Gas sensor element and method for manufacturing it

Abstract

Gas sensor element (1) comprising: a solid electrolyte plate (2) exhibiting oxygen ion conductivity; a measuring gas chamber (41) into which a measuring gas (G) is introduced, which is configured in contact with a first surface of the solid electrolyte plate (2); a measuring electrode (3A) which is arranged on the first surface of the solid electrolyte plate (2) such that it is located in the measuring gas chamber (41); a reference gas chamber (42) into which a reference gas (A) is introduced, which is configured to be in contact with a second surface of the solid electrolyte plate (2); a reference electrode (3B) which is arranged on the second surface of the solid electrolyte plate (2) such that it is located in the reference gas chamber (42); wherein the measuring electrode (3A) and the reference electrode (3B) contain platinum-containing precious metal and a solid electrolyte made of a type of ceramic identical to a ceramic material forming the solid electrolyte plate (2), a solid electrolyte region (32) in which the solid electrolyte is agglomerated, a mixed region (33) in which the precious metal and the solid electrolyte are distributed, a precious metal region (31) in which the precious metal is agglomerated, and a void (34) in a section plane (X1) of a specific electrode, which is one of the measuring electrode (3A) and the reference electrode (3B), along a thickness direction (T), wherein: the mixed region (33) and the empty spaces (34) near the precious metal region (31) are present; the mixed region (33) is formed by mixing the precious metal in the precious metal region (31) and the solid electrolyte in the solid electrolyte region (32) or the solid electrolyte plate (2) at the boundary between the precious metal region (31) and the solid electrolyte region (32) or the solid electrolyte plate (2); a parameter value as a product of a mixed region ratio B / A, which specifies a ratio of an area B of the mixed region in the section plane (X1) to an area A of the specific electrode (3A, 3B) in the section plane (X1), and a void ratio C / A, which specifies a ratio of an area C of the void (34) in the section plane (X1) to the area A of the specific electrode (3A, 3B) in the section plane (X1), falls in a range from 0.001 to 0.01; and where the resistance value of the entire solid electrolyte plate (2) and the pair of electrodes (3A, 3B) is less than 80 Ω.

Description

[Technical field]

[0001] The present disclosure relates to a gas sensor element configured such that electrodes are provided on a solid electrolyte and to a method for producing the same. [State of the art]

[0002] A gas sensor is installed in the exhaust system of an internal combustion engine for a vehicle. This sensor is designed to measure the oxygen concentration or the concentration of a specific gas, such as NOx (nitrogen oxide), in the exhaust gas flowing through the system. The gas sensor includes an integrated gas sensor element, which is manufactured by layering a paste of electrode material containing a precious metal and a solid electrolyte onto the surface of a ceramic sheet, forming a solid electrolyte plate. The electrode material and the ceramic sheet are then baked together.

[0003] In the gas sensor element, oxygen ions are formed at the three-phase interface where oxygen comes into contact with the noble metal and the electrolyte fixed in an electrode. These ions are then conducted within the electrode and the solid electrolyte plate. Thus, the process of effectively ensuring that the noble metal, the solid electrolyte, and a measuring gas come into contact with each other within the electrode influences the oxygen-reducing activity of the gas sensor element.

[0004] For example, an electrode for a gas sensor, as described in patent literature 1, has in one section a precious metal region containing precious metal, a solid electrolyte region containing solid electrolyte, and a mixed region in which the precious metal and the solid electrolyte are mixed. The mixed region is formed along a boundary between the precious metal region and the solid electrolyte region. This therefore has the efficiency of ionizing oxygen molecules in an exhaust gas and stabilizes the sensor output signal.

[0005] Furthermore, patent literature 2 discloses a gas sensor electrode forming metal paste that is able to improve the oxygen-reducing activity of a gas sensor element.

