Gas sensor

Electrolysis: processes – compositions used therein – and methods – Electrolytic analysis or testing – For nitrogen or nitrogen containing compound

Reexamination Certificate

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Details

C204S424000, C204S425000, C204S427000, C205S784500, C205S785000

Reexamination Certificate

active

06277267

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to method for measuring oxygen and/or the air/fuel ratio lambda and nitrogen oxides in gas mixture. The sensor has a reference electrode representing a constant oxygen partial pressure, at least two electrode pairs, wherein two electrode pairs each have a common electrode associated therewith The gas sensors also have solid electrolytes arranged between the two electrodes of each electrode pair directly at the electrodes and electrical leads for connection and for take-away of electrical measurement signals.
A gas sensor of a this type is known, e.g., from German published patent application DE 195 34 918 A1. The sensor therein has two electrodes constructed as mutually engaging comb structures (see FIG.
1
), which are arranged on the side of the solid electrolyte facing he gas being measured (hereinafter “measurement gas”), and a reference electrode is provided opposite thereto on the reference air side. That invention is directed mainly to a reliable seal to ensure that no effects are exerted on the operation and performance of the two electrodes (sensor contacts) provided on the measurement gas side of the solid electrolyte. This construction makes possible a voltammetric measurement of two gas components in a gas mixture.
In addition, a gas sensor of the generic type is known from German published patent application DE 36 10 366 A1, in which a plurality of electrochemical measuring cells are arranged on a tubular support This device allows only gaseous pollutants to be measured (not oxygen). The evaluation of the measurement signals takes place based on the characteristics of the pollutant concentrations.
Furthermore, a gas sensor of this type is known from German Patent DE 41 09 516 C2. In this device, the solid electrolyte is constructed in the shape of a platelet, on one side of which an electrode is applied which functions as a reference electrode, and on the opposite side of which at least two measuring electrodes are applied, which interact with various components of a gas mixture. The platelet-shaped sensor is built into a housing, which is then to be installed as a gas probe in the exhaust gas duct of a motor vehicle, more specifically perpendicular to the flow direction of the exhaust gas.
This probe functions without a reference gas, which is required for obtaining an electrode potential independent of the environment. However, such electrodes are not stable with respect to their electrochemical potential, especially when the mixture composition changes from lean to rich. In addition, with configurations of the sensor design which are not rotationally symmetric, it is very hard to realize a stable and uniform temperature distribution over the entire surface.
The nitrogen oxide (NO
x
) measuring electrode and the negative pumping electrode are two electrodes, which are arranged locally separated from each other. It is difficult to establish a low oxygen concentration at the nitrogen oxide measuring electrode, since more oxygen is always present which cannot be removed by the pumping electrode, because it does not reach this pumping electrode.
Similar gas sensors, relatively complicated in construction, are also known from German published patent application DE 42 25 775 A1. In the sensors described there, a constant voltage is applied respectively between a reference electrode and a measuring electrode, as well as between the measuring electrode and a counter-electrode, and the current exiting from the counter-electrode is measured as a function of the nitrogen oxide concentration.
SUMMARY OF THE INVENTION
In view of the above, an object of the present invention is to provide a method for measurement of nitrogen oxides using a gas sensor with which at least two gaseous components can be reliably detected simultaneously over a wide range of gas mixtures, and which also ensures a stable reference signal with the aid of ambient air, which additionally and, if necessary, allows the influence of the oxygen concentration by adding or removing oxygen at the respective measuring electrodes.
According to the invention, these objectives are achieved with the solid electrolyte between a first electrode pair being gas-permeable and the solid electrolyte between a second electrode pair being gas-tight, wherein the second electrode pair is provided for potentiometric or amperometric measurement of the nitrogen oxide concentration, the first electrode pair is constructed for applying a current or a voltage for pumping oxygen, the common electrode forms the cathode of the first electrode pair, and the second electrode of the second electrode pair is constructed as a reference electrode on the reference gas side of the solid electrolytes of the second electrode pair. Advantageous amplifications of the invention and uses of the gas sensor according to the invention are described below and set forth in the dependent claims.
Advantageously the solid electrolyte, generally provided as a solid electrolyte body of virtually any desired shape, is constructed as a small tube closed at one end, which has on its inner wall a reference electrode, positioned as close as possible to the closed end, and a plurality of electrodes arranged on the outer side, exposed to the measurement gas. The solid electrolyte consists, e.g., of partially or filly stabilized ZrO
2
or of CeO
2
. The arrangement of at least two independent measuring electrodes on the solid electrolyte guarantees the simultaneous detection of at least two measurement signals which correspond to at least two different gas components. Since a tubular solid electrolyte with a circular cross-section is used, the disturbances at an installation point perpendicular to the exhaust gas flow are thereby minimized, so that the measurement gas flows around the sensor in a relatively uniform manner. Accordingly, the gas components being measured arrive at the measuring electrodes practically without a delay, and the disturbing turbulence is avoided.
If the gas sensor is used at temperatures below 400° C., it is advantageous to provide the sensor with a heating element The heating element for this purpose can be applied as a heating conductor, likewise on the outer side of the solid electrolyte, wherein, however, in order to avoid a short circuit, an electrically insulating layer is arranged between the heating conductor and the solid electrolyte.
Expediently, one of the electrodes used as a measuring electrode on the outer side of the solid electrolyte tube, which is closed at one end, is made of a catalytically active material and is consequently particularly suited for the potentiometric oxygen measurement according to the principle of a Nernst probe. In contrast, the second measuring electrode is made of a catalytically inactive material.
As materials for at least one of the electrodes, platinum or platinum alloys have proven satisfactory. Furthermore, rhodium, palladium, iridium, or their alloys are also suitable as (also in some cases catalytically active) electrode materials. Among the (catalytically inactive) materials, which should be used for the second measuring electrode, gold and gold alloys, as well as metal oxides have proven satisfactory. The catalytically inactive metal oxides are exemplified by mixed-conductivity perovskite compounds. Other simple metal oxides can also be used as (non-catalytic) electrode materials, e.g., SnO
2
, TiO
2
, V
2
O
5
, Fe
2
O
3
, NiO, ZnO, Sb
2
O
3
, Cr
2
O
3
, CuO, or MnO
2
. By varying the composition of the measuring electrodes, different gaseous components can be brought for interaction with the electrodes.
In using the gas sensor in a method for nitrogen oxide measurement, a voltage or current is applied to a pair of electrodes, which consist of at least two measuring electrodes or of one of two measuring electrodes and a reference electrode. A negative potential is applied at the commonly used electrode. This electrode is referred to as the “pumping electrode,” i.e., it pumps off oxygen, so that the measurement of nitrogen oxides at the res

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