GB2334785A - Gas sensor - Google Patents

Abstract

A gas sensing arrangement, e.g. for monitoring the exhaust channel 9 of an internal combustion engine, includes a gas sensitive element 3 (e.g. an oxygen sensitive strontium titanate sensor) and a layer 2 consisting of a solid electrolyte such as zirconia which carries three electrodes 4, 6 and 7, which may be connected to external circuitry such that a voltage across electrodes 6 and 7 gives a measure of the difference in oxygen concentration across the layer and a current applied through the layer via electrodes 4 and 6 causes oxygen to be pumped through the layer. The circuitry may be arranged such that a current is caused to flow between electrodes 4 and 6 in response to a voltage across electrodes 6 and 7 in such a way as to reduce the voltage to zero and so expose sensor 3 to the same oxygen concentration as in the exhaust channel without exposing the sensor to other components of the exhaust stream. In this way, the layer 2 acts as an oxygen selective filter.

Description

GAS SENSOR The present invention relates to a gas sensor arrangement and to a selective oxygen filter.

Gas sensors for oxygen measurement are often used under unfavourable conditions, such as for exhaust-gas measurement. In this connection, for example, it is to be established whether the air-fuel mixture supplied to an internal combustion engine contains too much or too little oxygen. In order that it is possible to react quickly to load alternation, the measurements must take place quickly.

One method of characterising the fuel mixture is to measure the residual oxygen content in the exhaustgas mixture. For this purpose, probes made of a gasimpermeable, cup-shaped ceramic body of zirconium dioxide are known, which ceramic body is provided, on the inside and on the outside, with a thin layer of platinum. One portion of the known probe is in contact with the outside air, and the exhaust gas of the engine flows around the other portion. The ceramic material is conductive for oxygen ions at approximately 300"C.

If the proportion of oxygen on either side of the known sensor is different, an electrical voltage results, which is a measure for the residual oxygen content and can be tapped by leads fastened to the platinum layers, which serve as electrodes.

It is further known to form oxygen sensors with a layer of strontium titanate. In this connection, DE 42 03 522 C1 suggests an oxygen sensor arrangement on the basis of semiconducting metal oxides, the conductivity of which at a raised temperature depends on the partial oxygen pressure, with the sensor arrangement having two individual metal oxide sensors which, in the intended measuring range, have different dependency of conductivity on the partial oxygen pressure, but on the other hand show a substantially equal temperature dependency of the conductivity, which accordingly stands out to a considerable extent in the quotient of the conductivity measurement signals of the two sensors that is formed.

A problem in sensors of this type is, however, the cross-sensitivity to gaseous components which are always present in combustion exhaust gases, such as CO, H2 or unburnt or only partially oxidised hydrocarbons, sulphur dioxide, etc, which cause cross-sensitivities or which chemically attack the sensor. In order to reduce such cross-sensitivities, it has been suggested to provide over the oxygen-sensitive strontium titanate layer a metal oxide covering layer which has high ionic conductivity at least at operating temperature. This covering layer acts as a selective filter, which lets through practically only oxygen, or in any case predominantly oxygen. It is assumed that this is caused by the following mechanism: the high ionic conductivity means that the oxygen or the vacancies thereof already present in the metal oxide crystal lattice of the covering layer are also easily movable, i.e. have a high mobility, If oxygen from the gas phase of a gas mixture is absorbed at the surface of the covering layer, it can, after dissociation of the oxygen molecules, also penetrate easily into the crystal interior. This generates, from the surface, a concentration gradient of oxygen, i.e. an oxygen gradient. In the attempt to balance this oxygen gradient, the oxygen travels through the metal oxide layer, something which occurs in particular by recombination of the adsorbed oxygen with the oxygen vacancies of the crystal lattice, whereupon an oxygenvacancy gradient develops in the lattice. The high ionic conductivity of the metal oxide, which usually makes the metal oxide semiconducting at least at high operating temperatures, is therefore accompanied by a high permeability for oxygen. The assumed mechanism of the oxygen transport barely comes into question for other gases, which explains why the covering layer acts as a selective filter for oxygen.

