JPH0980017A - Gas sensor and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method by which a gas sensor provided with an electrode which can be formed without using any metal-containing paste and is durable to such a corrosive gas as SOx , etc., and to which lead-out wires can be welded easily in an auxiliary electrode section. SOLUTION: In case of manufacturing a gas sensor in which an auxiliary electrode section 12 which contains an electrode 16 in the surface and is composed of a sulfate containing silver sulfate is joined to one surface of a substrate 10 that carries a platinum electrode 14 on the other surface and is composed of yttria-stabilized zirconia, the sulfate containing a prescribed amount of silver sulfate is placed on the joint surface of the substrate 10 which is joined to the section 12 and a network body is formed of platinum wires coated with silver layers so that the silver sulfate can be formed when the silver of the silver layers react to a gas to be detected. Then the auxiliary electrode section 12 using the network body as the electrode 16 is formed by baking the sulfate after the network body is placed on the surface of the sulfate placed on the joint surface of the substrate 10.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas sensor and a method for manufacturing the same, and more particularly, to a platinum-containing electrode provided on one surface of the substrate, the other surface of the substrate made of a solid electrolyte material,
The present invention relates to a gas sensor in which an electrode is provided on at least a part of a surface, and a sub-electrode portion made of a crystalline inorganic salt is bonded to the gas sensor and a method for manufacturing the gas sensor.

[0002]

2. Description of the Related Art A technique for measuring the concentration of SO _X , CO _X, etc. in flue gas discharged from a flue such as a boiler that uses petroleum as a fuel is an important technique from the viewpoint of pollution prevention. Take SO _X
Regarding the sensor used for measuring the concentration of CO, CO _x, etc., two of the present inventors previously proposed the gas sensor shown in FIG. 6 in Japanese Patent Application Laid-Open No. 7-103937. In this gas sensor, a sub-electrode unit 102 made of a crystalline inorganic salt such as sulfate or carbonate is provided on one surface of a substrate 100 made of yttria-stabilized zirconia (YSZ) which is a solid electrolyte material.
Are joined. Further, a platinum electrode 104 is formed on the other surface side of the substrate 100, and the sub electrode portion 1
On the surface of 02, an electrode composed of a silver-containing electrode 106 covered with a platinum film 108 that is chemically stable against SO _X , CO _X, etc. is formed. The platinum film 108 is formed to protect the silver-containing electrode 106 from SO _X , CO _{X and the} like. Each of the platinum electrode 104 and the platinum film 108 covering the silver-containing electrode 106 is connected to a voltmeter (not shown) via lead wires 112 and 114 made of platinum and having one end bonded to each electrode. . In such a gas sensor, the portion including the sub-electrode portion 102 is inserted into the cylindrical body 110 into which the flue gas containing SO _X , CO _{X and the} like is introduced.

[0003]

A cylindrical body 110 shown in FIG.
When a flue gas containing SO _X , CO _X, etc. is introduced into the platinum electrode, the electromotive force resulting from the electrochemical reaction in the gas sensor causes the platinum electrode 1
A potential difference is generated between 04 and the silver-containing electrode 106. Therefore, the concentration of SO _X , CO _X, etc. in the flue gas can be measured by measuring the potential difference generated between the platinum electrode 104 and the silver-containing electrode 106 with a voltmeter. By the way, when molding the sub-electrode portion 102 that constitutes the gas sensor shown in FIG. 6, first, a crystalline inorganic salt such as a sulfate or a carbonate placed on the substrate 100 is fired to obtain a fired body.
Next, a silver paste is applied to the surface of the fired body and dried, and then a platinum paste is further applied and dried. After that, by baking the silver paste and the platinum paste applied / dried to the fired body, the sub-electrode portion 102 having the silver-containing electrode 106 covered with the platinum film 108 as an electrode can be formed.

