JP7313314B2 - Gas sensor element and gas sensor - Google Patents

Description

The present invention relates to gas sensor elements and gas sensors.

A gas sensor is arranged in an exhaust pipe or the like of an internal combustion engine of a vehicle, and is used to detect the concentration of a specific gas component, oxygen concentration, etc. contained in the detection target gas, which is exhaust gas flowing through the exhaust pipe. As this type of gas sensor, for example, a gas sensor that detects a mixed potential generated by an electrochemical oxidation reaction of a specific gas component and an electrochemical reduction reaction of oxygen gas occurring at a detection electrode formed on the surface of a solid electrolyte body is known. As the detection electrode, a material such as gold, which has catalytic activity for a specific gas component and oxygen gas, is used. Specific gas components include ammonia gas, nitrogen dioxide gas, and hydrocarbon gas.

As a gas sensor using a mixed potential, there is a gas sensor described in Patent Document 1, for example. The gas sensor has a gas sensor element having a detection electrode formed to have a porosity of 10% or more and 40% or less and a thickness of 5 μm or more and 35 μm or less.

JP 2019-158495 A

Conventionally known mixed potential type gas sensors are merely those in which the porosity and thickness of the detection electrodes are empirically set based on the known electrode structures of air-fuel ratio sensors, NOx sensors, and the like. The porosity defined in this manner includes closed pores that do not contribute to the electrochemical oxidation-reduction reaction, in addition to the open pores that contribute to the electrochemical oxidation-reduction reaction. Therefore, it is difficult to stabilize the sensor output simply because the detection electrode has a certain ratio of pores.

SUMMARY OF THE INVENTION The present invention has been made in view of such problems, and an object thereof is to provide a gas sensor element capable of stabilizing sensor output, and a gas sensor having the gas sensor element.

One aspect of the present invention is a gas sensor element (2) used in a gas sensor (1) that utilizes a mixed potential,
a solid electrolyte body (21) having ionic conductivity; a detection electrode (22) formed on a first surface (201) of the solid electrolyte body and exposed to a detection target gas (G); and a reference electrode (23) formed at a position facing the detection electrode on the second surface (202) of the solid electrolyte body,
The detection electrode has a large number of pores (22c),
In the gas sensor element (2), the product of the surface opening ratio of the pores on the surface of the detection electrode and the thickness μm of the detection electrode is 0.3 μm or more and 4 μm or less.

Another aspect of the present invention is a gas sensor (1) utilizing mixed potentials, comprising:
A gas sensor (1) having the above gas sensor element.

In the gas sensor element, the detection electrode has a large number of pores, and the product of the surface opening ratio of the pores on the surface of the detection electrode and the thickness μm of the detection electrode is within the specific range. Therefore, the gas sensor element can stabilize the sensor output.

The gas sensor has the gas sensor element. Therefore, according to the above gas sensor, a mixed potential type gas sensor capable of stabilizing the sensor output can be obtained.

It should be noted that the symbols in parentheses described in the claims and means for solving the problems indicate the correspondence with specific means described in the embodiments described later, and do not limit the technical scope of the present invention.

FIG. 1 is an explanatory view showing the gas sensor and gas sensor element of Embodiment 1. FIG. FIG. 2 is an explanatory diagram showing a cross section taken along line II-II shown in FIG. 3A and 3B are explanatory diagrams for explaining the technical significance of the product of the surface opening ratio of pores on the surface of the detection electrode and the thickness of the detection electrode in the gas sensor element of Embodiment 1. FIG. 3A is a cross-sectional view of a detection electrode of a conventional gas sensor element, and FIG. FIG. 4 is a diagram showing an example of a backscattered electron image of the surface of the detection electrode obtained by a scanning electron microscope when measuring the surface opening ratio of pores on the surface of the detection electrode in the gas sensor element of Embodiment 1. FIG. 5 is a diagram showing an example of a binarized image obtained by binarizing the backscattered electron image of FIG. FIG. 6 is an explanatory diagram showing an exhaust system of an internal combustion engine in which the gas sensor of Embodiment 1 is arranged. FIG. 7 is an explanatory diagram for explaining definitions and measuring methods of the average thickness T _ave , the maximum thickness T _max , and the minimum thickness T _min of the detection electrodes in the gas sensor element of Embodiment 2. FIG. FIG. 8 is a graph showing the relationship between the product [μm] (horizontal axis) of the surface opening ratio of pores on the surface of the detecting electrode and the thickness of the detecting electrode and the ammonia output [V] (vertical axis) when the temperature of the detecting electrode is 400° C., obtained for the gas sensor element and the gas sensor produced in Experimental Example 1. FIG. 9 is a graph showing the relationship between the product [μm] (horizontal axis) of the surface opening ratio of pores on the surface of the detecting electrode and the thickness of the detecting electrode and the ammonia output [V] (vertical axis) when the temperature of the detecting electrode is 450° C., obtained for the gas sensor element and the gas sensor produced in Experimental Example 1. FIG. 10 is a graph showing the relationship between the product [μm] (horizontal axis) of the surface opening ratio of pores on the surface of the detecting electrode and the thickness of the detecting electrode and the ammonia output [V] (vertical axis) obtained for the gas sensor element and the gas sensor produced in Experimental Example 1 when the temperature of the detecting electrode is 500° C. FIG. 11 is a graph showing the relationship between the product [μm] (horizontal axis) of the surface opening ratio of the pores on the surface of the detecting electrode and the thickness of the detecting electrode and the nitrogen dioxide output [V] (vertical axis) when the temperature of the detecting electrode is 400° C., obtained for the gas sensor element and the gas sensor produced in Experimental Example 1. 12 is a graph showing the relationship between the product [μm] (horizontal axis) of the surface opening ratio of the pores on the surface of the detecting electrode and the thickness of the detecting electrode and the nitrogen dioxide output [V] (vertical axis) obtained for the gas sensor element and the gas sensor produced in Experimental Example 1 when the temperature of the detecting electrode is 450° C. FIG. 13 is a graph showing the relationship between the product [μm] (horizontal axis) of the surface opening ratio of the pores on the surface of the detecting electrode and the thickness of the detecting electrode and the nitrogen dioxide output [V] (vertical axis) when the temperature of the detecting electrode is 500° C., obtained for the gas sensor element and the gas sensor produced in Experimental Example 1. 14 is a graph showing the relationship between the value of |(T _max −T _ave )/T _ave | [−] (horizontal axis) and the ammonia output [V] (vertical axis) obtained in Experimental Example 2. FIG. FIG. 15 is a graph showing the relationship between the value of |(T _min −T _ave )/T _ave | [−] (horizontal axis) and the ammonia output [V] (vertical axis) obtained in Experimental Example 2.

(Embodiment 1)
A gas sensor element and a gas sensor according to Embodiment 1 will be described with reference to FIGS. 1 to 6. FIG.

