a measurement target gas in a measurement gas and has a function of obtaining a detection current value corresponding to the concentration of residual oxygen present in the measurement gas flow portion. [Explanation of symbols]

[0230] 1 First board layer 2 Second board layer 3 Third board layer 4 First solid electrolyte layer 5 Spacer layer 6 Second solid electrolyte layer 10 Gas inlet 11 First diffusion-controlled section 12 Buffer space 13 Second diffusion-controlled section 14 Internal void 15, 16 Measurement gas flow section 20 1st internal void 21 Main pump cell 22 Inner main pump electrode 22a (Inner main pump electrode) ceiling electrode part 22b Bottom electrode part (of inner main pump electrode) 23 Outer pump electrode 24 Variable power supply (for main pump cell) 30 Third diffusion-controlled section 40 Second internal void 41 Residual oxygen measurement pump cell 42 Reference electrode 43 Reference gas introduction space 44 Residual oxygen measurement electrode 46 Variable power supply (for pump cell for measuring residual oxygen) 48 Atmospheric introduction layer 50 Measurement target gas decomposition pump cell 51 Measurement target gas decomposition pump electrode 51a (of the measurement target gas decomposition pump electrode) ceiling electrode 51b Bottom electrode part (of the measurement target gas decomposition pump electrode) 52 Variable power supply (for the measurement target gas decomposition pump cell) 53 Oxygen sensing electrode 60 4th diffusion-controlled section 61 3rd internal void 70 Heater section 71 Heater electrode 72 Heater 73 through holes 74 Heater insulator 75 Pressure relief hole 80 Oxygen partial pressure detection sensor cell for main pump control 81, 481 Oxygen partial pressure detection sensor cell for measuring gas decomposition pump control 82 Oxygen partial pressure detection sensor cell for pump control for residual oxygen measurement 83 Sensor Cell 100, 900 gas sensor 101, 201, 301, 401, 901 sensor elements 102, 302, 402, 902 Base part 941 Measuring pump cell 944 Measuring electrode 946 Variable power supply (for measuring electrodes) 950 Auxiliary Pump Cell 951 Auxiliary pump electrode 951a (auxiliary pump electrode) ceiling electrode part 951b Bottom electrode part (of auxiliary pump electrode) 952 Variable power supply (for auxiliary pump electrodes) 981 Oxygen partial pressure detection sensor cell for auxiliary pump control 982 Oxygen partial pressure detection sensor cell for measuring pump control

Claims

1. a long plate-shaped substrate including a plurality of stacked oxygen ion conductive solid electrolyte layers; a measurement gas flow section for introducing a measurement gas from one longitudinal end of the base section and allowing the measurement gas to flow therethrough; a main pump cell including an inner main pump electrode disposed on an inner surface of the measurement target gas flow portion and an outer pump electrode disposed on the base portion at a position different from the measurement target gas flow portion and corresponding to the inner main pump electrode; a measurement target gas decomposition pump cell including: a measurement target gas decomposition pump electrode disposed on an inner surface of the measurement target gas flow portion at a position farther from the one longitudinal end of the base portion than the inner main pump electrode; and an outer pump electrode disposed at a position different from the measurement target gas flow portion of the base portion, corresponding to the measurement target gas decomposition pump electrode; a residual oxygen measurement pump cell including: a residual oxygen measurement electrode disposed on an inner surface of the measurement target gas flow portion at a position farther from the one longitudinal end of the base portion than the inner main pump electrode; and an outer pump electrode disposed at a position different from the measurement target gas flow portion of the base portion, corresponding to the residual oxygen measurement electrode; a reference electrode disposed inside the base portion so as to be in contact with a reference gas; Including, a metal material contained in the measurement target gas decomposition pump electrode has catalytic activity for decomposing the measurement target gas; a measuring electrode for measuring the residual oxygen in the measuring gas, the measuring electrode being disposed in series in this order in the longitudinal direction of the base portion at a position on the inner surface of the measuring gas flow portion farther from the one end of the base portion in the longitudinal direction than the inner main pump electrode;

2. 2. The sensor element according to claim 1, wherein the metal material contained in the residual oxygen measuring electrode does not have catalytic activity for decomposing the gas to be measured.

