JPH09236574A - Mixture electrode for solid electrolyte type gas sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance the response speed of a sensor and to prevent the expansion of a measuring range and a change of a characteristic value with the elapse of time by mixing a specific oxide powder with an Au powder or Au paste to paste the resulting mixture and coating a solid electrolyte with the obtained paste to baking the coated electrolyte. SOLUTION: A mixture electrode is produced by mixing an oxide powder excepting Al2 O3 such as NiO or LiAlO2 with an Au powder or Au paste to paste the resulting mixture and coating a solid electrolyte with the obtained paste to bake the coated electrolyte. Or, an oxide powder capable of forming carbonate is mixed with the Au powder or Au paste and the resulting mixture is pasted to coat the solid electrolyte to bake the coated electrolyte to produce the mixture electrode. Since this mixture electrode can hold a porous structure, the diffusion of gas to be inspected to the interface of the electrode and the electrolyte is smoothly generated and electrode raction becomes easy to advance. Further, by using this mixture electrode, the stability of the fine structure of the electrode is ensured over a long period of time and the lowering of the response speed of a sensor is suppressed and the stability of the response characteristics of the sensor is obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a mixture electrode (cermet electrode) suitable for a solid electrolyte type gas sensor for measuring CO ₂ gas concentration and the like, and more specifically to a reaction between an electrode and a solid electrolyte interface. A solid with low resistance that can suppress changes in the electrode structure over time due to the progress of the sintering reaction of the electrodes, improve the response speed of the sensor, expand the measurement range, and prevent changes in characteristic values over time. The present invention relates to a mixture electrode for an electrolytic gas sensor.

[0002]

Conventionally, a pair of electrodes formed on the solid electrolyte, and CO ₂ gas-sensitive electrodes by providing an alkali metal carbonate layer on one, CO ₂ and the other was used as a non-sensitive electrode There is known a practical CO ₂ gas sensor that has a gas detection portion and can be self-heated by a small heater (for example, Japanese Patent Laid-Open No. 1-213565). The solid electrolyte type CO ₂ gas sensor as described above is usually C
Commercially available Pt or Au paste is mainly used for the O ₂ gas detection electrode. However, since most solid electrolyte type CO ₂ gas sensors use carbonate as a CO ₂ gas detecting material, when a commercially available Pt paste as described above is used, a compound is formed between the compound and the carbonate. There is a problem that it is formed and changes over time, resulting in a lack of long-term stability.

Further, since the commercially available Au paste is not manufactured for the porous electrode in the gas sensor, and the emphasis is placed on the fineness and smoothness after baking,
It is not suitable as a porous electrode for gas sensors. That is, since the surface of the CO ₂ gas detecting portion that is in contact with the electrode of the solid electrolyte is densely covered with the gold film, there is little contact with the gas phase and there is a problem that the electrode reaction does not proceed smoothly. Furthermore, the sensor operating temperature of 300 to 600
At ° C, the Au paste is gradually sintered and the porosity of the electrode is lost. For this reason, the electrode reaction at the interface between the electrode and the solid electrolyte becomes difficult to proceed, and the response of the sensor deteriorates, that is, the response time becomes long.

[0004]

Therefore, according to the present invention, Au is used.
It is an object of the present invention to form a mixture electrode for a solid electrolyte type gas sensor by using an electrode material prepared by mixing a paste or Au powder with an oxide powder that does not react with NiO or Au, thereby solving the above-mentioned problems. Since the mixture electrode according to the present invention can maintain a porous structure,
The test gas diffuses smoothly to the interface between the electrode and the electrolyte, facilitating the electrode reaction. Further, by using this mixture electrode, it is possible to stabilize the fine structure of the electrode for a long period of time, suppress the decrease in the response speed of the sensor, and obtain the stability of the response characteristic of the sensor.

[0005]

Therefore, the means for solving the problem adopted by the present invention is to add Au powder or Au paste to N
A mixture electrode for a solid electrolyte type gas sensor, characterized by being formed by mixing oxide powders other than Al ₂ O ₃ such as iO and LiAlO ₂ , forming a paste, applying this to a solid electrolyte, and baking it. A solid electrolyte type gas sensor characterized in that it is constituted by mixing an Au powder or an Au paste with an oxide powder capable of forming a carbonate, forming a paste, applying the paste to a solid electrolyte, and baking the mixture. It is a mixture electrode.

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION An example of producing a mixture electrode according to the present invention will be described below. Since the structure of the solid electrolyte type gas sensor is the same as the conventional one, the description thereof will be omitted.
Average particle size 0.5-10 μm, preferably 0.5-2 μm
m of Au powder of 0.1 to 50 μm, preferably 0.5 to 20 μm of NiO powder is adjusted to a mixed amount of 5 to 60 wt%, and ethanol is added. To mix. After removing ethanol from the mixture, a small amount of a solvent such as turpentine oil or castor oil is added to form a paste. Apply this to solid electrolyte etc., about 40
A mixture electrode is formed by baking at 0 ° C or higher.

FIG. 1 shows the relationship between the interfacial conductivity and the NiO composition of the Au / NiO mixture electrode constructed as described above. As is clear from this figure, at 400 ° C, N
It can be seen that the interfacial conductivity increases (the resistance decreases) as the amount of iO mixed increases. Further, the electrode interface resistance changes depending on the average particle size of the mixed NiO, but the smaller the electrode interface resistance, the better (preferably 50Ω or less). For example, in the case of 600 ° C. as shown in FIG. For this reason, it is desirable that the average particle size of NiO be 30 μm or less.

