Gas sensor element and gas sensor unit

Abstract

Gas sensor element (100) comprising a solid electrolyte body (70) which has oxygen ion conductivity and detects a concentration of a specific gas in a measured gas based on an amount of oxygen ions which are conducted in the solid electrolyte body (70), wherein the gas sensor element (100) comprises: a measuring gas chamber (10) into which the measured gas is introduced; a pump cell (40) which has a pump electrode (41) which is provided on the solid electrolyte body (70); a sensor cell (20) comprising a sensor electrode (21) and arranged downstream of the pump cell (40) in the direction in which the measured gas is introduced, the sensor electrode (21) containing a precious metal, and the pump electrode (41) being placed on the same surface of the measuring gas chamber (10) and reducing the specific gas to detect the concentration of the specific gas; and a pump cell control (50) which sets a concentration of oxygen in the measured gas in the measuring gas chamber (10) by applying a voltage to the pump cell (40), wherein if the gas sensor element (100) is activated prior to the detection of the gas concentration in order to remove oxygen which is occluded in the sensor electrode (21), the pump cell control (50) decomposes a substance which is present in the measuring gas chamber (10) by applying a removal voltage to the pump cell (40) so that a reducing gas is produced, wherein the sensor electrode (21) has a plurality of precious metal areas (PM) which are made of the precious metal, and electrolyte areas which are distributed such that an interface is created between each of the electrolyte areas and each of the plurality of precious metal areas (PM), wherein the sensor electrode (21) has an open pore which extends from an electrode surface (S) of the sensor electrode (21) and reaches at least one of the plurality of precious metal regions (PM), and wherein a cross-section orthogonal to the electrode surface (S) of the sensor electrode (21) has a pore ratio of not less than 2% and not more than 15% when a section is cut at least 30 µm long in one direction along the electrode surface (S).

Description

CROSS-REFERENCE TO RELATED REGISTRATIONS

[0001] This application is based on and claims the priority benefit of Japanese patent application 2016-248147, which was filed on December 21, 2016. BACKGROUND OF THE INVENTION [Technical Field]

[0002] The present disclosure relates to a gas sensor element which detects a concentration of a specific gas in a measured gas, and a gas sensor unit which is configured to include the gas sensor element. [State of the art]

[0003] A gas sensor element is known which is configured to have a solid electrolyte body and an electrode provided on the solid electrolyte body, and which electrically detects the concentration of a specific gas in a measured gas by ionizing the measured gas. In order to make such a gas sensor element usable as quickly as possible, the solid electrolyte body and the electrode must be in an activated state early on.

[0004] The activated state, for example, includes a state in which the solid electrolyte body and the electrode are at temperatures suitable for the reaction of the specific gas, and a state in which a surface of the electrode is in a state suitable for the reaction of the specific gas. In particular, to achieve a surface in the suitable state, patent literature 1 discloses a gas concentration detection device comprising a sensor electrode and a pump electrode as electrodes. A specific gas to be measured is ionized in the sensor electrode, and the concentration of the specific gas is detected based on an electric current (sensor current) corresponding to a quantity of ionized specific gas.The gas concentration sensing device is configured such that, when a gas sensor element is activated, a removal voltage is applied to the pump electrode to remove oxygen, thus generating a reducing gas. Oxygen adsorbed on the sensor electrode reacts with this reducing gas and is removed. This allows the sensor electrode to operate in a state where noise superimposed on the sensor current, generated by an oxygen ion, is reduced. [List of citations][Patent literature]

[0005] [PTL 1] JP 2016-70922A

[0006] DE 10 2010 063 520 A1 discloses a method and a sensor device for diagnosing a sensor element. The method for diagnosing a sensor element for detecting at least a fraction of a gas component in a measuring gas space, in particular a sensor element for detecting nitrogen oxides in the exhaust gas of an internal combustion engine, is proposed. The sensor element comprises at least one solid electrolyte and at least two electrodes. The electrodes comprise at least two diagnostic electrodes and at least two response electrodes. At least one defined signal sequence is applied to the diagnostic electrodes. At least one response signal is detected at the response electrodes.

[0007] DE 11 2013 005 603 T5 discloses an electrode for use in a gas sensor and a gas sensor element. In the gas sensor element, which is capable of detecting the concentration of a specific gas contained in a target gas, the electrode, which is formed on a solid electrolyte body exhibiting oxygen ion conductivity, consists of a precious metal and a solid electrolyte. Considering a cross-sectional area of ​​the electrode, it has a precious metal portion, a solid electrolyte portion, and a mixed portion consisting of the precious metal and the solid electrolyte. The mixed portion is formed along an interface between the precious metal portion and the solid electrolyte portion.

