DE102007049716A1 - Sensor e.g. pump sensor, unit for determining physical characteristic i.e. oxygen concentration, of gas i.e. exhaust gas, in measuring gas chamber, has gas-tightly closed cavity provided such that electrode is arranged in cavity - Google Patents

Abstract

The sensor unit (110) has an electrode (114) arranged in a cathode cavity (116), another electrode (122) and a solid electrolyte (142), which is connected to the electrodes. Another solid electrolyte (146) is connected with a third electrode (148) and a fourth electrode (150). A gas-tightly closed cavity (138) is provided such that the third electrode is arranged in the cavity. The third electrode and the fourth electrode are electrically connected with each other by an electric conductor. An independent claim is also included for a method for determining a physical characteristic of a gas in measuring gas chamber.

Description

State of the art

The The invention is based on known sensor elements, which are based on electrolytic Properties of certain solids based, so the ability this solid to conduct certain ions. such Sensor elements are used in particular in motor vehicles, to measure air-fuel gas mixture compositions in which Case these sensor elements also called "lambda probe" are known and have an essential role in the reduction of pollutants in exhaust gases, both in gasoline engines and in diesel technology play.

With the so-called air ratio "lambda" (λ) is thereby generally in combustion engineering the relationship between an actually offered air mass and one for the combustion theoretically required (i.e., stoichiometric) Air mass designated. The air ratio is thereby by means of one or more Sensor elements usually at one or more locations in the exhaust system an internal combustion engine measured. Accordingly, "fat" Gas mixtures (ie gas mixtures with a fuel surplus) an air ratio λ <1 whereas "lean" gas mixtures (i.e., gas mixtures with a fuel deficiency) have an air ratio λ> 1. Next Automotive technology will be such and similar Sensor elements in other areas of technology (in particular combustion technology), for example in aviation technology or in the control of burners, z. B. in heating systems or power plants.

Such sensor elements are now known in numerous different embodiments. One embodiment is the so-called "jump probe" whose measuring principle is based on the measurement of an electrochemical potential difference between a reference electrode exposed to a reference gas and a measuring electrode exposed to the gas mixture to be measured Properties usually doped zirconia (eg yttrium-stabilized ZrO ₂ ) or similar ceramics used as solid electrolyte.Theoretically, the potential difference between the electrodes just at the transition between rich gas mixture and lean gas mixture on a characteristic jump, which can be used in order to actively regulate the gas mixture composition around the jump point λ = 1. Various embodiments of such jump probes, which are also referred to as "Nernst cells", are described, for example, in US Pat DE 10 2004 035 826 A1 . DE 199 38 416 A1 and DE 10 2005 027 225 A1 described.

alternative or in addition to jump probes also so-called "pump cells" used in which an electrical "pumping voltage" to two electrodes connected over the solid electrolyte is applied, with the "pumping current" through the pumping cell is measured. In contrast to the principle of the jump probes For pump cells usually both electrodes with the measured Gas mixture in conjunction. One of the two electrodes (usually over a permeable protective layer) immediately to be measured Exposed to gas mixture. However, the second of the two electrodes is designed such that the gas mixture is not directly to this Electrode can pass, but first penetrate a so-called "diffusion barrier" in order to penetrate into a cavity adjacent to this second electrode to get. As a diffusion barrier is usually a porous ceramic structure with specifically adjustable pore radii used. If lean exhaust gas passes through this diffusion barrier into the Cavity, so by means of the pumping voltage to oxygen molecules the second negative electrode electrochemically to oxygen ions reduced, through the solid electrolyte to the first, positive electrode transported and released there as free oxygen again. The Sensor elements are usually in the so-called limit current operation operated, that is in an enterprise, in which the Pumping voltage is chosen such that the through the diffusion barrier incoming oxygen is completely pumped to the counter electrode becomes. In this mode, the pumping current is approximately proportional to the partial pressure of the oxygen in the exhaust gas mixture, see above that such sensor elements often as proportional sensors be designated. In contrast to jump sensors can be such Proportionalsensoren as so-called broadband sensors on a comparatively wide range for the air ratio lambda deploy.

Such broadband probes are for example in DE 38 09 154 C1 and in DE 199 38 416 A1 described.

In many sensor elements, the sensor principles described above are also combined, so that the sensor elements contain one or more sensors ("cells") operating according to the jump sensor principle and one or more proportional sensors.For example, the principle described above for a pump cell To expand the principle of working "single cell" by adding a snap cell (Nernst cell) to a "double cell." Such a structure is, for example, in EP 0 678 740 B1 described. In this case, by means of a Nernst cell, the oxygen partial pressure in the above-described, to the second Measured electrode adjacent cavity and tracked the pumping voltage by a control so that in the cavity always the condition λ = 1 prevails.