First, the gas sensor element of this embodiment will be described. As illustrated in FIGS. 1, 2, and 6, the gas sensor element 2 of this embodiment is used in a gas sensor 1 of this embodiment (details will be described later) that utilizes a mixed potential.

The gas sensor element 2 includes a solid electrolyte body 21, a detection electrode 22 formed on a first surface 201 of the solid electrolyte body 21 and exposed to the detection target gas G, and a reference electrode 23 formed on the second surface 202 of the solid electrolyte body 21 at a position facing the detection electrode 22.

In the gas sensor element 2, the solid electrolyte body 21 has ionic conductivity. Specifically, the solid electrolyte body 21 can be composed of a solid electrolyte having oxygen ion conductivity. Moreover, the solid electrolyte body 21 can also be composed of a solid electrolyte having proton conductivity. The solid electrolyte body 21 having oxygen ion conductivity can be made of a zirconia-based material having oxygen ion conductivity. Examples of zirconia-based materials include stabilized zirconia or partially stabilized zirconia containing an oxide (stabilizer) of a rare earth metal element such as yttria (yttrium oxide) in zirconia. The solid electrolyte body 21 having proton conductivity can be made of, for example, strontium zirconate, barium zirconate, strontium cerate, barium cerate, lanthanum phosphate, or the like.

The detection electrode 22 has a large number of pores 22c, as illustrated in FIGS. That is, the detection electrode 22 is formed porous, and has a skeleton portion 220 forming a skeleton of the detection electrode 22 and pores 22c. The pores 22 c of the detection electrode 22 have surface openings 221 c on the surface of the detection electrode 22 . Pores 22c extend inwardly from surface openings 221c. Note that the detection electrode 22 may include closed pores (not shown) that do not open on the surface of the detection electrode 22 .

Sensing electrode 22 may include a noble metal 22a. Further, the detection electrode 22 may specifically include a solid electrolyte 22b having ionic conductivity. Specifically, the skeleton portion 220 of the detection electrode 22 can include a noble metal 22a and a solid electrolyte 22b having ionic conductivity. The noble metal 22a only needs to have catalytic activity with respect to the gas G to be detected. The noble metal 22a can be mainly composed of gold. Having gold as the main component means that 50 mol % or more of the metal component constituting the noble metal 22a is gold. Therefore, the noble metal 22a containing gold as a main component includes not only gold but also gold alloys in which 50 mol % or more of the composition is gold. Examples of the partner element to be alloyed with gold in the gold alloy include palladium (Pd), platinum (Pt), rhodium (Rh), iridium (Ir), nickel (Ni), silver (Ag), copper (Cu), zinc (Zn), and the like, and one or more of these may be contained. Specific examples of the gold alloy include an alloy of gold and palladium, an alloy of gold and platinum, an alloy of gold and rhodium, an alloy of gold and iridium, an alloy of gold and nickel, an alloy of gold and silver, an alloy of gold and copper, an alloy of gold and zinc, and the like. If 50 mol % or more of the gold alloy is gold, the properties of gold are superior to those of the other element alloyed with gold. The solid electrolyte 22b can be composed of a solid electrolyte having oxygen ion conductivity. Further, the solid electrolyte 22b can also be composed of a solid electrolyte having proton conductivity. The solid electrolyte 22b having oxygen ion conductivity can be made of a zirconia-based material having oxygen ion conductivity. As the zirconia-based material, stabilized zirconia containing an oxide of a rare earth metal element such as yttria (yttrium oxide) in zirconia, partially stabilized zirconia, or the like can be used. The solid electrolyte 22b having proton conductivity can be made of, for example, strontium zirconate, barium zirconate, strontium cerate, barium cerate, lanthanum phosphate, or the like. In addition, when the detection electrode 22 contains the noble metal 22a and the solid electrolyte 22b, the total content of the noble metal 22a and the solid electrolyte 22b in the entire detection electrode 22 can be 50% by mass or more. In addition, the detection electrode 22 may also contain, for example, aluminum (Al), alumina (Al ₂ O ₃ ), or the like.

As the solid electrolyte used for the detection electrode 22, the same type of solid electrolyte as that constituting the solid electrolyte body 21 described above can be used. Thereby, the solid electrolyte 22b used for the detection electrode 22 can not only form the pores 22c but also function as a common material when sintered with the solid electrolyte body 21 . In addition, in the zirconia-based materials described above, not only zirconia-based materials having the same type of stabilizer and the same solid-solution amount of the stabilizer, but also zirconia-based materials having the same type of stabilizer but different solid-solution amounts of the stabilizer are included in the same type of zirconia-based material.

Here, in the gas sensor element 2, the product of the surface opening ratio of the pores 22c on the surface of the detection electrode 22 and the thickness μm of the detection electrode 22 is set to 0.3 μm or more and 4 μm or less. The technical significance of defining the above product is as follows.

The commonly used porosity includes not only open pores that contribute to electrochemical redox reactions, but also closed pores that do not contribute to electrochemical redox reactions. Therefore, it is difficult to stabilize the sensor output of the mixed potential type gas sensor 1 simply by specifying the porosity of the detection electrode 22 . Therefore, the present inventors conducted various experiments and found out the following points as a result of trial and error. In other words, the easiness of taking in the gas in the detection electrode 22 and the easiness of discharging the gas are important for stabilizing the output of the mixed potential type gas sensor 1 . The easiness of gas intake and easiness of gas discharge can be arranged by the product of the surface opening ratio of the pores 22c on the surface of the detection electrode 22 and the thickness μm of the detection electrode. Specifically, the surface opening ratio (unit: none) of the pores 22c on the surface of the detection electrode 22 is a measure related to the ease with which gas is taken into the interior from the surface of the detection electrode 22 and the gas is discharged from the interior to the surface of the detection electrode 22. Further, the thickness (unit: μm) of the detection electrode 22 is a scale for making information on the surface of the detection electrode 22 into three-dimensional information. From these, the product (unit: μm) of the surface opening ratio of the pores 22c on the surface of the detection electrode 22 and the thickness of the detection electrode is significant as a scale representing the state in which the surface opening ratio of the pores 22c on the surface of the detection electrode 22 expands three-dimensionally. The sensor output of the mixed-potential gas sensor 1 can be ordered by a new measure, the above product. In particular, when obtaining a stable sensor output for a highly adsorbable gas such as ammonia gas, the ease with which the gas is taken in and discharged is an important factor.