[0006] Further state of the art can be found in JP2000-12042 A and in JP 2010-60343 A. [List of cited documents](patent literature) PTL 1 JP 2014-122878 A PTL 2 JP 2014-145607 A [Summary of the invention]

[0007] In the gas sensor element, the oxygen ions formed at the three-phase interface of the electrode are conducted, resulting in an electron flow. To improve the oxygen-reducing activity of the gas sensor element, the three-phase interface area must be increased. However, it has been found that if only the mixed region or a void is enlarged to increase the three-phase interface area, the electrode resistance increases, negatively impacting electron flow. Therefore, to improve the oxygen-reducing activity of the gas sensor element, the three-phase interface must be formed in a suitable region that keeps the resistance low.

[0008] It is an object of the present invention to provide a gas sensor element and a manufacturing method for the same which solve the problems explained above.

[0009] This problem is solved by the gas sensor element with the features of claim 1 and by the method of claim 3. An advantageous embodiment of the subject matter of claim 1 is found in dependent claim 2.

[0010] The present disclosure provides a gas sensor element configured such that mixed regions and voids are present in a section plane of an electrode, so that the resistance value of the electrode is kept low and the oxygen-degrading activity is improved, and a method for manufacturing the same.

[0011] A gas sensor element according to one aspect of the present disclosure comprises a solid electrolyte plate exhibiting oxygen ion conductivity, a measuring gas chamber into which a measuring gas is introduced and which is configured in contact with a first surface of the solid electrolyte plate, a measuring electrode arranged on the first surface of the solid electrolyte plate such that it is located in the measuring gas chamber, a reference gas chamber into which a reference gas is introduced and which is configured in contact with a second surface of the solid electrolyte plate, and a reference electrode arranged on the second surface of the solid electrolyte plate such that it is located in the reference gas chamber. The measuring electrode and the reference electrode contain a precious metal containing platinum and a solid electrolyte made of a type of ceramic material identical to the ceramic material forming the solid electrolyte plate.In a section plane of a given electrode, defined by the measuring electrode and the reference electrode, along a thickness direction, there exists a noble metal region where the noble metal is agglomerated, a solid electrolyte region where the solid electrolyte is agglomerated, a mixed region where the noble metal and solid electrolyte are distributed, and a void. Mixed regions and voids are located near the noble metal region. The mixed region is formed by the mixing of the noble metal in the noble metal region and the solid electrolyte in the solid electrolyte region or the solid electrolyte plate at the boundary between the noble metal region and the solid electrolyte region or the solid electrolyte plate.A parameter value as the product of a mixed space ratio B / A, which specifies the ratio of the area B of the mixed region in the section plane to the area A of the specific electrode in the section plane, and a void ratio C / A, which specifies the ratio of the area C of the void in the section plane to the area A of the specific electrode in the section plane, falls in a range of 0.001 to 0.01.

[0012] In the previously described gas sensor element, the mixed region ratio B / A and the void ratio C / A are measured in the section plane of the electrode along the thickness direction. Mixed regions and voids are arranged in the section plane of the electrode such that the parameter value falls within a range of 0.001 to 0.01. This can keep the electrode resistance low and improve the oxygen-reducing activity of the gas sensor element.

[0013] Conversely, if the parameter value is less than 0.001 or exceeds 0.01, the electrode resistance increases, and therefore the oxygen-reducing activity of the gas sensor element cannot be improved. The voids in the section plane include open voids connected to a surface of the electrode and insulated voids within the electrode. The area A of the electrode in the section plane is the total area of ​​the noble metal region, the solid electrolyte region, the mixed region, and the insulated voids. When area A is obtained, the solid electrolyte region is a solid electrolyte region that is separated and insulated from the solid electrolyte that forms the solid electrolyte plate.

[0014] The "mixed region" is a region that is neither the precious metal region nor the solid electrolyte region of a total region that excludes the void of the section plane of the electrode and contains both the precious metal and the solid electrolyte. In the mixed region, the precious metal and the solid electrolyte open up in such a complex three-dimensional form that they are entangled. Furthermore, in the mixed region, the phases of the precious metal and the solid electrolyte are entangled in a marbled manner. Additionally, at least a portion of the precious metal and the solid electrolyte in the mixed region exhibits a phase that is continuous with the precious metal in the precious metal region, the solid electrolyte in the solid electrolyte region, or the solid electrolyte plate.