[0008] JP 2016 - 130 724 A discloses a NOx detection sensor. The NOx detection sensor comprises: a solid electrolyte body; a pump electrode for adjusting the oxygen concentration of the gas to be measured; and a sensor electrode for detecting the NOx concentration of the gas to be measured. A metallic component of the sensor electrode comprises a Pt-Rh alloy. The mass ratio Pt:Rh in the entire sensor electrode is Pt:Rh = 70:30 to 35:65. The ratio of Rh to Pt-Rh alloy in a surface layer of the sensor electrode is 4 to 10 atomic percent higher than the ratio of Rh to Pt-Rh alloy in the entire sensor electrode. SUMMARY [Technical Problem]

[0009] It is taken into account that not all of the reducing gas described above reacts with the oxygen on the sensor electrode, and that some of the reducing gas remains on the surface of the sensor electrode. Although the reducing gas is gradually removed from the surface of the sensor electrode, and a sensor current caused by the reducing gas stabilizes at a zero level, it is necessary to achieve the removal of the reducing gas earlier through early activation of the sensor electrode.

[0010] With regard to this requirement, it is an object of the present disclosure to achieve a previous activated state of a gas sensor element and a gas sensor unit in which oxygen, which is sealed or occluded in a sensor electrode, is reduced by using a reducing gas. [Solution to the problem]

[0011] The problem is solved by a gas sensor element with the features of claim 1 and a gas sensor unit with the features of claim 4. Further advantageous embodiments and developments are the subject of the subsequent claims.

[0012] To achieve this objective, a gas sensor element of the present disclosure is a gas sensor element comprising a solid electrolyte body having oxygen ion conductivity and detecting a concentration of a specific gas in a measured gas based on an amount of oxygen ions conducted in the electrolyte body, wherein the gas sensor element comprises: a measuring gas chamber into which the measuring gas is introduced; a pump cell comprising a pump electrode provided on a surface of the solid electrolyte body; a sensor cell comprising a sensor electrode located downstream of the pump cell in the direction in which the measured gas is introduced, wherein the sensor electrode contains a noble metal and the pump electrode is located on the same surface of the measuring gas chamber and reduces the specific gas.to detect the concentration of the specific gas; and a pump cell controller which adjusts a concentration of oxygen in the measured gas in the measuring chamber by applying a voltage to the pump cell when the gas sensor element is activated prior to detecting the concentration of the gas, in order to remove oxygen which is occluded in the sensor electrode, wherein the pump cell controller decomposes a substance which is present in the measuring gas chamber by applying a removal voltage to the pump cell, so that a reducing gas is produced, wherein the sensor electrode has a plurality of noble metal areas which are made of the noble metal, and electrolyte areas which are distributed such that an interface is produced between each of the electrolyte areas and each of the plurality of noble metal areas, wherein the sensor electrode has open pores,which extend from an electrode surface of the sensor electrode and reach at least one of the majority of precious metal areas.

[0013] According to the configuration described above, the pores that are open on the electrode surface of the sensor electrode are present. This prevents the reducing gas from remaining on the surface of the sensor electrode and promotes early diffusion and removal of the reducing gas. In particular, since the open pores are formed to reach the noble metal region, it is possible to promote the diffusion and removal of the reducing gas near the noble metal more efficiently, which contributes to the ionization of the specific gas. This means that an early transition of the sensor electrode to an activated state is possible. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] In the attached drawings: Fig. Figure 1 is a side cross-sectional view showing a schematic structure of a gas sensor unit according to a first embodiment; Fig. Figure 2 is a cross-sectional view showing a detailed configuration of a gas sensor element; Fig. Figure 3 is a concept diagram illustrating how a reducing gas is generated by a pump cell and how a reduction is carried out in a sensor electrode by utilizing the reducing gas; Fig. Figure 4 is a graph showing changes over time in the voltage applied to a pump electrode and in the sensor current; Fig. Figure 5 shows a result of observing a detailed configuration near the sensor electrode under a scanning electron microscope; Fig. Figure 6 is a cross-sectional view of the sensor electrode; Fig. Figure 7 is a graph showing an activation time with respect to a pore ratio; and Fig. Figure 8 is a cross-sectional view showing a detailed configuration of the gas sensor element according to a different design. DETAILED DESCRIPTION OF PREFERRED EXECUTION FORMS

[0015] The following describes embodiments of the present disclosure with reference to the drawings. In each embodiment, sections corresponding to items described in a preceding embodiment are given the same reference numerals, and duplicate descriptions may be omitted. In each embodiment, if only part of a configuration is described, another embodiment described above is applicable to other parts of the configuration. Furthermore, possible combinations of sections specifically and explicitly mentioned in each embodiment are not explicitly mentioned; partial combinations of embodiments are also possible if such combinations do not present a particular problem. (First embodiment)

[0016] First, the following describes, with reference to Fig. Figures 1 to 4 show a schematic structure of a gas sensor element and a gas sensor unit according to the present embodiment.