Specifically, FIG. 3A schematically shows a cross section of the detection electrode 9 of the conventional gas sensor element, and FIG. 3B schematically shows a cross section of the detection electrode 22 of the gas sensor element 2 of this embodiment. It is assumed that the detection electrode 9 in FIG. 3(a) and the detection electrode 22 in FIG. 3(b) have the same porosity. However, even if the porosity is the same, the surface of the conventional detection electrode 9 has substantially no surface openings 221c due to the pores 22c, and the pores 22c are closed on the surface. On the other hand, the surface of the detection electrode 22 has many surface openings 221c formed by pores 22c. Therefore, in the detection electrode 22 having such a microstructure, the uptake of the gas g1 and the discharge of the generated gas g3 from the detection electrode 22 are promoted.

The fact that the product of the surface opening ratio of the pores 22c on the surface of the detection electrode 22 and the thickness μm of the detection electrode 22 (hereinafter sometimes simply referred to as “surface opening ratio×detection electrode thickness”) is relatively small means that the surface opening area of the pores 22c on the surface of the detection electrode 22 is small, and a sufficient gas flow path cannot be secured in the detection electrode 22. On the other hand, the fact that the surface opening ratio×detection electrode thickness is relatively large means that the surface opening area of the pores 22c on the surface of the detection electrode 22 is large, and a sufficient gas flow path is ensured in the detection electrode 22. In the gas sensor element 2, the sensor output can be stabilized when the surface opening ratio×detection electrode thickness is 0.3 μm or more and 4 μm or less. This is believed to be because the surface opening ratio×detection electrode thickness being within a specific range promotes gas uptake into the detection electrode 22 and discharge of gas from the detection electrode 22, and maintains the imbalance (non-equilibrium state) in the speed of the electrochemical oxidation-reduction reaction, which is a prerequisite for the expression of the mixed potential.

If the surface opening ratio×detection electrode thickness is less than 0.3 μm, it becomes difficult for gas to be taken into the detection electrode 22 and discharged from the detection electrode 22, resulting in unstable sensor output. On the other hand, when the surface opening ratio×detection electrode thickness becomes 0.3 μm or more, gas intake into the detection electrode 22 and gas discharge from the detection electrode 22 are activated. However, when the surface opening ratio×detection electrode thickness exceeds 4 μm, at the mixed potential, the electrochemical reduction reaction of oxygen or the electrochemical oxidation reaction of oxygen ions, which causes the sensor output to decrease, is activated rather than the electrochemical oxidation reaction or electrochemical reduction reaction of the specific gas component that contributes to the improvement of the sensor output. As a result, the sensor output tends to decrease. From the viewpoint of ensuring sensor output, stabilizing sensor output, etc., the surface opening ratio×detection electrode thickness is preferably 0.4 μm or more, more preferably 0.5 μm or more, still more preferably 0.6 μm or more, even more preferably 0.7 μm or more, even more preferably 0.8 μm or more, and most preferably 1 μm or more. On the other hand, the surface opening ratio×detection electrode thickness is preferably 3.9 μm or less, more preferably 3.8 μm or less, even more preferably 3.7 μm or less, even more preferably 3.6 μm or less, and even more preferably 3.5 μm or less, from the viewpoint of improving sensor output and stabilizing sensor output. The numerical values of the upper and lower limits of the ratio of the surface opening times the thickness of the detection electrode can be arbitrarily combined, including the values shown in the drawings according to the experimental examples to be described later.

The surface opening ratio of the pores 22c described above is measured as follows. First, a backscattered electron image of the surface of the detection electrode 22 exposed to the detection target gas G is acquired using a scanning electron microscope (hereinafter sometimes referred to as SEM). At this time, FEG-Quanta 250 manufactured by FEI (or its successor if the model is discontinued) can be used as the SEM. Further, the SEM observation conditions can be acceleration voltage: 10 kV, magnification: 10000 times, and degree of vacuum: 50 Pa. FIG. 4 shows an example of a backscattered electron image of the surface of the detection electrode 22. As shown in FIG. In the backscattered electron image of FIG. 4 , the black area is the surface opening 221 c of the pores 22 c on the surface of the detection electrode 22 , and the light gray area and white area are the skeleton 220 of the detection electrode 22 . Specifically, the light gray region is the particulate solid electrolyte 22b forming the skeleton 220, and the white region is the particulate noble metal 22a forming the skeleton 220. As shown in FIG. Next, the acquired backscattered electron image is subjected to binarization processing using image analysis software to extract the surface openings 221c of the pores 22c on the surface of the detection electrode 22, and the surface opening ratio is quantified. The main purpose of the binarization processing is to distinguish between the surface openings 221c of the pores 22c on the surface of the detection electrode 22 and the skeleton portion 220 on the surface of the detection electrode 22 . The surface opening 221c and the skeleton portion 220 have different luminances. The binarization process is performed after removing noise remaining in the captured image and setting a predetermined threshold value. Specifically, the binarization process is a binarization process using two thresholds, and the threshold for extracting the black area that is the surface opening 221c is set to 0 to 70 (overall: 0 to 255, that is, the threshold for the light gray area and the white area that is the skeleton 220 is set to 71 to 255). Note that isolated points are removed from both bright and dark points. In addition, expansion and contraction processing (so-called smoothing processing) is performed. FIG. 5 shows an example of a binarized image. In the binarized image of FIG. 5, the light gray area is the surface opening 221c of the pore 22c on the surface of the detection electrode 22, which is extracted. The area ratio in the field of view is quantified for the extracted surface aperture 221c. Specifically, the surface aperture ratio (unit: none) in one field of view is calculated by the formula of (total area of surface apertures 221c in one field of view)/(area of entire binarized image in one field of view). The arithmetic average value of the surface aperture ratios in each field of view obtained from each binarized image obtained from each of the binarized images obtained for arbitrary ten different locations on the surface of the detection electrode 22 using one gas sensor element 2 as described above is taken as the surface aperture ratio (unit: none) of the pores 22c on the surface of the detection electrode 22.

Further, the thickness of the detection electrode 22 when obtaining the product described above is measured as follows. First, a portion of the gas sensor element 2 including the detection electrode 22 and the solid electrolyte body 21 is embedded in epoxy resin and mirror-polished so that cross sections along the thickness direction of the detection electrode 22 and the solid electrolyte body 21 are exposed. This is used as a measurement sample for observation with a scanning electron microscope (hereinafter sometimes referred to as SEM). Note that the measurement sample is in a non-deposited state. Next, the measurement sample is observed with an SEM, and the distance from the interface of the detection electrode 22/solid electrolyte body 21 to the surface of the detection electrode 22 is measured in the length measurement mode of the SEM. For the SEM, FEG-Quanta250 manufactured by EFI (or its successor if the model is discontinued) can be used. The SEM imaging conditions are acceleration voltage: 10 kV, degree of vacuum: 50 Pa, observation mode: backscattered electron image, observation magnification: 1000 times, and number of imaging fields: 10 fields. Next, the thickness of the detection electrode 22 at three locations is measured for one field of view, and the arithmetic average value thereof is taken as the thickness measurement value of the detection electrode 22 in one field of view. Then, the measured values of the thickness of the detection electrode 22 in each of the 10 fields of view are obtained, and the arithmetic average value is taken as the thickness (unit: μm) of the detection electrode 22 when obtaining the above product.

