Chemistry: electrical and wave energy – Apparatus – Electrolytic
Patent
1991-05-16
1994-05-10
Tung, T.
Chemistry: electrical and wave energy
Apparatus
Electrolytic
204426, 204429, G01N 27419
Patent
active
053104722
DESCRIPTION:
BRIEF SUMMARY
PRIOR ART
The invention is based on a sensor element for limiting current sensors. In such sensor elements, which operate by the diffusion limiting current principle, the diffusion limiting current is generally measured at a constant voltage applied to both electrodes of the sensor element. In exhaust gas produced in combustion processes, current is dependent on the oxygen concentration, as long as the diffusion of the gas to the pump electrode determines the speed of the electrode reaction taking place. It is known to structure such sensors, which operate by the polarographic measuring principle, in such a way that both the anode and the cathode are exposed to the gas to be measured; the cathode has a diffusion barrier, to enable operating in the diffusion limiting current range.
As a rule, the known limiting current sensors serve to determine the lambda value of exhaust gas mixtures, which expresses the ratio between total oxygen and the oxygen required for complete combustion of the fuel in the fuel/air mixture burning in a cylinder.
Because of its simplified and relatively inexpensive manufacture, the production of probes and sensor elements that can be made by ceramic foil and screenprinting techniques has become established in the industry in recent years.
Planar polarographic probes can be produced simply and economically beginning with oxygen-carrying solid electrolytes in the form of small plates or foils, for instance of stabilized zirconium dioxide, which are coated on both sides with an (inner or outer) pump electrode and with the associated conductor track. The inner pump electrode is advantageously located at the end of a diffusion gap or diffusion channel, through which the measurement gas can diffuse in, and which serves as a gas diffusion resistor.
Sensor elements and detectors are also known from German Patent Disclosure Document 35 43 759 and from European Patent Applications 0 142 992, 0 142 993, 0 188 900 and 0 194 082, which have in common the fact that they each have a pump cell and a sensor cell, which comprise oxygen-carrying solid electrolytes in the form of small plates or foils, and two electrodes disposed on them, and have a common diffusion gap or diffusion channel.
German Patent Disclosure Document 38 34 987 also describes a sensor element for limiting current sensors for determining the lambda value of gas mixtures, in particular the exhaust gases of internal combustion engines, having a pair of pump electrodes disposed on a solid electrolyte that conducts O.sup.2 -ions, in which the inner pump electrode communicates with the measurement gas via a diffusion gap, and the diffusion gap is covered by a solid electrolyte layer produced by screenprinting.
A disadvantage of the known sensor elements, which are in particular manufactured by laminating a plurality of solid electrolyte foils together, particularly those based on stabilized ZrO.sub.2, is that a later change in the geometry of the diffusion layer, for instance for the sake of calibrating the sensor elements once the sensor elements have been finally sintered together, is as a rule difficult. The magnitude of the diffusion current in limiting current sensors in fact depends on the shaping of the diffusion layer, which can be made by various methods. In ceramic probes, such diffusion barriers are preferably applied to unsintered ceramic substrates by screenprinting and the entire assembly is then sintered. Only then can the limiting current be measured.
To preclude this disadvantage, it is known, for instance from European Patent Application 0 191 627, to use sensor elements having adjustable resistors on the sensor body. However, this has the disadvantage that manufacturing such a sensor element is comparatively complicated and expensive, and that additional electric leads are needed for this purpose on the sensor body.
ADVANTAGES OF THE INVENTION
The sensor element according to the invention has the substantial advantage over the element known from European Patent Application 0 191 627 that it is simpler to manufac
REFERENCES:
patent: 4502939 (1985-03-01), Holfelder et al.
patent: 4724061 (1988-02-01), Nyberg
patent: 4810529 (1989-03-01), Mantese et al.
patent: 4816749 (1989-03-01), Schmidtpott et al.
De La Prieta Claudio
Dietz Hermann
Grunwald Werner
Robert & Bosch GmbH
Tung T.
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