[0015] In the case of observing the section plane of the electrode, the mixed region is a region in which multiple interfaces exist between the precious metal and the solid electrolyte. Regions with multiple interfaces between the precious metal and the solid electrolyte are defined as regions in which two or more interfaces exist between the precious metal and the solid electrolyte. In other words, a region with multiple interfaces between the precious metal and the solid electrolyte indicates that the precious metal and the solid electrolyte are not simply divided in two by a single continuous curved line. [Brief description of the characters]

[0016] The previously described aims, characteristics, and beneficial effects of this disclosure will become clearer with reference to the detailed description provided below and the accompanying figures. These figures are: Fig. Figure 1 is a partial view to describe a gas sensor element with a measuring electrode and a reference electrode in one embodiment; Fig. 2 is a photograph of a section plane of the reference electrode in the embodiment; Fig. Figure 3 is a graph that shows a relationship between the product of a three-phase interface ratio and a void ratio in the reference electrode and a resistance value in the embodiment; and Fig. Figure 4 is a graph that shows a relationship between the void ratio in the reference electrode and the resistance value in the embodiment. [Description of the embodiments]

[0017] In the following, an embodiment of the previously described gas sensor element and a method for manufacturing it are described with reference to Fig. 1 to 4 described.

[0018] As in Fig. As shown in Figure 1, a gas sensor element 1 of the present embodiment comprises an electrolyte plate 2 exhibiting oxygen ion conductivity and a measuring electrode 3A and a reference electrode 3B provided on surfaces of the solid electrolyte plate 2. Each electrode 3A, 3B contains a precious metal containing platinum and a solid electrolyte made of the same type of ceramic material as the ceramic material forming the solid electrolyte plate 2. As shown in Figure 1, the gas sensor element 1 comprises an electrolyte plate 2 exhibiting oxygen ion conductivity and a measuring electrode 3A and a reference electrode 3B provided on surfaces of the solid electrolyte plate 2. Each electrode 3A, 3B contains a precious metal containing platinum and a solid electrolyte made of the same type of ceramic material as the ceramic material forming the solid electrolyte plate 2. Fig. As shown in Figure 2, in a section plane X1 of the reference electrode 3B along a thickness direction C there are noble metal regions 31 in which the noble metal is agglomerated, solid electrolyte regions 32 in which the solid electrolyte is agglomerated, mixed regions 33 in which the noble metal and the solid electrolyte are distributed and voids 34.

[0019] The ratio of an area B of the mixed regions 33 in section plane X1 to an area A of the reference electrode 3B in section plane X1 is here referred to as a "mixed region ratio B / A". Similarly, the ratio of an area C containing voids 34 in section plane X1 to the area A of the reference electrode 3B in section plane X1 is here referred to as a "void ratio C / A". Furthermore, a parameter value, as the product of the mixed region ratio B / A and the void ratio C / A, falls within a range of 0.001 to 0.01. Additionally, the void ratio C / A is 0.3 or less. It should be noted that the parameter value is expressed as a percentage: 0.1% when the parameter value is 0.001 and 1% when the parameter value is 0.01.

[0020] The gas sensor element 1 is arranged in an exhaust pipe of an internal combustion engine and uses the exhaust gas flowing through the exhaust pipe as a measuring gas G and atmospheric air as a reference gas A to obtain the oxygen concentration of the measuring gas G. In the present embodiment, the gas sensor element 1 forms an air-fuel ratio (A / F) sensor that utilizes limiting flow characteristics, based on the diffusion limitation of the measuring gas G, to quantitatively obtain the A / F (i.e., air-fuel ratio) of the engine. Alternatively, the gas sensor element 1 can form a concentration cell sensor designed to detect whether the air-fuel ratio, as the mixing ratio between fuel and air in the engine relative to the theoretical air-fuel ratio, is in a rich state with excess fuel or in a lean state with excess air.Alternatively, the gas sensor element 1 can form a NOx sensor designed to measure the NOx concentration in the sample gas G.