In the gas sensor element 2, the surface opening ratio of the pores 22c on the surface of the detection electrode 22 can be specifically set to 0.05 or more and 0.35 or less. According to this configuration, the intake of gas into the detection electrode 22 and the discharge of gas from the detection electrode 22 are promoted, and the bias in the speed of the electrochemical oxidation-reduction reaction (non-equilibrium state), which is a prerequisite for the expression of the mixed potential, is more likely to be maintained. Therefore, according to this configuration, it is possible to ensure the stabilization of the sensor output.

The surface opening ratio of the pores 22c is preferably 2% or more, more preferably 3% or more, and still more preferably 5% or more, from the viewpoint of promoting the intake of gas into the detection electrode 22 and the promotion of gas discharge from the detection electrode 22. In addition, the surface opening ratio of the pores 22c is preferably 25% or less, more preferably 20% or less, still more preferably 18% or less from the viewpoint of suppressing the electrochemical reduction reaction of oxygen and the suppression of the electrochemical oxidation reaction of oxygen ions. The numerical values of the upper and lower limits of the surface opening ratio of the pores 22c can be combined arbitrarily.

In the gas sensor element 2, the thickness of the detection electrode 22 when obtaining the product described above can be specifically set to 3 μm or more and 25 μm or less. According to this configuration, the intake of gas into the detection electrode 22 and the discharge of gas from the detection electrode 22 are promoted, and the bias in the speed of the electrochemical oxidation-reduction reaction (non-equilibrium state), which is a prerequisite for the expression of the mixed potential, is more likely to be maintained. Therefore, according to this configuration, it is possible to ensure the stabilization of the sensor output.

The thickness of the detection electrode 22 when obtaining the product described above is preferably 3 μm or more, more preferably 5 μm or more, and still more preferably 8 μm or more from the viewpoint of securing a reaction point for the electrochemical oxidation-reduction reaction of the detection target gas G. In addition, the thickness of the detection electrode 22 when obtaining the product described above can be preferably 25 μm or less, more preferably 20 μm or less, and even more preferably 15 μm or less from the viewpoint of ensuring responsiveness. It should be noted that the numerical values of the upper and lower limits of the thickness of the detection electrode 22 can be combined arbitrarily.

In the gas sensor element 2, the detection electrode 22 may be composed of a single layer, or may be composed of multiple layers.

The gas sensor element 2 can use ammonia (NH ₃ ) gas, nitrogen dioxide (NO ₂ ) gas, hydrocarbon gas, etc. as specific gas components contained in the gas G to be detected. When the specific gas component is ammonia gas, the partner gas of the specific gas component is oxygen (O ₂ ) gas. Similarly, when the specific gas component is nitrogen dioxide gas, the partner gas of the specific gas component is oxygen (O ₂ ) gas. When the specific gas component is a hydrocarbon gas, the partner gas of the specific gas component is oxygen (O ₂ ) gas.

The gas sensor element 2 can detect at least one of ammonia gas and nitrogen dioxide gas. In the case where the gas sensor element 2 detects ammonia gas, ammonia gas is a highly adsorbable gas, but the detection electrode 22 that promotes the intake of the gas into the detection electrode 22 and the discharge of the gas from the detection electrode 22 increases the stabilizing effect of the sensor output. Further, when the gas sensor element 2 detects nitrogen dioxide gas, the nitrogen dioxide gas is a highly adsorbable gas like ammonia gas, but the detection electrode 22 promotes the intake of the gas into the detection electrode 22 and the discharge of the gas from the detection electrode 22, thereby increasing the stabilizing effect of the sensor output.

An example of the detailed configuration of the gas sensor element 2 of this embodiment will be described below, but the gas sensor element 2 of this embodiment is not limited to the following configuration.

The gas sensor element 2 shown in FIGS. 1 and 2 is constructed by stacking a solid electrolyte body 21 provided with a detection electrode 22 and a reference electrode 23 and an insulator 3 in which a heating element 41 is embedded. The gas sensor element 2 is formed in an elongated shape. A portion of the gas sensor element 2 on the front end side X1 in the longitudinal direction X is accommodated in a cover that constitutes the gas sensor 1 and is arranged in an exhaust pipe 71 to be described later. In the gas sensor element 2, the direction perpendicular to the longitudinal direction X in which the solid electrolyte body 21 and the insulator 3 are laminated is called the lamination direction D, and the direction perpendicular to both the longitudinal direction X and the lamination direction D is called the width direction W.

Solid electrolyte body 21 is formed in a plate shape. A first surface 201 of the solid electrolyte body 21 exposed to the detection target gas G forms the outermost surface of the gas sensor element 2 . The detection electrodes 22 provided on the first surface 201 are formed in a state in which the gas G to be detected easily comes into contact. In FIGS. 1 and 2, the surface of the detection electrode 22 is not provided with a protective layer made of a ceramic porous material or the like. Therefore, the gas G to be detected can come into contact with the detection electrode 22 without being diffusion-controlled. It is also possible to provide a protective layer on the surface of the detection electrode 22 so as not to reduce the flow velocity of the detection target gas G as much as possible.

A reference gas duct (air duct) 24 into which air is introduced is formed adjacent to the second surface 202 of the solid electrolyte body 21 . The second surface 202 of the solid electrolyte body 21 and the reference electrode 23 provided on the second surface 202 of the solid electrolyte body 21 are exposed to the atmosphere as the reference gas A.

The detection electrode 22 is provided on the first surface 201 of the solid electrolyte body 21 exposed to the detection target gas G containing oxygen and ammonia. The detection electrode 22 includes a noble metal 22a and an ionically conductive solid electrolyte 22b. Here, a zirconia-based material having oxygen ion conductivity is used as the solid electrolyte 22b having ion conductivity, which is a common material when sintered with the solid electrolyte body 21. As shown in FIG. Stabilized zirconia containing an oxide of a rare earth metal element such as yttria (yttrium oxide) or partially stabilized zirconia is used as the zirconia-based material. A lead portion 221 connected to the detection electrode 22 is provided on the first surface 201 of the solid electrolyte body 21 . The lead portion 221 is used to electrically connect the detection electrode 22 to the outside of the gas sensor 1 .

