GB2313671A - Solid electrolyte oxygen sensor having two measurement ranges - Google Patents

Description

2313671 Oxygen partial pressure sensor having two measurement ranges The

invention relates to a sensor for measuring oxygen partial pressures, comprising an electrochemical oxygen pumping cell that has an oxygen ion-conducting solid electrolyte to which electrodes are fixed on mutually opposite surfaces, and one of the two electrodes is surrounded by a first gas diffusion barrier, connected in gas-tight fashion to the solid electrolyte.

In these oxygen sensors, electrodes are arranged on two opposite sides of a solid electrolyte, and a voltage supplied to the measuring cell produced in this way, as a result of which oxygen is "pumped" from the cathode to the anode of the cell since the charge trans- is port in the interior of the cell is carried out by oxygen ions. When the applied voltage is increased, the current reaches a saturation value that depends on the oxygen content of the atmosphere surrounding the cell.

In the case of this simple embodiment of the sensor, the saturation current is unstable since it depends directly on the decomposition of the cathode. The cathode is therefore surrounded by a diffusion barrier. The latter may be of gas-tight design and may be provided with a small hole through which the atmosphere can reach the cathode.

The hole is of such small design that, when a sufficiently high voltage is applied, the value of the_ saturation current attained at a specific oxygen concentration depends only on the gas diffusion through this hole. The geometry of the hole therefore establishes the saturation- current: oxygen- concentration ratio, and thus the sensitivity of the sensor.

It should also be noted that an oxygen sensor of the type described can only be operated with a specific maximum voltage. Above this voltage, an additional conductivity occurs, which does not depend on the oxygen concentration and the hole geometry, and would vitiate the result of the measurement.

The value of the saturation current which occurs at the maximum voltage is still just dependent on the hole geometry and the oxygen concentration; the size of the hole, together with the internal resistance of the cell, therefore establishes the highest oxygen concentration which can still be measured, and thus the upper limit of the measurement range.

If the known sensors which have been described are operated in gas mixtures or gases (apart from oxygen), then an effect on the sensor signal must be expected. These effects can be roughly assigned to gas components whose presence merely reduces the partial pressure (for example N., nitrogen) and those which cause an active change in the sensor signal (for example S02. sulphur dioxide or H20. water vapour).

So., sulphur dioxide, above all, is a component which leads to changes in the sensor signal and to a reduction in the sensor life. A cause of this change is the coating of the relatively large unprotected and exposed electrode underside with platinum- sulphur compounds, which cause lasting impairment to the platinum surface and the catalysis needed for unimpaired operation of the sensor can therefore no longer take place to full measure, or the active area therefor is continuously reduced. The consequence of these reactions is a greatly reduced life of the sensor, which is unacceptable for certain applications and therefore rules them out.

Since the gas component SO, is present, at least in a small amount, in all gas mixtures fed to combustion, an improvement in terms of the property described above is desirable and should be sought. Similar conditions also occur with other gas components, for example CO (carbon monoxide) and C02 (carbon dioxide) as well as No.

(nitrogen oxides).

The following further disadvantage arises with hitherto known sensors. If different measurement ranges are required in one application, then a plurality of sensors need to be operated in parallel with one another, since each sensor has a fixed, unchangeable measurement 3 range because of its structure. Measurement arrangements having a large number of sensors entail the disadvantage of the need for a large amount of sensor circuitry, and therefore greater outlay on components and need for 5 space.

The object of the invention is to avoid these disadvantages, and to provide a sensor of the type mentioned at the start, which can be operated in two different measurement ranges, simultaneously protects its two electrodes from chemical effects due to atmospheric components and which furthermore has a linear relationship between oxygen partial pressure and output signal.

According to the invention, this is achieved in that the second electrode is also surrounded by a second is gas diffusion barrier, connected in gas-tight fashion to the solid electrolyte, in that thetwo gas diffusion barriers have different values of gas permeability, and in that the electrodes are connected to a voltage source via a current detection device.