[0021] As in Fig. As shown in Figure 1, the solid electrolyte plate 2 is made of yttrium oxide-stabilized zirconium dioxide in a plate form. The electrodes 3A and 3B comprise the measuring electrode 3A, which is provided on a first surface of the solid electrolyte plate and exposed to the measuring gas G, and the reference electrode 3B, which is provided on a second surface of the solid electrolyte plate 2 and exposed to the reference gas A. A measuring gas chamber 41, into which the measuring gas G is introduced, is formed on the first surface of the solid electrolyte plate 2, and the measuring electrode 3A is arranged in the measuring gas chamber 41. The measuring gas chamber 41 is designed such that an insulator 43 and a diffusion resistance layer 44, which allows the measuring gas G to pass through at a predetermined diffusion rate, surround the measuring gas chamber 41.

[0022] A reference gas chamber 42, into which the reference gas A is introduced, is formed on the second surface of the solid electrolyte plate 2, and the reference electrode 3B is arranged in the reference chamber 42. A heater 5, which is laminated in the thickness direction T of the solid electrolyte plate 2, comprises heating elements 52, which are designed to generate heat when energy is supplied, and ceramic substrates 51, which embed the heating elements 52. The reference gas chamber 42 is designed such that the ceramic substrate 51 surrounds the reference gas chamber 42.

[0023] As shown in this figure, the gas sensor element 1 of the present embodiment is used as the A / F sensor in such a way that a predetermined voltage V, which exhibits the limiting current characteristics, is applied between the measuring electrode 3A and the reference electrode 3B. Oxygen molecules O2 in the measuring gas G receive electrons mainly at each interface between the noble metal and the solid electrolyte in the mixed regions of the measuring electrode 3A. - , to form oxygen ions O 2- to transform. Then the oxygen ions O pass through 2- the solid electrolyte plate 2. The oxygen ions O 2- Electrons that have passed through the solid electrolyte plate 2 transfer electrons at each interface between the noble metal and the solid electrolyte in the mixed regions of the reference electrode 3B. -, in order to transform back into oxygen molecules O2. It should be noted that the same applies to the conduction of oxygen ions O 2- In each electrode 3A, 3B, the discharge of oxygen contained in the measuring gas G applies in a case where the gas sensor element 1 forms the NOx sensor. Furthermore, in a case where the gas sensor element 1 forms the concentration cell sensor, the oxygen ions O 2- from the reference electrode 3B to the measuring electrode 3a.

[0024] The solid electrolyte plate 2 is designed such that many crystal particles made of yttrium oxide-stabilized zirconium dioxide are accumulated. The crystal particles are interconnected. Electrodes 3A and 3B contain crystal particles of platinum as the precious metal and crystal particles of yttrium oxide-stabilized zirconium dioxide as the solid electrolyte, i.e., the material they share with the solid electrolyte plate 2.

[0025] Fig. Figure 2 shows a photograph of the section plane X1 of the reference electrode 3B along the thickness direction T of the gas sensor element 1. This photograph was taken by photographing the section plane X1 of the reference electrode 3B through a scanning electron microscope (i.e., SEM).