[0017] The gas sensor element according to the present embodiment is, for example, a NOx sensor which detects a quantity of nitrogen oxide (NOx). Such a gas sensor element is arranged and used, for example, in an exhaust pipe through which exhaust gas flows in a machine with internal combustion. The measured gas is the exhaust gas, and the specific gas whose concentration is to be detected is NOx.

[0018] The gas sensor unit is a module that contains the gas sensor element. For example, the gas sensor unit is installed as a single unit in the exhaust pipe.

[0019] The following description is based on the assumption that the specific gas in the measured gas is NOx. However, it should be noted that the specific gas to be detected is not limited to NOx, and the gas sensor element can be implemented as a sensor that detects, for example, ammonia or other types of gas, depending on the constituent materials of a solid electrolyte body and an electrode.

[0020] As in Fig. As shown in Figure 1, a gas sensor unit 1 is arranged in an exhaust passage 2 of an internal combustion engine in a vehicle. Exhaust gas flowing through the exhaust passage 2 is introduced into the gas sensor unit 1 as the gas to be measured. The gas sensor unit 1 then measures the NOx concentration in the measured gas using a gas sensor element 100, which is contained within the gas sensor unit 1. In particular, in addition to the gas sensor element 100, the gas sensor unit 1 is configured to include a sensor housing 101, an insulator 102, element covers 103 and 104, a plurality of sensor cable strands or sensor wiring harnesses 105, and a sensor control circuit 106. It should be noted that F in Fig. 1 indicates a gas flow direction in which the measured gas flows within the gas sensor element 100.

[0021] The sensor housing 101 holds the gas sensor element 100 within the housing above the insulator 102. The covers 103 and 104 are attached to the sensor housing 101. An upstream section of the gas sensor element 100 is a first element end 100a, and a downstream section of the gas sensor element 100 is a second element end 100b in the direction of exhaust gas flow. The second element end 100b faces the sensor control circuit, and the first element end 100a is positioned opposite the second element end 100b. The element cover 103 covers one outer circumferential side of the first element end 100a. The element cover 103 has a gas inlet hole 103a for introducing the measured gas, i.e., exhaust gas from the exhaust pipe, into the first element end 100a, which is enclosed within the housing. The cover 104 covers an outer circumferential side of the second element end 100b.The cover 104 has an air inlet hole 104a for introducing atinospheric air into the second element end 100b, which is enclosed within the gas sensor unit 1. The plurality of sensor cable strands 105 are provided to extend over the inside and outside of the cover 104. The sensor control circuit 106 is connected to the gas sensor element 100 via the plurality of sensor cable strands 105 outside the sensor housing 101 and the element cover 103. The sensor control circuit 106 according to the present embodiment has a pump cell controller 50 (described later) for supplying a voltage to a pump cell 40 and controls a voltage supply to a sensor cell 20.

[0022] The following describes a detailed configuration of the gas sensor element 100 with reference to Fig. 2. The gas sensor element 100 comprises a measuring gas chamber 10, a reference gas chamber 11, the sensor cell 20, the monitoring cell 30, the pump cell 40, the pump cell controller 50, and a heater 60. The gas sensor element 100 is configured such that the heater 60, a solid electrolyte body 70, and an insulating layer 90 (described later) are layered. The measuring gas chamber 10 is formed as a space surrounded by the solid electrolyte body 70 and the insulating layer 90. The reference gas chamber 11 is formed as a space surrounded by the heater 60 and the solid electrolyte body 70. The following describes the components of the gas sensor element 100 in detail.

[0023] The measuring gas chamber 10 is a space into which the exhaust gas, which is the gas being measured, is introduced. The measuring gas chamber 10 is formed as a space inserted between the solid electrolyte body 70 and the insulating layer 90. The insulating layer 90 is plate-shaped and is layered onto the plate-shaped solid electrolyte body 70 via a first spacer 91. When viewed from the front, the first spacer 91 forms a C-shape with one side open, and thus the measuring gas chamber 10 has a box shape with an open portion. The open portion is an inlet port 10a for the exhaust gas. In the inlet port 10a according to the present embodiment, a diffusion resistance body 93 is arranged, and the exhaust gas is introduced from the inlet port 10a into the measuring gas chamber 10 by passing through the diffusion resistance body 93.This means that the exhaust gas is introduced into the measuring gas chamber 10 under a predetermined diffusion resistance through the diffusion resistance body 93.