This only unsubstantially complicates the sensor structure in comparison with the use of two separate sensors; by simple modification to the sensor circuitry, a sensor according to the invention can be operated in such a way that it fulfils the function of two separate sensors.

Connecting the electrodes to a voltage source allows the sensor according to the invention to be operated in accordance with the namperometric measurement method", as a result of which, primarily, a linear relationship between the oxygen pressure and the measurement signal (sensor current) is provided, and, in particular, this facilitates the further processing of the result of the measurement. Furthermore, sensors operated amperometrically exhibit a high capacity for resolution, connected with a high signal stability as well as a low temperature dependence of the measurement signal.

In a refinement of the invention, provision may be made that the gas diffusion barriers are made of a gastight material and each have at least one hole.

This allows the gas permeability of the two barriers to be set accurately.

It may be advantageous that the holes have 5 different diameters from each other.

As a result, the two gas diffusion barriers can be designed with the same thickness, which is advantageous during production because both barriers can be cut from the same plate of raw material.

According to another variant of the invention, provision may be made that the holes have different lengths from each other.

This makes it possible for both holes to be produced with the same diameter and therefore using the is same tool.

According to a particularly preferred embodiment of the invention, provision may be made that the solid electrolyte is fused in gastight fashion to the gas diffusion barriers by means of glass.

An absolutely gas-tight connection between the oxygen pumping cell and the diffusion barrier can thereby be obtained in simple fashion.

In connection with this, in a refinement of the inventionj provision may be made that the gas diffusion barriers consist of the material of the solid electrolyte.

The coefficients of expansion of the solid electrolyte and of the gas diffusion barriers are there-_ fore absolutely equal; deformations of the aforementioned components, caused by temperature changes, do not lead to their gas-tight connection being broken.

According to another configuration of the invention, provision may be made that the gas diffusion barriers consist of a substance chosen from the group glass, metal or glass ceramic.

These materials are much more favourable in terms of cost than a solid electrolyte such as zirconium oxide, and this leads to a lower overall production cost for the sensor. Furthermore, these materials are substantially easier to process, and in particular smaller holes can be made than in solid electrolytes. As a result, the operating temperature of the sensor according to the invention can be kept lower than with a sensor having diffusion barriers composed of solid electrolyte, and a lower power consumption as well as an increase in reliability can thereby be achieved. Nevertheless, the materials listed above have all the properties needed in order to be designed as gas diffusion barriers.

A further feature of the invention may reside in the fact that, as heater for the sensor, platinum tracks provided with platinum connecting wires are arranged on the outside of at least one of the diffusion barriers.

In this way, the heater is simple to produce and, is because of the direct arrangement on the diffusion barrier, allows the sensor to be heated effectively.

In connection with this, it may be advantageous that, as pull relief for the platinum connecting wires, elements respectively enclosing the platinum connecting wires and composed of glass paste are arranged on the surface of the diffusion barrier(s). This pull relief is simple to produce and offers effective protection against unacceptably high tensile stresses. 25 in a further configuration of the invention, provision may be made that the glass and the glass paste are made of a mixture of dif f erent glass powders and organic binders. By virtue of this measure, both materials can be processed together. 30 A preferred embodiment of the invention may consist in that the glass and the glass paste consist of substances chosen from the group Si02. Na203. BaO, K20, A1203 and B203 This composition provides a glass having a coefficient of thermal expansion that in matched particularly well to that of the oxygen pumping cell.

The invention in described in more detail below with reference to the drawings, in which:

Fig. 1 shows a sensor according to the prior art, - 6 in vertical section; Fig. 2 shows a characteristic current/voltage curve of a sensor according to Fig. 1; Fig. 3 shows a sensor according to the invention in vertical section; and Fig. 4 shows a characteristic current/voltage curve of a sensor, in accordance with the invention, according to Fig. 3.

A prior art sensor according to Fig. 1 comprises an oxygen pumping cell 1 that has a preferably cylindrical solid electrolyte 20. A material which is preferably used for this solid electrolyte is zirconium oxide stabilized with yttrium oxide.

