JP2017049051A - Gas sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a gas sensor that can facilitate oxygen ionization in a measuring electrode and thereby enhance sensor output performance.SOLUTION: A gas sensor 1 is equipped with an oxygen ion-conductive solid electrolyte body 2, a measuring electrode 21 formed on one surface 201 of the solid electrolyte body 2 and exposed to measurable gas G and a reference electrode 22 formed on the other surface 202 of the solid electrolyte body 2 and exposed to reference gas A. In the gas sensor 1, the average particle diameter of precious metal particle K1 contained in the measuring electrode 21 is smaller than the average particle diameter of precious metal particle K2 contained in the reference electrode 22.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a gas sensor that measures the concentration of oxygen, a specific gas, or the like contained in a gas to be measured.

In the gas sensor disposed in the exhaust pipe of the internal combustion engine, the exhaust gas exhausted from the internal combustion engine is the measured gas, the atmosphere is the reference gas, and the measured gas is based on the difference in oxygen concentration between the measured gas and the reference gas. The oxygen concentration in the measurement gas, the specific gas component concentration, etc. are measured. The gas sensor is formed by providing a measurement electrode that is exposed to the gas to be measured on one surface of the solid electrolyte body and a reference electrode that is exposed to the reference gas on the other surface of the solid electrolyte body.
For example, in Patent Document 1, the particle size distribution of zirconia crystal particles constituting the solid electrolyte has a specific peak in order to improve the bonding strength between the solid electrolyte and the platinum electrode. There are at least two specific peaks in this particle size distribution, the first peak is 0.15-0.25 μm, the second peak is 0.25-0.35 μm, and the difference between the two peaks Is 0.03 to 0.2 μm.

JP 2003-194766 A

  By the way, in order to improve the sensor output performance of the gas sensor, it is conceivable to improve the conductivity of oxygen ions at the interface between the solid electrolyte body and each electrode. However, in the conventional gas sensor including Patent Document 1 and the like, no special contrivance has been made with respect to the properties of the noble metal particles constituting the electrode, the porosity of the electrode, and the like.

  The present invention has been made in view of such problems, and has been obtained by providing a gas sensor that can promote oxygen ionization in a measurement electrode and improve sensor output performance.

One embodiment of the present invention includes an oxygen ion conductive solid electrolyte body, a measurement electrode formed on one surface of the solid electrolyte body and exposed to a gas to be measured, and formed on the other surface of the solid electrolyte body. And a reference electrode that is exposed to the reference gas,
The average particle diameter of the noble metal particles contained in the measurement electrode is in the gas sensor which is smaller than the average particle diameter of the noble metal particles contained in the reference electrode.

In the gas sensor, the average particle diameter of the noble metal particles contained in the measurement electrode is made smaller than the average particle diameter of the noble metal particles contained in the reference electrode.
In the measurement electrode and the reference electrode, solid electrolyte particles are included as a co-material for joining with the solid electrolyte body. Oxygen molecules in the gas to be measured that contact the measurement electrode become oxygen ions at the interface between the noble metal particles and the solid electrolyte particles, pass through the solid electrolyte body, and then again at the interface between the noble metal particles and the solid electrolyte particles at the reference electrode. It becomes an oxygen molecule.
At this time, since the average particle diameter of the noble metal particles in the measurement electrode is small, the surface area of the entire noble metal particles in the measurement electrode increases, and the gas to be measured easily comes into contact with the noble metal particles and the solid electrolyte particles in the measurement electrode. Thereby, in the measurement electrode, the interface reaction in which oxygen in the gas to be measured becomes oxygen ions is promoted, and the sensor output performance of the gas sensor can be improved.

On the other hand, even if the average particle diameter of the noble metal particles in the reference electrode is relatively larger than the average particle diameter of the noble metal particles in the measurement electrode, it is unlikely to cause a negative effect when oxygen ions become oxygen molecules. The reason is considered as follows.
In the measurement electrode, the composition of the gas to be measured and the oxygen concentration are appropriately changed, and an interface reaction is performed in which oxygen molecules become oxygen ions. In order to improve the sensor output characteristics, it is necessary to quickly respond to changes in the composition of the gas to be measured and the oxygen concentration and to promote the interface reaction, and it is preferable that the average particle diameter of the noble metal particles in the measurement electrode is small. On the other hand, in the reference electrode, the atmosphere or the like as a reference gas having a constant oxygen concentration is brought into contact, and the oxygen ions only need to be converted into oxygen molecules, so that the response for the reaction and the promotion of the reaction are not so required. Therefore, even if the average particle diameter of the noble metal particles in the reference electrode is larger than the average particle diameter of the noble metal particles in the measurement electrode, it is unlikely to cause a deterioration in sensor output characteristics.