[0024] For example, yttria-stabilized zirconia (YSZ) can be used as the solid electrolyte body 70 according to the present embodiment. YSZ exhibits solid electrolyte function at temperatures of not less than approximately 600 °C. The solid electrolyte body 70 is heated by the heater 60 (described later) to maintain its solid electrolyte function. A material such as calcium oxide-stabilized zirconia or aluminum oxide-stabilized zirconia can also be used as the solid electrolyte body 70. Aluminum oxide or other commonly known insulating materials can be used as the insulating layer 90 and the first spacer 91.

[0025] The reference gas chamber 11 is a chamber into which a reference gas is introduced. The reference gas is used to generate a reference potential for calculating the concentration of NOx, which is the specific gas. For example, atmospheric air is introduced into the reference gas chamber 11. The reference gas chamber 11 is formed as a chamber inserted between the heater 60 and the solid electrolyte body 70. The solid electrolyte body 70 is layered on the heater 60, which is formed in a plate shape, via a second spacer 92. When the solid electrolyte body 70 is viewed from the front, the second spacer 92 is located closer to the diffusion resistance body 93 on one side. An inlet port for atmospheric air (not shown) is open in the reference gas chamber 11 on one side opposite the second spacer 92.

[0026] Accordingly, the electrolyte body 70 is shaped to be placed between the measuring gas chamber 10 and the reference gas chamber 11 and is exposed to both. This allows ions to move within the solid electrolyte body 70 in accordance with a difference between a NOx concentration in the exhaust gas and a NOx concentration in the atmospheric air, thus generating a sensor current.

[0027] The sensor cell 20 comprises a sensor electrode 21, a solid electrolyte body 70, and a reference electrode 80. The sensor electrode 21 is formed on the solid electrolyte body 70 to be exposed to the measuring gas chamber 10. The reference electrode 80 is also formed on the solid electrolyte body 70 to be exposed to the reference gas chamber 11. This means that the solid electrolyte body 70 is positioned between the sensor electrode 21 and the reference electrode 80. According to the present embodiment, the sensor cell 20, the monitoring cell 30, and the pump cell 40 all utilize the solid electrolyte body 70 and the reference electrode 80.

[0028] The sensor electrode 21 is a precious metal catalyst containing platinum (Pt) and rhodium (Rh). Furthermore, the sensor electrode 21 contains a solid electrolyte made of YSZ, identical in composition to the YSZ of the solid electrolyte body 70. That is, the sensor electrode 21 is an electrode made of platinum and rhodium, which act as catalysts, and the solid electrolyte, which has ionic conductivity. The solid electrolyte contained in the sensor electrode 21 is integrally combined with the solid electrolyte body 70, which forms the sensor cell 20, so that ionic conduction between the solid electrolyte and the solid electrolyte body 70 is possible.

[0029] NOx contained in the exhaust gas introduced into the measuring gas chamber 10 is adsorbed on an exposed surface of a noble metal and ionized into nitrogen and oxygen ions by the catalyst activity. The oxygen ions are then passed through the solid electrolyte that forms the sensor electrode 21. These oxygen ions are further directed to the solid electrolyte body 70 and are measured as the sensor current. Based on the magnitude of this sensor current, the NOx concentration is determined. A detailed configuration of the sensor electrode 21 will be described later.

[0030] The monitoring cell 30 comprises a monitoring electrode 31, the solid electrolyte body 70, and the reference electrode 80. As described above, the monitoring cell 30 shares the solid electrolyte body 70 and the reference electrode 80 with the sensor cell 20. The monitoring electrode 31 is formed on a surface of the solid electrolyte body 70 to be exposed to the measuring gas chamber 10. The monitoring electrode 31 is an electrode containing, for example, platinum (Pt) and gold (Au). Although the monitoring electrode 31 is not capable of decomposing NOx, it does decompose oxygen molecules, causing an electric current to flow that is generated by oxygen ions.

[0031] The monitoring electrode 31 is positioned next to the sensor electrode 21 approximately perpendicular to the direction of exhaust gas flow from the inlet port 10a to the sensor cell 20. This means that the sensor electrode 21 and the monitoring electrode 31 are exposed to the exhaust gas in the same way, which is introduced into the measuring gas chamber 10 in a nearly uniform manner. The monitoring cell 30 detects the concentration of residual oxygen contained in the exhaust gas, in which the oxygen concentration has been adjusted by the pump cell 40. In particular, the monitoring cell 30 detects an electric current caused by the residual oxygen flowing into the solid electrolyte body 70.By subtracting an electric current caused by the oxygen ions and output by the monitoring cell 30 from an electric current caused by the oxygen ions and output by the sensor cell 20, the gas sensor element 100 is able to detect a concentration of NOx by eliminating an offset of an electric current caused by the oxygen ions and output by the sensor cell 20, where the electric current is caused by the residual oxygen.

