GB2399644A

GB2399644A - Zero point adjustment of the signal from a sensor element of a gas sensor

Info

Publication number: GB2399644A
Application number: GB0404212A
Authority: GB
Inventors: Berndt Cramer; Bernd Schumann; Thorsten Ochs; Helge Schichlein; Sabine Thiemann-Handler
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2003-02-27
Filing date: 2004-02-25
Publication date: 2004-09-22
Also published as: FR2851824A1; GB0404212D0; DE10308394A1

Abstract

A sensor element of a gas sensor is described, which serves for determining the concentration of at least one constituent of a gas mixture, in particular in exhaust gases from internal combustion engines. The sensor element comprises at least one auxiliary electrode 20, 20a, 24 and a measurement electrode 26 that are in direct contact with the gas mixture, wherein a signal generated by means of the measurement electrode 26 serves for the determination of the concentration of the constituent. In order to adjust the zero point of this signal, a potential is temporarily applied to the auxiliary electrode 20, 20a,24 at which the constituent contained in the gas mixture is completely removed from the said gas mixture. Then the resulting signal of the measurement electrode 26 is set equal to zero.

Description

Sensor Element of a Gas Sensor The present invention relates to a sensor

element of a gas sensor and a method for determining the concentration of a constituent of a gas mixture according to the precharacterising part of the independent claims.

Prior Art

Sensor technology as applied to exhaust gases from internal combustion engines has become ever more important in the wake of increasingly stringent environmental guidelines.

Solid electrolyte-based gas sensors are used in particular in this connection, which detect in a highly selective manner the relevant gaseous constituents in the exhaust gas. The aggressive ambient conditions in conjunction with high operating temperatures in the exhaust gas of internal combustion engines represent a particular challenge to gas sensors. As a result all components directly exposed to the gas mixture are subjected to a rapid ageing and degeneration process. Changes in signals and thus inaccurate measurement results occur after a relatively short time, in particular due to the ageing of the electrodes of the gas sensor.

A solid electrolyte-based gas sensor is known from EP 678 740 B1, which is used to detect nitrogen oxides. The measurement principle of the sensor is based on the fact that, within the gas sensor, excess oxygen is removed without interfering with the nitrogen oxide concentration, and after adjusting a constantly low oxygen atmosphere the 2 - content of nitrogen oxides is determined amperometrically.

A calibration or a zero setting of this sensor is not provided.

The object of the present invention on the other hand is to provide a sensor element for a gas sensor that enables a temporary zero setting to be effected in a simple manner and thus ensures over the long term an accurate determination of at least one constituent of a gas mixture.

Advantages of the Invention The sensor element according to the invention and the method on which the invention is based having the characterizing features of the independent claims have the advantage that it permanently allows the constituents of a gas mixture to be measured. To this end advantageously an at least temporary equilibration of the sensor element is carried out. The constituent to be determined is removed electrochemically by means of an auxiliary electrode from a gas mixture diffusing into the sensor element, and the gas mixture pretreated in this way is used to carry out a zero setting of the sensor element.

Advantageous modifications and improvements of the sensor element and method disclosed in the independent claims are possible due to the features itemised in the subclaims.

Thus, it is for example advantageous if a first and a second auxiliary electrode composed of different materials are provided, in which the materials differ as regards their catalytic activity with respect to a decomposition of the gas to be measured. Advantageously both auxiliary electrodes are electrically connected to one another so that only one connection has to be provided for both electrodes.

In a particularly advantageous embodiment the sensor element is then only operated in an equilibration (zero setting) mode if the signal of the sensor exceeds or does not reach a predetermined value. This avoids having to carry out a periodic equilibration of the sensor element and prolongs the time that the sensor element is available for measurement purposes.

Drawings Three examples of implementation of the invention are illustrated in the drawings and discussed in more detail in the following description. Fig. 1 is a cross-section through part of a sensor element according to the invention, on the measurement gas side, according to a first example of implementation, and Figs. 2 and 3 are cross-sections through parts of sensor elements, in each case on the measurement gas side, according to two further examples of implementation.

Examples of implementation Fig. 1 shows in outline the construction of a first embodiment of the present invention. 10 denotes a planar sensor element of an electrochemical gas sensor, which serves to determine a constituent of a gas mixture, in particular the content of nitrogen oxides, preferably in exhaust gases from internal combustion engines. The sensor element comprises a plurality of oxygen ion-conducting solid electrolyte layers lla, lib, lie, lid, lie, llf and llg, which for example are fabricated as ceramic sheets and form a planar ceramic body. They consist of an oxygen ion- conducting solid electrolyte material, such as for example ZrO2 fully or partially stabilised with Y203. The solid electrolyte layers lla-llg may alternatively be replaced by sheets of aluminium oxide, at least at places at which an ion conduction in the solid electrolyte is not important or is undesirable.