  Therefore, according to the gas sensor, oxygen ionization at the measurement electrode can be promoted, and the sensor output performance can be improved.

Explanatory drawing which shows typically the microstructure of a solid electrolyte body and each electrode concerning embodiment. Explanatory drawing which shows the structure of the sensor element of the gas sensor concerning embodiment by the cross section orthogonal to the longitudinal direction. The graph which shows the relationship between the average particle diameter of the noble metal particle | grains of a measurement electrode, and sensor output characteristic concerning embodiment. The graph which shows the relationship between the average particle diameter of the noble metal particle | grains of a reference electrode, and sensor output characteristic concerning embodiment. The graph which shows the relationship between the average particle diameter of the noble metal particle | grains of a measurement electrode, and output durability fluctuation amount concerning embodiment. The graph which shows the relationship between the average particle diameter of the noble metal particle | grains of a reference electrode, and output durability fluctuation amount concerning embodiment. The graph which shows the relationship between the porosity of a measurement electrode and sensor output characteristic concerning embodiment. The graph which shows the relationship between the porosity of a reference electrode and sensor output characteristic concerning embodiment. The graph which shows the relationship between the porosity of a reference electrode and output durability fluctuation amount concerning embodiment. Explanatory drawing which shows the process in which the gas sensor concerning embodiment is manufactured.

A preferred embodiment of the gas sensor described above will be described with reference to the drawings.
As shown in FIG. 1, the gas sensor 1 of the present embodiment includes an oxygen ion conductive solid electrolyte body 2, a measurement electrode 21 formed on one surface 201 of the solid electrolyte body 2 and exposed to the gas G to be measured, And a reference electrode 22 formed on the other surface 202 of the solid electrolyte body 2 and exposed to the reference gas A. In the gas sensor 1, the average particle diameter of the noble metal particles K <b> 1 included in the measurement electrode 21 is smaller than the average particle diameter of the noble metal particles K <b> 2 included in the reference electrode 22.

  Although not shown, the gas sensor 1 includes a sensor element 10 formed by laminating a heater 5 on a solid electrolyte body 2, a housing through which the sensor element 10 is inserted, and a base end side in the longitudinal direction of the sensor element 10 An insulator that holds the portion in the housing and a protective cover that is attached to the housing and covers the front end portion in the longitudinal direction of the sensor element 10 are provided. The detection unit configured by providing the measurement electrode 21 and the reference electrode 22 on the solid electrolyte body 2 is formed at the tip side portion of the sensor element 10. The protective cover is provided with a through hole for guiding the gas G to be measured to the detection unit.

The gas sensor 1 is disposed in an exhaust pipe of an engine as an internal combustion engine, and an exhaust gas passing through the exhaust pipe is a measured gas G, and the atmosphere is a reference gas A, and oxygen concentrations of the measured gas G and the reference gas A Based on the difference, the concentration of oxygen in the measurement gas G is obtained.
A heater 5 for heating the solid electrolyte body 2 is laminated on the sensor element 10 configured by providing the measurement electrode 21 and the reference electrode 22 on the solid electrolyte body 2. The sensor element 10 is formed in a long shape, and the measurement electrode 21 and the reference electrode 22 are arranged at the front end side portion of the sensor element 10 in the longitudinal direction. The lead portion connected to the measurement electrode 21 and the reference electrode 22 is drawn out to the rear end side in the longitudinal direction of the sensor element 10.

  FIG. 2 shows the sensor element 10 of the gas sensor 1 by a cross section orthogonal to its longitudinal direction. As shown in the figure, the solid electrolyte body 2 is formed of plate-shaped yttria-stabilized zirconia (simply referred to as zirconia). A porous protective layer 31 for protecting the measurement electrode 21 is laminated on one surface 201 of the solid electrolyte body 2 so as to cover the measurement electrode 21. The porous protective layer 31 suppresses poisoning of the measurement electrode 21 and has a property of allowing gas to pass therethrough but not allowing moisture to pass therethrough. In the sensor element 10 of this embodiment, an insulator such as alumina is not laminated on one surface 201 of the solid electrolyte body 2, and a measurement gas chamber is formed as a space for arranging the measurement electrode 21. Not.