In this example, when the amount of NiO mixed is large and the adhesion strength of the mixture electrode to the solid electrolyte is remarkably reduced, glass frit or the like may be added to maintain the strength. When using a glass frit, it is preferable to use as few as possible because the porosity of the electrode is not lost and adhesion strength is obtained, and an unnecessary reaction does not occur between the glass frit and the constituent materials such as the electrode and the electrolyte. Is 0 to 10 wt% or less). Also,
Since it is difficult to adjust the viscosity and fluidity of the paste when the paste is formed using only turpentine oil, the Au powder and NiO powder particles are easily separated, which causes unevenness of Au and NiO on the electrode after baking. Therefore, in order to eliminate such inconvenience, it is possible to form a mixture electrode by using an organic binder to form a paste in which Au and NiO powder are uniformly dispersed and baking the paste. Alternatively, a commercially available Au paste is used in place of the Au powder, and NiO is added to this Au paste.
A similar mixture electrode can be constructed by mixing powders. By the way, in the case of the Au + NiO mixed electrode as described above, for example, it is expected that Li ₂ CO ₃ and NiO of the solid electrolyte react to produce LiNiO ₂ , but the compound LiNiO produced by this reaction
_{Since 2} has a high activity for CO ₂ gas and has high conductivity, it is expected that the electrode characteristics will be improved, and the electrode performance can be stabilized.

Further, instead of NiO powder, Au and
Incompatible oxides such as Al_TwoO_ThreeExcluding LiAlO_2,
Use oxides such as BaO, MgO, SrO, MnO
You can also. Among the oxides that do not react with Au,
Al_TwoO_ThreeExplaining briefly the reason excluding, the present invention
Then, a carbonate (for example, Li_TwoCO_Three, Na
_TwoCO _Three, K_TwoCO_Three) Is used, so carbonate and A
l_TwoO_ThreeReact with and form a compound as shown in the following formula. Li_TwoCO_Three+ Al_TwoO_Three→ 2 LiAlO_Two+ CO_Two In this way, the aging of the electrode occurs and the porous structure of the electrode
I can't maintain. On the other hand, LiAlO_TwoPlace
In case of_TwoCO_ThreeAnd Al_TwoO_ThreeReacted with
Material LiAlO_TwoSince the use of
Instead, the porous structure can be maintained and the performance of the electrode can be maintained for a long time.
Stabilize. In addition, BaO, MgO, SrO, MnO, etc.
Oxides are capable of forming carbonates, so the porous structure of the electrode
In addition to playing the role of making structures, CO_TwoHigh activity for gas
You can expect a role to play.

With the mixture electrode constructed as described above,
Smooths the diffusion of the test gas to the interface between the electrode and the electrolyte.
Occurs in the sensor and the electrode reaction easily proceeds,
Improvement of response speed, expansion of measurement range, change of characteristic value over time
It can be prevented. It is also possible to form carbonates
If oxide is used, the CO_TwoActivity against gas
It is enhanced and the responsiveness is improved. This improvement in characteristics is due to the electrode
Observed as a reduction in interfacial resistance, the response speed of the sensor
Connected to the top. Mixture by complex impedance measurement
The interfacial resistance of the electrode is 1 / compared to the commercially available Au paste
Decrease to 10 to 1/50 and its resistance value changes with time.
It has been confirmed that it is unlikely to occur. Figure 3
An electrode using Au paste and a mixture electrode according to the present invention
The graph which compared the time-dependent change with a pole is shown. This graph
As can be seen from the authors, the mixture electrode is better than the Au paste electrode.
It is clear that the change with time is small in comparison. Also,
Electrode material of solid electrolyte type CO_TwoApplied to gas sensor
If the CO _TwoThe electromotive force proportional to the concentration is the response characteristic of the electrode.
The value according to Nernst's theoretical formula was obtained as
CO_TwoThe concentration detection range is expanded. Note that the above
Compound electrode is CO_TwoNot limited to gas sensors, O_Two, N
O_X, SO_X, Gas for measuring the concentration of CO, etc.
Of course, it can also be used as a sensor electrode.
You.

[0011]

As described in detail above, according to the present invention, A
Since the electrode material is a mixture of u paste or Au powder and oxide powders other than alumina that does not react with NiO or Au, the prepared mixture electrode can maintain a porous structure. As a result, the diffusion of the test gas to the interface between the electrode and the electrolyte occurs smoothly, and the electrode reaction easily proceeds. In other words, by using this mixture electrode, it is possible to stabilize the fine structure of the electrode for a long period of time, suppress the decrease in the response speed of the sensor, and obtain excellent effects such as the stability of the response characteristic of the sensor. Can be played.

[Brief description of drawings]

FIG. 1 Interfacial conductivity and Ni of Au / NiO mixture electrode
It is a graph which shows the relationship of O composition.

FIG. 2 is a graph showing the relationship between NiO average particle size and electrode interface resistance.

FIG. 3 is a graph comparing changes over time between an electrode using a conventional Au paste and a mixture electrode according to the present invention.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Zhang Masui 5-4-71 East, Hanyu City, Saitama Prefecture, Akebono Brake Central Technical Research Institute Co., Ltd. (72) Inventor Shigeaki Kobayashi 5-4-171 East, Hanyu City, Saitama Prefecture No. Akebono Brake Central Technology Research Institute

Claims (2)

[Claims] 1. Au powder or Au paste containing NiO, Li
A mixture electrode for a solid electrolyte type gas sensor, characterized by being formed by mixing oxide powders such as AlO ₂ excluding Al ₂ O ₃ , forming a paste, applying this to a solid electrolyte, and baking it. 2. A solid electrolyte type gas sensor, characterized in that an Au powder or an Au paste is mixed with an oxide powder capable of forming a carbonate, made into a paste, applied to a solid electrolyte and baked. Mixture electrode for.