The oxygen transport, however, effects the buildup of a spatial charge, because, after travelling through the covering-layer lattice, the oxygen, in ionic form or as oxygen vacancies, collects on one layer side, for instance because of a concentration gradient between the outside and inside of the covering layer. This builds up a potential which, as a result of the electrostatic field forces, opposes the further transport of oxygen ions, because this would lead to a further growth of potential.

In an unpublished German patent application related to the German application from which this United Kingdom application claims priority, it has been suggested that electrodes be arranged on either side of the covering layer, which electrodes can either be short-circuited in order to avoid the build-up of the voltage potential, or with which a pumping current can be provided through the covering layer in order actively to favour the transport of oxygen. For this purpose, there is provided a current through the covering layer that acts against the oxygen gradient and reduces it to as close as possible to zero. In this case, it is understood that in the present document, the term oxygen gradient" can mean both a gradient of the oxygen concentration and a gradient of the oxygen-vacancy concentration.

If the oxygen concentration on the inside and on the outside of the covering layer is the same, i.e. the gradient is reduced to zero, the oxygen concentration which is also present on the outside of the covering layer is present in the oxygen-sensitive region, i.e. at the strontium titanate sensor. In order to permit genuine measurements, it is necessary to ensure that the oxygen gradient is as close as possible to zero and is preferably identical to zero. Further, the adjustment of the oxygen gradient to zero with the provision of an active pumping current is to take place as quickly as possible in order to improve the response times of the gas sensor.

The present invention seeks to provide something new for industrial application and in particular to develop an improved gas sensor which responds more quickly and has increased accuracy and can also be used under unfavourable conditions.

According to a first aspect of the present invention, there is provided gas sensor arrangement having a layer, over which an oxygen-gradient-dependent voltage can be measured, and an electrode pair consisting of a first electrode arranged on the first layer side and a second electrode arranged on the second layer side, further comprising a third electrode on the first layer side that is separate from the electrode pair, in order in particular to measure between the second and third electrodes a voltage over the layer and to provide between the first and second electrodes a flow of current in response to the measured voltage.

According to a second aspect of the invention, there is provided selective oxygen filter having a layer, over which an oxygen-gradient-dependent voltage can be measured, and an electrode pair consisting of a first electrode arranged on the first layer side and a second electrode arranged opposite, on the second layer side, further comprising a third electrode on the first layer side that is separate from the electrode pair, in order to measure between the second and third electrodes a voltage and to provide between the first and second electrodes a flow of current in response to the measured voltage.

Preferred embodiments of the invention are set out in the dependent claims.

It is thus suggested that there be provided at least one further electrode, with which an oxygengradient-dependent voltage over the layer can be measured independently of a pumping current through the layer. This permits the individual electrodes to be designed and arranged optimally for their respective purpose. Thus, the third electrode can be arranged in such a way that it has only a small capacitance with respect to the other voltage-measuring electrode, while the electrodes of the electrode pair can be formed for a homogeneous pumping current through the layer that is as even as possible.

The layer preferably acts as a selective filter for oxygen, for which purpose, as a solid electrolyte which is selectively permeable to oxygen, it can be selected, in particular, from a metal oxide having high ionic conductivity, such as zirconium dioxide or cerium dioxide.

In such a case, the voltage over the layer generally responds to the Nernst equation, i.e. has the form: Unernst = (kT/4e) ln[p'(O2)/p'(O2)] where Unerrist = Nernst voltage k = Bolzmann constant T = absolute temperature e = elementary charge p' ( 2) = partial oxygen pressure on the one layer side p'(O2) = partial oxygen pressure on the other layer side, and where the factor 4 originates from the dissociation of the oxygen molecules and their doubled ionisation in the layer.

The layer usually separates completely from the gas mixture to be tested a region in which there is arranged an oxygen-sensitive semiconductor or suchlike in order to measure the oxygen concentration in the separated region. If, by applying a pumping current to the layer which is permeable to oxygen, so much oxygen is titrated through the layer that the oxygen gradient is zero, the same concentration is present on the inside and on the outside; if desired, the oxygen value measured with the oxygen-sensitive region can be corrected beforehand by bringing in the Nernst voltage which falls off over the layer.

The geometry of the electrode pair is preferably selected in such a way that the layer is at most slightly polarised. This is achieved in the case of a substantially homogeneous flow of current. The polarisation, which is dependent on the impressed electrical current, the geometry and morphology of the electrodes of the solid electrolytic chain, could otherwise lead to a reduction in the through-velocity of the transported oxygen through the layer and/or falsify the measurement of the oxygen-gradientdependent voltage.