As described above, when the silver paste and the platinum paste are applied, various components contained in each paste affect the electrochemical reaction in the sub-electrode portion 102 or the like to cause a side reaction. To do. Further, since one end of the pull-out wire 112 is joined to the applied dry platinum paste, it cannot be welded to the electrode of the sub-electrode section 102, and the joint strength between the pull-out wire 112 and the electrode of the sub-electrode section 102 is increased. Low and product yield is low. Further, if the electrode of the sub electrode portion 102 is exposed to a corrosive gas such as SO _X , the electrode portion may be corroded. Therefore, an object of the present invention is to be able to prepare without using a metal-containing paste, to easily weld a drawing wire, and to provide an electrode having durability against a corrosive gas such as SO _X as a sub electrode portion. An object of the present invention is to provide a gas sensor provided and a manufacturing method thereof.

[0005]

As a result of various studies to solve the above problems, the inventors of the present invention have found that, for example, in the case of a SO _X gas sensor, platinum having a metal layer made of a metal such as silver formed on its peripheral surface. By using the mesh made of wire for the electrode of the sub-electrode part made of sulfate containing silver sulfate, the electrode of the sub-electrode part can be formed without using the metal-containing paste, and one end of the extraction wire can be easily formed. The inventors have found that an electrode that can be welded and has durability against SO _X gas can be formed in the sub-electrode part, and has reached the present invention. That is, the present invention provides a sub-electrode part made of a crystalline inorganic salt, in which an electrode is provided on at least a part of the surface on the other side of the substrate made of a solid electrolyte material, on which the platinum-containing electrode is provided on one side. In the gas sensor joined to, the electrode provided on the surface of the sub-electrode portion is composed of a net body formed of a metal wire rod made of a metal that is chemically stable against a gas such as a detection gas, and the metal is The gas sensor is characterized in that a metal layer made of a metal capable of reacting with a detection gas to form a crystalline inorganic salt forming the sub-electrode is formed on the peripheral surface of the wire. Further, the present invention provides a sub-electrode part made of a crystalline inorganic salt, in which a platinum-containing electrode is provided on one surface side, and an electrode is provided on at least a part of the surface on the other surface side of the substrate made of a solid electrolyte material. When manufacturing a gas sensor in which is bonded, a predetermined amount of crystalline inorganic salt is placed on the bonding surface of the substrate bonded to the sub-electrode, and the crystalline inorganic salt is generated by reacting with the detection gas. A metal layer made of a metal that can be chemically stabilized against a gas such as a detection gas to form a net, and then placed on the bonding surface of the substrate. Forming a sub-electrode part using the net body as an electrode by placing the net body on the surface of the formed crystalline inorganic salt and then calcining the crystalline inorganic salt having the net body placed on the surface. And a method for manufacturing a gas sensor.

In the present invention having such a structure, the gas sensor can be easily manufactured by using the substrate made of yttria-stabilized zirconia as the substrate. Further, by using a platinum wire or a gold wire that is chemically stable against most gases as the metal wire forming the net, it is possible to obtain a net having durability against detection gas and the like. In some cases, platinum or gold forming the metal wire may exert a catalytic action on the electrochemical reaction in the sub-electrode part or the like. Further, after placing a net body in which one end of the drawing wire is welded by spot welding or the like is placed on the surface of the crystalline inorganic salt placed on the bonding surface of the substrate, the crystalline inorganic salt is fired to form the sub-electrode. It is possible to obtain the gas sensor in which the extraction wire is firmly welded to the electrode of the part. By using a platinum wire or a gold wire as the pull-out wire to be welded to the mesh body, it is possible to impart corrosion resistance to the detection gas and the like to the pull-out wire.