  A reference gas chamber 33 into which the reference gas A is introduced is formed on the other surface 202 of the solid electrolyte body 2, and the reference electrode 22 is disposed in the reference gas chamber 33. The reference gas chamber 33 is formed by a spacer 32 made of alumina or the like laminated on the other surface 202 of the solid electrolyte body 2. The heater 5 is laminated on the spacer 32, and is formed by a heating element 52 that generates heat when energized, and a ceramic substrate 51 made of alumina or the like in which the heating element 52 is embedded. The heating element 52 is formed containing platinum, alumina, or the like.

  As shown in FIG. 1, the solid electrolyte body 2 is formed by collecting a large number of solid electrolyte particles Z made of zirconia. The measurement electrode 21 and the reference electrode 22 contain noble metal particles K1 and K2 such as platinum and solid electrolyte particles Z as zirconia components that are co-materials with the solid electrolyte body 2. The solid electrolyte particles Z in the solid electrolyte body 2 and the solid electrolyte particles Z in the measurement electrode 21 and the reference electrode 22 have the same composition.

The gas sensor 1 according to the present embodiment is used as an oxygen sensor, and a voltage detection unit 11 converts a current flowing between the measurement electrode 21 and the reference electrode 22 in accordance with a difference in oxygen concentration between the measurement gas G and the reference gas A. Is detected as an electromotive force. As shown in FIG. 1, oxygen molecules O ₂ in the reference gas A pass through the solid electrolyte body 2 as oxygen ions O ^{2− at} the interface between the noble metal particles K 2 and the solid electrolyte particles Z in the reference electrode 22. . The oxygen ions O ²⁻ that have passed through the solid electrolyte body 2 become oxygen molecules O ₂ again at the interface between the noble metal particles K1 and the solid electrolyte particles Z in the measurement electrode 21. Further, an electromotive force is generated between the measurement electrode 21 and the reference electrode 22 when the oxygen ions O ²⁻ pass through the solid electrolyte body 2.

In the gas sensor 1, the average particle diameter of the noble metal particles K <b> 1 included in the measurement electrode 21 is smaller than the average particle diameter of the noble metal particles K <b> 2 included in the reference electrode 22.
When the oxygen in the measurement gas G contacting the measurement electrode 21 becomes oxygen ions, the surface area of the entire noble metal particle K1 in the measurement electrode 21 is increased due to the small average particle diameter of the noble metal particle K1 in the measurement electrode 21. The measurement gas G easily comes into contact with the noble metal particles K1 and the solid electrolyte particles Z in the measurement electrode 21. Thereby, in the measurement electrode 21, the interface reaction in which oxygen in the measurement gas G becomes oxygen ions is promoted, and the sensor output performance of the gas sensor 1 can be improved.

On the other hand, even if the average particle diameter of the noble metal particles K2 at the reference electrode 22 is relatively larger than the average particle diameter of the noble metal particles K1 at the measurement electrode 21, it is unlikely to cause an adverse effect when oxygen ions become oxygen molecules again. The reason is considered as follows.
In the measurement electrode 21, the composition of the gas G to be measured and the oxygen concentration are appropriately changed, and an interface reaction in which oxygen molecules become oxygen ions is performed. In order to enhance the sensor output characteristics, it is necessary to quickly respond to changes in the composition of the gas G to be measured and the oxygen concentration and to promote the interface reaction, and the average particle diameter of the noble metal particles K1 in the measurement electrode 21 is smaller. Is preferred. On the other hand, the reference electrode 22 is in contact with the atmosphere as the reference gas A having a constant oxygen concentration, and the oxygen ions only need to be converted into oxygen molecules, so that the response and the acceleration of the reaction are not so required. . Therefore, even if the average particle diameter of the noble metal particles K2 in the reference electrode 22 is larger than the average particle diameter of the noble metal particles K1 in the measurement electrode 21, it is unlikely to cause a deterioration in sensor output characteristics.