The integrated form of the planar ceramic body of the sensor element 10 is produced by laminating together the ceramic sheets printed with function layers and then sistering the laminated structure in a manner known per se.

The sensor element 10 contains for example an inner gas space 12 and a reference gas channel 18. The reference gas channel 18 is in contact, through a gas inlet that projects at one end from the planar body of the sensor element 10, with a reference gas atmosphere that is formed for example by ambient air.

The inner gas space 12 has an opening 15 that permits contact with the gas mixture to be determined. The opening 15 is arranged in the solid electrolyte layer lla perpendicular to the surface of the sensor element 10, but may however also be provided in the solid electrolyte layer lib. 5

A first auxiliary electrode 20 is preferably formed in duplicate in the inner gas space 12. A further auxiliary electrode 24, preferably likewise in duplicate, is arranged in a subordinate role in the diffusion direction of the gas mixture. An outer electrode 22, which may be covered with a porous protective layer (not shown), is located on the outer side of the solid electrolyte layer lla, directly facing the measurement gas.

The auxiliary electrodes 20, 24 form in each case with the outer electrode 22 electrochemical pump cells. If the sensor element is used to determine reducible gases such as nitrogen oxides or sulfur oxides, then an in each case constant oxygen partial pressure is adjusted in the inner gas space 12 by means of the pump cells 20, 22 and 24, 22.

In this connection, in a first step a first, low oxygen partial pressure is adjusted by means of the pump cell 20, 22, and a second, lower oxygen partial pressure is adjusted by means of the pump cell 24, 22. In order to monitor the adjusted oxygen partial pressure at least one of the auxiliary electrodes 20, 24 is additionally connected up to a reference electrode 30, which is arranged in the reference gas channel 18 and may also be provided in duplicate, to form a so-called Nernst or concentration cell. This permits a direct comparison of the potential of the auxiliary electrodes 20, 24 dependent on the oxygen concentration in the inner gas space 12, with the constant potential of the reference electrode 30 in the form of a measurable electrical voltage. The value of the pump voltages to be applied to the pump cells is chosen so that 6 - a constant voltage is adjusted between the electrodes 20, and 24, 30 of the concentration cells.

A measurement electrode 26, which preferably together with the reference electrode 30 forms a further pump cell, is furthermore arranged in the inner gas space 12 in the diffusion direction of the gas mixture and subordinate to the auxiliary electrodes 20, 24. This pump cell serves to detect the gas to be measured, in which connection the reducible gas to be measured is specifically reduced at the surface of the measurement electrode 26 and the released oxygen is pumped off. The pump stream flowing to the measurement electrode 26 serves as a measure of the concentration of the gas to be determined.

In order to ensure that no decomposition of the gas to be determined occurs at the first auxiliary electrodes 20, the said first auxiliary electrodes 20 are fabricated from a catalytically inactive material. This may for example be a platinum alloy, preferably a gold/platinum alloy with a gold content of up to 2 wt.%. The potential at the first auxiliary electrode is then preferably -200 to -400 mV.

The further auxiliary electrode 24 is preferably fabricated from the same material as the first auxiliary electrode 20.

At the further auxiliary electrode 24 the oxygen content of the gas mixture diffusing thereto is reduced further without causing a reduction of the nitrogen oxides contained in the gas mixture. To this end a potential of -200 to -500 mV is adjusted at the further auxiliary electrode 24. - 7

The measurement electrode 26 is on the other hand designed to be catalytically active and consists for example of rhodium, a platinum/rhodium alloy or another suitable platinum alloy. The outer electrode 22 as well as the reference electrode 30 likewise consist of a catalytically active material such as for example platinum. The electrode material for all electrodes is used in a manner known per se as a cermet, in order to be able to be sintered with the ceramic sheets.

In addition, a resistance heater 35 is embedded between two electrical insulating layers 32, 33 in the ceramic base body of the sensor element TO. The resistance heater 35 serves to heat up the sensor element 10 to the necessary operating temperature of for example 600 to 900 C.

