Chemistry: electrical and wave energy – Apparatus – Electrolytic
Patent
1992-09-14
1994-04-12
Niebling, John
Chemistry: electrical and wave energy
Apparatus
Electrolytic
204425, 204426, 204427, G01N 2726
Patent
active
053022750
DESCRIPTION:
BRIEF SUMMARY
STATE OF THE ART
The invention is based on a sensor element for an oxygen limiting current probe of the type including a pumping cell where oxygen supply is provided by a short-circuit cell disposed upstream of the pumping cell. Such sensor elements, which operate according to the diffusion limiting current principle, measure the diffusion limiting current at a constant voltage applied to the two electrodes of the sensor element. In an oxygen containing measuring gas, this current is a linear function of the oxygen partial pressure 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 sensor elements so that the anode as well as the cathode are exposed to the gas to be measured, with the cathode including a diffusion barrier so as to ensure operation within the diffusion limiting current range.
The prior art oxygen limiting current probes generally serve to determine the .lambda. value of exhaust gas mixtures from internal-combustion engines. This .lambda. value represents the ratio of "total oxygen to the oxygen required for a complete combustion of the fuel" for the air-fuel mixture being combusted in a cylinder.
Due to a simplified and economical manner of manufacturing, the production of sensor elements that can be produced in ceramic sheet and screen-printing technology has become popular in practice in recent years.
Planar sensor elements can be produced in a simple and economical manner from plate or sheet shaped oxygen conducting solid electrolytes, for example of stabilized zirconium dioxide. These sensor elements are coated on both sides with electrodes and leads, namely with an inner pumping electrode on the one side and an outer pumping electrode on the other side. The inner pumping electrode is here advantageously disposed at the end of a diffusion gap or diffusion channel through which measuring gas is able to diffuse in and which serves as a gas diffusion resistance or diffusion barrier.
In addition, German Unexamined Published Patent DE-OS 3,543,759 and EP-A 0,142,992, 0,142,993, 0,188,900 and 0,194,082 disclose sensor elements and detectors which have in common that they each include a pumping cell and a sensing cell composed of plate or sheet shaped oxygen conducting solid electrolytes and two electrodes disposed thereon, and they have a common diffusion gap or diffusion channel.
Finally, DE-OS 3,728,618 discloses a sensor element for polarographic probes for a determination of the .lambda. value of gas mixtures. This sensor element includes a plate or sheet shaped solid electrolyte that is conductive for O.sup.2- ions and is equipped with outer and inner pumping electrodes, with the inner pumping electrode on the plate or sheet shaped solid electrolyte being disposed in a diffusion channel for the measuring gas. The sensor element further includes conductor paths for the pumping electrodes. At least one second inner pumping electrode is disposed in the diffusion channel on the side facing the inner pumping electrode and this second inner pumping electrode is short-circuited with the first inner pumping electrode.
A lecture by B.Y. Liaw and W. Weppner of the Max-Planck-Institut fur Festkorperforschung [Max Planck Institute For Solid State Research] in Stuttgart, held on the occasion of the "7th International Conference on Solid State Ionics," Nov. 5-11, 1989, in Hakone, Japan, disclosed an oxygen limiting current probe based on tetragonal ZrO.sub.2 in which a short-circuit cell and an oxidic mixed conductor are attached upstream of an inner pumping electrode (Cathode) for measuring the limiting current. The authors represent the opinion that the upstream connected short-circuit cell and the oxidic mixed conductor simultaneously constitute a diffusion barrier and they expect the oxygen permeation to obey the diffusion laws so that, consequently, it is a linear function of the external oxygen partial pressure and thus generates a limiting current at the subsequently connected pumping cell. However, in f
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Beyer Barbara
Da La Prieta Claudio
Dietz Hermann
Gruenwald Werner
Bell Bruce F.
Niebling John
Robert & Bosch GmbH
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