Gas sensor device

Abstract

To provide a NOx sensor capable of suppressing the influence of peripheral environment temperature, accurately performing correction based on the moisture content contained in a measurement target gas, and improving the detection accuracy of NOx concentration.SOLUTION: A gas sensor device S comprises a gas sensor element 1, a porous protective layer 11, and a sensor control unit 10. The gas sensor element 1 includes: a measurement target gas chamber 2 into which the measurement target gas is introduced through a diffusion resistance layer; a pump cell 1p which has a pump electrode 21 on a surface of a solid electrolyte body 20 and adjusts the oxygen concentration in the measurement target gas chamber; and a sensor cell 1s which has a sensor electrode 22 on a surface of the solid electrolyte on the downstream side relative to the gas flow, and generates an output corresponding to the concentration of a specific gas. The porous protective layer 11 has an adsorption amount of organic components in the use environment of the gas sensor element 1 within a range of 100 mg / g to 400 mg / g.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a gas sensor device for detecting the concentration of a specific gas in a gas to be measured. [Background technology]

[0002] Gas sensors equipped with solid electrolyte gas sensor elements have been used in the exhaust systems of internal combustion engines, such as vehicle engines. The gas sensor elements include a pump cell for adjusting the oxygen concentration in the measurement gas and a sensor cell for detecting specific gases, such as nitrogen oxides (NOx). Gas sensors are required to start gas detection quickly after startup, and the solid electrolyte bodies and electrodes constituting each cell can be heated to a predetermined activation temperature by a built-in heater or the like.

[0003] Furthermore, Patent Document 1 proposes early activation control using a pump cell, focusing on the fact that sensor output fluctuates due to oxygen (O2) being absorbed into the electrode material while the sensor is stopped. Specifically, a pump cell control unit is provided that sets the control voltage of the pump cell to a removal voltage higher than the normal voltage for a certain period of time prior to gas concentration detection. When the removal voltage is applied, reducing gases such as hydrogen (H2) are generated by decomposition of moisture (H2O) and the like in the measured gas, making it possible to remove the oxygen absorbed in the sensor cell. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-70922 Summary of the Invention [Problem to be solved by the invention]

[0005] Exhaust gas from an internal combustion engine, which is the gas to be measured, contains various substances in addition to the specific gases such as NOx that are the target of detection. Gas sensors generally have a porous protective layer covering the outer surface of the gas sensor element, which is configured to protect the gas sensor element from inorganic or organic poisons that may penetrate into the interior of the gas sensor element. For example, when an internal combustion engine is started or stopped, hydrocarbon (HC)-based organic components are more likely to be emitted, and if they adhere to the sensor cell, they may interfere with the detection of NOx. However, in the lean (air-rich) region, where NOx is more likely to be emitted, the emission of organic components decreases, so the impact of organic components on sensor output has not been studied much in the past.

[0006] Furthermore, it is known that in the lean region, the oxygen concentration in the exhaust gas G increases, which reduces the response of NOx detection. Therefore, conventionally, the pump cell has been controlled to maintain a predetermined low oxygen concentration with respect to the oxygen entering the gas sensor element. Alternatively, in addition to such normal control, early activation control, as described in Patent Document 1, can be employed to further improve the initial response. However, there is a limit to how much the control operation of the gas sensor element can improve the response.

[0007] The present invention has been made in consideration of such problems, and aims to provide a gas sensor device that can suppress the influence of components not to be detected on the sensor output and improve responsiveness in detecting specific gases. [Means for solving the problem]

[0008] One aspect of the present invention is a gas sensor element (1) for detecting a specific gas contained in a measurement gas (G); a porous protective layer (11) for protecting the outer surface of the gas sensor element; a sensor control unit (10) that controls the operation of the gas sensor element and detects the specific gas based on an output of the gas sensor element, The gas sensor element is a diffusion resistance layer (31) disposed inside the porous protective layer; a measurement gas chamber (2) into which the measurement gas is introduced via the diffusion resistance layer; a pump cell (1p) having a pump electrode (21) on a surface of a solid electrolyte body (20) facing the measurement gas chamber, for adjusting the oxygen concentration in the measurement gas chamber; a sensor cell (1s) having a sensor electrode (22) on a surface of the solid electrolyte body downstream of the pump electrode with respect to the gas flow in the measurement gas chamber, and producing an output corresponding to the concentration of the specific gas; The porous protective layer is In the gas sensor device, the amount of adsorption of organic components in the usage environment of the gas sensor element is in the range of 100 mg / g to 400 mg / g. [Effects of the Invention]

[0009] In the gas sensor device having the above configuration, when a measurement gas containing a specific gas to be detected reaches the gas sensor element, poisonous substances including organic components are captured in the porous protective layer formed on the outer surface. The measurement gas passes through the porous protective layer located outside the diffusion resistance layer, which serves as a gas inlet to the interior of the gas sensor element, and diffuses through the interior to reach the measurement gas chamber. To improve the responsiveness of the sensor output, it is generally desirable for the measurement gas to pass through the porous protective layer quickly. However, this also increases the likelihood of poisonous substances reaching the sensor cell, hindering the detection of the specific gas.

[0010] On the other hand, the organic components trapped in the porous protective layer affect the responsiveness of the sensor output. Specifically, it has been found that the responsiveness of the sensor output can be improved when the amount of organic components adsorbed by the porous protective layer is 100 mg / g or more and 400 mg / g or less in the atmosphere in which the gas sensor element is used. The reason for this is not entirely clear, but the presence of a predetermined amount or more of organic components in the porous protective layer promotes reaction with oxygen contained in the measured gas, increasing the amount of oxygen consumed as the gas passes through the porous protective layer. Furthermore, keeping the amount of adsorbed organic components below a predetermined amount suppresses an increase in the diffusion resistance of the flow path leading to the measured gas chamber.

[0011] This allows the poisonous organic components to be trapped in the porous protective layer without impeding the flow of the measurement gas passing through the porous protective layer, consuming oxygen that affects the output of the sensor cell and reducing the amount of oxygen flowing into the measurement gas chamber. This prevents components that are not the target of detection from flowing into the measurement gas chamber and reaching the sensor cell, stabilizing control by the sensor control unit and enabling responsive detection of the specific gas.