[0026] As in Fig. As shown in Figure 2, the mixed regions 33 of platinum and solid electrolyte are distributed over substantially the entirety of the reference electrode 3B in the section plane X1. The mixed regions 33 are formed in the reference electrode 3B from a boundary position 301 with the solid electrolyte plate 2 to a surface position 302 over the entire area, except for a portion with the noble metal regions 31 in the solid electrolyte regions 32 and the voids 34. The mixed regions 33 are formed as parts in which multiple boundaries between the platinum and the solid electrolyte exist. In each mixed region 33, the platinum and the solid electrolyte are arranged in such a complex three-dimensional form that the platinum and the solid electrolyte are entangled with each other.Furthermore, a portion of the platinum in the mixed region 33 exhibits a phase that is continuous with the platinum in the precious metal region 31, and a portion of the solid electrolyte in the mixed region 33 exhibits a phase that is continuous with the solid electrolyte in the solid electrolyte region 32 or the solid electrolyte plate 2. It should be noted that, although not shown in the figures, a material distribution similar to that of the reference electrode 3B is also shown in a section plane of the measuring electrode 3A.

[0027] As in Fig. As shown in Figure 2, the voids 34 in the section plane X1 of the reference electrode 3B comprise open voids 34A, which are recessed into the surface of the reference electrode 3B, and closed voids 34B, which are formed within the reference electrode 3B. Each void 34 is continuously formed in three dimensions, and the closed void 34B can be the open void 34A when viewed in a different section plane X1. Furthermore, the surface position 302 of the reference electrode 3B is formed in a complex uneven shape.

[0028] Many voids 34, comprising the open voids 34A and the isolated voids 34B, are formed in areas adjacent to the mixed regions 33. Many three-phase interfaces between the precious metal and the solid electrolyte in the mixed regions 33 and atmospheric air in the voids 34 are formed at the periphery of the mixed regions 33. Each mixed region 33 is formed such that an electrical process is performed for the gas sensor element 1 to mix the precious metal in the precious metal region 31 and the solid electrolyte in the solid electrolyte plate 2 or the solid electrolyte region 32 at an interface between the precious metal region 31 and the solid electrolyte region 32 or the solid electrolyte plate 2.

[0029] In the gas sensor element 1 of the present embodiment, the extent of three-phase interface formation in the reference electrode 3B is determined by the parameter value as the product of the mixture region ratio B / A, which represents the proportion of the area B of the mixture regions 33 in which the noble metal and the solid electrolyte are distributed, and the void ratio C / A, and the proportion of the area C of the voids 34 into which oxygen is introduced. The mixture region ratio B / A and the void ratio C / A are measured in the section plane X1 of the reference electrode 3B along its thickness direction T.

[0030] Fig. Figure 3 shows a relationship between the parameter value (-) as the product of the mixed region ratio B / A and the void ratio C / A, and a resistance value (Ω). A formation area of ​​the reference electrode 3B in the gas sensor element 1, as described here, is an area in the section plane X1 of the gas sensor element 1 in which the solid electrolyte regions 32, which are separate and insulated from the solid electrolyte forming the solid electrolyte plate 2, the noble metal regions 31, the mixed regions 33, and the insulated voids 34B are formed. The area A of the reference electrode 3B in the section plane X1 is a total area that includes the noble metal regions 31, the solid electrolyte regions 32, which are separate and insulated from the solid electrolyte plate 2, the mixed regions 32, and the insulated voids 34B.The area B of the mixed regions 33 in section plane X1 can be easily obtained as the portions insulated by the solid electrolyte plate 2, the noble metal regions 31, and the solid electrolyte regions 32. The area C of the voids 34 in section plane X1 is obtained in section plane X1 as the area of ​​the insulating voids 34B present in the reference electrode 3B.

[0031] The parameter value is represented by the product of the mixed region ratio B / A and the vacancy ratio C / A and is a value that can be changed by altering either the mixed region ratio B / A or the vacancy ratio C / A. It should be noted that in the reference electrode 3B, most of the mixed regions 33 and the isolated vacancies 34B are formed adjacent to one another, and the area B of the mixed regions 33 and the area C of the isolated vacancies 34B are essentially proportionally related. Thus, there is a low probability that only one of the mixed region ratios B / A or the vacancy ratio C / A will vary significantly, and the parameter value is a value that indicates the extent of three-phase interface formation (i.e., a perimeter in section plane X1 of the reference electrode 3B as the product of the mixed region ratio B / A and the vacancy ratio C / A).

