Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
1999-01-29
2001-08-07
Warden, Sr., Robert J. (Department: 1744)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C204S426000, C204S427000
Reexamination Certificate
active
06270639
ABSTRACT:
BACKGROUND INFORMATION
Conventional electrochemical sensors generally include an electrochemical element, which has an electrochemical pump cell having a preferably planar, first solid electrolyte body and a first and second preferably porous electrode. These conventional sensors also include an electrochemical sensor cell having a preferably planar, second solid electrolyte body and a third and fourth preferably porous electrode. The sensor has a gas supply opening which includes a gas supply channel which, on the one hand, is connected to a measuring gas chamber, and on the other hand, is connected to a hollow space surrounded by the two solid electrolyte bodies.
A diffusion resistance device, which can contain a porous filling, is arranged in the measuring gas chamber.
The measuring gas arrives in the measuring gas chamber via the gas supply opening and the gas supply channel, the first and the second electrodes of the pump cell acting to regulate the admission of the measuring gas into the gas chamber and thus assure a controlled partial pressure of the gas components to be measured. The electrochemical difference in potential between the electrodes of the second solid electrolyte body arises due to the varying gas partial pressures in the diffusion resistance device and in a reference gas chamber, arranged, for example, in the second solid electrolyte body. The difference in potential can be measured by a voltmeter situated outside the electrochemical element.
The conventional sensors described above, referred to as planar wideband-lambda probes, have been used, for example, in the technology of catalytic exhaust emission control systems for internal combustion engines. A typical design of one such conventional electrochemical sensor is described in German Patent Application No. 38 11 713. The conventional sensors have a disadvantage that they have a so-called lambda=1 ripple, particularly in high operating temperatures. This leads to problems in control processes in which the lambda value represents the control variable. As a result of the ripple of the lambda signal, it is in many cases impossible to make an adjustment for an output quantity value to be sufficiently stable.
SUMMARY OF THE INVENTION
The present invention provides an electrochemical sensor for determining the concentration of a gas (e.g., a concentration of oxygen) in a measuring gas, and having an electrochemical element. The sensor includes a first solid electrolyte body having an electrochemical pump cell and a first (external pump) and a second (internal pump) electrode. In addition, the sensor has a gas chamber, which is connected via the gas supply opening and a gas supply channel to the measuring gas chamber, and in which one of the two electrodes is arranged. Furthermore, a second solid electrolyte body is provides which has an electrochemical sensor cell (e.g., Nernst cell), which includes a third and fourth electrode. Each electrode has a lead for the purpose of electrical contacting.
According to the present invention, leads of the first and second electrodes are capacitively decoupled from the lead of at least the fourth electrode with the assistance of a device. Thus, capacitive couplings of the electrode leads in conventional electrochemical sensors can lead to a reaction of the pump voltage on the Nernst voltage of the sensor cell and that this, in turn, particularly at high temperatures, is one cause for the undesirable phenomenon of the lambda=1 ripple (i.e., undershooting or overshooting of the output signal in response to violent fluctuations in the gas exchange).
As a result of the capacitive decoupling of the electrode leads according to the present invention, the lamdba=1 ripple is advantageously reduced or even prevented.
In an exemplary embodiment of the present invention, a device causing the decoupling is formed by the lead of the second electrode. In particular, the leads of the first and second electrodes are arranged with a clearance to be situated one on top of the other. In this manner, a coupling of the pump voltage into the sensor cell is avoided, so that the lamdba=1 ripple is at least reduced.
In another embodiment of the present invention, the leads of the first and second electrodes are arranged in the center in the electrochemical element. In this manner, the one-over-the-other positioning of the two electrode leads (as described above) is attained in a simple manner.
An exemplary embodiment of the present invention provides that the leads of all electrodes are arranged with a clearance to be one on top of the other. In this manner, a coupling of a voltage into the sensor cell is avoided. In particular, this holds true for a sensor heating element. This means that the leads for the heating element can also be situated below or above the leads of the electrodes.
In another exemplary embodiment of the present invention, the device for the capacitive decoupling is formed through an electron-conductive layer, which can also be a foil binder layer. This foil binder layer preferably joins the first and second electrolyte bodies to each other and is therefore a part of the solid electrolyte body that is performing the coupling.
Another exemplary embodiment of the present invention provides that the layer is connected in an electrically conductive manner to the second electrode. Alternatively, it is also possible that the layer has its own lead extending out from the electrochemical element as a contact.
In another exemplary embodiment of the present invention, the foil binder layer has a doping using cerium dioxide or titanium dioxide.
Electrochemical sensors according to the present invention and their electrochemical elements are advantageously manufactured starting with plateshaped or foil-shaped oxygen-conducting solid electrolytes, for example, made of stabilized zirconium dioxide, and coating them on both sides with an interior and exterior pump electrode, respectively, having the appropriate printed circuit traces. In this manner, the inner pump electrode is located advantageously in the edge area of a diffusion or gas supply channel, through which the measuring gas is delivered, and which functions as the gas diffusion resistance. The pump cell obtained in this manner can then be laminated together with a sensor cell (e.g., the Nernst cell) that is manufactured in a similar way and is composed of a second, formed solid electrolyte foil, and can be sintered, for example, at 1300 to 1550° C.
For manufacturing porous fillers, the process starts, for example, with providing porously sintering foil inserts made of a ceramic material and which has suitable thermal properties of expansion that closely correspond to those of the solid electrolyte foils used. It is advantageous if, for the filling, a foil insert is used made of the ceramic material which the solid electrolyte foils are also made of, it being possible to induce the porosity of the insert using pore-forming materials such as thermal carbon powder, organic plastics, or salts, which, during the sintering process, burn, decompose, or evaporate. The output materials are used in concentration such that, after the sintering, porosities of 10 to 50%, preferably 40%, are achieved, the average pore diameter being approximately 5 to 50 &mgr;m, preferably 10 &mgr;m.
The present invention also relates to wideband-lambda probes for determining the lambda value of gas mixtures in internal combustion engines. The lambda value or the “air number”, in this context, is defined as the relationship of the prevailing air-fuel ratio to the stoichiometric air-fuel ratio. The probes measure the oxygen content of the exhaust gas above a change in the limiting current.
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Diehl Lothar
Jach Olaf
Lenfers Martin
Neumann Harald
Riegel Johann
Kenyon & Kenyon
Olsen Kaj K.
Robert & Bosch GmbH
Warden, Sr. Robert J.
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