[0012] As described above, according to the above aspect, it is possible to provide a gas sensor device that can suppress the influence of components other than the detection target on the sensor output and improve the responsiveness in detecting a specific gas. In addition, the symbols in parentheses in the claims and the means for solving the problems indicate the correspondence with the specific means described in the embodiments described below, and do not limit the technical scope of the present invention. [Brief explanation of the drawings]

[0013] [Figure 1] 1A and 1B are a longitudinal cross-sectional view and a cross-sectional view taken along line II, showing a schematic configuration of a gas sensor element constituting the gas sensor device according to a first embodiment. [Figure 2] 1 is a cross-sectional view showing the configuration of a main part of a gas sensor device and an attached state thereof according to a first embodiment. [Figure 3]1A and 1B are a cross-sectional view showing the overall configuration of a gas sensor device and a schematic configuration diagram of a sensor control unit according to a first embodiment. [Figure 4] FIG. 3 is a graph showing the relationship between the amount of organic components adsorbed in the porous protective layer of the gas sensor element and the responsiveness in the first embodiment. [Figure 5] FIG. 4 is a graph showing the relationship between the amount of organic components adsorbed in the porous protective layer of the gas sensor element and the amount of organic components adsorbed in the sensor electrode, and the responsiveness, in the first embodiment. [Figure 6] 3A and 3B are schematic diagrams for explaining the state of an organic component adsorbed to a porous protective layer of the gas sensor element and the effect thereof in the first embodiment. [Figure 7] FIG. 3 is a graph showing the relationship between the average pore diameter of the porous protective layer of the gas sensor element and the amount of adsorption and responsiveness in the first embodiment. [Figure 8] FIG. 3 is a graph showing the relationship between the film thickness of the porous protective layer of the gas sensor element and the volume and amount of adsorption in the first embodiment. [Figure 9] FIG. 4 is a graph showing the relationship between the film thickness of the sensor electrode of the gas sensor element and the poisoning rate and responsiveness in the first embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0014] (Embodiment 1) A first embodiment of the gas sensor device will be described with reference to the drawings. 2 and 3, the gas sensor device S of this embodiment includes a sensor main body 1A that is installed in an exhaust gas passage EX of an internal combustion engine such as a vehicle engine, and a sensor control unit 10 that is connected to the sensor main body 1A. The sensor main body 1A incorporates a gas sensor element 1 for detecting a specific gas contained in a gas to be measured. The sensor control unit 10 controls the operation of the gas sensor element 1 and detects the specific gas based on its output.

[0015] In this embodiment, the measurement gas is exhaust gas G discharged into an exhaust gas passage EX, and the gas sensor device S can detect, for example, NOx in the exhaust gas G as a specific gas. NOx is a general term for nitrogen oxides, including nitric oxide (NO) and nitrogen dioxide (NO2). In addition to the NOx to be detected, the exhaust gas G also contains various gas components that are not the target of detection, such as atmospheric components and gas components derived from engine combustion.

[0016] As shown enlarged in FIG. 1 , the gas sensor device S includes a porous protective layer 11 that protects the outer surface of a gas sensor element 1. The gas sensor element 1 can be configured, for example, as a solid electrolyte limiting current sensor. Specifically, the gas sensor element 1 includes a diffusion resistance layer 31, a measurement gas chamber 2 into which exhaust gas G is introduced via the diffusion resistance layer 31, and a pump cell 1p and a sensor cell 1s that are disposed in the measurement gas chamber 2. The diffusion resistance layer 31 is disposed inside the porous protective layer 11 and is configured to introduce the exhaust gas G that has passed through the porous protective layer 11 into the measurement gas chamber 2 at a predetermined diffusion resistance.

[0017] The pump cell 1p has a pump electrode 21 on the surface of the solid electrolyte body 20 facing the measurement gas chamber 2, and is configured to adjust the oxygen concentration in the measurement gas chamber 2. The sensor cell 1s has a sensor electrode 22 on the surface of the solid electrolyte body 20 facing the measurement gas chamber 2, and is configured to generate an output corresponding to the NOx concentration in the exhaust gas G. The sensor electrode 22 is disposed downstream of the pump electrode 21 with respect to the gas flow in the measurement gas chamber 2. The solid electrolyte body 20 can be preferably made of a solid electrolyte material such as zirconia.

[0018] At this time, the pump cell 1p is controlled to perform pumping, which decomposes and discharges oxygen at the pump electrode 21, and the exhaust gas G adjusted to a predetermined low oxygen concentration is introduced into the sensor cell 1s located downstream. The sensor electrode 22 outputs a signal based on the oxygen generated by the decomposition of NOx, and the sensor control unit 10 detects the NOx concentration based on the output of the sensor cell 1s. The control of each cell by the sensor control unit 10 will be described in detail later.

[0019] The porous protective layer 11 is formed to cover the outer surface of one end of the gas sensor element 1 and is configured to adsorb poisoning substances, including organic components, contained in the exhaust gas G. This allows poisoning substances that reach the periphery of the gas sensor element 1 to be captured by the porous protective layer 11 and prevent them from penetrating into the element. A portion of the porous protective layer 11 is located in contact with the outside of the diffusion resistance layer 31 and, together with the diffusion resistance layer 31, constitutes the gas inlet 3 to the measurement gas chamber 2.

[0020] In this embodiment, the poisoning substances refer to components derived from fuel, engine oil, etc., contained in the exhaust gas G emitted from the engine combustion chamber, and which inhibit NOx detection when adsorbed onto the sensor electrode 22 in the sensor cell 1s. Among the components not to be detected, typical poisoning substances that are captured by the porous protective layer 11 are known to be compounds such as inorganic oxides and acids containing sulfur (S) and phosphorus (P), or salts thereof. In addition, the exhaust gas G also contains oxygen, water vapor, etc. as other gas components not to be detected.

[0021] In this embodiment, among these poisoning substances, attention is focused on hydrocarbon-based organic components, including hydrocarbons (HC) derived mainly from unburned fuel and compounds such as oxides thereof, and the amount to be adsorbed by the porous protective layer 11 is specified. Specifically, the adsorption amount of organic components in the usage environment of the gas sensor element 1 (hereinafter referred to as protective layer adsorption amount as appropriate) is in the range of 100 mg / g or more and 400 mg / g or less.