Reference electrode 23 is provided on second surface 202 of solid electrolyte body 21 opposite to first surface 201 . The reference electrode 23 is formed at a position facing the detection electrode 22 with the solid electrolyte body 21 interposed therebetween. The second surface 202 of the solid electrolyte body 21 and the reference electrode 23 provided on the second surface 202 are exposed to the atmosphere as the reference gas A. The reference electrode 23 contains a noble metal having catalytic activity against oxygen, and a zirconia-based material that serves as a common material when sintered with the solid electrolyte body 21 . Platinum (Pt) or the like can be used as the noble metal forming the reference electrode 23 . A lead portion 231 connected to the reference electrode 23 is provided on the second surface 202 of the solid electrolyte body 21 . The lead portion 231 is used to electrically connect the reference electrode 23 to the outside of the gas sensor 1 .

In the gas sensor element 2, the detection electrode 22, the reference electrode 23, and the portion of the solid electrolyte body 21 sandwiched between the detection electrode 22 and the reference electrode 23 form a detection cell that conducts oxygen ions. The temperature of the gas sensor element 2 due to the heat generated by the heat generating portion 411 of the heat generating body 41 is controlled so that the temperature of the detection cell becomes a predetermined operating temperature.

The insulator 3 is formed by a spacer insulator portion 31 provided with a notch portion forming the reference gas duct 24 and a heater insulator portion 32 in which the heating element 41 is embedded. The insulator 3 is made of an insulating ceramic material such as alumina. The reference gas duct 24 is formed from the position where the reference electrode 23 is arranged to the position on the base end side X2 in the longitudinal direction X. As shown in FIG. Air as a reference gas A is introduced into the reference gas duct 24 from an opening 241 formed at a position on the base end side X2 in the longitudinal direction X. As shown in FIG.

A heating element 41 that generates heat when energized is embedded in the heater insulator portion 32 of the insulator 3 . The heating element 41 is formed of a heating portion 411 and a heating element lead portion 412 connected to the heating portion 411 . The heat generating portion 411 is arranged at a position facing the detection electrode 22 and the reference electrode 23 in the stacking direction D. As shown in FIG. An energization control unit 52 for energizing the heating element 41 is connected to the heating element 41 . The energization control unit 52 is formed using a drive circuit or the like that applies a voltage subjected to PWM (Pulse Width Modulation) control or the like to the heating element 41 . The energization control section 52 is formed within the sensor control unit 5 .

The cross-sectional area of the heat generating portion 411 is smaller than the cross-sectional area of the heat generating lead portion 412 , and the resistance value per unit length of the heat generating portion 411 is higher than the resistance value per unit length of the heat generating lead portion 412 . This cross-sectional area means a cross-sectional area in a plane orthogonal to the direction in which the heat generating portion 411 and the heat generating lead portion 412 extend. Then, when a voltage is applied to the heating element lead portion 412 , the heating portion 411 generates heat due to Joule heat, and this heat generation heats the periphery of the detection electrode 22 and the reference electrode 23 .

A mixed potential is developed by a bias in the rate of electrochemical oxidation-reduction reaction between a specific gas component and its partner gas at a reaction point within the detection electrode. Therefore, if the intake of the gas before reaching the reaction point in the electrode and the discharge of the generated gas after the electrochemical oxidation-reduction reaction at the reaction point are delayed, the mixed potential will be destabilized. That is, the sensor output becomes unstable. In contrast, in the gas sensor element 2 of the present embodiment, the detection electrode 22 has a large number of pores 22c, and the product of the surface opening ratio of the pores 22c on the surface of the detection electrode 22 and the thickness μm of the detection electrode 22 is within the above specific range. Therefore, the gas sensor element 2 can stabilize the sensor output.

Next, the gas sensor 1 of this embodiment will be described. As illustrated in FIGS. 1, 2, and 6, the gas sensor 1 of this embodiment is of a mixed potential type and has the gas sensor element 2 of this embodiment.

An example of the detailed configuration of the gas sensor 1 of this embodiment will be described below, but the gas sensor 1 of this embodiment is not limited to the following configuration.

Gas sensor 1 shown in FIGS. The detection unit 51 is configured to detect a mixed potential generated between the detection electrode 22 and the reference electrode 23 based on the specific gas component concentration and the oxygen concentration.

The gas sensor 1 is arranged and used in an exhaust pipe 71 of an internal combustion engine (engine) 7 of a vehicle, as illustrated in FIG. The gas G to be detected by the gas sensor 1 is exhaust gas discharged from the internal combustion engine 7 to the exhaust pipe 71 . The gas sensor 1 is arranged downstream of the NOx reducing catalyst 72 arranged in the exhaust pipe 71 in the exhaust gas flow, and detects the concentration of ammonia gas flowing out from the catalyst 72 .

The gas sensor 1 detects ammonia gas concentration as a specific gas component concentration and a mixed potential based on the oxygen gas concentration, corrects this mixed potential with the oxygen gas concentration, and detects the ammonia gas concentration. The detection unit 51 is configured to detect, as a mixed potential, the potential difference ΔV between the detection electrode 22 and the reference electrode 23 that occurs when the reduction current due to the electrochemical reduction reaction of oxygen and the oxidation current due to the electrochemical oxidation reaction of ammonia are equal.

The detector 51 is formed in the sensor control unit 5 connected to the engine control unit 50 of the vehicle. The detection unit 51 includes a potential difference detection circuit 511 that detects the potential difference ΔV generated between the detection electrode 22 and the reference electrode 23, an arithmetic processing unit 512 that corrects the potential difference ΔV detected by the potential difference detection circuit 511 with the oxygen concentration to determine the ammonia concentration, and the like. Arithmetic processing unit 512 can determine the ammonia concentration by comparing the potential difference ΔV and the oxygen concentration against the relationship map using the relationship map obtained by obtaining the relationship between the potential difference ΔV and the ammonia concentration using the oxygen concentration as a parameter.

In the detection electrode 22, when ammonia and oxygen are present in the detection target gas G in contact with the detection electrode 22, an electrochemical oxidation reaction of ammonia and an electrochemical reduction reaction of oxygen proceed simultaneously. The electrochemical oxidation reaction of ammonia is typically represented by 2NH ₃ +3O ²⁻ →N ₂ +3H ₂ O+6e ⁻ . The electrochemical reduction reaction of oxygen is typically represented by O ₂ +4e ⁻ →2O ²⁻ . A mixed potential of ammonia and oxygen in the detection electrode 22 is generated as a potential when the electrochemical oxidation reaction (rate) of ammonia and the electrochemical reduction reaction (rate) of oxygen are equal to each other in the detection electrode 22.

Although illustration is omitted, the oxygen gas concentration is detected by an oxygen sensor different from the gas sensor 1 . The oxygen sensor is arranged downstream of the catalyst 72 in the exhaust pipe 71 . Then, in the detection unit 51, the oxygen gas concentration obtained by the oxygen sensor is used to correct the mixed potential and obtain the ammonia concentration.

