Planar polarographic probe for determining the .lambda. value of

Details Planar polarographic probe for determining the .lambda. value of Planar polarographic probe for determining the .lambda. value of

1990-09-18

1992-12-08

Niebling, John

Chemistry: electrical and wave energy

Apparatus

Electrolytic

204424, 204425, 204427, G01N 2726

Patent

active

051695120

DESCRIPTION:

BRIEF SUMMARY
PRIOR ART

The invention is based on a planar polarographic probe of the generic type of the main claim. In such polarographic probes, which operate in accordance with the diffusion resistance principle, the diffusion current at a constant voltage present at the two electrodes of the probe, or the diffusion limit current is measured. In an exhaust gas produced during combustion processes, this current is dependent on the oxygen concentration for as long as the diffusion of the gas to the pumping electrode determines the velocity of the reaction occurring. It is known to construct such polarographic probes operating in accordance with the polarographic measuring principle, in such a manner that both anode and cathode are exposed to the gas to be measured, the cathode exhibiting a diffusion barrier.
As a rule, the known polarographic probes are used for determining the .lambda. value of gas mixtures which designates the ratio between "total oxygen/oxygen needed for the complete combustion of the fuel" of the fuel/air mixture burning in a cylinder, the probes determining the oxygen content of the exhaust gas via an electro-chemical change in potential.
Due to a simplified and inexpensive method of production, the production of probes and sensor elements which can be produced in ceramic foil and screen printing technique has become successful in practice in recent years.
Starting with platelet or foil-shaped oxygen-conducting solid electrolytes, for example of stabilized zirconium dioxide, planar polarographic probes can be produced in a simple and economical manner which are coated on both sides with an inner and outer pumping electrode each with the associated conductor tracks. In this arrangement, the inner pumping electrode is advantageously located in the edge region of a diffusion channel through which the gas to be measured is supplied and which serves as gas diffusion resistance.
Furthermore, sensor elements and detectors are known from German Offenlegungsschrift 3,543,759 and EP-A 0,142,992, 0,142,993, 0,188,900 and 0,194,082, which have in common that they exhibit in each case a pumping cell and a sensor cell which consist of platelet- or foil-shaped oxygen-conducting solid electrolytes and two electrodes arranged thereupon and exhibit a common diffusion channel.
A certain disadvantage of known polarographic probes and sensor elements consists in the fact that the front part of the inner pumping electrode, which faces the supplied gas to be measured, is utilized to a greater extent than the rear part of the pumping electrode which faces away from the supplied gas to be measured. This leads to a high electrode polarization which requires a high pumping voltage. The latter, in turn, entails the risk of electrolyte decomposition in the region of the inner pumping electrode.
It is therefore proposed in German Offenlegungsschrift 3,728,618 to arrange, in a sensor element for polarographic probes for determining the .lambda. value of gas mixtures with an (sic) outer and inner pumping electrodes which are arranged on platelet- or foil-shaped solid electrolytes conducting O.sup.2 -ions, of which the inner pumping electrode is arranged on the platelet- or foil-shaped solid electrolyte in a diffusion channel for the gas to be measured, and with conductor tracks for the pumping electrodes, at least a second inner pumping electrode which is short-circuited with the first inner pumping electrode on the side opposite to the inner pumping electrode in the diffusion channel.
It is furthermore disadvantageous in known planar polarographic probes that their method of production is frequently complicated and therefore expensive, that the diffusion resistance is uncontrollably affected in subsequent process steps and thus the reproducibility of the diffusion resistance is unsatisfactory and that furthermore its constancy under severe use in the exhaust gas of internal combustion engines is inadequate.


ADVANTAGES OF THE INVENTION

By comparison, the polarographic probe according to the invention, having the characteriz

REFERENCES:
patent: 4305803 (1981-12-01), Beyer et al.
patent: 4312736 (1982-01-01), Menth et al.
patent: 4334510 (1982-06-01), Croset et al.
patent: 4579643 (1986-04-01), Mase et al.
patent: 4645572 (1987-02-01), Nishizawa et al.
patent: 4647364 (1987-03-01), Mase et al.

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