[0022] 4, when the gas sensor element 1 is exposed to the environment in which it is used, the protective layer adsorption amount within a specific range contributes to improving responsiveness. Preferably, the porous protective layer 11 is adjusted so that the adsorption amount of organic components is in the range of 130 mg / g or more and 350 mg / g or less. This enhances the effect of improving the responsiveness of the sensor output, allowing for a more rapid and stable sensor output. The relationships shown in FIG. 4 will be described in detail later.

[0023] The organic components in the exhaust gas G, which is the engine combustion gas, generally increase when the engine is started or stopped, are present around the gas sensor element 1, and are adsorbed to the porous protective layer 11 when the sensor operation is stopped or started. During normal operation, the engine enters a lean (excess air) region where NOx is likely to be emitted, and the amount of oxygen, which is not a target gas for detection, increases, while the emission of organic components decreases. In this case, the amount of organic components adsorbed in the porous protective layer 11 remains almost constant during the operating period after the start of sensor operation, and is determined depending on the structure of the porous protective layer 11.

[0024] At this time, the organic components in the exhaust gas G in the porous protective layer 11 react with oxygen in the exhaust gas G introduced into the porous protective layer 11, thereby reducing the amount of oxygen passing through the diffusion resistance layer 31. Therefore, when the amount of oxygen consumed for the oxidation of the organic components increases, in other words, when the amount of adsorption of the organic components becomes sufficient to consume oxygen, the amount of oxygen introduced into the measurement gas chamber 2 becomes sufficiently small, and the effect on the sensor output is thought to be suppressed. However, since the diffusion resistance in the porous protective layer 11 increases as the amount of adsorption of the organic components increases, it is desirable to form the porous protective layer 11 so that the amount of adsorption falls within a range in which a desired responsiveness is obtained.

[0025] Preferably, the gas sensor device S can include an early activation control unit 101 in the sensor control unit 10. The early activation control unit 101 performs early activation control, which controls the voltage applied to the pump cell 1p at the start of sensor operation to a starting voltage V2 higher than the normal control voltage V1. By performing such early activation control prior to the normal detection control, reducing gas is supplied to the sensor electrode 22, allowing the stored oxygen to be quickly removed. Furthermore, even if organic components diffuse through the gas inlet 3 and are adsorbed on the sensor electrode 22, the early activation control facilitates removal of the organic components, thereby improving responsiveness.

[0026] Preferably, as shown in FIGS. 4 and 5, during the sensor operation period, the amount of organic components adsorbed on the sensor electrode 22 (hereinafter referred to as the electrode adsorption amount) is equal to or less than the normal electrode area of ​​the sensor electrode 22 (for example, 1.0 mm 2 It is desirable to maintain the adsorption amount of the organic components in the porous protective layer 11 at a level of 0.1 mg or less (approximately or less than that). In this case, by adjusting the amount of adsorption of the organic components in the porous protective layer 11, together with the amount of adsorption of the protective layer, to fall within the above-mentioned predetermined range, the responsiveness of the sensor output can be improved. On the other hand, it has been found that when the amount of adsorption of the organic components in the sensor electrode 22 increases beyond 0.1 mg, the effect of the protective layer adsorption amount in improving responsiveness tends to decrease. Therefore, it is desirable to adjust the protective layer adsorption amount and the electrode adsorption amount to fall within appropriate ranges.

[0027] 5 is a test comparison of the amount of organic components adsorbed on the sensor electrode 22. When the gas sensor device S is in a normal operating state, the sensor control unit 10 controls the operation of the gas sensor element 1, thereby maintaining the amount of organic components adsorbed on the sensor electrode 22 at or below a predetermined value at which the effect of improving responsiveness can be obtained.

[0028] Preferably, the amount of organic components adsorbed in the porous protective layer 11 is adjusted by the average pore diameter and layer thickness of the porous protective layer 11. For example, the particle size and composition of the raw material particles and additives are adjusted so as to obtain an average pore diameter capable of capturing organic components in an expected molecular weight range, and the layer thickness is further increased or decreased to obtain the porous protective layer 11 with the desired adsorption amount. The average pore diameter can be, for example, the average value of pore diameters measured by observing the cross section of the porous protective layer 11 using a microscope. The layer thickness can be formed to a desired uniform thickness, for example, by a dipping method described below.

[0029] In this way, by appropriately adjusting the amount of organic components adsorbed in the porous protective layer 11 and further in the sensor electrode 22 in the environment in which the gas sensor element 1 is used, it is possible to suppress the influence of gas components that are not the detection target and improve the responsiveness of NOx detection. The configuration of the gas sensor device S and an example of control of the gas sensor element 1 by the sensor control unit 10 will be described in detail below.

[0030] (Overall configuration of gas sensor device S) 2, the sensor main body 1A of the gas sensor device S has a cylindrical housing S1, a gas sensor element 1 inserted and held inside the housing S1, an element cover S2, and an atmosphere cover S3. The gas sensor element 1 has its both ends protruding outward from the housing S1, with the axial direction of the housing S1 being the longitudinal direction X (i.e., the vertical direction in FIG. 2). An element cover S2 and an atmosphere cover S3 are fixed to both ends of the housing S1, respectively. The element cover S2 is arranged to cover the outer periphery of the tip end side (i.e., the lower end side in FIG. 2) of the gas sensor element 1, and the atmosphere cover S3 is arranged to cover the outer periphery of the base end side (i.e., the upper end side in FIG. 2) of the gas sensor element 1.

[0031] In this embodiment, the gas sensor device S is configured as a NOx sensor device and is incorporated, for example, in a NOx purification system installed in an exhaust system to detect the NOx concentration in exhaust gas G. The sensor main body 1A has an outer circumferential thread portion of a housing S1 fixed to a passage wall EX1 of the exhaust gas passage EX, and the tip side of the gas sensor element 1 is housed inside an element cover S2 and is positioned to protrude into the exhaust gas passage EX.

[0032] The element cover S2 has a double-tube structure with a bottom surface to prevent, for example, water damage. A plurality of side gas circulation holes S21 are provided on the side surfaces of the outer and inner covers, and a bottom gas circulation hole S22 is further provided on the bottom surface of the inner cover. The cylindrical atmosphere cover S3 also has a plurality of gas circulation holes (not shown) on its side surface. This allows the exhaust gas G flowing through the exhaust gas passage EX to be taken into the element cover S2, where moisture and other components are separated as the gas passes through the flow path between the outer and inner covers. Some of the separated gas reaches the tip side of the gas sensor element 1.