As illustrated in FIG. 6 , an exhaust pipe 71 of the internal combustion engine 7 is provided with a catalyst 72 for reducing NOx and a reducing agent supply device 73 for supplying a reducing agent K containing ammonia to the catalyst 72 . The catalyst 72 has a catalyst carrier on which ammonia as a reducing agent K for NOx is adhered. The amount of ammonia deposited on the catalyst carrier of the catalyst 72 decreases with the reduction reaction of NOx. Then, when the amount of ammonia adhering to the catalyst carrier becomes small, ammonia is newly replenished from the reducing agent supply device 73 to the catalyst carrier. The reducing agent supply device 73 is arranged in the exhaust pipe 71 upstream of the catalyst 72 in the flow of the exhaust gas, and supplies the ammonia gas generated by injecting the urea water to the exhaust pipe 71 . Ammonia gas is produced by hydrolysis of urea water. A urea water tank 731 is connected to the reducing agent supply device 73 .

Specifically, the internal combustion engine 7 can be a diesel engine that performs combustion operation using self-ignition of light oil. Also, the catalyst 72 is a selective reduction catalyst (SCR) that chemically reacts NOx (nitrogen oxide) with ammonia (NH ₃ ) to reduce it to nitrogen (N ₂ ) and water (H ₂ O). Although not shown, an oxidation catalyst (DOC) that converts (oxidizes) NO to NO ₂ and reduces CO, HC (hydrocarbons), etc., a filter (DPF) that collects fine particles, etc. may be arranged in the exhaust pipe 71 upstream of the catalyst 72.

The gas sensor element 2 and gas sensor 1 of the present embodiment are useful for detecting the concentration of ammonia in order to precisely control the injection amount of urea water in an SCR system, which is a system for purifying exhaust gas from diesel engines. The gas sensor element 2 and gas sensor 1 of this embodiment can also be configured to detect the concentration of nitrogen dioxide in the exhaust gas. In the SCR system, detecting the nitrogen dioxide concentration is effective in controlling the urea water injection amount, detecting failure of the selective reduction catalyst, and the like.

The gas sensor 1 of this embodiment has the gas sensor element 2 of this embodiment. Therefore, according to the gas sensor 1 of the present embodiment, the mixed potential type gas sensor 1 capable of stabilizing the sensor output can be obtained.

(Embodiment 2)
A gas sensor element and a gas sensor according to Embodiment 2 will be described with reference to FIG. It should be noted that, of the reference numerals used in the second and subsequent embodiments, the same reference numerals as those used in the previously described embodiments represent the same components and the like as those in the previously described embodiments, unless otherwise specified.

In the gas sensor element 2 of the present embodiment, when the average thickness of the detection electrode 22 is T _ave , the maximum thickness is T _max , and the minimum thickness is T _min ,
|(T _max −T _ave )/T _ave |≦0.5 and |(T _min −T _ave )/T _ave |≦0.5. Note that "||" means taking an absolute value.

The average thickness T _ave of the sensing electrodes 22 , the maximum thickness T _max of the sensing electrodes 22 , and the minimum thickness T _min of the sensing electrodes 22 are collectively referred to as thickness parameters of the sensing electrodes 22 . A thickness parameter of the sensing electrode 22 is measured and defined as follows. Note that the average thickness T _ave of the detection electrodes 22 is based on a concept different from the thickness of the detection electrodes 22 when obtaining the product described above in the first embodiment.

A thickness parameter of the detection electrode 22 is acquired by a laser displacement meter. Specifically, it is as follows. The position where the thickness of the detection electrode 22 is measured by the laser displacement meter is the position where the length L of the detection electrode forming portion 22L, which will be described later, is the longest among straight lines that bisect the electrode area of the detection electrode 22 . As shown in FIG. 7, the measurement is started from the outside of the detection electrode 22 with the laser displacement meter, and the thickness at the starting position P0 is set to 0 μm. A position P2 is a position further advanced by 200 μm from the position P1 where the laser displacement meter first detects a thickness of 1 μm. Then, the position where the laser displacement meter next detects the thickness of 1 μm is defined as P1′. A position 200 μm before the position P1′ is defined as P2′. A sensing electrode forming portion 22L is defined from the position P2 to the position P2', and the length from the position P2 to the position P2' is defined as the length L of the sensing electrode forming portion 22L. The average value of the thickness obtained by the laser displacement meter in the detection electrode forming portion 22L is taken as the average thickness T _ave of the detection electrode 22 . The maximum thickness _Tmax of the detection electrode 22 is the maximum thickness obtained by the laser displacement meter in the detection electrode forming portion 22L. The minimum value of the thickness obtained by the laser displacement meter in the detection electrode forming portion 22L is taken as the minimum thickness T _min of the detection electrode 22 . The reason why the detection electrode forming portion 22L is defined as described above is to eliminate the influence of the large variation in thickness of the end portion of the detection electrode 22, which normally occurs.

The thickness parameter of the detection electrode 22 defined as described above is significant as an index of thickness variation in the detection electrode 22 . It means that the smaller the value of the thickness parameter of the detection electrode 22 is, the smaller the thickness variation in the detection electrode 22 is. When the thickness variation in the detection electrode 22 is small, the gas G to be detected enters through the surface openings 221c of the pores 22c on the surface of the detection electrode 22, the specific gas component undergoes an electrochemical oxidation-reduction reaction, and the mixed potential is stably developed. On the other hand, if the thickness variation in the detection electrode 22 becomes large, the electrochemical oxidation-reduction reaction in the detection electrode 22 becomes biased, making it difficult to develop a stable mixed potential.

When the gas sensor element 2 satisfies |(T _max −T _ave )/T _ave |≦0.5 and |(T _min −T _ave )/T _ave |≦0.5, the sensor output is easily stabilized. This is because the use of the detection electrode 22 with small thickness variation promotes the intake of gas into the detection electrode 22 and the discharge of the generated gas from the detection electrode 22, and the imbalance in the rate of the electrochemical oxidation-reduction reaction (non-equilibrium state), which is a prerequisite for the expression of the mixed potential, is more likely to be maintained.

Other configurations and effects are the same as those of the gas sensor element 2 of the first embodiment.

The gas sensor 1 of this embodiment has the gas sensor element 2 of this embodiment. Since the gas sensor 1 of this embodiment has the gas sensor element 2 of this embodiment, it is easy to stabilize the sensor output for the above reasons. Other configurations and effects are the same as those of the gas sensor 1 of the first embodiment.

(Experimental example 1)
-Production of gas sensor elements and gas sensors-
An insulator-forming slurry was prepared by kneading Al ₂ O ₃ powder, a binder, and a plasticizer. The insulator-forming slurry was placed in a sheet molding machine and dried to produce three insulator-forming sheets. A heating element forming paste was prepared by kneading Pt powder, Al ₂ O ₃ powder as a common material, and a binder. A heating element forming part for forming a heating element was printed on one surface of one insulator forming sheet with a heating element forming paste and dried. A part of another insulator forming sheet was punched out to form a reference gas duct forming portion for forming a reference gas duct. The reference gas duct formation portion was filled with carbon.