There can be provided in the region surrounding the third electrode, which is used for the voltage measurement, an increased resistance. In this way, a substantial flow of current over the third electrode is also avoided if the second electrode on the opposite layer side, which electrode is connected to a lowresistance current source, is used as reference electrode. Because only a very low measuring current has to flow for the voltage measurement, which current is dependent on the internal resistance of the voltage measuring circuit which is used, the voltage measurement is barely influenced by the increased resistance around the third electrode, although the parasitic current flow is substantially eliminated.

For a better understanding of the present invention, and to show how it may be brought into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 shows a gas sensor arrangement according to the present invention; and Figure 2 shows an equivalent circuit diagram for a gas sensor arrangement of the present invention.

According to Figure 1, a gas sensor arrangement, denoted in general with 1, comprises a cup-shaped layer 2 and an actual gas sensor 3, i.e. a gas probe 3, arranged therein.

The layer 2 consists of a solid electrolyte which is selectively permeable to oxygen. An oxygen gradient over the layer 2 effects a voltage over the layer 2.

On the outside, the layer 2 carries an electrode 4 made of platinum or another material which is resistant to high temperatures and has good conductivity, which electrode surrounds the cup wall, leaving the tip free.

Provided on the electrode 4 is a terminal 4a. The cup interior 5 of the cup-shaped layer 2 is lined, opposite the electrode 4 and continuously to the cup base, with a second platinum layer as a second electrode 6.

Provided on the second electrode 6 in the cup interior 5 is a terminal 6a to an external lead.

In order to increase its ionic conductivity, the layer 2 is doped at least in the regions between the first electrode 4 and the second electrode 6. If the layer is made of zirconium dioxide ZrO2, yttrium is preferably used for the doping. If a layer of cerium dioxide is used, gadolinium is preferably used for the doping.

Provided on the outer tip of the cup is a third electrode 7 having a terminal 7a, the third electrode 7 not being in direct electrical contact with the first electrode 4. In the region surrounding the third electrode 7, an area 2a is provided in the layer 2, which area is doped in order to lower the conductivity there. For this purpose, the whole layer structure can first of all be provided with a homogenous doping in order to increase the conductivity, and then a further doping can be placed specifically on the cup tip in order to reduce the conductivity.

Provided inside the cup space 5 which is surrounded by the layer 2 is the actual gas sensor 3, which, as an oxygen sensor, has, for example, oxygensensitive strontium titanate layers, the conductivities of which change with the oxygen content of an ambient gas atmosphere. In the usual way, the actual gas sensor 3 is arranged on a carrier 3a with heating structure, temperature sensors and suchlike. On the side which faces away from the base of the cup, the actual gas sensor 3 is connected in a gas-tight manner to the cup-shaped layer 2, as indicated by a sealing plate 8. The gas sensor arrangement 1 is installed in an exhaust-gas channel 9 or suchlike.

It is understood that the heating elements can, for example, be realised by heating structures and suchlike mounted on the layer, or the sensor can be heated to the required operating temperature by the hot exhaust gases in the exhaust-gas channel or combustion channel 9.

Figure 2 shows the arrangement 10 for the interconnection of the electrodes 4, 6 and 7 which are mounted on the layer 2. The electrode 7 at the cup tip is connected via the terminal 7a to the first measurement input ila of a high-resistance voltagemeasuring device 11, the second measurement input lib of which is connected to the terminal 6a of the electrode 6 on the inside of the cup-shaped layer 2.

The terminal 6a of the electrode 6 is further connected to a current output 12a of a controllable current source 12, the second current output 12b of which is connected to the terminal 4a of the electrode 4. The controllable current source 12 has a control input 12c, which receives a desired current signal from the voltage-measuring device 11 in response to a voltage over the layer 2 that is measured between the electrodes 6 and 7. This voltage is caused by an oxygen gradient over the layer 2. The desired current signal at the control input 12c is thus dependent on an oxygen gradient between the outside of the layer and the inside of the layer.