According to the present invention, a metal which is chemically stable to a gas such as a detection gas and which can react with the detection gas on the peripheral surface to form the crystalline inorganic salt containing the metal component. The net body formed of the metal wire having the metal layer made of is placed on the surface of the crystalline inorganic salt and fired to form the sub-electrode portion. Therefore, the electrode of the sub-electrode part can be formed without using a metal-containing paste that affects the electrochemical reaction in the sub-electrode part, etc., and also in the manufacturing process of the gas sensor, the surface of the crystalline inorganic salt contains metal-containing material. The step of applying and drying the paste can be omitted. One end of the draw wire can be welded to the net body forming the electrode of the sub electrode portion by spot welding or the like, and the joining strength between the net body and the draw wire can be improved.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in more detail with reference to the drawings. FIG. 1 is an explanatory diagram for explaining an SO _X gas sensor, which is an example of a gas sensor according to the present invention. In this SO _X gas sensor, a sub-electrode portion 12 made of a sulfate containing a crystalline inorganic salt of silver sulfate is bonded to one surface of a substrate 10 made of yttria-stabilized zirconia (YSZ) which is a solid electrolyte material. It is a thing. Furthermore, the substrate 1
A platinum electrode 14 is formed on the other surface side of 0, and a silver-containing electrode 16 is also formed on the surface of the sub electrode portion 12. Each of the platinum electrode 14 and the silver-containing electrode 16 is
One end is connected to a voltmeter (not shown) via platinum wires 22 and 24 joined to the respective electrodes. SO like this
_{In the X} gas sensor, the portion including the sub electrode portion 12 is SO _X.
It is inserted into the cylinder 20 into which the contained flue gas is introduced.

In the SO _X gas sensor shown in FIG. 1, the silver-containing electrode 16 provided on the sub-electrode portion 12 is formed by a net 28 made of a platinum wire as a metal wire, as shown in FIG. . Therefore, one end of the platinum wire 22 and the net 28 can be connected by welding such as spot welding, and the connection strength between the two can be made high. Further, as shown in FIG. 3, the platinum wire rod forming the net 28 has a metal layer 32 made of silver formed on the peripheral surface of the platinum wire rod 30 by silver plating. This metal layer 3
The silver forming 2 reacts with SO _X gas to form silver sulfate, which is a crystalline inorganic salt, as described later. As the metal wire rod forming the mesh body 28, a gold wire rod may be used instead of the platinum wire rod 30.

In the SO _X gas sensor shown in FIG. 1, platinum 18 produced by thermally decomposing a platinum compound is present at the joint interface between the sub-electrode portion 12 and the substrate 10. Due to the presence of the platinum 18, the SO _X gas concentration changes
The response time of the _X gas sensor can be shortened. It is presumed that the reaction at the bonding interface between the sub electrode portion 12 and the substrate 10 can be promoted by the presence of platinum 18. Here, the platinum fine particles are blended with a sulfate to prepare the auxiliary electrode portion 12
However, the obtained SO _X gas sensor was unable to shorten the response time to changes in the SO _X gas concentration. Further, it was obtained even when a platinum thin film was formed by sputtering, vapor deposition, etc., over the entire surface of one side of the substrate 10.
The SO _X gas sensor could not shorten the response time to changes in SO _X gas concentration. In addition, the sub-electrode portion 12 in FIG.
As the sulfate salt containing silver sulfate for forming, silver sulfate and barium sulfate were mixed in an amount of 50 mol%. In this way, the silver sulfate and barium sulfate are mixed to form the sub-electrode portion 12.
By forming the SO _x gas sensor, not only the cost of the SO _x gas sensor can be reduced, but even if the SO _x gas sensor is used in a high temperature atmosphere above the melting point (645 ° C.) of silver sulfate, it has a melting point higher than that of silver sulfate. Barium sulfate (melting point; 1580 ° C)
Is present in the sub-electrode portion 12, it is possible to prevent the molten silver sulfate from flowing in the sub-electrode portion 12.

In the SO _X gas sensor shown in FIG. 1, the bonding surface (3.
A 5 mm square) is coated with the platinum compound solution. At this time, hexachloroplatinic acid (H ₂ PtCl ₆
An aqueous solution can be used as a solution by using 6H ₂ O). In the case of the SO _X gas sensor shown in FIG.
4 μl of an aqueous solution in which 5N (N) hexachloroplatinic acid was dissolved was used. In this way, the coated surface of the substrate 10 coated with the solution in which the platinum compound is dissolved is heated at a temperature at which the platinum compound is completely thermally decomposed to generate platinum, and platinum is attached to the bonding surface of the substrate 10. Since hexachloroplatinic acid was used as the platinum compound when the SO _X gas sensor shown in FIG. 1 was prepared, it was heated at 300 to 750 ° C., which is the temperature at which hexachloroplatinic acid thermally decomposes into platinum.
Then, a sulfate powder containing a predetermined amount of silver sulfate is placed on the bonding surface of the substrate 10 to which platinum is attached. In this case, by forming a frame 28 on the substrate 10 as shown in FIG. 1, it is possible to fill the frame 28 with a powder of sulfate containing silver sulfate. The frame 18 may be made of yttria-stabilized zirconia (YSZ) integrally with the substrate 10 or may be made of another material. As the sulfate containing silver sulfate used in FIG. 1, a sulfate obtained by mixing 50 mol% each of silver sulfate and barium sulfate was used.