Platinum electrodes 4, 5 are arranged on the top and underside of the solid electrolyte 20 and are connected to a voltage source 11 via platinum wires 6, 12. This source outputs a constant voltage, as a result of which it is possible to obtain information regarding the value of the oxygen partial pressure to be measured by evaluating the current which the voltage source 11 delivers to the sensor. To this end, the electrodes 4, 5 are connected to the voltage source 11 via a current detection device 13 which, in the embodiment represented in the drawing, is formed by an ammeter 13.

This way of operating the sensor, which is also referred to as the amperometric measurement method, has the following decisive advantages over a likewise possible potentiometric mode of operation, in which a constant current is applied to the sensor and the potential difference which is then set up between the two electrodes 4, 5 represents a measure of the oxygen content in the surrounding atmosphere:

In the case of a potentiometrically operated sensor, there is a logarithmic relationship between the measured quantity (= oxygen content) and the measurement signal (= voltage between the electrodes), whereas the measurement signal for the amperometric measurement (= sensor current) exhibits a linear relationship with the oxygen content for a relatively high range of oxygen pressures (above about 0 "% 02). Evaluation and further processing of the measurement signal is therefore substantially simpler than with a potentiometric method.

In the measurement range, of interest for many applications, starting at 0.1% 02 (combustion processes) to 96% (medical technology), the amperometric sensor exhibits a high resolution capacity in conjunction with high signal stability. Although a potentiometrically operated sensor does indeed span a broad measurement range because of its logarithmic characteristic, it is as a result rather unsuitable for measuring high oxygen concentrations in view of accuracy and resolution.

In contrast to the potentiometric sensor, the measurement signal of an amperometic sensor has a par- is ticularly low temperature dependence. The accurate temperature control, which is needed for proper potentiometric operation and in a large number of casesis associated with considerable technical complexity, may then be omitted.

Finally, with potentiometric operation of a sensor, it must be noted that one of the two electrodes must be exposed to a reference atmosphere having accurately known oxygen content, and the other electrode must be exposed to the atmosphere to be measured. The reference atmosphere and the atmosphere to be measured must be kept separated from each other in gas-tight fashion, or the reference atmosphere must, in addition to this, be produced with corresponding design outlay.

In contrast, an amperometrically operated sensor makes do entirely without a reference atmosphere of this type, which saves technical outlay and allows the overall size of the sensor to be kept small.

Its simpler structure, the lower outlay on material and the properties also referred to allow the use of the amperometric sensor in application cases for which it has not yet been possible to provide a satisfactory solution with a potentiometric sensor for technical or economic reasons.

Fig. 2 represents the current/voltage curve of an amperometric sensor of this type. In the first quadrant, the curve shows three different regions A, B, C. In region A, the sensor current firstly rises as a result of the internal resistance of the solid electrolyte 20.

Thereafter, in region B, a saturation value is reached, the value of which depends on the geometry of the diffusion barrier (cross -sectional area/length) and on the oxygen partial pressure in the surrounding atmosphere. If the sensor voltage is increased further to a critical voltage value VD, a strong increase in the sensor current then occurs, due to additional electronic conductivity (region C). This region C can no longer be employed for stable evaluation.

In the third quadrant, the curve only has the regions A and C. The described representation shows that the maximum output current is determined by the internal resistance of the cell and by the voltage value V,. In order to make the sensitivity of the sensor as great as possible, the geometry of the diffusion barrier (hole diameter, hole length) is set in such a way that the upper limit of the measurement range corresponds to this maximum saturation current. The sensitivity is given by the ratio of I., to the upper limit of the measurement range. For various applications, sensors are therefore needed which have different upper limits for the measurement range, for example 0-1%, 0-5%, 0-25%, 0-95%.

The sensor according to the invention for measuring oxygen partial pressures, which is represented in Fig. 3, has a structure essentially identical to that of the sensor according to Fig. 1. It likewise comprises an electrochemical oxygen pumping cell 1, formed by an oxygen ion-conducting solid electrolyte 20 to which electrodes 4, 5 are fixed on mutually opposite surfaces.