3. the measurement target gas decomposition pump electrode and an oxygen detection electrode are disposed on an inner surface of the measurement target gas flow portion at positions farther from the one longitudinal end of the base portion than the inner main pump electrode, The sensor element according to claim 1 or 2, further comprising an oxygen partial pressure detection cell including the oxygen sensing electrode and the reference electrode.

4. 4. The sensor element according to claim 1, wherein the gas to be measured is NOx.

5. A sensor element according to any one of claims 1 to 4, wherein the metal material contained in the measurement target gas decomposition pump electrode includes at least one metal selected from the group consisting of platinum and rhodium as a metal having catalytic activity for decomposing the measurement target gas.

6. A sensor element as described in any one of claims 1 to 5, wherein the metal material contained in the measurement target gas decomposition pump electrode does not contain gold or contains gold to an extent that does not inhibit the catalytic activity of decomposing the measurement target gas.

7. The sensor element according to any one of claims 1 to 6, wherein the metal material contained in the residual oxygen measuring electrode contains platinum, and contains at least one metal selected from the group consisting of gold and silver as a metal that reduces catalytic activity for decomposing the gas to be measured.

8. the metal material contained in the residual oxygen measuring electrode contains gold, 8. The sensor element according to claim 1, wherein the metal material contains 0.3% by weight or more of gold.

9. A sensor element as described in any one of claims 1 to 8, wherein at least two of the outer pump electrode corresponding to the inner main pump electrode, the outer pump electrode corresponding to the measurement target gas decomposition pump electrode, and the outer pump electrode corresponding to the residual oxygen measurement electrode are formed as an integrated electrode.

10. a long plate-shaped substrate including a plurality of stacked oxygen ion conductive solid electrolyte layers; a measurement gas flow section for introducing a measurement gas from one longitudinal end of the base section and allowing the measurement gas to flow therethrough; a main pump cell including an inner main pump electrode disposed on an inner surface of the measurement target gas flow portion and an outer pump electrode disposed on the base portion at a position different from the measurement target gas flow portion and corresponding to the inner main pump electrode; a measurement target gas decomposition pump cell including: a measurement target gas decomposition pump electrode disposed on an inner surface of the measurement target gas flow portion at a position farther from the one longitudinal end of the base portion than the inner main pump electrode; and an outer pump electrode disposed at a position different from the measurement target gas flow portion of the base portion, corresponding to the measurement target gas decomposition pump electrode; a residual oxygen measurement pump cell including: a residual oxygen measurement electrode disposed on an inner surface of the measurement target gas flow portion at a position farther from the one longitudinal end of the base portion than the inner main pump electrode; and an outer pump electrode disposed at a position different from the measurement target gas flow portion of the base portion, corresponding to the residual oxygen measurement electrode; a reference electrode disposed inside the base portion so as to be in contact with a reference gas; Including, a sensor element in which the metal material contained in the measurement object gas decomposition pump electrode has catalytic activity for decomposing the measurement object gas; an oxygen concentration adjusting step of adjusting the oxygen concentration in the measurement gas flowing into the measurement gas flow section to a predetermined concentration by the main pump cell, thereby obtaining the measurement gas whose oxygen concentration has been adjusted to the predetermined concentration; a current value control step of decomposing the measurement target gas in the measurement target gas at the measurement target gas decomposition pump electrode by the measurement target gas decomposition pump cell, and discharging a predetermined amount of all oxygen in the measurement target gas, including oxygen generated by decomposition of the measurement target gas, from the measurement target gas flow portion so that the current value flowing through the measurement target gas decomposition pump cell is kept constant at a preset value; a detecting step of obtaining a detection current value corresponding to the concentration of residual oxygen present in the measurement target gas flow portion by the residual oxygen measurement pump cell; a concentration calculation step of calculating a concentration of the measurement target gas based on the detected current value; A method for detecting a gas to be measured in a gas to be measured, comprising:

11. 11. The detection method according to claim 10, wherein in the current value control step, the set value of the current value is determined based on a total amount of the measurement target gas that reaches the measurement target gas decomposition pump electrode of the sensor element.

12. the current value control step includes a plurality of set values ​​for the current value, 12. The detection method according to claim 10, wherein the current value control step further comprises a set value determination step of determining which set value to use from among the plurality of set values.

13. 13. The detection method according to claim 12, wherein the step of determining a set value determines which of the plurality of set values ​​to use based on a predicted concentration of the gas to be measured in the measurement target gas. 。

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