Broadband sensor elements in a single-cell arrangement with two electrodes exposed to the gas mixture have various problems in practice. Thus, a positive pumping current (lean pumping current) with a clear relationship to the oxygen content of the gas mixture is usually measured at a fixed pumping voltage in a lean gas mixture. In the rich gas mixture, however, a positive pumping current is usually also measured, even if the applied pumping voltage (usually about 400-700 mV, for example 500 mV) is well below the decomposition voltage of water (about 1.23 V). This positive pumping current is essentially due to the molecular hydrogen contained in the gas mixture, which influences the electrochemical potential of the anode, ie the first electrode, since water can now be formed on the first electrode from the oxygen ions leaving the solid electrolyte instead of molecular oxygen. The energy released at the anode during H ₂ O formation thus compensates for the energy required for H ₂ O decomposition at the cathode, which is why the pumping voltage is generally below 1.23 V. Similar effects also play a role for other oxygen-supplying redox systems present in the gas mixture, for example CO ₂ / CO. The current is thus in the range of rich mixtures (fat pump current) by the hydrogen content in the region of the first electrode (eg anode) and the water vapor content (ie in particular the access of the water vapor through the above-described diffusion barrier) in the region of the second electrode (z. B. cathode) limited. The problem now consists, in particular, in that the fat pumping current and the lean pumping current have the same direction electrically, so that a conclusion on the composition of the gas mixture is scarcely possible from the pumping current. In addition to the problems described in the field of rich mixtures, a falsification of the pumping current by the hydrogen is also to be observed in the area of slightly lean exhaust gases, which is already present in this area and provides a positive contribution to the pumping current.

A further problem of known sensor elements in single cell or Mehrzelleranordnung is that the electrodes used in practical use strong pollution due to contamination, liquid and / or gaseous species exposed are. In particular, this problem is noticeable in electrodes, which are used as reference electrodes to a potential difference to eat. Here the pollution leads to a change of the electrode potential and thus influences the measured Nernst voltages. In pump cells, the contamination of the second pump electrode due to an additional potential drop to an increased cause effective pumping voltage. In particular, it can be at this contamination to contamination of a reference through Moisture and / or organic contaminants, such as Fuel vapors act. This problem is common too known as "CSD" (Continuous Shift Down) may particularly affect newer planar probe generations, in which particularly short probe elements are used, ie For example, probe elements where contaminants have shorter paths have to travel back to the reference electrode than in previous Probe elements.

to There are a variety of solutions to this problem of contamination Ways. In particular, the relevant electrode (in particular the Reference electrode) can be effectively sealed to prevent contamination Keep away from the electrode. So could a sealed Reference air space can be generated around the relevant electrode, the shielded against contamination. Disadvantageous This idea, however, is that in this case the reference airspace not or insufficiently supplied with fresh oxygen. In addition, a shielding of the reference air space is technically complicated and expensive, since this costly sealing materials higher Temperature resistance would have to be used.

Disclosure of the invention

It Therefore, a sensor element is proposed which for the determination at least one physical property of a gas in at least a measuring gas space, in particular for determining an oxygen concentration in the exhaust of an internal combustion engine, can be used and which has the disadvantages the above-described, known from the prior art sensor elements at least largely avoids. In particular, it is with the following proposed possible on unwanted Electrode reactions attributable to non-objectivity problematic to avoid the characteristic of sensor elements in broadband arrangement. Also, the described CSD problem, especially in unicellulars or multicellular with reference electrodes, can by means of the proposed Sensor element concept are largely avoided. The concept is thus applicable to both pump and broadband sensors as also on sensor elements in the jump cell concept.

Proposed is a sensor element for determining at least one physical property of a gas in at least one measurement gas space, in particular a sensor element according to the above description for determining an oxygen concentration in the exhaust gas of an internal combustion engine. However, other applications are conceivable. The sensor element comprises at least four electrodes: a first electrode, a second electrode, at least one first solid electrolyte connecting the first electrode and the second electrode, a third electrode, a fourth electrode, and at least one second solid electrolyte connecting the third electrode and the fourth electrode. In this respect, the proposed arrangement can be regarded as a sensor element with a first electrochemical measuring cell (first electrode, second electrode, first solid electrolyte) and a second electrochemical measuring cell (third electrode, fourth electrode and second solid electrolyte).

Under a solid electrolyte is, as already stated above, a To understand material which has ion-conducting properties, preferably solid electrolyte with oxygen ion-conducting Properties such as yttrium-stabilized zirconia (YSZ) are preferred. However, other materials are used. The electron-conducting properties of such solid electrolytes are usually negligible and can for example, four orders of magnitude below the Ion-conducting properties are, for example, at least two or even three orders of magnitude in the usual Working temperatures of over 600 ° C.

The Sensor element is preferably designed such that at least one of the electrodes of the first electrochemical measuring cell (ie the first electrode and / or the second electrode) with gas from the sample gas space can be acted upon. For this purpose, the first electrode and / or the second electrode, for example, with the sample gas chamber directly, via a channel or a narrowed opening, over a diffusion barrier, via a protective layer or the like Way communicate with the sample gas space, so that at least to a limited extent a gas exchange between a surrounding area the first electrode and / or the second electrode and the sample gas space is possible.