[0032] In the gas sensor element 1 of the present embodiment, a suitable three-phase interface formation extent for improving the oxygen-decomposing activity of the gas sensor element 1 is represented by using the parameter value as a scale. It was found that the magnitude of the parameter value influences the resistance value of the reference electrode 3B and thus the resistance value of the gas sensor element 1.

[0033] As in Fig. As shown in Figure 3, the resistance value is provided as a value obtained by measuring the current flowing between the pair of electrodes 3A and 3B, which are provided on the solid electrolyte plate 2, when a voltage is applied. That is, the resistance value in Fig. Value 3 is provided as the resistance value of the entire solid electrolyte plate 2 and the pair of electrodes 3A, 3B. It should be noted that a change in the resistance value reflects a change in the resistance value of the reference electrode 3B. Thus, a change in the resistance value of the entire solid electrolyte plate 2 and the pair of electrodes 3A, 3B can be viewed as a scale indicating a change in the resistance value of the reference electrode 3B.

[0034] A in Fig. Graph 3 shown is obtained by measuring the mixed region ratio B / A and the vacancy ratio C / A in the section plane X1 of the reference electrode 3B for each of the gas sensor element 1 samples with different energy supply times and voltages applied during the current processing, and by measuring the resistance value. The mixed region ratio B / A and the vacancy ratio C / A as described here can be obtained by sectioning the reference electrode 3B with a laser, etc., and observing such a section plane X1 with a scanning electron microscope (i.e., SEM).

[0035] The resistance value in Fig. 3 is maintained at a low value of 80 Ω or less when the parameter value falls within the range of 0.001 to 0.01. Furthermore, in a case where the parameter value is less than 0.001, a smaller parameter value becomes a larger resistance value. The reasons for this are assumed to be as follows: The proportions of the mixed regions 33 and the isolated voids 34B in the reference electrode 3B decrease; and as a result, the extent of three-phase interface formation in the reference electrode 3B decreases, as does the movement of oxygen ions O. 2- and the electrons e - decreases.

[0036] Conversely, if the parameter value exceeds 0.01, a higher parameter value results in a higher resistance value. The reasons for this are assumed to be as follows: The proportion of insulated voids 34B in the reference electrode 3B increases, and as a result, the movement of electrons e - influenced.

[0037] The resistance value of each electrode 3A, 3B indicates the ease of electron movement. - on, when the oxygen ions O 2- The value is directed into the gas sensor element 1 and serves as a scale that influences the quality of the oxygen-reducing activity of the gas sensor element 1. It was found that an extremely low or extremely high three-phase interface fraction in the reference electrode 3B leads to a high resistance value of the reference electrode 3B and therefore to lower oxygen-reducing activity of the gas sensor element 1. On the other hand, the following was found: The three-phase interface fraction in the reference electrode 3B is set such that the previously described parameter value falls within the range of 0.001 to 0.01; in this way, the resistance value of the reference electrode 3B is kept low and the oxygen-reducing activity of the gas sensor element 1 can be kept high.

[0038] Furthermore, in the present embodiment, a suitable range of the proportion of the insulated voids 34B in the reference electrode 3B was checked.

[0039] Fig. Figure 4 shows a relationship between the void ratio C / A (-) and the resistance value (Ω). A graph shown in this figure is obtained by measuring the void ratio C / A in the section plane X1 of the reference electrode 3B and measuring the resistance value for each of the gas sensor element 1 samples with different current durations and applied voltages during current processing.

[0040] As in Fig. As shown in Figure 4, in a case where the vacancy ratio C / A, which indicates the proportion of isolated vacancies 34B, exceeds 0.3, a larger vacancy ratio C / A results in a larger resistance value. The reasons are assumed to be as follows: The movement of the electrons e -will be as in the case of Fig. 3 is affected. On the other hand, if the void ratio C / A is 0.3 or less, the resistance value is kept at a low value of 80 Ω or less. This result shows that a void ratio C / A of 0.3 or less keeps the resistance value of the reference electrode 3B low and can maintain a high oxygen-depleting activity of the gas sensor element 1.

