Sensor element for limiting current sensors for determination of

Details Sensor element for limiting current sensors for determination of Sensor element for limiting current sensors for determination of

1990-08-30

1992-08-11

Niebling, John

Chemistry: electrical and wave energy

Apparatus

Electrolytic

204429, 204432, 204410, 204425, G01N 2700

Patent

active

051376156

DESCRIPTION:

BRIEF SUMMARY
PRIOR ART

The present invention relates generally to limiting current sensors. In the case of such sensor elements, which operate on the diffusion limiting current principle, the diffusion limiting current is measured with a constant voltage applied to the two electrodes of the sensor element. In an exhaust gas produced in combustion processes, this current is dependant on the oxygen concentration as long as the diffusion of the gas to the pumping electrode determines the speed of the reaction taking place. It is known to construct such sensors operating on the polarographic measuring principle in such a way that both anode and cathode are exposed to the gas to be measured, the cathode having a diffusion barrier, in order to achieve an operation in the diffusion limiting current range.
The known limiting current sensors serve as a rule for determination of the .lambda. value of gas mixtures which designates the "total oxygen/oxygen required for complete combustion of the fuel" relationship of the air/fuel mixture burning in a cylinder, the sensors determining the oxygen content of the exhaust gas via an electrochemical pumping current measurement.
On the basis of a simplified and inexpensive production method, in recent years the production of sensor elements which can be produced by ceramic foil and screen printing techniques has established itself in practice.
Planar sensor elements can be produced in a simple and cost-effective way, starting with oxygen-conducting solid electrolytes in platelet or foil form, for example from stabilized zirconium dioxide, which are coated on both sides with an inner and an outer pumping electrode with associated conductor tracks. The inner pumping electrode is in this case advantageously located in the edge region of a diffusion channel, through which the measuring gas is fed, and which serves as gas diffusion resistance.
German Offenlegungsschrift 3,543,759, YAMADA, as well as EP-A 0,142,993, EP-A 0,188,900 and EP-A 0,194,082 also disclose sensor elements and detectors, which have in common that they each have a pumping cell and a sensor cell, which consist of oxygen-conducting solid electrolytes in platelet or foil form and two electrodes arranged thereupon, and have a common diffusion channel.
A disadvantage of sensor elements of the generic type of the main claim is that the front part of the inner pumping electrode, facing the fed measuring gas, is subjected to greater stress than the rear part of the pumping electrode, facing away from the fed measuring gas. This leads to a high electrode polarization, which requires a high pumping voltage. The latter in turn entails the risk of an electrolyte decomposition in the region of the inner pumping electrode.
It is therefore proposed in German Offenlegungsschrift 3,728,618, in a sensor element for limiting current sensors for determination of the .lambda. value of gas mixtures with outer and inner pumping electrodes arranged on a solid electrolyte in platelet or foil form, conducting O.sup.2- ions, of which the inner pumping electrode is arranged on the solid electrolyte in platelet or foil form in a diffusion channel for the measuring gas, as well as with conductor tracks for the pumping electrodes, to arrange in the diffusion channel on the side opposite the inner pumping electrode at least a second inner pumping electrode, which is short-circuited with the first inner pumping electrode.


ADVANTAGES OF THE INVENTION

The sensor element according to the invention with the characterizing features of the main claim has, in comparison, the advantage that, due to the special design of the inner pumping electrode, its load-bearing capacity is increased by virtue of its greater electrode area and the service life of the sensor element is improved. A further advantage arises from the fact that the inner pumping electrode takes on a supporting function during lamination and compression in the case of production of the sensor element by ceramic foil and screen printing techniques.
The sensor element according to the invention c

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