Electrochemical sensor

Details Electrochemical sensor Electrochemical sensor

1998-12-24

2001-08-28

Tung, T. (Department: 1743)

Electrolysis: processes, compositions used therein, and methods

Electrolytic analysis or testing

For oxygen or oxygen containing compound

C204S425000, C204S426000, C204S427000

Reexamination Certificate

active

06280605

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to an electrochemical sensor, and to a use of the electrochemical sensor in determining a lambda value of a gas mixture.
BACKGROUND INFORMATION
Conventional electrochemical sensors generally include an electrochemical element which has an electrochemical pump cell with a preferably flat first solid electrolyte body and first and second preferably porous electrodes. These conventional sensors further include an electrochemical sensor cell which interacts with the pump cell and has a preferably flat second solid electrolyte body and a third and a fourth preferably porous electrode. The electrochemical sensor also has a gas inlet opening and a gas inlet duct, which is connected at one end to a measured gas chamber. The other end of the gas inlet duct opens into a cavity, also referred to as a gas chamber, lying inside the electrochemical element. The second and third electrodes and preferably one diffusion resistor arrangement are located in the gas chamber. The diffusion resistor arrangement can be formed by a porous filling. The measured gas enters the cavity via the gas inlet opening and the gas inlet duct, with the first and second electrodes of the pump cell regulating the entry of the measured gas into the gas chamber. This produces a controlled partial pressure of the gas component to be measured. The electrochemical potential difference between the electrodes of the second solid electrolyte body (which is due to the different partial pressures of the gas in the diffusion resistor arrangement and in a reference gas chamber located, for example, in the second solid electrolyte body) can be detected by a detecting device, such as a voltmeter unit, positioned outside the electrochemical element.
Conventional electrochemical sensors are also used in applications such as catalytic emission control in internal combustion engines under. These electromechanical sensors are designated in the industry as “flat broadband lambda sensors”.
One of the disadvantages of the conventional electrochemical sensors is that they demonstrate elevated ripple during a lambda=1 pass, especially at high operating temperatures. This leads to problems, especially in control processes where the lambda value represents the controlled variable. Due to the ripple in the lambda signal, it is sometimes not possible to set an adequately stable output quantity.
SUMMARY OF THE INVENTION
An electrochemical sensor according to the present invention has an electrochemical element for measuring a gas concentration of a measured gas. The sensor includes an electrochemical pump cell with a first solid electrolyte body, first and second electrodes, and a gas chamber which is connected to a measured gas chamber via a gas inlet opening. An electrochemical sensor cell (e.g., a Nernst cell) is also provided which has a second solid electrolyte body, a third electrode, and a reference gas chamber in which a fourth electrode is located. The electrodes have a supply conductor for an electrical contacting.
According to the present invention, the supply conductor to the fourth electrode is provided with an electrically insulating layer to insulate this supply conductor against the second solid electrolyte body. A resistive coupling of the electrode supply conductors in conventional electrochemical sensors can cause the pump voltage to interfere with the Nernst voltage of the sensor cell. Especially at high operating temperatures, this can be one reason for the known (however unwanted) phenomenon of lambda=1 ripple (e.g., transients during abrupt gas change).
According to the present invention, a resistive decoupling of the supply conductor to the fourth electrode from the solid electrolyte body, and therefore from the other electrode supply conductors as well, advantageously reduces the lambda=1 ripple and can also eliminate it. This improves the controller dynamics of the electrochemical sensor according to the present invention as compared to that of the conventional electrochemical sensors.
In another embodiment of the present invention, the layer is made of aluminum oxide or contains aluminum oxide.
In another embodiment of the present invention, the layer (e.g., an insulation material used for resistive decoupling) is attached to the solid electrolyte body or the electrode supply conductor in the form of a printed layer.
In another embodiment of the present invention, the layer is at least as wide as the supply conductor of the fourth electrode. Alternatively, the layer can be the same width as a reference gas duct containing the supply conductor of the fourth electrode and assigned to the reference gas chamber. The electrically insulating layer is positioned between the supply conductor and a wall of the reference gas chamber located in the second solid electrolyte body.
In another embodiment of the present invention, the supply conductor of the fourth electrode is much narrower than the reference gas duct. This additionally prevents the pump voltage from interfering with the Nernst voltage, since the supply conductor has a small surface area.
The electrochemical sensor according to the present invention and its electrochemical element are produced in a suitable manner by starting with oxygen-conducting, wafer-shaped or film-like solid electrolytes, made for example of stabilized zirconium dioxide, and coating both sides with an inner and an outer pump electrode having conductor paths which represent the supply conductors for electrical contacting. The resistive layer according to the present invention is applied between the conductor paths and the solid electrolyte film. Thus, the conductor paths are preferably applied to the layer. The inner pump electrode is advantageously located in the edge region of a diffusion or gas inlet duct through which the measured gas is supplied. The gas inlet duct can be designed as a gas diffusion resistance. The pump cell obtained in this manner can then be laminated and sintered to a sensor cell (Nernst cell), produced in a similar manner and made from a second solid electrolyte film and to a third, solid electrolyte film that may be designed as a heater unit.
The porous fillings (for example, the diffusion barriers in the gas chamber) are produced by using, in particular, porously sintered film inserts made of a ceramic material which has suitable thermal expansion characteristics which are identical or similar to the those of the solid electrolyte films used. A film insert made of the same ceramic material from which the solid electrolyte films are produced is preferably used for the filling. The porosity of the insert can be generated by pore-forming substances such as thermal carbon black powder, organic plastics, or salts. These pore-forming substances burn up, decompose, or evaporate during sintering.
The present invention also relates to broadband lambda sensors for determining the lambda value of gas mixtures in internal combustion engines. The lambda value, or “air ratio”, is defined as the ratio between the instantaneous air/fuel ratio and the stoichiometric air/fuel ratio. The sensors measure the oxygen content of the exhaust gas via a limit current variation.


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