[0041] Next, the method for manufacturing the gas sensor element 1 of the present embodiment will be described.

[0042] In the fabrication of the gas sensor element 1, electron material paste is first layered onto both surfaces of a ceramic sheet to form the solid electrolyte plate 2, forming each of the electrodes 3A, 3B. Then the insulator 43, the diffusion resistance layer 44 and each of the heater sheets 5 are stacked onto the ceramic sheet of the solid electrolyte plate 2, forming an element intermediate.

[0043] The intermediate element is then heated in a baking step and baked at a temperature of 1400 to 1500 °C, with pressure being applied in one direction of the intermediate element stacking. In this way, a baked element body is obtained such that the electrodes 3A, 3B are each formed on the solid electrolyte plate 2. In each electrode 3A, 3B of the baked element body, there are many noble metal regions 31 in which the noble metal is agglomerated and many solid electrolyte regions 32 in which the solid electrolyte is agglomerated, and the proportion of vacancies 34 is small. Furthermore, there are almost no mixed regions 33 in each electrode 3A, 3B of the baked element body.

[0044] Finally, a current is applied to the area between the pair of electrodes 3A, 3B, which sandwich-like enclose the solid electrolyte plate 2 in the baked element body. In this current-energizing step, the baked element body is placed under a temperature environment of 800 to 900 degrees C, and a voltage of 1.5 to 2.5 V is applied between the pair of electrodes 3A, 3B for a predetermined time (e.g., 5 to 300 seconds).

[0045] Here, it is assumed that the precious metal penetrates the solid electrolyte in the solid electrolyte regions 31 of the reference electrode 3B and the solid electrolyte in the solid electrolyte plate 2. Furthermore, it is assumed that the current application forms the mixed regions 33, in which the precious metal and the solid electrolyte are unevenly distributed, and that the movement of the precious metal forms the voids 34 near the area where the precious metal was present before the movement. Additionally, the mixed regions 33 and the voids 34 are also formed in the measuring electrode 3A, just as in the reference electrode 3B.

[0046] In the current-energizing step, the baked element body is placed under a temperature environment of 800 to 900 °C, which is lower than a baking temperature. This prevents oxygen from escaping (i.e., blackening) from the solid electrolyte in the solid electrolyte plate 2, while promoting the movement of the noble metal into the noble metal regions 31. Additionally, a voltage of 1.5 to 2.5 V is applied between the electrode pair 3A, 3B for a predetermined time. This voltage is higher than the voltage (e.g., 0.1 to 1.0 V) applied to the electrode pair 3A, 3B during use (i.e., measurement) of the gas sensor element 1. This also prevents the solid electrolyte plate 2 from blackening, while promoting the movement of the noble metal into the noble metal regions 31.

[0047] If the ambient temperature is below 800°C or if the voltage applied during energization is lower than 1.5 V, not only must the energization time be increased, but there is also a probability that the mixed regions 33 and the voids 34 will not form sufficiently. Furthermore, if the ambient temperature exceeds 900°C or if the voltage applied during energization exceeds 2.5 V, not only is the movement of the precious metal in each of the electrodes 3A, 3B promoted, but so is the blackening of the solid electrolyte plate 2. This can lead to the deterioration of the solid electrolyte plate 2.

[0048] The present disclosure is not limited to the embodiment described above, and various embodiments can be implemented without deviating from the scope of this disclosure. For example, an electrode configured such that the parameter value described above falls within a range of 0.001 to 0.01 can be used as the measuring electrode 3A. In this case, the precious metal in the measuring electrode 3A can include gold in addition to platinum. In a case where the gas sensor element 1 is used for the NOx sensor, the precious metal in the measuring electrode 3A can include rhodium, etc., in addition to platinum.