As described above, the metal layer 3 made of silver is provided on the peripheral surface of the surface of the powder of the sulfate containing silver sulfate filled in the frame 26.
The net body 2 formed by the platinum wire rod 30 on which 2 is formed
At 8, the silver-containing electrode 16 having one end of the platinum wire 22 welded by spot welding or the like is placed. The metal layer 32 made of silver was formed by silver plating. Further, with the silver-containing electrode 16 made of the mesh body 28 placed, the powder of sulfate containing silver sulfate is fired to form the sub-electrode portion 12. The auxiliary electrode portion 12 of the SO _X gas sensor shown in FIG. 1 was fired at about 790 ° C. In addition, the platinum electrode 14 of the substrate 10
In the process different from the firing of the sub-electrode part 12.
It is formed by firing the platinum paste applied to the side surface of the No. 0 side surface that is opposite to the surface to be joined to the sub electrode portion 12.

1 to 3, the metal layer 32 formed on the peripheral surface of the platinum wire 30 forming the net 28 is formed by silver plating, but may be formed by spatter or vapor deposition instead of plating. Good. In addition, the sub-electrode portion 12 is formed by firing a sulfate containing silver sulfate with the net 28 placed on the surface.
After forming, the one end of the platinum wire 22 may be welded to the net 28 by spot welding or the like. Further, a powder of sulfate containing silver sulfate may be formed into a sheet with a binder or a solvent and then placed on the substrate 10. In this case, the formation of the frame 26 shown in FIG. 1 can be omitted.

The SO _X gas sensor thus obtained is
When the silver-containing electrode 16 is formed, the silver-containing electrode 16 can be formed without using a silver paste. Therefore, the influence of the components contained in the paste on the electrochemical reaction in the sub-electrode part 12 is eliminated. it can. Furthermore, platinum wire 2
2 and the net 28 can be connected by welding such as spot welding, and the connection strength between the two can be improved as compared with the gas sensor shown in FIG. In addition, the SO _X gas sensor shown in FIG. 1 in which platinum 18 is present at the bonding boundary surface between the substrate 10 and the sub-electrode part 12 has the substrate 100 and the sub-electrode part 10.
The conventional SO shown in Fig. 6 in which platinum is not present at the joint interface with 2
The adhesive strength between the substrate 10 and the sub electrode portion 12 is improved as compared with the _X gas sensor. This is the SO shown in FIG.
_{In the X} gas sensor, the platinum wire 112 bonded to the sub-electrode portion 102 is pulled to separate the substrate 100 and the sub-electrode 102 from each other, but in the SO _X gas sensor shown in FIG. 1, the substrate 10 and the sub-electrode portion 12 are separated from each other. Is also clear from the fact that they do not peel off.

In the SO _X gas sensor shown in FIG. 1, when a flue gas containing SO _X gas is introduced into the cylindrical body 20,
Platinum electrode 14 of substrate 10 and silver-containing electrode 1 of sub-electrode part 12
In 6 and the sub-electrode part 12, the following electrochemical reaction occurs.

Embedded image Therefore, the electromotive force due to the electrochemical reaction can be measured by measuring the potential difference between the platinum electrode 14 and the silver-containing electrode 16, and the SO _X gas concentration can be calculated based on the Nernst equation shown below. it can.