The average particle diameter of the noble metal particles K1 included in the measurement electrode 21 is 0.05 to 0.5 μm, and the average particle diameter of the noble metal particles K2 included in the reference electrode 22 is 1 to 10 μm.
Here, the particle diameter of the noble metal particles K1 and K2 refers to the maximum particle diameter which is the length of the longest part of the noble metal particles K1 and K2 in any shape. The maximum particle diameter of the noble metal particles K1 and K2 is, for example, the diameter when the noble metal particles K1 and K2 are spherical, and when the noble metal particles K1 and K2 are plate-shaped, It refers to the length of the corner portion, and when the noble metal particles K1 and K2 have a complicated shape, it refers to the length of the longest portion.
Further, the average particle diameter of the noble metal particles K1 and K2 refers to the average value of the particle size distribution of the noble metal particles K1 and K2. Specifically, the maximum particle diameter of 100 noble metal particles K1 and K2 is arbitrarily set. This refers to the number average particle diameter when measured.

The maximum particle diameter of the noble metal particles K1 and K2 can be measured by observation using an optical microscope or an electron microscope. The maximum particle diameter can be measured by exposing a cross section of the measurement electrode 21 or the like by ion beam processing or the like, and observing the cross section by SEM (scanning electron microscopy) or the like.
The noble metal particles K1 and K2 included in the measurement electrode 21 and the reference electrode 22 are in a state where adjacent particles are bonded to each other because the measurement electrode 21 and the reference electrode 22 are fired at a predetermined temperature. However, even if adjacent particles are joined, the outer shape of each particle can be observed by observation using an optical microscope or an electron microscope. Therefore, the maximum particle diameter of the noble metal particles K1 and K2 in each of the electrodes 21 and 22 can be measured, and the average particle diameter can be obtained based on the measured maximum particle diameter.

When the average particle diameter of the noble metal particles K1 contained in the measurement electrode 21 exceeds 0.5 μm, it becomes difficult to increase the surface area of the entire noble metal particles K1 in the measurement electrode 21, and the gas G to be measured becomes noble metal particles in the measurement electrode 21. It becomes difficult to come into contact with K1. On the other hand, when the average particle diameter of the noble metal particles K1 is less than 0.05 μm, the durability of the measurement electrode 21 may be deteriorated.
When the average particle diameter of the noble metal particles K2 included in the reference electrode 22 is less than 1 μm, the noble metal particles K2 are small, and due to the evaporation of the noble metal particles K2 generated when contacting the atmosphere or the like as the reference gas A, Durability may deteriorate. If the average particle diameter of the noble metal particles K2 exceeds 10 μm, the area where the atmosphere as the reference gas A contacts with the noble metal particles K2 may be reduced, which may affect the sensor output characteristics.

FIG. 3 is a graph showing the relationship between the average particle diameter (μm) of the noble metal particles K1 in the measurement electrode 21 and the sensor output characteristics (V).
The sensor output characteristic indicates an electromotive force (potential difference) generated between the measurement electrode 21 and the reference electrode 22, and the sensor output characteristic is good when the electromotive force is maintained at a certain value or more. When obtaining the sensor output characteristics, nitrogen containing 100 ppm of carbon monoxide is brought into contact with the measurement electrode 21, and the atmosphere is brought into contact with the reference electrode 22, and the electromotive force between the measurement electrode 21 and the reference electrode 22 is determined. It was measured. The temperature of the measurement electrode 21 and the reference electrode 22 was 400 ° C.

  In the figure, it can be seen that when the average particle diameter of the noble metal particles K1 in the measurement electrode 21 is 0.5 μm or less, the sensor output characteristics are maintained satisfactorily. On the other hand, it can be seen that the sensor output characteristics deteriorate as the average particle diameter of the noble metal particles K1 in the measurement electrode 21 becomes larger than 0.5 μm. From this, it can be said that the average particle diameter of the noble metal particles K1 in the measurement electrode 21 is preferably 0.5 μm or less.

FIG. 4 is a graph showing the relationship between the average particle diameter (μm) of the noble metal particles K2 at the reference electrode 22 and the sensor output characteristic (V). The conditions for obtaining the sensor output characteristics are the same as those for the measurement electrode 21 in FIG.
In the figure, it can be seen that when the average particle diameter of the noble metal particles K2 at the reference electrode 22 is 10 μm or less, the sensor output characteristics are maintained satisfactorily. On the other hand, it can be seen that as the average particle diameter of the noble metal particles K2 at the reference electrode 22 becomes larger than 10 μm, the sensor output characteristics deteriorate. From this, it can be said that the average particle diameter of the noble metal particles K2 in the reference electrode 22 is preferably 10 μm or less.
The reason why the sensor output characteristics are hardly deteriorated even if the average particle diameter of the noble metal particles K2 in the reference electrode 22 is larger than the average particle diameter of the noble metal particles K1 in the measurement electrode 21 is as described above.