[0032] The pump cell 40 is positioned closer to the insertion port 10a on one side than the sensor cell 20 and the monitoring cell 30. The pump cell 40 comprises a pump electrode 41, the solid electrolyte body 70, and the reference electrode 80. The pump electrode 41 is formed on the solid electrolyte body 70 to be exposed to the measuring gas chamber 10. The pump electrode 41 is an electrode containing platinum (Pt) and gold (Au), as is the monitoring electrode 31. The pump electrode 41 reduces oxygen, generating oxygen ions. The oxygen ions are directed into the solid electrolyte body 70 to move towards the side of the reference electrode 80 and are discharged into the reference gas chamber 11. Thus, the pump cell 40 is a cell that adjusts the oxygen concentration in the measuring gas chamber 10 by its pumping action. This means that on the side of the inlet port 10a, the pump cell 40 adjusts the oxygen concentration in the exhaust gas.With regard to the exhaust gas in which the oxygen concentration has been adjusted, the sensor cell 20 and the monitoring cell 30 each output an electrical current caused by NOx and an electrical current caused by the residual oxygen.

[0033] In addition, according to the present embodiment, the pump cell 40 has a function for decomposing a substance contained in the exhaust gas, so that a reducing gas (hydrogen) is produced. In particular, the pump cell 40 decomposes water molecules contained in the exhaust gas, so that hydrogen gas is produced. The hydrogen is reduced, and therefore oxygen, which is occluded in the sensor electrode 21, is reduced and removed by the hydrogen.

[0034] The pump cell controller 50 is a section for applying a predetermined voltage to the pump cell 40 at a predetermined time. The pump cell controller 50 forms part of the sensor control circuit 106; which is located in Fig. Figure 1 shows that applying a voltage to the pump cell 40 creates a potential difference between the reference electrode 80 and the pump electrode 41. In the pump cell controller 50, a normal voltage V1 can be applied to the pump cell 40, causing it to pump to adjust the oxygen concentration. Furthermore, a displacement voltage V2 can be applied to the pump electrode 41, generating hydrogen, the reducing gas, through the decomposition of water molecules. The displacement voltage V2, relative to the electrical potential of the reference electrode 80, is set to be approximately 0.5 V to 2.0 V, which is higher than the 0.3 V to 0.4 V for the normal voltage V1.

[0035] As in Fig. As shown in Figure 3, while the removal voltage V2 is applied to the pump electrode 41, water molecules are decomposed, producing hydrogen. Hydrogen, being a reducing agent, produces water molecules by reducing oxygen ions that combine with a rhodium atom, which forms the sensor electrode 21, to form rhodium oxide (III) or oxygen ions, which are adsorbed on a surface of the sensor electrode 21.

[0036] As in Fig. As shown in Figure 4, when the gas sensor element 100 is activated (time t0), the pump cell controller 50 applies the remote voltage V2 to the pump cell electrode 41 until time t1, at which time the normal voltage V1 is applied. While the remote voltage V2 is applied to the pump electrode 41, hydrogen generated in the pump cell 40 reaches the sensor cell 20, and a sensor current changes. Specifically, the sensor current has a lower value compared to a normal state in which the normal voltage V1 is applied to the pump cell 40, and the pump cell 40 is operating in its normal state. The lower sensor current value, compared to the normal state, indicates that the hydrogen is interacting with the sensor electrode 21.

[0037] At time t1, the pump cell controller 50 changes the voltage applied to the pump cell 40 from the distance voltage V2 to the normal voltage V1. Consequently, the sensor current is increased and the pump cell 40 returns to its normal state. After the transition to the normal state, i.e., after a time t2, the sensor current changes depending on the amount of residual oxygen or the concentration of NOx and is therefore not constant. Fig. However, in figure 4, a value of the sensor current is shown as a constant value for a simple explanation.

[0038] The heater 60 maintains a temperature of the solid electrolyte body 70 of at least approximately 600 °C, so that YSZ acts as the solid electrolyte. The heater 60 is configured such that a conductive layer 62, which generates heat by supplying power, is provided between ceramic substrates 61. The conductive layer 62 is configured such that, when viewed from the front, the conductive layer 62 overlaps the solid electrolyte body 70, and thus temperatures at least in and near the sections where the electrodes 21, 31, 41, and 80 are formed can be maintained at activation temperatures. The temperature distribution in the solid electrolyte body 70 achieved by the heater 60 must be adjusted appropriately depending on the required power output.The arrangement of the conductor layer 62 can be set in accordance with a required temperature distribution.

