Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
2002-10-09
2004-09-07
Olsen, Kaj K. (Department: 1753)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C204S426000, C204S427000, C204S429000
Reexamination Certificate
active
06787014
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a gas-detecting element and a gas-detecting device for measuring the concentration of a detection object gas in a gas atmosphere, particularly to a gas-detecting element and a gas-detecting device suitable for directly measuring the concentration of nitrogen oxides in a combustion exhaust gas emitted from automobiles, etc.
BACKGROUND OF THE INVENTION
So-called gas sensors with high gas selectivity capable of electrochemically detecting a particular gas by using solid electrolyte substrates have recently been proposed actively. Particularly, gas sensors capable of measuring the concentration of total NOx in an exhaust gas from automobiles without affected by other gases are strongly demanded.
Thus, the inventors previously proposed a mixed-potential-type NOx sensor comprising an oxygen-ion-conductive zirconia solid electrolyte operable at high temperatures in JP 9-274011 A. This NOx sensor has a basic structure, which comprises a NOx-sensing electrode and a reference electrode formed on an opposite or same surface of a zirconia solid electrolyte substrate as the NOx-sensing electrode. In this NOx sensor, a sensing electrode is, of course, exposed to a detection gas (gas to be detected), and a reference electrode can be simultaneously exposed to a detection gas, if the reference electrode is active with only oxygen. Because the NOx-sensing electrode is active with NOx and oxygen, and because the reference electrode is active only with oxygen, output (potential difference) can be obtained due to the difference in chemical potential between both electrodes. Accordingly, the measurement of potential difference between both electrodes leads to the detection of the NOx concentration in the detection gas. Incidentally, when the reference electrode is also active with NOx, the same NOx sensitivity can be obtained if isolated from the detection gas.
At the time of detecting a gas by the sensing electrode of the above mixed-potential-type NOx sensor, however, NO is subjected to reactions represented by the following formulae (1) and (2):
O
2
+4e
−
→2O
2−
(1), and
2NO+2O
2−
→2NO
2
+4e
−
(2),
and NO
2
is subjected to reactions represented by the following formulae (3) and (4):
2O
2−
→O
2
+4e
−
(3), and
2NO
2
+4e
−
→2NO+2O
2−
(4).
As a result, sensor outputs with NO and NO
2
at the time of detecting a gas are just opposite in polarity. When the concentration of total NOx is detected in an exhaust gas emitted from vehicles, the coexistence of NO and NO
2
causes interference if no measure is taken, failing to detect the concentration of total NOx precisely.
Accordingly, JP 9-274011 A proposes a laminate-type gas-detecting device. According to the principle of this laminate-type gas-detecting device, oxygen from air is introduced into a gas detection chamber using an electrochemical oxygen pump. As a result, reducing gases such as HC (hydrocarbons), CO (carbon monoxide), etc. in the detection gas are oxidized to be harmless. Simultaneously, NO in NOx is electrochemically converted to NO
2
, so that NOx becomes consisting only of NO
2
. After this treatment for turning a detection gas to contain only one detection object gas, the NO
2
concentration is measured from the potential difference between the NOx-sensing electrode and the reference electrode, thereby determining the concentration of total NOx.
In such NOx-detecting element or such laminate-type NOx gas-detecting device, its detection performance, namely sensitivity and its stability and response, is particularly governed by the performance of a sensing electrode. Conventionally reported as the sensing electrodes of such mixed-potential-type NOx sensors are, for instance, NiCr
2
O
4
(SAE Paper No. 961130), Pt—Rh alloys or cermet electrodes comprising Pt—Rh alloys to which a zirconia solid electrolyte is added (JP 11-72476 A). These sensing electrodes have excellent sensitivity. However, further improvement is needed with respect to the stability of sensitivity of sensing electrodes. For this purpose, it is important to improve the stability of an electrode material per se, and the bonding stability of interface (electrode interface) between a solid electrolyte substrate and a sensing electrode. Particularly when metal oxides are used for the electrodes, it has conventionally been difficult to control the bonding stability of this electrode interface. This is because there is generally weak bonding between metal oxides and solid electrolyte substrates, resulting in the likelihood that peeling and cracking occur in their interface.
To improve the response of gas detection, it is necessary to reduce the interface impedance of the electrode in the gas sensor. For this purpose, increase in an electrode area and the elevation of operation temperatures have been investigated. However, in a mixed-potential-type sensor, the higher the temperature, the lower the gas sensitivity. In addition, to increase an electrode area, it is necessary to make a sensor element larger. The increase of the sensor element deteriorates the uniformity of the temperature distribution of the sensor element, resulting in the variation of performance and instability.
As described above, though there are materials excellent in sensitivity for the mixed-potential-type NOx sensor, further improvement is needed with respect to the stability of sensitivity. Particularly when a metal oxide electrode is used as a sensing electrode, there is poor bonding stability with the solid electrolyte substrate, resulting in the variation of detection performance and decrease in yield. Therefore, it is desired not only to improve interface stability between the sensing electrode and the solid electrolyte substrate, but also to reduce the variation of characteristics that are caused during a production process for some reasons. Further, it is desired to improve gas response without making the sensor element larger, and without accompanying decrease in gas sensitivity.
Though the importance of stability and response of the sensing electrode has been described above, such characteristics are not required only to the sensing electrode. In the case of the NOx sensor, for instance, it is important to improve the stability and response of a reference electrode serving as a reference for electrode potential, and an oxygen-sensing electrode for making compensation for oxygen concentration, etc., because these characteristics also affect the performance of the NOx sensor.
In the case of a NOx sensor mounted onto a vehicle, oxygen concentration in the detection gas widely varies, and thus the influence of the concentration of coexisting oxygen cannot be neglected. In the NOx sensor of this type, a reference electrode active only with oxygen is disposed in a portion close to the NOx-sensing electrode, and the measurement of potential between the NOx-sensing electrode and the reference electrode leads to the determination of NOx concentration in the detection gas. By mounting an oxygen-sensing electrode active with oxygen but inactive with NOx near the NOx-sensing electrode within a detection chamber, by measuring potential difference E
2
between the reference electrode and the oxygen-sensing electrode disposed in an air duct, and potential difference E
1
between the reference electrode and the NOx-sensing electrode, and by arithmetically treating these difference (E
1
−E
2
), it is possible to compensate the variation of oxygen concentration. The measurement using such electrode inactive with a detection gas enables high-precision measurement of the concentration of a detection object gas even with a detection gas such as an exhaust gas from automobiles, etc. in which oxygen concentration varies.
However, if the reference electrode or the oxygen-sensing electrode becomes considerable active with NOx (exhibits mixed potential), its influence decreases the precision of NOx detection. To imp
Hasei Masaharu
Kunimoto Akira
Ono Takashi
Yan Yongtie
Browdy and Neimark , P.L.L.C.
Kabushiki Kaisha Riken
Olsen Kaj K.
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