The durability is improved by increasing the average particle diameter of the noble metal particles K2 in the reference electrode 22. FIG. 5 shows the relationship between the average particle diameter (μm) of the noble metal particles K1 in the measurement electrode 21 and the output durability fluctuation amount (V), and FIG. 6 shows the average particle diameter of the noble metal particles K2 in the reference electrode 22 ( μm) and the output durability fluctuation amount (V).
The output durability fluctuation amount indicates a difference between the sensor output characteristic in the initial state and the sensor output characteristic after use after measuring the oxygen concentration by the gas sensor 1 in a temperature environment of 950 ° C. for 800 hours by a potential difference. The smaller the output durability fluctuation amount, the better the durability of the reference electrode 22. When calculating the output durability fluctuation amount, nitrogen containing 100 ppm of carbon monoxide is brought into contact with the measurement electrode 21, and the atmosphere is brought into contact with the reference electrode 22, so that an electromotive force between the measurement electrode 21 and the reference electrode 22 is obtained. Was measured. The temperature of the measurement electrode 21 and the reference electrode 22 was 400 ° C.

  In FIG. 5, it can be seen that when the average particle diameter of the noble metal particles K1 in the measurement electrode 21 is 0.05 μm or more, the output durability fluctuation amount is maintained well. On the other hand, it can be seen that the output durability fluctuation amount deteriorates as the average particle diameter of the noble metal particles K1 in the measurement electrode 21 becomes smaller than 0.05 μm. From this, it can be said that the average particle diameter of the noble metal particles K1 in the measurement electrode 21 is preferably 0.05 μm or more.

  In FIG. 6, it can be seen that when the average particle diameter of the noble metal particles K2 in the reference electrode 22 is 1 μm or more, the output durability fluctuation amount is well maintained. On the other hand, it can be seen that as the average particle diameter of the noble metal particles K2 at the reference electrode 22 becomes smaller than 1 μm, the output durability fluctuation amount deteriorates. From this, it can be said that the average particle diameter of the noble metal particles K2 in the reference electrode 22 is preferably 1 μm or more.

The reason why the average particle diameter of the noble metal particles K1 in the measurement electrode 21 for maintaining a good output durability fluctuation amount may be significantly smaller than the average particle diameter of the noble metal particles K2 in the reference electrode 22 is as follows. Think.
In general, transpiration of noble metal particles K1, K2 (such as platinum) that causes electrode deterioration is caused by contact with oxygen molecules. Since the reference electrode 22 comes into contact with the atmosphere or the like as the reference gas A, a device for preventing deterioration is required. On the other hand, the measurement electrode 21 is in contact with the gas G to be measured such as exhaust gas whose oxygen concentration has decreased due to combustion, so that deterioration due to oxidation and transpiration hardly occurs. Therefore, for the measurement electrode 21, even if the average particle diameter of the noble metal particles K1 is reduced to 0.05 μm, the output durability fluctuation amount hardly increases.

In the gas sensor 1, the porosity of the measurement electrode 21 is larger than the porosity of the reference electrode 22.
Here, the porosity means the ratio of the volume represented by the pores (cavities) H other than the noble metal particles K1, K2 and the solid electrolyte particles Z with respect to the entire volume of the measurement electrode 21 or the reference electrode 22. . In the measurement electrode 21 and the reference electrode 22, there are pores H appearing in a groove shape on the surfaces of the electrodes 21 and 22, pores H appearing inside the electrodes 21 and 22, and the like. These pores H are often connected in a complicated shape from the surface of each electrode 21, 22 to the inside. In the measurement electrode 21, the noble metal particle K <b> 1, the solid electrolyte particle Z and the measurement gas G are brought into contact with each other through the pores H, and an interfacial reaction is performed in which oxygen in the measurement gas G becomes oxygen ions.

