Sensor element

Details Sensor element Sensor element

1999-01-29

2001-07-17

Tung, T. (Department: 1744)

Chemistry: electrical and wave energy

Apparatus

Electrolytic

C204S408000, C204S426000, C204S429000

Reexamination Certificate

active

06261429

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a sensor element for an electrochemical sensor for determining oxygen content in exhaust gases from an internal combustion engine.
BACKGROUND OF THE INVENTION
Sensor elements for determining oxygen content are composed of ceramic foils made of, for example, stabilized ZrO
2
, which are imprinted with electrodes having electrode leads and a heating device having a heating conductor and heating conductor leads, and which, after being laminated together and then sintered, yield a planar sensor element. In the case of electrochemical sensors, the heating device is embedded between two electrically insulating layers, for example, made of Al
2
O
3
, which, in turn, are arranged, in each case, with regard to the adjoining oxygen-ion-conducting ceramic sheets.
In electrochemical sensor elements that operate using a reference gas, an electrode is disposed in a reference gas channel, which extends, for example, in the longitudinal direction of the sensor element and is in contact with ambient air. This reference gas channel runs in a plane of the layer between the reference electrode and the heating device.
In those areas in which the heating conductor spans the reference gas channel, there is poor heat dissipation for the heat generated, so that in this area it is possible that the heating conductor may overheat. In addition, the poor heat conduction in the area of the reference gas channel means that it is only possible to heat the sensor elements to their operating temperature using increased heating energy or after a correspondingly long warm-up period.
German Patent No. 196 09 323 describes that improved heat conduction may be achieved by arranging the heat conductor of the heating device outside the projection of the reference gas channel. For this purpose, the reference gas channel has been designed in two parts, in the area of the electrodes and the heat conductor.
SUMMARY OF THE INVENTION
The sensor element according to the present invention has the advantage that the heat transfer from the heating device to the sensitive, electrode-bearing area of the sensor element is markedly improved. An overheating of the heating conductor in the area of the reference gas channel is avoided with the result that long-term stability of the heating device and thus of the sensor element is increased.
Loop-shaped extension segments of the heating circuit trace segments, that are designed on a narrow end of a measuring gas side on a large surface, provide a resistance relationship of the heating conductor leads and the heating conductor shifted to the advantage of the heating conductor. In this way, it is also possible to more quickly heat up the sensor element (quick-start heater). Moreover, as a result of the improved heating of the narrow end of the measuring gas side, the resistance to thermal shock of the sensor element is increased.
In addition, by heating the electrode leads, a capacitive and resistance coupling of the electrode leads increases, which ultimately leads to a distortion of the measuring signal as a result of over and undershooting in response to changes of concentration in the measuring gas. By moving the heat generation to the tip of the sensor element, i.e., to the sensitive area of the electrodes, the capacitive and resistance coupling of the leads in the sensor element is reduced.
As a result of the tapering and expanding configuration of the transitions from the heating conductor leads to the heating circuit trace segments and from the heating circuit trace segments to a connecting segment, the danger of leakage currents in response to local overheating is reduced. Leakage currents arise as a result of the formation of a current path between the heating device and an electrode, resulting ultimately in distorting the measuring signal received from the electrodes. By enlarging the insulated areas between the meandering branches, the high potential differences in areas between the meandering branches are at the same time attenuated.
Widening the reference gas channel inside the area that is not covered by the heating conductor also has the advantage that the reference electrode may be enlarged accordingly and that an improved reference gas exchange is possible.


REFERENCES:
patent: 4300990 (1981-11-01), Maurer
patent: 4861456 (1989-08-01), Mase et al.
patent: 5529677 (1996-06-01), Schneider et al.
patent: 5658445 (1997-08-01), Hafele et al.
patent: 5895591 (1999-04-01), Kojima et al.
patent: 5902470 (1999-05-01), Schneider et al.
patent: 5985118 (1999-11-01), Makino et al.
patent: 196 09 323 A1 (1997-09-01), None
patent: 0 343 533 (1989-11-01), None

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