Also, a ZrO ₂ powder containing Y ₂ O ₃ , a binder, and a plasticizer were kneaded to prepare a slurry for forming a solid electrolyte. The slurry for forming a solid electrolyte body was placed in a sheet molding machine and dried to prepare a sheet for forming a solid electrolyte body. The Pt powder, the ZrO ₂ powder as a common material, and a binder were kneaded to prepare a paste for forming a reference electrode. A reference electrode/lead forming portion for forming a reference electrode and a lead portion was printed on one side of the solid electrolyte forming sheet with a reference electrode forming paste and dried.

Next, an insulator-forming sheet on which a heating element-forming portion was printed, an insulator-forming sheet, a reference gas duct-forming portion filled with carbon, and a solid electrolyte-forming sheet on which a reference electrode/lead forming portion were printed were laminated in this order, pressed, and then cut into an element shape. Then, this laminate was fired at 1400° C. to obtain a sintered body.

Next, the Au powder, the ZrO ₂ powder as a common material, and a binder were kneaded to prepare a paste A for forming a detection electrode. Further, the Au powder, the ZrO ₂ powder as a common material, a binder, and an aqueous solution of ZrO(NO ₃ ) _{2.2H 2} O (zirconyl nitrate dihydrate) were kneaded to prepare a paste B for forming a detection electrode. The amount of ZrO(NO ₃ ) ₂ ·2H ₂ O added was 0.05 wt % with respect to the Au powder.

Next, a sensing electrode forming portion (part) for forming a portion of the sensing electrode and a lead portion for forming a lead portion were printed and dried using the sensing electrode forming paste A at predetermined locations of the solid electrolyte body in the sintered body formed in the form of an element. After that, the detecting electrode forming portion (remaining portion) for forming the remaining portion of the detecting electrode was printed on the detecting electrode forming portion (part) with the detecting electrode forming paste B, and dried. That is, the sensing electrode forming portion (entire) for forming the entire sensing electrode has a two-layer structure of the sensing electrode forming portion (part) formed by the sensing electrode forming paste A and the sensing electrode forming portion (remainder) formed by the sensing electrode forming paste B, which is laminated on the surface of the sensing electrode forming portion (part).

Next, the sintered body on which the detection electrode forming portion (whole) was printed was fired at 800° C. to obtain a gas sensor element. The detection electrode of the obtained gas sensor element had a large number of pores, and a large number of surface openings due to the pores were confirmed on the surface of the detection electrode, as exemplified in FIG. The reason why such surface openings of pores are formed is considered as follows. In the detection electrode forming portion (remaining portion) described above, Zr, which is not sintered at a firing temperature of about 800° C. to 900° C., is finely and highly dispersed. In this experimental example, by adding zirconyl nitrate in an aqueous solution during the preparation of the detection electrode forming paste B, Zr can be highly dispersed at the atomic level compared to adding powder. As a result, during firing of the detection electrode forming portion (remaining portion), the highly dispersed Zr inhibits the movement of the Au to agglomerate due to heating, suppresses the grain growth of the Au, and is thought to form surface openings due to pores on the surface of the detection electrode.

- Measurement of the product of the surface opening ratio of pores on the surface of the detection electrode and the thickness of the detection electrode -
For the produced gas sensor element, a backscattered electron image of the surface of the detection electrode was obtained by SEM according to the above-described measurement method, the obtained backscattered electron image was subjected to binarization processing using image analysis software, the surface openings of the pores on the surface of the detection electrode were extracted, and the surface opening ratio was quantified. In addition, the thickness of the detection electrode was measured when obtaining the above product according to the above-described measurement method. At this time, FEG-Quanta250 manufactured by FEI was used as the SEM. In addition, the brightness was set to 77.8 and the contrast was set to 82.7 during SEM imaging of the detection electrode surface. Also, WinROOF Ver. 7.4 was used.

In the detection electrode of the gas sensor element produced above, the product of the surface opening ratio of the pores on the surface of the detection electrode and the thickness of the detection electrode was 1.98 μm.

In the gas sensor element described above, a plurality of gas sensor elements having different values of the product of the surface opening ratio of the pores on the surface of the detection electrode and the thickness of the detection electrode were produced by changing the addition amount of the zirconyl nitrate dihydrate aqueous solution and the thickness of the detection electrode. However, the sintering temperature of the detection electrodes was kept constant at 800°C. When the amount of zirconyl nitrate dihydrate added is increased, the surface open ratio of pores is increased, and when the amount of zirconyl nitrate dihydrate added is decreased, the surface open ratio of pores is decreased.

Further, each gas sensor incorporating each gas sensor element was produced.

－Measurement of ammonia output and nitrogen dioxide output－
Ammonia output and nitrogen dioxide output were measured for a plurality of gas sensors incorporating gas sensor elements having different values of the product of the surface opening ratio of pores on the surface of the detection electrode and the thickness of the detection electrode. At this time, the ammonia output was measured under the following conditions: detection electrode temperature: 400° C., 450° C. or 500° C.; supply gas concentration: 10 vol % O ₂ +100 ppm (volume ratio) NH ₃ . In addition, the nitrogen dioxide output was measured under the following conditions: detection electrode temperature: 400° C., 450° C., or 500° C.; supply gas concentration: 10 vol % O ₂ +100 ppm (volume ratio) NO ₂ . The measurement results of the ammonia output are shown in FIGS. 8 to 10. FIG. 11 to 13 show the measurement results of the nitrogen dioxide output.

First, FIGS. 8 to 10 will be described. The mechanism of ammonia output in the mixed potential type gas sensor is as follows. The mixed potential of ammonia is developed by an electrochemical oxidation-reduction reaction between ammonia and oxygen.
2NH ₃ +3O ²⁻ →N ₂ +3H ₂ O+ ^6e (1)
O ₂ +4e ⁻ →2O ²⁻ (2)
The more accelerated the electrochemical oxidation reaction of _NH3 in equation (1) above, the higher the output of the mixed potential, and the more accelerated the electrochemical reduction reaction of O2 in equation ( ₂ ) above, the lower the output of the mixed potential. By the way, _NH3 is a highly reactive substance, so it is easily oxidized in the vapor phase and disappears on the detection electrode according to the following reaction formula.
4NH ₃ +3O ₂ →2N ₂ +6H ₂ O (3)
4NH ₃ +5O ₂ →4NO+6H ₂ O (4)
Therefore, it is important for improving the sensor output to promote the electrochemical reactions of the above equations (1) and (2) while suppressing the vapor phase oxidation reactions of the above equations (3) and (4). In principle, however, the promotion of the above formula (2) leads to a decrease in the output of the mixed potential, so it is important to promote the reaction of the above formula (1).

