Gas sensor

Details Gas sensor Gas sensor

1998-02-19

2002-02-05

Tung, T. (Department: 1743)

Chemistry: electrical and wave energy

Apparatus

Electrolytic

C204S426000, C204S427000, C205S781000

Reexamination Certificate

active

06344119

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a gas sensor for measuring oxides such as NO, NO
2
, SO
2
, CO
2
, and H
2
O contained in, for example, atmospheric air and exhaust gas discharged from vehicles or automobiles, and inflammable gases such as H
2
, CO, and hydrocarbon (CnHm). Preferably, the present invention relates to a gas sensor for measuring NO and NO
2
.
2. Description of the Related Art
Exhaust gas, which is discharged from vehicles or automobiles such as gasoline-fueled automobiles and diesel powered automobiles, contains nitrogen oxides (NOx) such as nitrogen monoxide (NO) and nitrogen dioxide (NO
2
), as well as carbon monoxide (CO), carbon dioxide (CO
2
), water (H
2
O), hydrocarbon (CnHm), hydrogen (H
2
), oxygen (O
2
) and so on. In such exhaust gas, about 80% of the entire NOx is occupied by NO, and about 95% of the entire NOx is occupied by NO and NO
2
.
The three way catalyst, which is used to clean HC, CO, and NOx contained in the exhaust gas, exhibits its maximum cleaning efficiency in the vicinity of the theoretical air fuel ratio (A/F=14.6). If A/F is controlled to be not less than 16, the amount of produced NOx is decreased. However, the cleaning efficiency of the catalyst is lowered, and consequently the amount of discharged NOx is apt to increase.
Recently, in order to effectively utilize fossil fuel and avoid global warming, the market demand increases, for example, in that the discharge amount of CO
2
should be suppressed. In order to respond to such a demand, it becomes more necessary to improve the fuel efficiency. In response to such a demand, for example, the lean burn engine and the catalyst for cleaning NOx are being researched. Especially, the need for a NOx sensor increases.
A conventional NOx analyzer has been hitherto known in order to detect NOx as described above. The conventional NOx analyzer is operated to measure a characteristic inherent in NOx, based on the use of chemical luminous analysis. However, the conventional NOx analyzer is inconvenient in that the instrument itself is extremely large and expensive. The conventional NOx analyzer requires frequent maintenance because optical parts are used to detect NOx. Further, when the conventional NOx analyzer is used, any sampling operation should be performed for measurement of NOx, wherein it is impossible to directly insert a detecting element itself into a fluid. Therefore, the conventional NOx analyzer is not suitable for analyzing transient phenomena such as those occur in the exhaust gas discharged from an automobile, in which the condition frequently varies.
In order to dissolve the inconveniences as described above, there has been suggested a sensor for measuring a desired gas component in exhaust gas by using a substrate composed of an oxygen ion-conductive solid electrolyte.
FIG. 10
shows a cross-sectional arrangement of a gas analyzer disclosed in International Publication WO 95/30146. This apparatus comprises a first chamber
4
for introducing a measurement gas containing NO through a small hole
2
thereinto, and a second chamber
8
for introducing the measurement gas from the first chamber
4
through a small hole
6
. Wall surfaces for constructing the first chamber
4
and the second chamber
8
are composed of zirconia (ZrO
2
) partition walls
10
a
,
10
b
through which oxygen ion is transmittable. A pair of measuring electrodes
12
a
,
12
b
,
14
a
,
14
b
for detecting the partial pressure of oxygen in the respective chambers are disposed on one of the ZrO
2
partition walls
10
a
of the first chamber
4
and the second chamber
8
respectively. Pumping electrodes
16
a
,
16
b
,
18
a
,
18
b
for pumping out O
2
in the respective chambers to the outside of the chambers are disposed on the other ZrO
2
partition wall
10
b
respectively.
In the gas analyzer constructed as described above, the partial pressure of oxygen contained in the measurement gas G introduced into the first chamber
4
via the small hole
2
is detected by a voltmeter
20
as a difference in electric potential generated between the measuring electrodes
12
a
,
12
b
. A voltage in a range of 100 to 200 mV is applied between the pumping electrodes
16
a
,
16
b
by the aid of a power source
22
so that the difference in electric potential has a predetermined value. Accordingly, O
2
in the first chamber
4
is pumped out to the outside of the apparatus. The amount of oxygen pumped out as described above can be measured by using an ammeter
24
.
On the other hand, the measurement gas G, from which almost all of O
2
has been removed, is introduced into the second chamber
8
via the small hole
6
. In the second chamber
8
, a difference in electric potential, which is generated between the measuring electrodes
14
a
,
14
b
, is detected by using a voltmeter
26
. Thus, the partial pressure of oxygen in the second chamber
8
is measured. Further, NO contained in the measurement gas G introduced into the second chamber
8
is decomposed as follows by the aid of the voltage applied between the pumping electrodes
18
a
,
18
b
by means of a power source
28
:
NO→(½)N
2
+(½)O
2
O
2
is generated during this process, which is pumped out to the outside of the chamber by the aid of the pumping electrodes
18
a
,
18
b
. At this time, a generated current value is detected by using an ammeter
30
. Thus, the concentration of NO contained in the measurement gas G is measured.
In the case of the gas analyzer constructed as described above, the partial pressure of oxygen in the chamber is adjusted by measuring the minute voltage between the measuring electrodes
12
a
,
12
b
and between the measuring electrodes
14
a
,
14
b
, and the concentration of NO contained in the measurement gas G is measured by measuring the minute current between the pumping electrodes
18
a
,
18
b
. In this case, in order to maintain the measurement accuracy in the gas analyzer, it is necessary to sufficiently ensure the insulation performance between lead wires connected to the respective measuring electrodes
12
a
,
12
b
,
14
a
,
14
b
and the pumping electrodes
18
a
,
18
b
so that the variation in detection signal due to cross talk and disturbance is avoided as less as possible.
In general, the insulation performance between the lead wires is ensured in accordance with such methods as disclosed, for example, in Japanese Patent Publication Nos. 4-26055 and 5-62297, in which a porous insulative material is used to make insulation between the pumping cell and the sensor cell or make insulation between electrode lead wires. Those generally used as the material for ensuring the insulation performance as described above include alumina and spinel.
Further, in order to improve the pumping ability or improve the response performance when the electromotive force is measured, the respective electrodes used for the gas analyzer are produced by using porous materials.
FIG. 11
shows an illustrative pattern of an electrode lead wire
34
which is wired from a through-hole
32
connected to an external connector to the measuring electrode
14
b
. In the illustrative arrangement shown in
FIG. 11
, porous insulative layers
36
a
,
36
b
are formed over and under the electrode lead wire
34
respectively to make insulation from other lead wires.
However, in the case of the conventional gas analyzer, the porous insulative layers
36
a
,
36
b
are formed to extend up to the through-hole
32
. For this reason, a problem arises in that O
2
, which makes invasion from the outside through the through-hole
32
, invades the second chamber
8
through the insulative layers
36
a
,
36
b
, and it increases the oxygen concentration in the vicinity of the measuring electrode
14
b
disposed near to the insulative layers
36
a
,
36
b.
Further, the electrode lead wire
34
is composed of a porous material. For this reason, a problem arises in that O
2
invades the second chamber
8
through the electrode lead wire
34
from the connector side of the e

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