[0033] 3, the base end side of the gas sensor element 1 is housed inside the atmosphere cover S3 that protrudes to the outside of the exhaust gas passage EX. As a result, the atmosphere A taken into the atmosphere cover S3 reaches the base end side of the gas sensor element 1 as a reference gas.

[0034] The sensor main body 1A is electrically connected via lead wires S4 to a sensor control unit 10 located outside the exhaust gas passage EX. The sensor control unit 10 incorporates terminals connected to various parts of the gas sensor element 1, a communication unit for transmitting and receiving signals, a microcomputer, etc., and controls the entire gas sensor device S. It is also capable of receiving control commands from an engine control unit (ECU) (not shown) or transmitting detection results. The ECU determines the operating state of the engine based on information input from various sensors mounted on the engine, and controls the entire vehicle.

[0035] (Configuration of gas sensor element 1) In Fig. 1 (upper diagram), the gas sensor element 1 is a stacked element with a three-cell structure, and includes a pump cell 1p, a sensor cell 1s, and a monitor cell 1m. In Fig. 1 (lower diagram), the gas sensor element 1 contains a measurement gas chamber 2 into which exhaust gas G is introduced and a reference gas chamber 4 into which air A as a reference gas is introduced, each of which has a solid electrolyte body 20 as a chamber wall. The gas sensor element 1 also contains a built-in heater H, which can heat each cell to a temperature suitable for detection.

[0036] A porous protective layer 11 of approximately uniform thickness is formed on the outside of the gas sensor element 1, covering the entire outer surface at the tip end. A diffusion resistance layer 31 is embedded in the end face at the tip end of the gas sensor element 1, and a part of the porous protective layer 11 in contact with the outside thereof, together with the diffusion resistance layer 31, constitutes the gas inlet 3. As a result, the exhaust gas G that has reached the tip end of the gas sensor element 1 passes through the part of the porous protective layer 11 that constitutes the gas inlet 3, diffuses through the inner diffusion resistance layer 31, and is taken into the measurement gas chamber 2.

[0037] The gas sensor element 1 has a stacking direction perpendicular to the gas flow direction X (i.e., the vertical direction in the lower diagram of FIG. 1), and includes a shielding layer 13, a spacer layer 12, a solid electrolyte body 20, and a heater insulating layer 51 arranged in this order. The spacer layer 12 and the shielding layer 13 are arranged on one surface of the solid electrolyte body 20, forming a space that will become the measurement gas chamber 2. On the other surface of the solid electrolyte body 20, a space that will become the reference gas chamber 4 is formed between the solid electrolyte body 20 and the heater insulating layer 51 that constitutes the heater H.

[0038] The pump cell 1p, sensor cell 1s, and monitor cell 1m each have a pair of electrodes facing each other with a solid electrolyte body 20 sandwiched therebetween. In the measurement gas chamber 2, a pump electrode 21 of the pump cell 1p is disposed on the upstream side of the surface of the solid electrolyte body 20 in the gas flow direction X, and a sensor electrode 22 of the sensor cell 1s and a monitor electrode 23 of the monitor cell 1m are disposed downstream of the pump electrode 21. In the reference gas chamber 4, a common reference electrode 41 is disposed on the surface of the solid electrolyte body 20 at a position opposite to the electrodes 21, 22, and 23.

[0039] The reference gas chamber 4, in which the common reference electrode 41 is disposed, extends to the base end side of the gas sensor element 1 (i.e., the right end side in the lower view of FIG. 1), and communicates with the space inside the atmosphere cover S3 (see FIG. 3, for example), in which the atmosphere A is present, via a reference gas inlet opening at the end face (not shown). A heating element 52 that generates heat when electricity is applied is embedded inside a heater insulating layer 51 that forms the bottom wall of the reference gas chamber 4, thereby constituting a heater H. The heating element 52 is disposed corresponding to the positions where the electrodes of the pump cell 1p, sensor cell 1s, and monitor cell 1m are formed, and is capable of heating the entire tip side of the element, which serves as the gas detection section.

[0040] As a result, one electrode 21, 22, 23 of the pair of electrodes of each cell is exposed to the exhaust gas G introduced into the measurement gas chamber 2, and the other electrode 41 is exposed to the atmosphere A introduced into the reference gas chamber 4. At this time, by applying a voltage between the pair of electrodes of each cell, oxygen pumping is possible, in which oxygen contained in the exhaust gas G is pumped into or out of the reference gas chamber 4 through the solid electrolyte body 20 having oxide ion conductivity. In addition, by operating the heater H, each cell can be heated to a temperature above the activation temperature, thereby enabling stable oxygen pumping.

[0041] A predetermined voltage can be applied to each cell of the gas sensor element 1 from the sensor control unit 10 via a common terminal COM+ connected to the common reference electrode 41. A detection terminal P- is connected to the pump electrode 21 of the pump cell 1p, and a detection terminal S- and a detection terminal M- are connected to the sensor electrode 22 and the monitor electrode 23, respectively, so as to detect the current output from each cell. Both ends of the heating element 52 of the heater H are connected to a pair of heater terminals H+ and H-.

[0042] In the measurement gas chamber 2, the pump electrode 21 of the pump cell 1p is formed with a large area on the upstream side in the gas flow direction X. When the exhaust gas G that has passed through the diffusion resistance layer 31 comes into contact with the pump electrode 21, oxygen is converted into oxide ions (O 2- ), which permeates the solid electrolyte body 20 and is discharged to the reference electrode 41 side. The exhaust gas G adjusted to a low oxygen concentration by the pump cell 1p reaches the downstream sensor electrode 22 and monitor electrode 23, and at the sensor electrode 22, oxygen resulting from NOx is discharged by oxygen pumping together with the remaining oxygen in the exhaust gas G. At the monitor electrode 23, the remaining oxygen in the exhaust gas G is discharged by oxygen pumping.

[0043] The sensor electrode 22 and the monitor electrode 23 are arranged side by side at the same position in the gas flow direction X, and are formed to have the same shape and smaller area than the pump electrode 21. This puts the sensor electrode 22 and the monitor electrode 23 under the same conditions with respect to the flow of exhaust gas G, and by comparing the outputs of the sensor cell 1s and the monitor cell 1m, the influence of the residual oxygen contained in the NOx output can be eliminated.