[0039] The following describes a detailed configuration of the sensor electrode 21 with reference to the Fig. 5 and Fig. 6.

[0040] Fig. Figure 5 shows a result of the observation under a scanning electron microscope (SEM) of a cross-section of the sensor electrode 21 and the solid electrolyte body 70, which is a substrate of the sensor electrode 21, in the same direction as in the side view shown in Fig. 2 is shown. The cross-section in Fig. Figure 5 shows a result obtained when a cross-section orthogonal to an electrode surface of the sensor electrode 21, with a size of approximately 30 µm in one direction along the electrode surface, is cut and observed.

[0041] The sensor electrode 21 is formed on the surface of the solid electrolyte body 70, which is exposed to the measuring gas chamber 10. As described above, the sensor electrode 21 is an electrode that functions as the noble metal catalyst containing Pt and Rh, and the sensor electrode 21 contains YSZ for ion conduction.

[0042] In Fig. Five areas, observed to have a light gray color, are precious metal areas (PM) containing Pt and Rh. These areas form multiple clusters and constitute the sensor electrode 21. A dark gray area, observed to have a solid electrolyte (SE), is another. The solid electrolyte SE enters the spaces between the scattered precious metal areas (PM) and forms a path for ionic conduction. Sections observed to have a black color are spaces that do not contain Pt, Rh, or YSZ. Some of these sections communicate with an external space, while others are embedded within the sensor electrode 21 or the solid electrolyte body 70.

[0043] In Fig. 5 is a region formed by sections that do not contain any precious metal and which appear dark gray or black as an empty space, the solid electrolyte body 70, which is the substrate. The solid electrolyte region SE of the sensor electrode 21 is made of the same material as that of the solid electrolyte body 70, which is the substrate. After the sensor electrode 21 is formed, the solid electrolyte region SE and the solid electrolyte body 70 are integrally connected, so that there is no difference between the solid electrolyte region SE and the solid electrolyte body 70.

[0044] An area located on one side opposite the solid electrolyte body 70 with respect to the sensor electrode 21, and observed to have a black color, is the space of the measuring gas chamber 10. That is, a boundary between the sensor electrode 21 and the space of the measuring gas chamber 10 is an electrode surface S of the sensor electrode 21.

[0045] The sensor electrode 21 has a plurality of pores as sections, which are observed to give it a black color under the scanning electron microscope. Specifically, among the plurality of pores, reference is made to a pore extending from the electrode surface S and reaching a noble metal region PM, referred to as an open pore H. The open pore H has several structures. As in pore H1 in Fig. As shown in Figure 6, a pore is formed when a single pore open on the electrode surface S reaches a single precious metal region PM, thus exposing the precious metal region PM. Pores H2 and H3 are formed when multiple pores open on the electrode surface S reach a single precious metal region PM, thus exposing the precious metal region PM. A pore H4 is formed when a single pore open on the electrode surface S reaches multiple precious metal regions PM, thus exposing the precious metal regions PM. In a broader sense, the open pore H also includes any form in which a precious metal region PM is exposed to the electrode surface S. Such an open pore is designated H5 in Figure 6. Fig. 6. However, the open pore H does not have a pore that is open on the electrode surface S but does not reach a noble metal region PM. This means that the open pore H has pores H1 to H5, but no pore H6 in Fig. does not have 6.

[0046] Assume that the open pore H is defined as above, and that a cross-section is cut which is orthogonal to the electrode surface S of the sensor electrode 21 and has a size of at least 30 µm in one direction along the electrode surface S. In such a case, when reference is made to a proportion of an area (cross-sectional area) of the open pore H to a total area of ​​the sensor electrode 21 as a pore ratio, the pore ratio is preferably not less than 2% and not more than 15%. For example, in the example shown in Fig. As shown in Figure 5, the pore ratio is approximately 10%.

[0047] Although the open pore H is like the pore H5 in Fig. 6. An open pore in a broad sense is a percentage of the area of ​​such an open pore to the total area of ​​the sensor electrode 21.

[0048] The following briefly describes a procedure for forming the sensor electrode, which is configured as described above.