[Equation 1]

The response and recovery times of the SO _X gas sensor shown in FIG. 1 are shown in FIGS. 4 and 5. As shown in FIG.
Platinum electrode 14 due to introduction / stop of flue gas containing _X gas
3 shows a change in potential difference between the electrode 16 and the silver-containing electrode 16. In addition, FIG. 5 shows the platinum electrode 14 and the silver-containing electrode 1 when the flue gas is introduced.
The time until the potential difference between 6 and 6 is stabilized is shown. As is clear from FIG. 4, in the SO _X gas sensor shown in FIG. 1, the potential difference between the platinum electrode 14 and the silver-containing electrode 16 is stable after the flue gas containing the SO _X gas is introduced into the cylindrical body 20. It takes about 5 minutes, and is significantly shortened by the stabilization time (FIG. 7) of about 30 minutes in the conventional SO _X gas sensor shown in FIG. Furthermore, even in the return time,
The return time of the SO _X gas sensor shown in FIG. 1 is about 5 minutes, which is significantly shorter than the return time of the conventional SO _X gas sensor shown in FIG. 6 (FIG. 7). Also, platinum electrode 1
As for the time (90% response time) from when a potential difference is generated between No. 4 and the silver-containing electrode 16 to reach 90% of the equilibrium state, the SO _X gas sensor shown in FIG. As shown, it takes about 3 minutes.
SO _X gas sensor 90% response time is significantly shorter than about 10 minutes (Figure 8).

Thus, the SO shown in FIG._XThe gas sensor
Conventional SO shown in FIG._XSO compared to gas sensors_Xgas
Details that shortened response time to changes in concentration
The reason for this is not clear yet, but it can be considered as follows.
That is, the conventional SO shown in FIG._XIn the gas sensor, the substrate 1
00 at the bonding interface between the substrate 00 and the sub-electrode 102
2 platinum electrode 104 and sub-electrode 102 silver-containing electrode 106
Since it is slower than the reaction in_XAgainst changes in gas concentration
Response / recovery time becomes longer. In this respect, the SO shown in Fig. 1_XMoth
In the sensor, at the bonding interface between the substrate 10 and the sub-electrode part 12,
The existing platinum 18 promotes the reaction at this bonding interface.
As a result of the progressive catalytic action, SO_XFor changes in gas concentration
To SO_XBecause the response time of the gas sensor was shortened
You. Therefore, the SO shown in FIG._XThe gas sensor is shown in FIG.
Conventional SO_XSO compared to gas sensors_XFor changes in gas concentration
The response time to SO can be significantly shortened, and SO in flue gas _XMoth
It can be used for monitoring, etc.

Although the above description has been made on the SO _X gas sensor, the present invention can be applied to the CO _X gas sensor and the NO _X gas sensor. However, when the gas sensor according to the present invention is used as a CO _X gas sensor, the metal layer 32 covering the peripheral surface of the platinum wire 30 of the net 28 forming the electrode of the sub-electrode part 12 is made of lead (Pb) or zinc. (Zn) and the like, and the sub-electrode portion 12 is made of lead carbonate (PbCO ₃ ) or zinc carbonate (ZnC).
O ₃ ) and the like. Further, the gas sensor according to the present invention,
When used as a NO _X gas sensor, auxiliary electrode portion 12
The metal layer 32 that covers the peripheral surface of the platinum wire 30 of the net 28 that forms the electrode is formed of silver (Ag), lead (Pb), or the like, and the sub-electrode portion 12 is silver nitrate [Ag (NO ₃ ) ₂ ]. Alternatively, lead nitrate [Pb (NO ₃ ) ₂ ] or the like is used. In the SO _X gas sensor, the net 28
The metal layer 32 covering the peripheral surface of the platinum wire rod 30 may be made of nickel. In this case, the sub electrode portion 12 is made of nickel sulfate.

[0019]

According to the present invention, since the electrode of the sub-electrode portion which constitutes the gas sensor can be formed without using the metal paste, the electrode in the sub-electrode portion due to the components contained in the paste can be formed. The influence on the electrochemical reaction can be eliminated, and the detected gas concentration and the like can be measured as accurately as possible. In addition, a net body obtained by welding a lead wire such as a platinum wire by spot welding or the like can be used as the electrode of the sub-electrode part, and the metal paste is used to form the electrode of the sub-electrode part. The manufacturing process of the gas sensor can be shortened as compared with the process. Further, the connection strength between the extraction wire and the electrode of the sub electrode portion can be improved, and the yield of the gas sensor can be improved.