Within the inner gas space 12 a porous diffusion barrier 19 is installed in front of the first auxiliary electrodes 20 in the diffusion direction of the gas mixture. The porous diffusion barrier 19 forms a diffusion resistance with respect to the gas mixture diffusing to the first auxiliary electrodes 20. A further porous diffusion barrier may additionally be provided in the inner gas space 12 between the first auxiliary electrodes 20 and the further auxiliary electrodes 24, in order to stabilise the adjustment of different oxygen concentrations in various regions of the inner gas space 12.

In addition to the aforedescribed procedure for the detection of a reducible gas to be measured in a first time - 8 period, a procedure may be provided in a second time period that ensures that the sensor element can be calibrated or equilibrated. In this case the potential applied to the electrodes 20 is altered in the second time period so that not only is oxygen reduced, as in the measurement operation, but in addition the reducible gases to be measured, such as for example nitrogen oxides, are electrochemically removed. At the further auxiliary electrodes 24, during the second time period not only is the oxygen content of the gas mixture diffusing therein reduced further, as in the measurement operation, by applying a suitable potential, but in addition residual amounts of gas to be measured that are possibly still present are removed from the gas mixture. The gas mixture freed from the gas to be measured finally reaches the measurement electrode 26. The pump flow flowing to the measurement electrode 26 is adjusted to a concentration of the gas to be measured of 0.

The equilibration mode is carried out until a sufficiently accurate zero setting is obtained. Following this the detection of the gas to be measured is continued further.

The equilibration may be carried out periodically or, preferably, whenever the measurement signal of the sensor element exceeds or drops below a predetermined value.

If the aforedescribed sensor element is used to determine oxidisable gases such as for example ammonia, hydrogen, hydrogen sulfide, sulfur monoxide or alkylamines, then the determination procedure is carried out as follows. - 9 -

The potential that is adjusted at the auxiliary electrodes is chosen so that not only is a constant oxygen partial pressure adjusted in the inner gas space 12, but that in addition oxidising gases such as nitrogen oxides or sulfur oxides possibly contained in the gas mixture are likewise reduced and removed from the gas mixture. This reduces the danger of a reaction of the gas to be measured with oxidising gases within the sensor element.

The oxidizable gas to be determined is then electrochemically oxidised at the surface of the further auxiliary electrode 24 and following this is specifically reduced at the surface of the measurement electrode 26, and the oxygen released is pumped off. The pump flow flowing to the measurement electrode 26 serves as a measure of the concentration of the gas to be determined. Over and above this, in addition or alternatively the pump flow flowing to the further auxiliary electrode 24 can be used as a measure of the concentration of the gas to be determined.

In order to ensure that no decomposition of the gas to be measured takes place at the first auxiliary electrodes 20 the potential applied to the first auxiliary electrodes 20 is preferably adjusted to -400 to -900 mV.

The oxidation of the gas to be determined then takes place at the further auxiliary electrode 24, for which purpose at the further auxiliary electrode 24 a more positive potential compared to the potential of the first auxiliary electrodes 20 is chosen in the range from -200 to -700 mV. - 10

An operation in equilibration mode is also possible in the determination of further reducing gases. For this, the described operation for the detection of the gas to be measured is carried out in a first time period, and an equilibration mode is carried out in a second time period.

In this connection the potential applied to the electrodes 20 is altered in the second time period so that not only are oxygen, and nitrogen oxides and sulfur oxides reduced, as in the measurement operation, but in addition the gas to be determined is electrochemically oxidised. At the further auxiliary electrodes 24 not only is a constantly low oxygen partial pressure adjusted in the second time period by applying a suitable potential, as in the measurement operation, but in addition the oxidised form of the gas to be measured is reduced and removed from the gas mixture. The gas mixture freed from the gas to be measured as well as from nitrogen oxides and sulfur oxides then reaches the measurement electrode 26. The pump flow flowing to the measurement electrode 26 is set to a concentration of the gas to be measured of 0 since the gas mixture reaching the measurement electrode 26 at this point in time is completely free from the gas to be measured. A precondition for this is that in the reduction of the oxidised form of the gas to be determined, a compound is formed that does not correspond to the gas originally to be determined. If for example the sensor element is operated as an ammonia sensor, then nitrogen oxides are formed at the first auxiliary electrode 20 by oxidation of the ammonia, which are reduced to nitrogen at the further auxiliary electrode 24.

- 11 - In order to ensure the oxidation of the gas to be determined at the first auxiliary electrodes 20 during the equilibration mode, these electrodes may additionally comprise a metal that catalyses the oxidation of the relevant gas. Thus, additions of silver, cobalt, rhodium, palladium or gold are for example suitable for the oxidation of ammonia.

