Solid electrolyte type carbon dioxide gas sensor element

Details Solid electrolyte type carbon dioxide gas sensor element Solid electrolyte type carbon dioxide gas sensor element

2000-01-27

2001-12-04

Tung, T. (Department: 1743)

Chemistry: electrical and wave energy

Apparatus

Electrolytic

C204S426000

Reexamination Certificate

active

06325905

ABSTRACT:

DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to a solid electrolyte-use gas sensor element which is incorporated in air cleaner system or environmental instrumentation system to measure the concentration of carbon dioxide gas contained in an atmosphere. More specifically, it relates to a solid electrolyte type carbon dioxide gas sensor element which has attained the improvement of stability in a high-humidity environment and a reduction in the stabilization time of electromotive force in particular.
In recent years, along with a growing interest in environmental problems, monitoring sensors for controlling the concentration of carbon dioxide gas discharged into the atmosphere have been attracting much attention. Out of the sensors, a solid electrolyte type carbon dioxide gas sensor element making use of a change in the electromotive force of a solid electrolyte is small in size, simple in structure and inexpensive. Therefore, the implementation of this solid electrolyte type carbon dioxide gas sensor element has been strongly desired.
The solid electrolyte type carbon dioxide gas sensor element whose implementation is now under study generally comprises a solid electrolyte layer which is an ionic conductor, a work electrode layer which contains an electron conducting material and an auxiliary electrode material, a reference electrode layer containing an electron conducting material and a heater for heating these layers.
This sensor element is generally heated at a fixed temperature of 100 to 600° C. to start operation and generates fixed electromotive force between the work electrode layer and the reference electrode layer through the solid electrolyte layer according to the concentration of carbon dioxide gas when it is left in an atmosphere containing carbon dioxide gas. When the concentration of carbon dioxide gas contained in an atmosphere where the sensor element is left changes, a dissociation equilibrium reaction between the auxiliary electrode material contained in the work electrode layer and carbon dioxide gas proceeds until equilibrium is reached and the concentration of movable ions in the solid electrolyte layer changes in the vicinity of the work electrode layer.
Since this concentration change appears as a change in electromotive force, it is possible to know the concentration of carbon dioxide gas by measuring electromotive force at that time by a voltmeter and using a prepared analytical curve showing the correlation between electromotive force and the concentration of carbon dioxide gas.
The solid electrolyte layer of this carbon dioxide gas sensor is formed of a cation conductor such as NASICON (Na
1+A
Zr
2
Si
A
P
3−A
O
12
, provided that 0≦A≦3) a complex oxide containing lithium oxide such as Li
2
SiO
3
, LiCoO
2
and Li
2
ZrSi
6
O
15
, or &bgr;-Al
2
O
3
.
The electron conducting material contained in the work electrode layer is required to detect electromotive force and selected from previous metal materials having excellent heat resistance and oxidation resistance, such as gold and platinum.
The auxiliary electrode material contained in the work electrode layer can cause an equilibrium reaction between it and carbon dioxide gas in an atmosphere containing carbon dioxide gas and is selected from alkali metal carbonates and alkali earth metal carbonates which have dissociation equilibrium with carbon dioxide gas.
Further, the electron conducting material contained in the reference electrode layer is identical to the electron conductive layer contained in the work electrode layer.
The solid electrolyte type carbon dioxide gas sensor which operates with the above constitution has such advantages that it can measure the concentration of carbon dioxide gas contained in an atmosphere very accurately and that it is small in size and can be produced at a low cost. Therefore, it is widely accepted as a sensor element having high general applicability.
JP-A 5-80021 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) discloses a carbon dioxide gas sensor which comprises a detection electrode (work electrode) and a standard electrode (reference electrode) with an ionic conductor therebetween, wherein a coating material for the detection electrode is a solid solution of an alkali earth metal carbonate and an alkali metal carbonate and does not contain alkali metal carbonate crystals.
However, the above solid electrolyte type carbon dioxide gas sensor in which an alkali metal carbonate or alkali earth metal carbonate is used as an auxiliary electrode material has such a problem that when it is left in an atmosphere having a high water concentration such as a high-humidity atmosphere or condensation atmosphere after heating is stopped to suspend the operation of the sensor and then its operation is resumed, electromotive force greatly falls below a value before it is left.
Further, the time interval from the start of operation to the time when electromotive force becomes stable is generally 6 to 8 hours whereas it is 20 to 40 hours when the gas sensor is left in the above atmosphere without being heated.
These problems are factors of preventing the implementation of a solid electrolyte type carbon dioxide gas sensor. Therefore, the development of a solid electrolyte type carbon dioxide gas sensor whose electromotive force does not change even when it is left in an atmosphere having a high water concentration such as high humidity and condensation for a long time without being heated and whose time interval from the start of operation to the time when electromotive force becomes stable is short has been desired.
It is therefore an object of the present invention to provide a solid electrolyte type carbon dioxide gas sensor element which comprises a work electrode layer having novel composition.
It is another object of the present invention to provide a solid electrolyte type carbon dioxide gas sensor element whose a value of electromotive force remains unchanged even when it is left in an atmosphere having a high water concentration such as high humidity and condensation without being heated and whose time interval from the start of operation to the time when electromotive force becomes stable is less than 2 hours.
Other objects and advantages of the present invention will become apparent from the following description.
According to the present invention, firstly, the above objects and advantages of the present invention are attained by a solid electrolyte type carbon dioxide gas sensor element (may be referred to as “first carbon dioxide gas sensor element of the present invention” hereinafter) which comprises a solid electrolyte layer, a work electrode layer formed on one side of the solid electrolyte layer and containing an electron conducting material and an auxiliary electrode material, and a reference electrode layer formed on the same side or the other side of the solid electrolyte layer and containing an electron conducting material, wherein the work electrode layer further containing a rare earth metal oxide.
Secondly, the above objects and advantages of the present invention are attained by a solid electrolyte type carbon dioxide gas sensor element (may be referred to as “second carbon dioxide gas sensor element of the present invention” hereinafter) which comprises a solid electrolyte layer, a work electrode layer formed on one side of the solid electrolyte layer and containing an electron conducting material and an auxiliary electrode material, and a reference electrode layer formed on the same side or the other side of the solid electrolyte layer and containing an electron conducting material, wherein at least part of the auxiliary electrode material of the work electrode layer is a rare earth metal oxide carbonate.


REFERENCES:
patent: 5194134 (1993-03-01), Futata et al.
patent: 5413691 (1995-05-01), Kaneyasu et al.
patent: A580021 (1993-03-01), None

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