With this interconnection, there result through the layer 2 resistances and capacitances between the electrodes as follows: resistance R (7.4) and capacitance C (7.4) between the electrode 7 and the electrode 4; resistance R (4.6) and capacitance C (4.6) between the electrodes 4 and 6; and resistance R (7.6) and capacitance C (7.6) between the electrode 7 and the electrode 6.

The gas sensor is operated as follows: After installation and connection to the measuring devices and control devices, the actual gas sensor 3 and the layer 2 are brought to the required operating temperature.

If, at the start of the operation, there are equal oxygen concentrations on the inside and outside of the layer, the voltage measured with the voltage-measuring device 11 is zero and consequently, the pumping current which is summoned between the electrode 4 and the electrode 6 and is dependent on this voltage and is provided by the current source 12 likewise amounts to zero.

If the combustion process is then initiated, the oxygen concentration in the exhaust-gas channel 9 falls, whilst first of all remaining substantially constant inside the cup-shaped layer 2 and thus in the oxygen-sensitive region of the actual gas sensor 3.

The presence of an oxygen gradient over the layer 2 causes the formation of a voltage over the layer 2, which is detected by the electrodes 6 and 7 and given to the voltage-measuring device 11 as an input signal.

The voltage-measuring device 11 thereupon outputs a desired current signal to the desired current input 12c of the controllable current source 12, whereupon the latter provides a pumping current over the electrode 4, the layer 2 and the electrode 6, in such a way that oxygen is titrated out of the cup interior and goes into the exhaust-gas channel 9. This continues until the voltage between the electrodes 7 and 6 has been reduced to zero. At this instant, the voltagemeasuring device 11 instructs the current source 12 with a desired current zero at its current input 12c.

This is the case if there are equal oxygen concentrations in the exhaust-gas channel 9 and in the cup interior 5. The oxygen concentration in the oxygen-sensitive region of the actual gas sensor 3, which oxygen concentration is now measured, thus corresponds to the concentration in the exhaust-gas channel 9. During the measurement with the oxygensensitive region of the actual gas probe, a crosssensitivity to other gas components, such as carbon monoxide, partially oxidised hydrocarbons, hydrogen, sulphur dioxide, nitric oxides, etc, contained in the exhaust gas is not to be feared, because the layer 2 acts as a selective oxygen filter.

In this connection, the partial oxygen pressures on the inside and outside of the layer are always identical if the voltage over the layer amounts to zero, irrespective of the temperature, as the formula given in the introduction to the description immediately shows. The adjustment of this state takes place quickly and precisely, so that measurements are possible with high resolution.

As a result of the doping of the layer 2, the resistance R (4.6) between the pumping electrodes is low. At the same time, as a result of the specific local doping in the region surrounding the electrode 7, the resistance R (7.4) between the third (measuring) electrode and the first (pumping) electrode is high, something which reduces interference currents, which would lead to falsifications of the measured values at the voltage-measuring device 11; the resistance R (7.6) between the electrode 7 and the second electrode 6 is also so high that the voltage measurement is influenced by interferences only in a virtually negligible way.

The geometry of the electrodes is chosen in such a way that even as a result of the stray capacitances C (6.7) and C (7.4), signals are not coupled into the voltage measurement even if the pumping current is provided in pulsed form or is quickly readjusted. As a result of large-surface electrodes 4 and 6, a high overall current is possible at the same time without generating a current density which increases beyond all measure. In this connection, the geometry which is described is suitable for providing a high current density without polarisation of the layer. In this way, and as a result of the low resistance R (4.6), a very fast adjustment of an oxygen balance between the inside and outside of the layer is possible.

If desired, corrections are also made to the measuring resistances R (7.4), C (7.4), R (7.6) and C (7.6), which can be measured and calculated easily.

Thus, the parasitic influences can be avoided to a considerable extent by the controlled current from the current source 12.

Although the actual probe 3 having the strontium titanate layers was shown and described as separate from the layer 2, a construction of the two together on a substrate and directly one on top of the other is easily possible.

Claims (21)

1. Gas sensor arrangement having a layer, over which an oxygen-gradient-dependent voltage can be measured, and an electrode pair consisting of a first electrode arranged on the first layer side and a second electrode arranged on the second layer side, further comprising a third electrode on the first layer side that is separate from the electrode pair, in order in particular to measure between the second and third electrodes a voltage over the layer and to provide between the first and second electrodes a flow of current in response to the measured voltage.