As shown in FIGS. 8 to 10, when the surface opening ratio×detection electrode thickness is less than 0.3 μm, it becomes difficult for gas to be taken into and discharged from the detection electrode, and ammonia output is unstable. On the other hand, when the surface opening ratio×detection electrode thickness is 0.3 μm or more, the intake of gas into the detection electrode and the discharge of gas from the detection electrode become active. However, when the surface opening ratio×detection electrode thickness exceeds 4 μm, the ammonia output tends to decrease. This is because the electrochemical reduction reaction of oxygen (equation (2) above), which causes a decrease in ammonia output, is activated rather than the electrochemical oxidation reaction of ammonia (equation (1) above), which contributes to the increase in ammonia output. On the other hand, when the surface opening ratio×detection electrode thickness is in the range of 0.3 μm or more and 4 μm or less, the ammonia output is maintained high and stabilized. This is because gas-phase oxidation of _NH3 (equation (3) and equation (4) above) caused by a decrease in gas flow velocity (gas retention) is less likely to occur, and the electrochemical oxidation reaction of ammonia (equation (1) above) is promoted.

Next, FIGS. 11 to 13 will be described. The mechanism of nitrogen dioxide output in the mixed potential type gas sensor is as follows. The mixed potential of nitrogen dioxide is developed by an electrochemical oxidation-reduction reaction between nitrogen dioxide and oxygen ions.
NO ₂ +2e ⁻ → NO + O ²⁻ (5)
2O ²⁻ →O ₂ +4e ⁻ (6)
When the electrochemical reduction reaction of NO ₂ in the above equation (5) is further promoted, the mixed potential output increases, and when the electrochemical oxidation reaction of O ²⁻ in the above equation (6) is further promoted, the mixed potential output decreases.

As shown in FIGS. 11 to 13, when the surface opening ratio×detection electrode thickness is less than 0.3 μm, it becomes difficult for gas to enter and exit the detection electrode, resulting in unstable nitrogen dioxide output. On the other hand, when the surface opening ratio×detection electrode thickness becomes 0.3 μm or more, gas intake into the detection electrode and gas discharge from the detection electrode are activated. However, when the surface opening ratio×detection electrode thickness exceeds 4 μm, the nitrogen dioxide output tends to decrease. This is because the electrochemical oxidation reaction of oxygen ions (equation (6) above), which causes a decrease in nitrogen dioxide output, is activated rather than the electrochemical reduction reaction of nitrogen dioxide (equation (5) above), which contributes to an increase in nitrogen dioxide output. On the other hand, the nitrogen dioxide output is maintained at a high level and stabilized when the surface opening ratio×detection electrode thickness is in the range of 0.3 μm or more and 4 μm or less. This is because the removal of gas diffusion rate-limiting portions due to pore clogging on the surface of the detection electrode makes it difficult for the decomposition of NO ₂ (formula (7) below) caused by a decrease in gas flow rate (gas retention) to occur, thereby promoting the electrochemical reduction reaction of nitrogen dioxide (formula (5) above).
2NO ₂ →2NO+O ₂ (7)

According to these results, it was confirmed that a mixed potential type gas sensor element and gas sensor capable of stabilizing the sensor output can be obtained by setting the product of the surface opening ratio of the pores on the surface of the detection electrode and the thickness of the detection electrode to 0.3 μm or more and 4 μm or less.

(Experimental example 2)
In the same manner as in Experimental Example 1, a plurality of gas sensor elements were produced in which the product of the surface opening ratio of the pores on the surface of the detection electrode and the thickness of the detection electrode was 0.3 μm or more and 4 μm or less.

-Measurement of the thickness parameter of the detection electrode-
The thickness parameter of the sensing electrode of the manufactured gas sensor element was measured according to the measurement method described above. LT-9010M manufactured by Keyence Corporation was used as a laser displacement meter when measuring the thickness parameter of the detection electrode. In this experimental example, the thickness of the detection electrode on a straight line along the longitudinal direction passing through the center of the detection electrode in the width direction was measured with a laser displacement meter.

Each gas sensor incorporating each gas sensor element was produced, and the ammonia output was measured in the same manner as in Experimental Example 1. The ammonia output was measured under the following conditions: detection electrode temperature: 450°C; supply gas concentration: 10 vol% O ₂ +100 ppm (volume ratio) NH ₃ . The results are shown in FIGS. 14 and 15. FIG. The detection electrode thicknesses shown in FIGS. 14 and 15 mean the average thickness T _ave of the detection electrodes measured with a laser displacement meter.

According to FIGS. 14 and 15, when |(T _max −T _ave )/T _ave |≦0.5 and |(T _min −T _ave )/T _ave |≦0.5 are satisfied, the ammonia output is easily stabilized. This is because the use of the detection electrode 22 with a small thickness variation promotes gas uptake into the detection electrode 22 and gas discharge from the detection electrode 22, thereby making it easier to maintain the imbalance (non-equilibrium state) in the electrochemical oxidation-reduction reaction rate, which is a prerequisite for the expression of the mixed potential.

The present invention is not limited to the above-described embodiments and experimental examples, and various modifications can be made without departing from the scope of the invention. Moreover, each configuration shown in each embodiment and each experimental example can be combined arbitrarily.

1 gas sensor 2 gas sensor element 21 solid electrolyte body 201 first surface 202 second surface 22 detection electrode 22c pores 23 reference electrode G gas to be detected

Claims (6)

A gas sensor element (2) for use in a gas sensor (1) that utilizes a mixed potential,
a solid electrolyte body (21) having ionic conductivity; a detection electrode (22) formed on a first surface (201) of the solid electrolyte body and exposed to a detection target gas (G); and a reference electrode (23) formed at a position facing the detection electrode on the second surface (202) of the solid electrolyte body,
The detection electrode has a large number of pores (22c),
The gas sensor element (2), wherein the product of the surface opening ratio of the pores on the surface of the detection electrode and the thickness [mu]m of the detection electrode is 0.3 [mu]m or more and 4 [mu]m or less. 2. The gas sensor element according to claim 1, wherein the surface opening ratio of said pores is 0.05 or more and 0.35 or less. When the average thickness of the detection electrode is T _ave , the maximum thickness is T _max , and the minimum thickness is T _min ,
3. The gas sensor element according to claim 1, wherein |(T _max −T _ave )/T _ave |≦0.5 and |(T _min −T _ave )/T _ave |≦0.5 are satisfied. The gas sensor element according to any one of claims 1 to 3, wherein the detection electrode contains a noble metal (22a), and the noble metal is mainly composed of gold. The gas sensor element according to any one of claims 1 to 4, which detects at least one of ammonia gas and nitrogen dioxide gas. A gas sensor (1) utilizing a mixed potential,
A gas sensor (1) comprising a gas sensor element according to any one of claims 1 to 5.

Images