[0044] The solid electrolyte body 20 is made of a zirconia-based solid electrolyte material having oxide ion conductivity. For example, stabilized zirconia containing a stabilizer such as yttria can be used as the zirconia-based solid electrolyte material. The diffusion resistance layer 31 and the porous protective layer 11 are made of a porous ceramic material such as alumina. The heater insulating layer 51, the shielding layer 13, and the spacer layer 12 can be made of an insulating ceramic material such as alumina.

[0045] The electrodes of the pump cell 1p, the sensor cell 1s, and the monitor cell 1m may be porous cermet electrodes containing a precious metal or a precious metal alloy material and a zirconia-based solid electrolyte. The sensor electrode 22 of the sensor cell 1s is made of an electrode material that has decomposition activity for the NOx to be detected. For example, an electrode containing platinum and rhodium (hereinafter referred to as a Pt-Rh electrode) may be used as such an electrode.

[0046] The pump electrode 21 of the pump cell 1p is made of an electrode material that has oxygen decomposition activity but does not have NOx decomposition activity. For example, an electrode containing platinum and gold (hereinafter referred to as a Pt-Au electrode) can be used as such an electrode. The monitor electrode 23 of the monitor cell 1m is also made of the same electrode material as the pump electrode 21. The reference electrode 41 can be made of an electrode containing platinum (hereinafter referred to as a Pt electrode).

[0047] By heating the gas sensor element 1 to a predetermined temperature with the heater H and applying a predetermined voltage between the electrodes of each cell, the sensor cell 1s can detect NOx in the exhaust gas G, which has been adjusted to a predetermined low oxygen concentration by the pump cell 1p. Furthermore, by utilizing the difference in gas adsorption properties between the Pt-Rh electrode used in the sensor electrode 22 and the Pt-Au electrode used in the monitor electrode 23, the difference in current output from the sensor cell 1s and the monitor cell 1m can be used as the NOx output, thereby canceling the effect of oxygen remaining in the exhaust gas G.

[0048] (Configuration of sensor control unit 10) 3, the sensor control unit 10 includes an early activation control unit 101 that performs early activation control when the sensor is started, a detection control unit 102 that performs NOx detection control, and a heater control unit 103 that controls energization of the heater H. When the sensor control unit 10 receives a control command from the ECU and starts a detection operation using the gas sensor element 1, the heater control unit 103 first starts controlling energization to the heater H. Then, the early activation control unit 101 starts early activation control, and thereafter the detection control unit 102 performs normal detection control.

[0049] Specifically, the detection control unit 102 includes a pump cell control unit 102A, a sensor cell detection unit 102B, and a monitor cell detection unit 102C. The pump cell control unit 102A controls the voltage applied to the pump cell 1p via a common terminal COM+ (see, for example, FIG. 1) during normal operation and detects the current output by the pump cell 1p. The sensor cell detection unit 102B detects the current output by the sensor cell 1s, and the monitor cell detection unit 102C detects the current output by the monitor cell 1m.

[0050] The detection control unit 102 calculates the NOx concentration based on the pump cell current Ip, the sensor cell current Is, and the monitor cell current Im, which are detected via the detection terminals P-, S-, and M- of each cell (see, for example, FIG. 1). Specifically, the detection control unit 102 calculates the difference between the sensor cell current Is and the monitor cell current Im (Is-Im), and references a pre-stored map or the like to calculate the NOx concentration.

[0051] The heater control unit 103 includes, for example, a switch circuit connected to the heater terminals H+ and H-, and controls the power supply to the heating element 52 of the heater H to heat each cell of the gas sensor element 1 to a predetermined activation temperature or higher. Furthermore, the heater control unit 103 can control the amount of power supplied based on the detected impedance by utilizing the temperature characteristics of the impedance of each cell. This allows the heater H to control heating so that each cell is maintained at a predetermined temperature suitable for NOx detection.

[0052] Before the gas sensor device S starts its detection operation, the oxygen concentration of the exhaust gas G introduced into the measurement gas chamber 2 tends to be high. Furthermore, the Pt—Rh electrode used for the sensor electrode 21 of the sensor cell 1s tends to store oxygen. Therefore, when starting the sensor, it is desirable to quickly adjust the oxygen concentration to a predetermined low level and remove the stored oxygen to quickly activate the sensor.

[0053] Therefore, when the sensor is started, the early activation control unit 101 performs early activation control in addition to the temperature rise control of the gas sensor element 1. In the early activation control, a starting voltage V2 higher than the normal control voltage V1 is applied to the pair of electrodes 21, 41 of the pump cell 1p. This generates reducing gas and supplies it to the sensor electrode 21 of the sensor cell 1s, quickly establishing a state suitable for normal detection operation and improving initial responsiveness.

[0054] As mentioned above, when the sensor starts up, hydrocarbon (HC)-based organic components are emitted and easily infiltrate the interior of the gas sensor element 1. If such organic components adhere to the sensor electrode 21 of the sensor cell 1s, they can become poisonous substances that inhibit electrode activity. Herein, the organic components contained in the exhaust gas G include, for example, saturated or unsaturated HCs derived from unburned fuel, engine oil, etc., which can be emitted into the exhaust system together with the combustion gas, as well as compounds such as their oxides. Hereinafter, these hydrocarbon-based compounds will be collectively referred to as HC components.

[0055] HC components include a variety of compounds ranging from those with a small carbon number (i.e., small molecular weight) to those with a large carbon number (i.e., large molecular weight). HC components are captured by the porous protective layer 11, but components with a relatively small molecular weight (e.g., molecular weight of about 100 or less) may pass through the gas inlet 3 and reach the sensor electrode 21. Even in this case, by keeping the amount of HC components adsorbed in the porous protective layer 11 within a predetermined range, the amount of HC components adsorbed in the sensor electrode 21 is appropriately suppressed, and in combination with early activation control, the effect on the detection operation can be suppressed.

[0056] In the early activation control, a starting voltage V2 higher than the normal control voltage V1 is applied to the pair of electrodes 21, 41 of the pump cell 1p. The starting voltage V2 is set to a voltage that allows oxygen in the exhaust gas G to decompose and water vapor to decompose on the pump electrode 21. Specifically, the control voltage V1 is determined (e.g., 0.3 V to 0.4 V) so as to exhibit a limiting current characteristic in which the current flowing through the pump cell 1p changes little with respect to the applied voltage, and the starting voltage V2 is set (e.g., 0.5 V to 2 V) higher than the voltage value that exhibits the limiting current characteristic.