At this time, since the porosity of the measurement electrode 21 is large, the noble metal particles K1, the solid electrolyte particles Z, and the gas G to be measured are easily brought into contact with each other through the pores H in the measurement electrode 21. Thereby, the interface reaction of oxygen ionization in the measurement electrode 21 is promoted, and the sensor output performance of the gas sensor 1 can be improved.
On the other hand, even if the porosity of the reference electrode 22 is relatively smaller than the porosity of the measurement electrode 21, there is no serious problem. The reason is the same as in the case of the average particle diameter. At the reference electrode 22, the atmosphere as the reference gas A having a constant oxygen concentration is in contact, and the oxygen ions only have to be oxygen molecules again. This is because the responsiveness and the promotion of the reaction are not required. For this reason, even if the porosity of the reference electrode 22 is smaller than the porosity of the measurement electrode 21, it is unlikely to cause a deterioration in sensor output characteristics.

  The measurement electrode 21 has a porosity of 30 to 60%, and the reference electrode 22 has a porosity of 5 to 20%. Here, the porosity of the measurement electrode 21 and the reference electrode 22 can be measured by observation using an optical microscope or an electron microscope. This porosity can be measured, for example, by exposing a cross section of the measurement electrode 21 or the like by ion beam processing or the like, and observing the cross section by SEM (scanning electron microscopy) or the like.

When the porosity of the measurement electrode 21 is less than 30%, it becomes difficult to increase the interface between the noble metal particles K1, the solid electrolyte particles Z, and the measurement gas G in the measurement electrode 21, and the measurement gas G becomes noble metal particles K1. And it becomes difficult to contact the solid electrolyte particles Z. On the other hand, when the porosity of the measurement electrode 21 exceeds 60%, the resistance value of the measurement electrode 21 increases, and the sensor output characteristics may be deteriorated.
When the porosity of the reference electrode 22 exceeds 20%, the ratio of the pores in the reference electrode 22 is large, and the durability of the reference electrode 22 may be deteriorated. On the other hand, when the porosity of the reference electrode 22 is less than 5%, the interface between the noble metal particles K2, the solid electrolyte particles Z, and the reference gas A is reduced, and the reference gas A is less likely to come into contact with the noble metal particles K2 and the solid electrolyte particles Z. .

  FIG. 7 is a graph showing the relationship between the porosity (%) of the measurement electrode 21 and the sensor output characteristic (V), and FIG. 8 is a graph showing the porosity (%) of the reference electrode 22 and the sensor output characteristic (V). Is shown in a graph. The contents indicated by the sensor output characteristics and the conditions for obtaining the sensor output characteristics are the same as described for the average particle diameter of the noble metal particles K1 of the measurement electrode 21 in FIG.

  In FIG. 7, it can be seen that when the porosity of the measurement electrode 21 is 30% or more, the sensor output characteristics are maintained satisfactorily. On the other hand, it can be seen that the sensor output characteristics deteriorate as the porosity of the measuring electrode 21 becomes smaller than 30%. It can also be seen that the sensor output characteristics deteriorate as the porosity of the measurement electrode 21 becomes larger than 60%. From these facts, it can be said that the porosity of the measurement electrode 21 is preferably 30 to 60%.

  In FIG. 8, it can be seen that when the porosity of the reference electrode 22 is 5% or more, the sensor output characteristics are maintained satisfactorily. On the other hand, it can be seen that the sensor output characteristics deteriorate as the porosity of the reference electrode 22 becomes smaller than 5%. From this, it can be said that the porosity of the reference electrode 22 is preferably 5% or more.

  The durability is improved by reducing the porosity of the reference electrode 22. FIG. 9 shows the relationship between the porosity (%) of the reference electrode 22 and the output durability fluctuation amount (V). The contents indicated by the output durability fluctuation amount and the conditions for obtaining the output durability fluctuation amount are the same as described for the average particle diameters of the noble metal particles K1 and K2 in FIGS.

In FIG. 9, it can be seen that when the porosity of the reference electrode 22 is 20% or less, the output durability fluctuation amount is maintained well. On the other hand, it can be seen that as the porosity of the reference electrode 22 becomes larger than 20%, the output durability fluctuation amount deteriorates. From this, it can be said that the porosity of the reference electrode 22 is preferably 20% or less.
Note that a graph of the relationship between the porosity of the measurement electrode 21 and the output durability fluctuation amount is omitted. This is because the measurement electrode 21 is in contact with the gas G to be measured such as exhaust gas whose oxygen concentration has decreased due to combustion, and therefore, deterioration due to oxidation and transpiration hardly occurs.