Claims

Gas sensor element (1) comprising: a solid electrolyte plate (2) exhibiting oxygen ion conductivity; a measuring gas chamber (41) into which a measuring gas (G) is introduced and which is configured in contact with a first surface of the solid electrolyte plate (2); a measuring electrode (3A) arranged on the first surface of the solid electrolyte plate (2) such that it is located in the measuring gas chamber (41); a reference gas chamber (42) into which a reference gas (A) is introduced and which is configured in contact with a second surface of the solid electrolyte plate (2); a reference electrode (3B) arranged on the second surface of the solid electrolyte plate (2) such that it is located in the reference gas chamber (42);wherein the measuring electrode (3A) and the reference electrode (3B) contain platinum-containing noble metal and a solid electrolyte made of a type of ceramic identical to a ceramic material forming the solid electrolyte plate (2), a solid electrolyte region (32) in which the solid electrolyte is agglomerated, a mixed region (33) in which the noble metal and the solid electrolyte are distributed, a noble metal region (31) in which the noble metal is agglomerated, and a void (34) in a section plane (X1) of a particular electrode, which is one of the measuring electrode (3A) and the reference electrode (3B), along a thickness direction (T), wherein: the mixed region (33) and the voids (34) are located near the noble metal region (31);the mixed region (33) is formed by mixing the precious metal in the precious metal region (31) and the solid electrolyte in the solid electrolyte region (32) or the solid electrolyte plate (2) at the boundary between the precious metal region (31) and the solid electrolyte region (32) or the solid electrolyte plate (2); a parameter value as a product of a mixed region ratio B / A, which specifies a ratio of an area B of the mixed region in the section plane (X1) to an area A of the specific electrode (3A, 3B) in the section plane (X1), and a void ratio C / A, which specifies a ratio of an area C of the void (34) in the section plane (X1) to the area A of the specific electrode (3A, 3B) in the section plane (X1), falls in a range of 0.001 to 0.01; and wherein the resistance value of the entire solid electrolyte plate (2) and the pair of electrodes (3A, 3B) is less than 80 Ω.; Gas sensor element according to claim 1, wherein the void ratio C / A is 0.3 or less. A method for manufacturing a gas sensor element (1) comprising a solid electrolyte plate (2) exhibiting oxygen ion conductivity and a pair of electrodes (3A, 3B) provided on both surfaces of the solid electrolyte plate (2), comprising: a layering step in which a paste for forming the pair of electrodes (3A, 3B) is layered onto both surfaces of a ceramic sheet for forming the solid electrolyte plate (2), thereby forming an element intermediate; a baking step in which the element intermediate is baked to form a baked element body; and an energizing step in which an electric current flows through a portion between the pair of electrodes (3A, 3B) in the baked element body, wherein the paste contains platinum-containing noble metal and a solid electrolyte made of a type of ceramic material identical to a ceramic material forming the solid electrolyte plate (2).In the current-energizing step, the baked element body is placed under a temperature environment of 800 to 900 degrees C and a voltage of 1.5 to 2.5 V is applied between the pair of electrodes (3A, 3B) in the baked element body, and in this way a noble metal region (31) in which the noble metal is agglomerated, a solid electrolyte region (32) in which the solid electrolyte is agglomerated, and a mixed region (33) in which the noble metal and the solid electrolyte are distributed, and a void (34) are present in a plane (X1) of the section of the pair of electrodes (3A, 3B) along a thickness direction (T), a parameter value as a product of a mixed region ratio B / A, which is a ratio of an area B of the mixed region of a particular electrode of the pair of electrodes (3A, 3B) in the section plane (X1) to an area A of the particular electrode (3A, 3B) in the Section level (X1) and an empty space ratio C / A,which specifies a ratio of an area C of the void (34) of the specific electrode (3A, 3B) in the section plane (X1) to the area A of the specific electrode (3A, 3B) in the section plane (X1), falls into a range of 0.001 to 0.01; and wherein a resistance value of the entirety of the solid electrolyte plate (2) and the pair of electrodes (3A, 3B) is less than 80 Ω.

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