The electrode 4 is surrounded by a first gas diffusion barrier 2, connected in gas-tight fashion to the solid electrolyte 20, and the second electrode 5 is, in similar fashion thereto, surrounded by a second gas diffusion barrier 3, connected in gas-tight fashion to the solid electrolyte 20.

9 Both gas diffusion barriers 2, 3 have the purpose of limiting direct access of the surrounding atmosphere to the electrodes 4, 5, and both thus have a low gas permeability. This being the case, for the intended production of two different upper limits for the measurement range, it is essential for the two barriers 2, 3 to have different values of gas permeability rates from each other.

If the operating voltage is applied in the fashion represented in Fig. 3, then the 02 ions are transported from the electrode 4 in the direction of the electrode 5. The diffusion barrier 2 with its hole 7 is effective for the surrounding atmosphere flowing by, so that the upper limit of the measurement range is also determined by this hole 7.

When the polarity of the operating voltage is reversed, the direction of flow of the 0. ions is also reversed, and the barrier 3 becomes effective for the surrounding atmosphere and determines the measurement range.

Fig. 4, to which reference will be made below, shows the current/voltage curve of such a sensor according to the invention. The regions A, B and C as described above can be seen both in the f!rat and in the third quadrants. It is clear that, in the third quadrant, the upper limit of the measurement range is only 1/3 that in the first quadrant, although the sensitivity is about three times as great. This shows that it is possible to combine two arbitrary upper limits of the measurement range.

By virtue of this configuration, and in a simple fashion, the possibility of a variable measurement range is provided and, at the same time, a decisive improvement is achieved, above all, in terms of the ef f ect due to the gas component SO,o sulphur dioxide. This embodiment thereby makes it possible to set two measurement ranges on one and the same sensor element, in such a way that, in order to change the range, only the polarity of the operating voltage needs to be changed. At the same time, the platinum electrode 5, which is mentioned above and according to the invention is now no longer exposed, is protected.

The already mentioned gas permeability of the diffusion barriers 2, 3 is preferably achieved by using a gas-tight material in which at least one hole 7, 15 is made.

Various possibilities can be proposed for setting the different gas permeability rates of the two barriers 2, 3. The two holes 7, 15 may, as represented by dashed lines in Fig. 3, have different diameters from each other; however, it would equally be possible to provide a first number of holes 7 in the first barrier 2, but a second number of holes 15 in the second barrier 3, it being possible for the individual holes 7, 15 to have equal or different diameters.

Another way of setting the gas permeability is based on the length of the holes 7, 15. As indicated with dot-dashed lines in Fig. 3, the holes 7, 15 may have dif - ferent angles of inclination from each other with respect to the electrodes 4, 5, which produces different hole lengths and therefore different gas diffusion resistances. In connection with this, it is also possible within the scope of the invention to design the two diffusion barriers 2, 3 with different thicknesses, and thereby to achieve different hole lengths.

Materials which may be used for the diffusion barriers 2, 3 are the substance constituting the solid electrolyte 20, i.e. Zr02 preferably stabilized with Y2031 or a material chosen from the group glass, metal and glass ceramic.

When using one of the materials glass, metal or glass ceramic, it is, in particular, advantageous that they can be obtained at substantially lower cost and are easier to process than Zr02. Glass ceramic is particularly preferred, since holes can be made in this material with smaller diam ters than in zirconium oxide. Glass ceramic also has particularly good wettability with the glass 14, which avoids possible sealing defects between the glass 14 and the diffusion barriers 2, 3.

In addition to the use, described so f ar, of components which are inherently gas-tight but have holes in them, as the diffusion barriers 2, 3, it is also possible to use bodies which are inherently porous. In the spirit of the invention, it is also important in this case that the porosities, and therefore the gas permeability rates, of the two barriers 2, 3 are chosen to have different values.

The gas-tight connection of the solid electrolyte to the gas diffusion barriers 2, 3 preferably takes place by means of a glass 14. During production, this glass 14 is melted and, when it cools, binds the abovementioned components together particularly reliably.