[0057] At this time, decomposition of water vapor in the measurement gas chamber 2 generates hydrogen, a reducing gas, which is supplied to the sensor cell 1s located downstream in the gas flow. As a result, the oxygen occluded in the sensor electrode 22 is reduced by the hydrogen to form water, which is then removed. Furthermore, as the temperature of the sensor electrode 22 increases, the adsorbed HC components become more easily desorbed, and are oxidized and removed by oxygen in the atmosphere. Alternatively, oxide ions are conducted from the reference electrode 41 side in the atmospheric air, which causes the HC components to be oxidized and removed.

[0058] By repeatedly performing this early activation control, the amount of HC components adsorbed on the sensor electrode 22 is maintained at or below the predetermined value shown in Fig. 4, thereby suppressing the effect on NOx detection. Furthermore, when the amount of HC components adsorbed on the porous protective layer 11 is within a predetermined range, the reaction between the HC components and oxygen in the exhaust gas G passing through the porous protective layer 11 can reduce the amount of oxygen introduced into the measurement gas chamber 2. In this way, gas components that are not to be detected are prevented from reaching the sensor electrode 22, thereby further improving the responsiveness of NOx detection.

[0059] (Test example) The relationship between the amount of adsorption of HC components and responsiveness shown in Fig. 4 was investigated by carrying out the following test. First, for the gas sensor element 1 having the above configuration, a number of test elements were prepared in which the average pore diameter and layer thickness of the porous protective layer 11 were adjusted, and after HC components were adsorbed, the responsiveness of the output to NOx was evaluated.

[0060] The test element was manufactured by firing a laminate of ceramic sheets that would become the shielding layer 13, spacer layer 12, solid electrolyte body 20, and heater insulating layer 51, and then forming the porous protective layer 11. A hole that would become the measurement gas chamber 2 was previously formed in the spacer layer 12, and a porous material that would become the diffusion resistance layer 31 was placed at a predetermined position on the tip side facing the hole. The pump electrode 21, sensor electrode 22, monitor electrode 23, and common reference electrode 41 were previously formed at predetermined positions on the solid electrolyte body 20 using electrode paste. The heater insulating layer 51 was also constructed by previously stacking multiple ceramic sheets with the heating element 52 sandwiched between them.

[0061] The porous protective layer 11 was formed by a dipping method on the entire outer surface of the obtained sintered body, which was to be the tip side. Specifically, a raw material slurry containing alumina particles was prepared as the raw material for the porous protective layer 11, and a predetermined area on the tip side of the gas sensor element 1 was immersed in a dipping tank containing the raw material slurry and then pulled out repeatedly to adjust the layer thickness to the desired thickness. The raw material slurry was prepared by adding a pore-forming material to the raw material alumina particles, and dispersing them together with a binder in a dispersion medium to form a slurry. The pore-forming material is carbon or resin beads, etc., and the binder can be an inorganic binder such as alumina sol, or an organic binder.

[0062] In the immersion step, the gas sensor element 1 was lifted out of the dipping bath, then inverted and left to stand, and air-dried. This process was repeated several times (e.g., about three times) to adjust the thickness of the layer to a uniform thickness. After that, a drying step (e.g., 45°C) and a baking step (e.g., 900°C) were performed to form the porous protective layer 11 on the outer surface of the tip side of the gas sensor element 1.

[0063] In this case, the porosity and volume of the porous protective layer 11 can be changed by adjusting the average pore diameter and layer thickness, thereby changing the surface area of ​​the flow path that becomes the gas inlet section 3. For example, the average pore diameter (e.g., 5 μm to 10 μm) can be adjusted by adjusting the particle size of the raw alumina particles and the particle size and ratio of the added pore-forming material, and the layer thickness (e.g., 200 μm to 1500 μm) can be adjusted by adjusting the viscosity of the raw material slurry and the number of immersions. In this way, various test elements with adjusted average pore diameters and layer thicknesses were produced, and the amount of HC components that could be adsorbed when exhaust gas G passed through them was changed.

[0064] The adsorption of HC components onto the test element was performed using a test gas simulating exhaust gas G. Specifically, the test element was exposed to the test gas in a desiccator and stored for one hour to allow the HC components to be adsorbed. Then, using an evaluation test device in which the test element was attached to a gas flow path equipped with a gas supply unit, an evaluation gas containing NOx was passed through to evaluate the responsiveness. Specifically, using the sensor control unit 10, early activation control was performed in the atmosphere, and then NO was introduced to a predetermined concentration, and detection control was performed in an NO-containing atmosphere. The output value of the test element was measured, and the time (unit: seconds) until the actual NO concentration was indicated was defined as the responsiveness.

[0065] The test gas was a gas containing mainly HC components with a molecular weight of 100 to 300, and the HC components included, for example, saturated HC, unsaturated HC, and HC oxides formed by partial oxidation of these. The amount of HC components adsorbed in the porous protective layer 11 was measured by collecting the material (3.0 g) of the porous protective layer 11 from the test element, followed by temperature-programmed desorption of the adsorbed gas, using a mass spectrometer. Based on these results, the relationship between the amount of HC components adsorbed per unit mass (mg / g) in the porous protective layer 11 and the responsiveness of the sensor output is shown in Figure 4 (Test Example 1).

[0066] The effect of the amount of electrode adsorption on the responsiveness of the sensor output was also investigated within the range of protective layer adsorption amounts shown in Fig. 4. Specifically, after the test element was exposed to the test gas, a waiting period was set before the responsiveness was evaluated, and adjustments were made so that some of the HC components adsorbed on the porous protective layer 11 could diffuse through the porous protective layer 11 and the diffusion resistance layer 31 and reach the sensor electrode 22. Test elements were prepared with different amounts of electrode adsorption within the range shown in Fig. 5 by varying the waiting period.