2. Gas sensor arrangement according to claim 1, wherein the layer is permeable to oxygen, preferably selectively permeable to oxygen.

3. Gas sensor arrangement according to the preceding claim, wherein the layer comprises a solid electrolyte and is formed in particular from a metal oxide having high ionic conductivity at least at operating temperature.

4. Gas sensor arrangement according to the preceding claim, wherein the layer preferably consists of yttrium-doped zirconium dioxide ZrO2, and/or preferably gadolinium-doped cerium dioxide CeO2.

5. Gas sensor arrangement according to one of the preceding claims, wherein the layer is formed in such a way that the measurable oxygen-gradient-dependent voltage is generally proportional to the difference between the logarithmized partial oxygen pressure on the one layer side and the logarithmized partial oxygen pressure on the other layer side.

6. Gas sensor arrangement according to one of the preceding claims, having at least one oxygen-sensitive region which is different from the layer.

7. Gas sensor arrangement according to the preceding claim, wherein the oxygen-sensitive region which is different from the layer is, by means of the layer, separated from direct contact with the gas mixture to be measured.

8. Gas sensor arrangement according to the preceding claim, wherein the layer is arranged as a selective oxygen filter over, around or on the oxygensensitive region.

9. Gas sensor arrangement according to one of claims 6 to 8, wherein the oxygen-sensitive region which is different from the layer comprises a strontium titanate layer, in particular doped, and preferably comprises two layers of strontium titanate SrTiO3 which are differently doped for alternate temperature compensation.

10. Gas sensor arrangement according to one of the preceding claims, wherein the geometry of the electrode pair is selected in such a way that a current which flows over it through the layer causes a layer polarisation which is at most slight.

11. Gas sensor arrangement according to one of the preceding claims, wherein there is provided an increased resistance of the third electrode with respect to the first and/or second electrode, for which purpose there is preferably provided a local doping of the layer, in particular in the region surrounding the third electrode, in order to reduce the specific electrical conductivity of the layer.

12. Gas sensor arrangement according to the preceding claim, wherein the layer is formed from yttrium-doped zirconium dioxide and in the region surrounding the third electrode is doped with a cation, in particular Ta and/or Nb, which is of higher valency in comparison with Zr4±ions.

13. Gas sensor arrangement according to the preceding claim, wherein the third and first electrodes are arranged on the side of the gas mixture to be tested and the second electrode is separated by the layer from the gas mixture to be tested.

14. Gas sensor arrangement according to one of the preceding claims, having a controllable current source connected to the first and second electrodes for the provision of a current which preferably reduces to zero the oxygen gradient over the layer.

15. Gas sensor arrangement according to one of the preceding claims, having a voltage-measuring device, the measurement inputs of which are connected to the first and third electrodes and which has a signal output which is connected to the control input of the current source.

16. A gas sensor arrangement, substantially as herein described, with reference to the accompanying drawings.

17. Selective oxygen filter having a layer, over which an oxygen-gradient-dependent voltage can be measured, and an electrode pair consisting of a first electrode arranged on the first layer side and a second electrode arranged opposite, on the second layer side, further comprising a third electrode on the first layer side that is separate from the electrode pair, in order to measure between the second and third electrodes a voltage and to provide between the first and second electrodes a flow of current in response to the measured voltage.

18. Filter according to claim 17, wherein the layer comprises a solid electrolyte and is formed in particular from a metal oxide having high ionic conductivity at least at operating temperature, with the layer consisting in particular of preferably yttrium-doped zirconium dioxide ZrO2, so that the measurable oxygen-gradient-dependent voltage is generally proportional to the difference between the logarithmized partial oxygen pressure on the one layer side and the logarithmized partial oxygen pressure on the other layer side.

19. Filter according to one of claims 17 or 18, wherein the layer is doped in the region surrounding the third electrode in order to reduce the specific electrical conductivity of the layer for the purpose of increasing the resistance of the third electrode with respect to the first and/or second electrode.

20. A selective oxygen filter substantially as herein described, with reference to the accompanying drawings.

21. A motor vehicle including a gas sensor as claimed in any of claims 1-16 and/or a selective oxygen filter as claimed in any of claims 17-20.