Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
1998-11-13
2001-11-20
Tung, T. (Department: 1744)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C204S426000, C204S427000, C204S421000, C204S293000
Reexamination Certificate
active
06319377
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a nitrogen oxide sensor for detecting a concentration of a nitrogen oxide. More particularly, the invention relates to a nitrogen oxide device for converting NO into NO
2
or NO
2
into NO electrochemically.
2. Prior Art
A solid element type of nitrogen oxide sensor has been representatively disclosed in the Japanese Patent Laid-Open Publication No. Hei 4-142455 (1983). This sensor is equipped on an ion conductor with a nitrate electrode and a reference electrode to measure an electromotive force to be generated between the electrodes, so that it has sensitivity to NO and NO
2
. Since the sensitivities of the electrodes to NO and NO
2
are different, however, the NOx concentration cannot be detected in an environment to be measured where the two gases (NO and NO
2
) coexist, or the concentration of neither NO nor NO
2
can be detected.
In order to improve the sensitivities to NO and NO
2
, there has been proposed (in the Japanese Patent Laid-Open Publication No. Hei 6-123726(1995)) an electromotive type sensor in which an oxide catalyzer of NO is applied to or mixed with an auxiliary electrode. According to this method, the NO gas in the coexisting gas of NO and NO
2
can be oxidized into a single gas of NO
2
so that the NOx concentration can be detected. Like the analyzing method of the prior art, however, the accuracy is decided by the oxidizing ability of the catalyst so that the NOx concentration is different from the actual value. Moreover, these sensors are troubled by their moisture and heat resistances because they employ a nitrate in the auxiliary electrode, so that they are substantially difficult for the practical purposes from the standpoint of long-term stability.
On the other hand, there has also been reported a sensor for measuring a change in the electric conductivity, as based on the NOx concentration, by making use of semiconductor characteristics of various oxides. In the Japanese Patent Laid-Open Publication No. Hei 6-160324 (1995), for example, there has been proposed a sensor using tin oxides as a gas sensitive member. However, this sensor is also different in the sensitivity to NO and NO
2
so that it cannot detect the NOx concentration in an environment to be measured where the two gases coexist.
In recent years, there has been proposed (in SAE TECHNICAL PAPER 960334 or the Japanese Patent Laid-Open Publication No. Hei 8-271476 (1997)) a method for electrolyzing NOx to detect the NOx concentration from the electrolytic current value. The detection principle itself of this sensor is developed from the electrolytic current type sensor which has been widely applied in the prior art to other gases. Specifically, an ion conductor is provided with two chambers, the first one of which lowers the oxygen concentration in the environment to be measured substantially to zero and reduces NO
2
to NO by an oxygen pump and the second one of which is equipped with electrodes for ionizing the oxygen produced from the reduced NO in the measured environment by applying a voltage to the electrodes thereby to detect the NOx concentration. The NOx concentration to be detected by the sensor highly depends on the performance of the oxygen pump. When the concentration of the gas to be detected is low, the interference of the residual oxygen concentration in the measured environment is high, and the signal current is minute. In a noisy circumstance as in an automobile, the S/N ratio is so poor that the NOx concentration is difficult to detect accurately.
We have proposed a mixed potential type NOx sensor and have filed patent applications published as Japanese Patent Laid-Open Publication Nos. Hei 6-194605 (1995), Hei 6-216698 (1995) and Hei 6-216699 (1995). However, these constructions are excellent in sensitivity to NO or NO
2
but are troubled by the mutual interference of NO or NO
2
or by the interference of a reducing gas.
SUMMARY OF THE INVENTION
The gas responsive characteristics of the mixed potential type sensor to the nitrogen oxide gases, especially to NO and NO
2
have opposite sensitive outputs to the concentration thereby to establish the mutual interference in the environment where the two gases coexist. Since the oxygen concentration is controlled by the oxygen pump, it is difficult to make a conversion into the single NO gas or the single NO
2
gas, and the mutual interference cannot be sufficiently avoided. There is demanded the construction of a sensor which has a high sensitivity output of the NOx to be detected and a high concentration dependency so that it can detect not only the NOx concentration accurately even in the noisy circumstance of the automobile or the like but also the total of nitrogen oxides contained in the environment to be detected. Therefore, the invention has an object to satisfy that demand.
In the total nitrogen oxide sensor combining the nitrogen oxide converting device and the electromotive force type or mixed potential type nitrogen oxide sensor, moreover, a stable sensor output cannot be produced due to the deterioration in the converting ability in a nitrogen oxide converting electrode, and the nitrogen oxide conversion ability has to be further improved to measure a low concentration of the nitrogen oxide accurately.
The present invention has an object to provide a nitrogen oxide converting device which has a higher nitrogen oxide converting ability free from being deteriorated even when the power is ON, and a nitrogen oxide sensor which has a high and stable sensor output by combining the converting device with the electromotive force type or mixed potential type nitrogen oxide sensor.
A nitrogen oxide sensor of the present invention is constructed such that there are integrated an oxygen pump portion, which employs an ion conductive solid electrolyte body and which is equipped with at least one pair of electrodes for sucking or discharging an oxygen gas electrochemically, and a NOx gas detecting portion which is equipped with a detecting electrode and a counter electrode, which is also a reference electrode, for detecting a NOx gas, and such that one of the electrodes of the oxygen pumping portion and either the detecting electrode or the detecting electrode and the counter electrode of the NOx gas detecting portion are arranged in a gas chamber communicating with an environment to be detected, so that the NOx gas concentration is detected in terms of a potential difference between the detecting electrode and the counter electrode which is heated to a predetermined temperature range by a heating mechanism. With respect to the electrodes composing the oxygen pumping portion, at least the electrode, which is formed in the gas chamber having the detecting electrode arranged therein, is a NOx converting electrode having an electrolytic performance in which the electrolytic current for NOx is higher than that for oxygen within a predetermined applied voltage range, so that on the NOx converting electrode, mainly the nitrogen oxide gas in the detected atmosphere, especially, NO is oxidized into NO
2
, or NO
2
is reduced into NO thereby to detect the total of the nitrogen oxide gas.
Here, the nitrogen peroxide gas (e.g., N
2
O
5
) over NO
2
raises no trouble because its direction of the sensor output in the system of the invention is identical to that of NO
2
, that is, the direction in which the sensor output increases with the increase in the concentration. On the other hand, the electrolytic direction on the NOx converting electrode depends upon the material and quality of the detecting electrode. In the case of the detecting electrode exhibiting a higher sensitivity to NO
2
than to NO, more specifically, it is preferable to detect NOx by applying a voltage to the NOx converting electrode as a positive electrode to effect a conversion into NO
2
. In the case of a detecting electrode exhibiting a higher sensitivity for NO than for NO
2
, on the other hand, it is preferable to detect NOx by applying a voltage to the NOx converting electrode a
Gao Yunzhi
Hasei Masaharu
Kunimoto Akira
Ono Takashi
Yan Yongtie
Kabushiki Kaisha Riken
Kubovcik & Kubovcik
Tung T.
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