[0067] The sensor output responsiveness was then investigated in the same manner as in the test element shown in FIG. 4 . The results are shown in FIG. 5 (Test Examples 2 and 3) along with the results shown in FIG. 4 (Test Example 1). The amount of electrode adsorption was evaluated by measuring the amount of adsorbed material on the surface of the sensor electrode 22 using infrared spectroscopy. In FIG. 5 , the amount of electrode adsorption in Test Examples 2 and 3 was determined by adjusting the standing period to a period sufficient for some of the HC components adsorbed on the porous protective layer 11 to diffuse through the porous protective layer 11 and the diffusion resistance layer 31 and move into the measurement gas chamber 2. The standing period in Test Example 3 was longer than that in Test Example 2, so that the amount of electrode adsorption in Test Example 3 was greater than that in Test Example 2. The standing periods in Test Examples 2 and 3 were sufficiently longer than the period during which an engine is normally stopped. Therefore, when the sensor is repeatedly operated together with the engine under normal operating conditions, the amount of electrode adsorption is maintained within the range of that in Test Example 1.

[0068] In Figure 4, the output responsiveness to NOx is significantly improved when the amount of HC components adsorbed in the porous protective layer 11 is within a specific range. That is, in the range where the protective layer adsorption amount is relatively small, the responsiveness improves as the adsorption amount increases, but the change is small. As the protective layer adsorption amount increases further, the responsiveness changes sharply, and high responsiveness is obtained in the range of 100 mg / g to 400 mg / g, and a nearly constant high responsiveness is shown in the range of 130 mg / g to 350 mg / g. When the protective layer adsorption amount increases beyond this range, the responsiveness again changes sharply in the decreasing direction, falling to the same level as in the range where the protective layer adsorption amount is small.

[0069] In Test Example 1 of FIG. 4, the amount of HC components adsorbed on the surface of the sensor electrode 22 measured based on the infrared spectroscopy spectrum was 0.1 mg (electrode area: 0.94 mm 2 5 were 0.5 mg and 1.0 mg, respectively. From the results of infrared spectroscopy analysis of the material adsorbed on the sensor electrode 22, it was confirmed that the HC components that can reach the sensor electrode 22 are relatively low molecular weight HC components with a molecular weight of about 100 or less (for example, a molecular weight of about 16 to 114), and that the HC components that poison the sensor electrode 22 are oxygen-containing HCs with molecular weights of 53 to 56 and 67 to 91.

[0070] The results of Figures 4 and 5 show that in the gas sensor S, the outer surface of the gas sensor element 1 is protected by a porous protective layer 11 with an HC component adsorption amount within a predetermined range, and the gas sensor S is in a normal operating state, and an improved responsiveness can be achieved. That is, when the gas sensor element 1 is controlled by the sensor control unit 10, the electrode adsorption amount is maintained at a predetermined value or less (≦0.1 mg), as in Test Example 1, and the responsiveness of NOx detection can be improved. In contrast, when the electrode adsorption amount exceeds the predetermined value (>0.5 mg), as in Test Example 3, due to a longer than normal storage period, the responsiveness cannot be sufficiently improved even if the HC component adsorption amount is within the predetermined range. Furthermore, as in Test Example 2, the improved responsiveness tends to be reduced when the electrode adsorption amount is between these values ​​(more than 0.1 mg and 0.5 mg or less).

[0071] As shown in Figure 6 (lower diagram), when the gas sensor S is started, HC components present around the porous protective layer 11 penetrate into the layer through pores (not shown) opening to the outer surface of the porous protective layer 11 and are adsorbed onto the inner surface of the pores. The same is true on the outside of the diffusion resistance layer 31 leading to the measurement gas chamber 2; HC components with a molecular weight large enough to penetrate the pores (e.g., a molecular weight of about 300 to 400 or less) are easily captured depending on the average pore diameter and pore size distribution. As a result, a predetermined amount of HC components depending on the structure of the porous protective layer 11 is quickly adsorbed and held by the porous protective layer 11, and migration to the diffusion resistance layer 31, which has a higher diffusion resistance, is prevented.

[0072] On the other hand, the amount of oxygen contained in the exhaust gas G in the gas inlet 3 leading to the measurement gas chamber 2 is small at the time of sensor startup, and the oxygen reacts relatively easily with the surrounding HC components or the HC components adsorbed in the pores and is reduced and removed while passing through the porous protective layer 11. Even during subsequent normal operation, a predetermined amount of HC components is adsorbed in the porous protective layer 11, so oxygen is consumed to oxidize the HC components before they reach the measurement gas chamber 2. In other words, the amount of oxygen itself in the measurement gas chamber 2 can be reduced, and the deterioration of responsiveness due to the influence of oxygen is suppressed in the early activation control by the sensor control unit 10 and in the normal detection control, thereby achieving a higher responsiveness than conventional methods.

[0073] 6 (upper diagram), when the amount of HC components in the porous protective layer 11 is below the predetermined range, it is considered that the amount of HC components that can react with oxygen in the exhaust gas G introduced into the gas inlet 3 is insufficient. As a result, the amount of oxidation reaction in the porous protective layer 11 decreases, and the effect of reducing the amount of oxygen flowing into the measurement gas chamber 2 cannot be obtained. Furthermore, when the amount of HC components adsorbed exceeds the predetermined range, it is considered that the diffusion resistance in the porous protective layer 11 becomes larger than that of the diffusion resistance layer 31. Therefore, the diffusion of not only oxygen but also NOx, which is the detection target, is suppressed, making it difficult for them to pass through the gas inlet 3, resulting in a decrease in responsiveness.

[0074] Here, the porous protective layer 11 may have any configuration as long as the pores formed therein communicate with each other to allow the flow of exhaust gas G and the amount of HC components adsorbed is within a predetermined range. Generally, as shown in Figure 7 (left diagram), if the average pore diameter of the porous protective layer 11 (hereinafter referred to as protective layer pore diameter as appropriate) is small, HC components with large molecular weights cannot penetrate inside, and the amount of adsorption is unlikely to increase. Therefore, it is desirable to set the average pore diameter of the porous protective layer 11 to a certain extent large so that the amount of protective layer adsorption is equal to or greater than a predetermined lower limit within a typical layer thickness range.

[0075] As a result, as shown in Figure 7 (right), as the pore diameter of the protective layer increases, the amount of protective layer adsorption increases, making it possible to improve responsiveness. Preferably, the average pore diameter of the porous protective layer 11 is 0.4 µm or more, which makes it easier to capture HC components with relatively large molecular weights. There is no upper limit to the average pore diameter, but it is desirable that it be a value that is sufficiently small relative to the layer thickness of the porous protective layer 11, and can be, for example, 10 µm or less.