[Brief description of drawings]

FIG. 1 is an explanatory diagram for explaining an example of a gas sensor according to the present invention.

FIG. 2 is an explanatory diagram for explaining a mesh body that forms an electrode provided in a sub electrode portion of the gas sensor shown in FIG.

3 is a cross-sectional view of a metal wire rod forming the net body shown in FIG.

4 is an explanatory diagram illustrating a response time of the gas sensor illustrated in FIG.

5 is an explanatory diagram illustrating a response time of the gas sensor illustrated in FIG.

FIG. 6 is an explanatory diagram for explaining a conventional gas sensor.

FIG. 7 is an explanatory diagram illustrating a response time of the conventional gas sensor shown in FIG.

FIG. 8 is an explanatory diagram illustrating a response time of the conventional gas sensor shown in FIG.

[Explanation of symbols]

 10 Substrate 12 Sub-electrode part 14 Platinum electrode (platinum-containing electrode) 16 Electrode of sub-electrode part 12 (silver-containing electrode) 18 Platinum 20 Cylindrical body 22, 24 Lead wire 26 Frame body 28 Net body forming electrode 16 30 Net body Metal wire rod forming 28 32 Metal layer formed on the surface of metal wire rod 30

Claims (10)

[Claims] 1. A sub-electrode portion made of a crystalline inorganic salt, having an electrode provided on at least a part of the surface thereof, on the other surface side of a substrate made of a solid electrolyte material, the platinum-containing electrode being provided on one surface side. In the joined gas sensor, the electrode provided on the surface of the sub-electrode portion is composed of a mesh formed of a metal wire rod which is chemically stable against a gas such as a detection gas, and the metal wire rod. A gas sensor, wherein a metal layer made of a metal capable of reacting with a detection gas to form a crystalline inorganic salt forming the sub-electrode is formed on the peripheral surface of the. 2. The gas sensor according to claim 1, wherein the substrate is a substrate made of yttria-stabilized zirconia. 3. The gas sensor according to claim 1, wherein the metal wire rod forming the mesh is a platinum wire rod or a gold wire rod. 4. A wire body provided on the surface of the sub-electrode portion, wherein one end of the extraction wire is welded by spot welding or the like.
4. The gas sensor according to claim 3. 5. The gas sensor according to claim 4, wherein the extraction wire is a platinum wire or a gold wire. 6. A sub-electrode portion made of a crystalline inorganic salt, having an electrode provided on at least a part of the surface thereof, on the other surface side of the substrate made of a solid electrolyte material, the platinum-containing electrode being provided on one surface side. When manufacturing a bonded gas sensor, a predetermined amount of crystalline inorganic salt is placed on the bonding surface of the substrate bonded to the sub-electrode, and the crystalline inorganic salt is generated by reacting with the detection gas. A metal layer made of a metal that is chemically stable to a gas such as a detection gas, on which a metal layer made of the obtained metal is formed, forms a net body, and then placed on the bonding surface of the substrate. After placing the net body on the surface of the crystalline inorganic salt, by firing the crystalline inorganic salt having the net body placed on the surface, to form a sub-electrode portion using the net body as an electrode. A method for manufacturing a characteristic gas sensor. 7. The method of manufacturing a gas sensor according to claim 6, wherein a substrate made of yttria-stabilized zirconia is used as the substrate. 8. The method of manufacturing a gas sensor according to claim 6, wherein a platinum wire or a gold wire is used as the metal wire forming the mesh. 9. The crystalline inorganic salt is fired after placing a net body having one end of a draw wire welded by spot welding or the like on the surface of the crystalline inorganic salt placed on the bonding surface of the substrate. The method for manufacturing a gas sensor according to any one of 6 to 8. 10. The method for manufacturing a gas sensor according to claim 9, wherein a platinum wire or a gold wire is used as the extraction wire.