A second example of implementation of the present sensor element is shown in Fig. 2. Here the same reference numerals apply to the same components as in Fig. 1. In contrast to the first example of implementation described above, the sensor element shown in Fig. 2 comprises in addition to the first auxiliary electrodes 20 a second auxiliary electrode 20a, for example in duplicate. The materials from which the electrodes 20 and 20a are fabricated preferably differ as regards their catalytic activity with respect to an electrochemical conversion of the gas to be determined. Thus, for example, when the sensor element is operating as a nitrogen oxide sensor the electrodes 20 do not contain any additives that catalyse the reduction of nitrogen oxides. If the sensor element is operated as a sensor for the determination of oxidisable gases, then no additives are provided that catalyse the oxidation of the oxidisable gases.

The second auxiliary electrodes 20a on the other hand include such additives. For the determination of reducible gases the second auxiliary electrodes 20a are made from a suitable platinum/noble metal alloy such as for example a platinum/gold alloy with a relatively low gold content of less than 1 wt.%. For the determination of oxidisable gases the second auxiliary electrodes preferably contain the already described additives that catalyse the oxidation of oxidizable gases.

The electrodes 20, 20a are preferably electrically connected to one another so as to form a common combination electrode. During the measurement operation in the first time period, a constant oxygen partial pressure is adjusted on the surfaces of the first auxiliary electrodes 20 as well as on the surfaces of the second auxiliary electrodes 20a. In the determination of oxidisable gases, in addition during the measurement operation nitrogen oxides and sulfur oxides are electrochemically reduced and removed from the gas mixture.

During the equilibration mode in the second time period, the electrochemical processes taking place on the surfaces of the first auxiliary electrodes 20 in the measurement operation are continued by a suitable choice of the applied potential. The potential of the second auxiliary electrodes 20a is however chosen so that, during operation of the sensor element as a nitrogen oxide sensor, nitrogen oxides are also reduced and removed from the gas mixture.

In the determination of oxidisable gases the gas to be determined is however in addition oxidised at the second auxiliary electrodes 20a. In this way the auxiliary electrodes 20 are exclusively available for the removal of oxygen or of oxygen, nitrogen and sulfur oxides during the equilibration mode, without additional loading due to a reaction of the gas to be determined.

If the first auxiliary electrodes 20 are not electrically connected to the second auxiliary electrodes 20a, then this opens up the possibility of applying different potentials to the auxiliary electrodes 20 and 20a. This enables a strongly negative potential to be applied to the first auxiliary electrodes 20 during the equilibration mode, which potential can be used effectively to achieve a substantial removal of oxygen and, in the determination of gases having a reducing effect, of oxygen, nitrogen and sulfur oxides, whereas on the other hand at the second auxiliary electrodes 20a the potential is chosen so as selectively to achieve as quantitative a conversion as possible of the gas to be determined. This improves the accuracy of the zero setting for the sensor element.

A further procedure for performing the zero setting of the sensor element consists, in the case of first and second auxiliary electrodes 20, 20a that are not electrically connected to one another, in that the potential at the first auxiliary electrodes 20 in the determination of nitrogen oxides should be chosen so that oxygen is reduced, and that at the second auxiliary electrode 20a oxygen and nitrogen oxides are reduced. In the determination of oxidisable gases a potential at the first auxiliary electrode 20 is chosen at which oxygen, and nitrogen and sulfur oxides are reduced, and in addition the gas to be determined is quantitatively oxidised. In this case at the second auxiliary electrodes 20a a potential is applied at which the gas to be determined is reduced and removed from the gas mixture. This permits the further auxiliary electrode 24 to be switched off.

A further improvement in the measurement accuracy can be achieved if the inner gas space 12 is subdivided by a second diffusion barrier 21 into a first inner gas space 12a and a second inner gas space 13. The diffusion barrier 21 is in this case arranged in the diffusion direction of the gas mixture between the second electrodes 20a and the further auxiliary electrodes 24.

A further example of implementation of the present invention is illustrated in Fig. 3, in which connection furthermore the same reference numerals denote the same structural components. Here the diffusion barrier 21 is arranged between the first auxiliary electrodes 20 and the second auxiliary electrodes 20a. This has the effect that, on the one hand, a diffusion of metal vapour, for example due to heating of the sensor element during the production process, and thus a contamination of the auxiliary electrodes 20, 20a with constituents of the in each case other auxiliary electrode is avoided. Here too it is possible either to electrically connect the auxiliary electrodes 20, 20a to one another, or to omit this for the sake of a better measurement accuracy.