The heater f or the sensor is arranged on the outside of at least one of the diffusion barriers 2, 3, the greatest effectiveness being, of course, achieved by using one heater in each case on each of the two barriers 2, 3. Like the electrodes 4, 5 of the oxygen pumping cell 1, the heaters consist of platinum tracks 8 which are connected via platinum wires 9 to a voltage source.

As pull relief for the platinum connecting wires, elements respectively enclosing the platinum connecting wires 9 and composed of glass paste 10 are arranged on the surface -of the diffusion barrier(s) 2, 3. The pull relief provided, according to Fig. 1, for the connecting wire 6 of the exposed electrode 5 is formed by the glass 14 itself an a result of the fixing according to the invention of the second diffusion barrier 3.

The compositions of the glass 14 and of the glass paste 10 forming the pull reliefs are chosen in such a way that these materials have similar thermal properties, matched to the diffusion barriers 2, 3. The effect achieved by this is that both materials, glass paste 10 and glass 14, can be processed together in one production step, i.e. fusing.

The glass 14 and the glass paste 10 are formed by a mixture of various glass powders and organic binders.

It has proved favourable for the glass 14 and the glass paste 10 to consist of substances, chosen from the group SiO., Na203, BaO, K20, A1203 and B203, a mixing ratio of 55% SiO2, 5% Na2031 17% BaO, 6% K20, 3% A12031 14% B203 providing a glass whose coefficient of thermal expansion is matched particularly well to the coefficient of thermal expansion of the oxygen pumping cell 1. About 5% of Plextol is added as binder to render this powder mouldable.

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Claims (11)

PATENT CLAIMS

1. Sensor for measuring oxygen partial pressures. comprising an electrochemical oxygen pumping cell (1) that has an oxygen ion-conducting solid electrolyte (20) to which electrodes (4, 5) are fixed on mutually opposite surfaces, and one of the two electrodes (4) is surrounded by a first gas diffusion barrier (2), connected in gastight fashion to the solid electrolyte (20), characterized in that the second electrode (5) is also sur- rounded by a second gas diffusion barrier (3), connected in gas-tight fashion to the solid electrolyte (20), in that the two gas diffusion barriers (2, 3) have different values of gas permeability, and in that the electrodes (4, 5) are connected to a voltage source (11) via a current detection device (13).

2. Sensor according to Claim 1, characterized in that the gas diffusion barriers (2, 3) are made of a gastight material and each have at least one hole (7, 15).

3. Sensor according to Claim 2, characterized in that the holes (7, 15) have different diameters from each other.

4. Sensor according to Claim 2 or 3, characterized in that the holes (7, 15) have different lengths from each other.

5. Sensor according to one of Claims 1 to 4, characterized in that the solid electrolyte (20) is fused in gas-tight fashion to the gas diffusion barriers (2, 3) by means of glass (14).

6. Sensor according to one of Claims 1 to 5, charac30 terized in that the gas diffusion barriers (2, 3) consist of the material of the solid electrolyte (20).

7. Sensor according to one of Claims 1 to 5, characterized in that the gas diffusion barriers (2, 3) consist of a substance chosen from the group glass, metal or glass ceramic.

8. Sensor according to one of Claims 1 to 7, characterized in that, as heater for the sensor, platinum tracks (8) provided with platinum connecting wires (9) are arranged on the outside of at least one of the diffusion barriers (2, 3).

g.- Sensor according to Claim 8, characterized in that, as pull relief for the platinum connecting wires (9), elements respectively enclosing the platinum connecting wires (9) and composed of glass paste (10) are arranged on the surface of the diffusion barrier(s) (2, 3).

10. Sensor according to Claim 9, characterized in that the glass (14) and the glass paste (10) are made of a mixture of different glass powders and organic binders.

11. Sensor according to Claim 10, characterized in that the glass (14) and the glass paste (10) consist of substances chosen from the group SiO., Na203. BaO, K.O.

A1,03 and B.03 -