[0049] First, YSZ, in a paste form containing precious metal powder prepared so that the mass percentage of Rh to the total mass of Pt and Rh is approximately 40%, is placed on the solid electrolyte body 70, which is the substrate. Then, the components of the gas sensor element 100 are assembled appropriately. The assembled gas sensor element 100 is placed under a reducing atmosphere, and an energy input process is carried out between the sensor electrode 21 and the reference electrode 80. During this process, a Pt-Rh alloy is distributed within the sensor electrode 21 onto the solid electrolyte body 70, forming pores into which the measured gas flows. These pores exhibit the open pore H. Similarly, within the reference electrode 80, Pt is distributed onto the solid electrolyte body 70, forming a pore into which the reference gas (atmospheric air according to the present embodiment) flows.

[0050] The pore ratio is determined based on the interrelationships between various power supply parameters, such as voltage, electric current, time, temperature, and the Pt to Rh composition ratio. The pore ratio for the open pore H created by the power supply process is preferably no more than 15%. This is because a pore ratio exceeding 15% can increase the contact resistance between the electrode surface S and a conductor used to connect the sensor electrode 21 to another external device, such as the sensor control circuit 106, potentially leading to a conductor malfunction.

[0051] The energy supply process takes place under a nitrogen atmosphere, which is a hypoxic environment and therefore allows RhO2 to be easily reduced. Consequently, a small amount of open H₂ pores is generated when oxygen, which forms RhO2, is removed, and this allows the open H₂ pore to be constantly present at a pore ratio of approximately 2%.

[0052] When the energy supply process is carried out, some of the Rh₂O₃ formed near the surface of the sensor electrode 21 is reduced. A phenomenon also occurs in which oxygen is removed from the solid electrolyte 70. Therefore, a reoxidation process is carried out by adding µm of oxygen to the solid electrolyte 70. During the reoxidation process, the measuring gas chamber 10 is under a reducing gas atmosphere in which hydrogen, as the reducing gas, constitutes a predetermined volume percentage, and nitrogen is the remaining volume percentage. Instead of hydrogen, the reducing gas atmosphere can contain carbon monoxide, hydrocarbons, or the like. The reference gas chamber 11 is under atmospheric air.

[0053] The gas sensor element 100 is then left stationary for a predetermined period while the heater 60 is supplied with energy, so that the solid electrolyte 70 is heated to a suitable temperature at which it exhibits oxygen ion conductivity. At this time, due to a difference in oxygen concentration between the atmospheric air and the reducing gas atmosphere, oxygen ions are conducted from the reference gas chamber 11 to the measuring gas chamber 10 via the solid electrolyte 70. This allows oxygen to be added to the solid electrolyte 70.

[0054] Since the reference gas chamber 11 is formed in the gas sensor element 100, sufficient oxygen is supplied to the reference electrode 80 during the reoxidation process. This makes it easier for the oxygen ions to be conducted from the reference electrode 80 to the sensor electrode 21.

[0055] The following describes, with reference to Fig. 7 Effects which are achieved by inserting the gas sensor element 100 and the gas sensor unit according to the present embodiment.

[0056] The gas sensor element 100 has a pore ratio of approximately 10% for the open pore H on the sensor electrode 21, which forms the sensor cell 20.

[0057] This prevents a reducing gas (hydrogen), generated by applying the removal voltage to the pump cell 40, from remaining on the surface of the sensor electrode 21 and allows for early diffusion and removal of the reducing gas from the pore opening. This means that early diffusion of the reducing gas can be promoted. In particular, since the open pore H is formed to reach the noble metal region PM, it is possible to more efficiently promote the diffusion and removal of the reducing gas near the noble metal, which contributes to the ionization of the specific gas. This means that an early transition of the sensor electrode 21 to an activated state is possible.

[0058] In particular, the inventor investigated by experiment the activation time required until the sensor electrode 21 reaches the activated state with respect to the pore ratio for the open pore H. Fig. Figure 7 shows a result of the investigation. The activation time shows a tendency to decrease as the pore ratio increases. (With reference to...) Fig. 7 It can be assumed that, as described above, if the pore ratio is higher, hydrogen, which is the reducing gas, is less likely to remain on the surface of the sensor electrode 21, and this promotes the activation of the sensor electrode 21.

[0059] In a range where the pore ratio for the open pore H is not less than 10%, the activation time is nearly saturated and does not change significantly. An excessively high pore ratio can cause an increase in the contact resistance between the sensor electrode 21 and another element (e.g., a conductor). Therefore, the pore ratio is preferably approximately 10% and within an optimal range of 2% to 15%. (Other embodiments)

[0060] Although the gas sensor element 100 including the monitoring cell 30 has been described in the embodiment mentioned above, the monitoring cell 30 is not an essential component for detecting the concentration of the specific gas in the measured gas. However, it should be noted that the gas sensor element 100, which includes the monitoring cell 30, is preferable for accurately detecting the oxygen concentration in the gas, in which the oxygen concentration has been adjusted by the pump cell 40, and for correcting the background for the output of the sensor cell 20.