[0076] Furthermore, as shown in FIG. 8 (right), as the thickness of the porous protective layer 11 (hereinafter referred to as protective layer thickness) increases, the volume of the porous protective layer 11 (hereinafter referred to as protective layer volume) increases cubically. At this time, as shown in FIG. 8 (left), the amount of HC components adsorbed also increases with the increase in protective layer volume. Therefore, the protective layer thickness can be adjusted according to the average pore diameter of the porous protective layer 11, or the average pore diameter can be adjusted for a set protective layer thickness, so that the protective layer adsorption amount corresponding to the protective layer volume is equal to or greater than a predetermined lower limit.

[0077] Preferably, the thickness of the porous protective layer 11 can be 50 μm or more. In this case, the protective layer volume can be set to a desired size in combination with the protective layer pore diameter, making it easy to increase the protective layer adsorption amount. If the layer thickness is thinner than 50 μm, HC components contained in the exhaust gas G will easily pass through the porous protective layer 11, which may reduce the ability to capture HC components. The upper limit of the layer thickness is not necessarily limited, but it is desirable to appropriately set it to, for example, a range of 3000 μm or less so as not to increase the diffusion resistance when the exhaust gas G passes through.

[0078] As shown in Fig. 9 (left diagram), the responsiveness of the output to NOx correlates with the poisoning rate of the electrolyte region in the sensor electrode 22. Here, the porous electrode film constituting the sensor electrode 22 has an electrolyte region in which a solid electrolyte and a precious metal are dissolved in solid solution, and as the poisoning rate of this region increases, the region forming the three-phase interface that contributes to electrode activity decreases, thereby reducing the responsiveness. Also, as shown in Fig. 9 (right diagram), the poisoning rate correlates with the film thickness of the sensor electrode 22 (hereinafter referred to as electrode film thickness, as appropriate), and the thicker the electrode film thickness, the lower the poisoning rate.

[0079] Therefore, it is desirable to set the electrode film thickness so that a desired responsiveness can be obtained even when a certain amount of poisoning substance is adsorbed within the range of the allowable electrode adsorption amount on the sensor electrode 22. Preferably, the porous electrode film that becomes the sensor electrode 22 is formed to a film thickness of 5 μm or more, which can keep the poisoning rate of the electrolyte region that forms the three-phase interface low, stabilize the electrode activity, and improve the responsiveness.

[0080] The upper limit of the electrode film thickness is not necessarily limited as long as it is within a range that does not reduce the responsiveness of the sensor electrode 22, and can be set to, for example, 50 μm or less. 9 shows the results of investigating the responsiveness when a certain amount of poisoning substance is adsorbed by changing the film thickness of the sensor electrode 22 of the test element. The poisoning rate was calculated based on the thickness of the electrolyte region and the proportion of poisoning by observing the cross section of the porous film that becomes the sensor electrode 22 with a scanning electron microscope (SEM).

[0081] As described above, according to this embodiment, in the gas sensor device S for detecting NOx contained in exhaust gas G, the HC components contained in the exhaust gas G are adsorbed into the porous protective layer 11 provided on the gas sensor element 1, the amount of adsorbed HC components is optimized, and appropriate control is performed to suppress the influence of gas components that are not the detection target, thereby improving the responsiveness of NOx detection.

[0082] In the above-described embodiment, the gas sensor device S is used to detect NOx, but the specific gas is not limited to NOx, and the gas sensor device S can be applied to the detection of any gas contained in the exhaust gas G. Furthermore, the configurations and shapes of the gas sensor device S and the gas sensor element 1 are not limited to those shown in the drawings, and can be modified as appropriate.

[0083] The present invention is not limited to the above-described embodiments, and further different embodiments can be configured without departing from the spirit of the present invention. The present invention also includes various modifications, modifications within the scope of equivalents, etc. Furthermore, various combinations of components, forms, etc. envisioned from the present invention are also included in the technical spirit of the present invention. [Explanation of symbols]

[0084] S Gas sensor device 1 Gas sensor element 11 Porous protective layer 2. Measurement gas chamber 20 Solid electrolyte body 31 Diffusion resistance layer 1p Pump cell 1s sensor cell 10 Sensor control unit 101 Early activation control section

Claims

1. a gas sensor element (1) for detecting a specific gas contained in a measurement gas (G); a porous protective layer (11) for protecting the outer surface of the gas sensor element; a sensor control unit (10) that controls the operation of the gas sensor element and detects the specific gas based on an output of the gas sensor element, The gas sensor element is a diffusion resistance layer (31) disposed inside the porous protective layer; a measurement gas chamber (2) into which the measurement gas is introduced via the diffusion resistance layer; a pump cell (1p) having a pump electrode (21) on a surface of a solid electrolyte body (20) facing the measurement gas chamber, for adjusting the oxygen concentration in the measurement gas chamber; a sensor cell (1s) having a sensor electrode (22) on a surface of the solid electrolyte body downstream of the pump electrode with respect to the gas flow in the measurement gas chamber, and producing an output corresponding to the concentration of the specific gas, The porous protective layer is The gas sensor device has an adsorption amount of organic components in the usage environment of the gas sensor element in the range of 100 mg / g to 400 mg / g.

2. the sensor control unit includes an early activation control unit (101) that controls a voltage applied to the pump cell at the start of sensor operation to a startup voltage (V2) that is higher than a normal control voltage (V1); 2. The gas sensor device according to claim 1, wherein the amount of the organic component adsorbed on the sensor electrode is maintained at 0.1 mg or less during the sensor operation period.

3. 3. The gas sensor device according to claim 1, wherein the amount of the organic component adsorbed in the porous protective layer is adjusted by adjusting an average pore diameter and a thickness of the porous protective layer.

4. 4. The gas sensor device according to claim 3, wherein the porous protective layer has an average pore diameter of 0.4 μm or more and a thickness of 50 μm or more.

5. 3. The gas sensor device according to claim 1, wherein the sensor electrode is made of a porous cermet electrode film containing a solid electrolyte and a precious metal, and the thickness of the porous cermet electrode film is 5 [mu]m or more.

6. 3. The gas sensor device according to claim 1, wherein the measurement gas is exhaust gas from an internal combustion engine, the specific gas is nitrogen oxides, and the organic component is a hydrocarbon-based organic component including a hydrocarbon-based compound.

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