If the diffusion barrier 21 in addition comprises a metal such as platinum that absorbs metal vapour, then the diffusion barrier may for example be insulated with respect to the surrounding solid electrolyte layers lla, lib, tic by providing an intermediate layer 23 of an insulating ceramic material, such as for example aluminium oxide, between the diffusion barrier 21 and the surrounding solid electrolyte layers.

As an alternative to an amperometric determination of the gas to be measured, by means of the pump cell 22, 26, a potentiometric determination may also be carried out. In this case oxygen or additionally nitrogen oxides and sulfur oxides are selectively removed as already described at the first and second auxiliary electrodes 20, 20a, without the content of the gas to be determined altering. The further auxiliary electrode 24 performs no function in this procedure and may also be omitted. If the measurement electrode 26 is made catalytically inactive by suitable platinum, silver and palladium alloys, then a non- equilibrium potential is established at its surface, the magnitude of which depends directly on the content of gas to be measured. This procedure is particularly suitable for the determination of oxidisable gases. The potential that is established at the measurement electrode 26 may be determined as a measurable voltage with respect to the constant potential of the reference electrode 30. - 16

Claims

Sensor Element of a Gas Sensor Claims 1. Sensor element of a gas sensor

for determining the concentration of at least one constituent of a gas mixture, in particular in exhaust gases from internal combustion engines, with at least one auxiliary electrode and a measurement electrode that are in direct contact with the gas mixture, wherein a signal generated by means of the measurement electrode serves for determining the concentration of the constituent, characterized in that for the equilibration of the sensor element there is temporarily applied to the auxiliary electrode (20, 20a, 24) a potential at which the constituent contained in the gas mixture is at least largely removed from the gas mixture.
2. Sensor element according to claim 1, characterized in that the auxiliary electrode (20, 20a, 24) is arranged in front of the measurement electrode (26) in the diffusion direction of the gas mixture.
3. Sensor element according to one of claims 1 or 2, characterized in that at the auxiliary electrode (20, 20a, 24) in a first time period a potential is applied at which the oxygen contained in the gas mixture is reduced, and that in a second time period for the equilibration of the sensor element a potential is - 17 applied at which oxygen and the nitrogen oxides contained in the gas mixture are reduced.
4. Sensor element according to one of claims 1 to 3, characterized in that the potential applied in the first time period to the auxiliary electrode (20, 20a, 24) is greater than the potential applied in the second time period.
5. Sensor element according to one of the preceding claims, characterized in that a first and a second auxiliary electrode (20, 20a) are provided that comprise different materials, in which the materials of the first and second auxiliary electrode (20, 20a) differ as regards their catalytic activity with respect to a decomposition of the constituent to be measured.
6. Sensor element according to claim 5, characterized in that the first and the second auxiliary electrode (20, 20a) are electrically connected to one another.
7. Sensor element according to claim 1, characterized in that the constituent to be measured is a nitrogen oxide and/or ammonia.
S. Method for the determination of the concentration of at least one constituent of a gas mixture, in particular in exhaust gases from internal combustion engines, by means of a sensor element of a gas sensor, characterized in that the sensor element is operated - 18 alternately in a measurement mode and an equilibration mode, in which in the measurement mode the concentration of the constituent is determined electrochemically at a measurement electrode (26), and in the equilibration mode in a first region of the sensor element the constituent of the gas mixture to be determined is removed electrochemically, and in a second region the content of the constituent in the gas mixture freed from the said constituent is determined and the resultant signal is set equal to zero.
9. Method according to claim 8, characterized in that in the measurement mode a first potential is adjusted at an auxiliary electrode (20, 20a, 24) so that a predetermined oxygen concentration is established within the sensor element, and that in the equilibration mode a second potential is adjusted at the auxiliary electrode (20, 20a, 24) at which in addition the constituent to be determined is removed.
10. Use of a method according to one of claims 8 and 9 for the determination of hydrogen, ammonia, hydrocarbons and/or nitrogen oxides.
11. Exhaust gas purification system for internal combustion engines, characterized in that a sensor element according to one of claims 1 to 7 is provided.
12. A sensor element substantially as herein described with reference to the accompanying drawings. À 19
13. A method for the determination of the concentration of at least one constituent of a gas mixture substantially as herein described with reference to the accompanying drawings.
14. Use of a method according to claim 13, substantially as herein described with reference to the accompanying drawings.
15. An exhaust gas purification system for internal combustion engines substantially as herein described with reference to the accompanying drawings.