[0061] Furthermore, although in the foregoing embodiment the precious metal containing Pt and Rh has been described as the precious metal forming the sensor electrode 21, Pt or Rh can be substituted by palladium (Pd) or ruthenium (Ru), or these elements can be added to Pt and Rh.

[0062] The measuring gas chamber 10 can be configured such that a space in which the sensor cell 20 is formed and a space in which the pump cell 40 is formed are separated from each other, so that the measured gas can move between these spaces. In particular, for example, in the gas sensor element 200, which is located in Fig. As shown in Figure 8, a diffusion rate determiner 94 is formed between the space in which the sensor cell 20 is located and the space in which the pump cell 40 is located. The diffusion rate determiner 94 separates the sensor cell 20 from the pump cell 40 in order to subdivide the space of the measuring gas chamber 10. In this case, while adjusting the diffusion resistance, the diffusion rate determiner 94 allows the measured gas to pass through it.

[0063] According to the embodiment mentioned above, the result of the observation under the scanning electron microscope, which is in Fig. Figure 5 is used to calculate the pore ratio. Fig. Figure 5 shows the result obtained when a cross-section orthogonal to the electrode surface of sensor electrode 21, with a size of approximately 30 µm, is sectioned along the electrode surface and observed under a scanning electron microscope. However, the cross-sectional size is not limited to 30 µm. For calculating the pore ratio, the cross-section is preferably sectioned to have a size of 30 µm or more along the electrode surface.

[0064] According to the embodiment described above, although the pore ratio is defined as the percentage of the open pore area to the total area of ​​the sensor electrode, the pore ratio can be defined differently. For example, the pore ratio can be defined as a proportion or ratio of a curve length of a section forming the open pore to a curve length along the electrode surface, which has convex and concave parts including the open pore H, when the cross-section is viewed perpendicular to the electrode surface of the sensor electrode 21. Alternatively, the pore ratio can be defined as a proportion of the open pore to the total electrode surface S of the sensor electrode 21 when the electrode surface S of the sensor electrode 21 is viewed from the front.

Claims

[1] Gas sensor element (100) comprising a solid electrolyte body (70) having oxygen ion conductivity and detecting a concentration of a specific gas in a measured gas based on an amount of oxygen ions which are conducted in the solid electrolyte body (70), wherein the gas sensor element (100) comprises: a measuring gas chamber (10) into which the measured gas is introduced; a pump cell (40) which has a pump electrode (41) which is provided on the solid electrolyte body (70); a sensor cell (20) comprising a sensor electrode (21) and arranged downstream of the pump cell (40) in the direction in which the measured gas is introduced, the sensor electrode (21) containing a precious metal, and the pump electrode (41) being placed on the same surface of the measuring gas chamber (10) and reducing the specific gas to detect the concentration of the specific gas; and a pump cell control (50) which sets a concentration of oxygen in the measured gas in the measuring gas chamber (10) by applying a voltage to the pump cell (40), wherein if the gas sensor element (100) is activated prior to the detection of the gas concentration in order to remove oxygen which is occluded in the sensor electrode (21), the pump cell control (50) decomposes a substance which is present in the measuring gas chamber (10) by applying a removal voltage to the pump cell (40) so that a reducing gas is produced, wherein the sensor electrode (21) has a plurality of precious metal areas (PM) which are made of the precious metal, and electrolyte areas which are distributed such that an interface is created between each of the electrolyte areas and each of the plurality of precious metal areas (PM), wherein the sensor electrode (21) has an open pore which extends from an electrode surface (S) of the sensor electrode (21) and reaches at least one of the plurality of precious metal regions (PM), and wherein a cross-section orthogonal to the electrode surface (S) of the sensor electrode (21) has a pore ratio of not less than 2% and not more than 15% when a section is cut at least 30 µm long in one direction along the electrode surface (S). [2] Gas sensor element (100) according to claim 1, wherein the precious metal contains at least platinum and rhodium. [3] Gas sensor element (100) according to claim 2, wherein the mass percentage of rhodium contained in the precious metal to a total mass of platinum and rhodium is not less than 30% and not more than 80%. [4] Gas sensor unit (1) comprising: the gas sensor element (100) which is referenced in claims 1 to 3; a sensor housing (101) which holds the gas sensor element (100) inside; an element cover (103, 104) which is attached to the sensor housing (101) to cover a first element end (100a) of the gas sensor element (100) and introduces the measured gas into the first element end (100a), the first element being oriented towards a stream of the measured gas; and a sensor control circuit (106) which controls a voltage which is applied to the pump electrode (41) and the sensor electrode (21).

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