The sensor element 10 can be manufactured as follows.
First, as shown in FIG. 10, a paste-like electrode material for forming the reference electrode 22 is disposed on the other surface 202 of the zirconia sheet forming the solid electrolyte body 2. This electrode material contains noble metal particles K2, solid electrolyte particles Z, organic solvents, and the like. A paste-like electrode material for forming the lead portion 211 connected to the measurement electrode 21 can be disposed on one surface 201 of the zirconia sheet.
In addition, a conductor material for forming the heating element 52 is disposed on the ceramic sheet forming the ceramic substrate 51 of the heater 5. And the zirconia sheet | seat which forms the solid electrolyte body 2, the ceramic sheet | seat which forms the spacer 32, and the ceramic sheet | seat which forms the ceramic substrate 51 are laminated | stacked, and a laminated body is formed. Next, this laminate is fired in an environment having a temperature of 1300 ° C. or higher.

  By firing the electrode material for forming the reference electrode 22 at a temperature of 1300 ° C. or higher, the firing of the noble metal particles K2, the solid electrolyte particles Z, etc. in the electrode material proceeds deeper. Thereby, the average particle diameter of the noble metal particles K2 constituting the reference electrode 22 is increased, and the porosity of the reference electrode 22 is decreased.

  Next, as shown in FIG. 10, a paste-like electrode material for forming the measurement electrode 21 is disposed on one surface 201 of the zirconia sheet in the laminate. This electrode material contains noble metal particles K1, solid electrolyte particles Z, organic solvents, and the like. This electrode material can be arranged by electroless plating. A porous material for forming the porous protective layer 31 is disposed on one surface 201 of the zirconia sheet in the laminate so as to cover the electrode material. And the laminated body by which the electrode material and the porous material are arrange | positioned is baked in the environment of the temperature of 1200 degrees C or less.

  The electrode material for forming the measurement electrode 21 is fired at a temperature of 1200 ° C. or lower, thereby suppressing the progress of firing of the noble metal particles K1, the solid electrolyte particles Z, and the like in the electrode material. Thereby, the average particle diameter of the noble metal particle K1 which comprises the measurement electrode 21 is restrained small, and the porosity of the measurement electrode 21 becomes large.

  Thus, by making the firing temperature of the measurement electrode 21 lower than the firing temperature of the reference electrode 22, the average particle diameter of the noble metal particles K <b> 1 included in the measurement electrode 21 is changed to the average of the noble metal particles K <b> 2 included in the reference electrode 22. It can be made smaller than the particle diameter. In addition, by making the firing temperature of the measurement electrode 21 lower than the firing temperature of the reference electrode 22, the porosity of the measurement electrode 21 can be made larger than the porosity of the reference electrode 22.

  Further, the electrode material for forming the measurement electrode 21 and the electrode material for forming the reference electrode 22 can contain a burnout agent such as carbon for forming pores. The burnout agent forms pores in the electrodes 21 and 22 by burning out when the electrode materials are fired.

As described above, according to the gas sensor 1, the sensor output performance can be improved and the durability can be improved.
Further, the present invention is not limited to the present embodiment, and can be applied to further different forms without departing from the gist thereof.

DESCRIPTION OF SYMBOLS 1 Gas sensor 2 Solid electrolyte body 201 One surface 202 The other surface 21 Measuring electrode 22 Reference electrode K1, K2 Noble metal particle G Gas to be measured A Reference gas

Claims (4)

An oxygen ion conductive solid electrolyte body (2), a measurement electrode (21) formed on one surface (201) of the solid electrolyte body (2) and exposed to the gas to be measured (G), and the solid electrolyte A reference electrode (22) formed on the other surface (202) of the body (2) and exposed to the reference gas (A),
The gas sensor, wherein an average particle diameter of the noble metal particles (K1) included in the measurement electrode (21) is smaller than an average particle diameter of the noble metal particles (K2) included in the reference electrode (22).   The average particle diameter of the noble metal particles (K1) contained in the measurement electrode (21) is 0.05 to 0.5 μm, and the average particle diameter of the noble metal particles (K2) contained in the reference electrode (22) is The gas sensor according to claim 1 which is 1-10 micrometers.   The gas sensor according to claim 1 or 2, wherein the porosity of the measurement electrode (21) is larger than the porosity of the reference electrode (22).   The gas sensor according to claim 3, wherein the porosity of the measurement electrode (21) is 30 to 60%, and the porosity of the reference electrode (22) is 5 to 20%.

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