The fourth electrode is similarly at least a gas space in connection. This connection can turn directly (ie via a channel, an opening, by direct arrangement of the fourth electrode in the gas space, via a Protective layer, a diffusion barrier or similar Done way, which is a gas exchange between an environment the fourth electrode and the gas space permits. The gas space may be a space separate from the sample gas space, for example an environmental space of the sensor element (for example, an engine room a motor vehicle) or at least partially with the measuring gas space be identical. In that sense, the term "gas space" is far to grasp, since the gas space, for example, in a pumping cell arrangement only can be used as a room in which from the solid electrolyte escaping gases are discharged without another Function, for example, a reference function in the electrochemical Meaning, would be perceived. Various embodiments, which implement one of the two concepts will become more detailed below explained.

One The basic idea of the present invention is at least one of the electrodes which is for one or both of the above described problems (CSD and / or unwanted electrode reactions) is responsible, in a gas-tight cavity of the Arrange sensor element. Under "gas-tight Cavity "is preferred in the context of the present invention to understand a cavity which completely gas-tight is completed. The term "gas-tight" is however far to grasp, so that also a slight influx of gas may be possible in the gas-tight cavity, however, this inflow should be low (for example, around at least one or two orders of magnitude smaller) against a gas flow, which in normal operation of Sensor element (i.e., at common voltages of some 100 mV to a maximum of 1 V and operating temperatures of, for example, over 600 ° C) enter the cavity through one or more of the solid electrolytes can or can get out of this cavity. This ensures that in the cavity disposed electrode or in the cavity arranged electrodes both with respect to the sample gas space as well as reliably shielded from the gas space are.

Such gas-tight closed cavities are already known from the prior art in another context. So shows, for example DE 102 19 881 A1 such a gas-tight closed cavity as a sample gas storage. By means of a pair of pump electrodes, measurement gas can be introduced via an ion conductor, which can be used in various measurement cycles for determining the gas composition in the measurement gas space.

In contrast to the known arrangements, however, the invention proposes to connect the gas-tight cavity with the gas space characterized in that the above-described second electrochemical measuring cell, which arranged in the gas-tight arranged third electrode, the second solid electrolyte and with the gas space in combination standing fourth electrode, designed as a short-circuited Nernst cell or as a short-circuited fuel cell. For this purpose, according to the invention, at least one electrical conductor is provided which electrically connects the third electrode and the fourth electrode to one another.

Consequently can via the second solid electrolyte, which, for example may be configured as a membrane, although gas (oxygen) from the gas-tight sealed cavity discharged into the gas space or from the gas space the gas-tight sealed cavity (depending on the concentration gradient, see below), but usually with a thermodynamic equilibrium between the cavity and the gas space can not adjust. On This way may vary depending on the mode of operation of the first electrochemical Measuring cell (see the various, listed below Possibilities) a slightly increased gas partial pressure (For example, oxygen partial pressure) or a slightly reduced gas partial pressure set in the cavity relative to the gas space. In contrast to mixed conducting materials (so-called Mixed Ionic-Elektronic Conductors, MIEC materials) become the ion conduction and electronic lead taken over by separate elements according to the invention, namely the second solid electrolyte or the electric Ladder.

On this way, all in the gas-tight sealed cavity arranged electrodes efficiently shielded. impurities such as water or electrode poisons, can not penetrate to the electrode and there the above-described CSD problem cause. This is particularly advantageous if a in the gas-tight cavity arranged as the electrode Reference electrode is used. Will one in the gas-tight closed Cavity electrode as expansion electrode of a pump cell used, the shielding is positively noticeable, that no unwanted decomposition effects on this shielded Electrode can occur, which in turn for a Production of a unique characteristic can be used.

Of the at least one electrical conductor should in particular be designed in this way be that it has an electrical resistance, which the electrical resistance of the second solid electrolyte considerably falls below, for example by at least one, preferably two, three or more orders of magnitude. Especially For example, the electrical conductor may be an electrical resistance from about 0.5 ohms to 5 ohms. Thus, this concludes electrical conductors, the third electrode and the fourth electrode preferably almost completely short. Alternatively, however, are In principle, other embodiments possible, For example, embodiments with resistances between a kiloohm and 100 kilohms. Preferably, the electrical Ladder be configured in the form of a conductor, which, for example can be arranged in and / or on the sensor element, and which preferably has a different material than the solid electrolyte. This at least one conductor track may preferably be around the second Solid electrolyte led around and / or through the second solid electrolyte be passed through, so spatially from this to be different. Another, alternative or additional realizable possibility for the electric The conductor is a solid electrolyte material of the second solid electrolyte (Ion conductor) with a metal material (electronic conductor) to mix to a ceramic-metal composite material. For example, yttrium-stabilized Zirconia be mixed with egg nem platinum material, for example in the form of a cermet. This can be done, for example, by that an electrode paste containing YSZ and platinum, is used.

As described above, several possibilities are conceivable, the gastight shielded cavity for shielding one or more electrodes use. Some of these possibilities can also be combined. So there is a possibility in it, even the least to arrange a second electrode in the cavity. This can be, for example be used in a jump cell arrangement in which the first electrochemical measuring cell is used as a jump cell. The first electrode is then preferably the gas in the sample gas space exposed (for example via a gas-permeable Protective layer), and the second electrode serves as a reference electrode, which is arranged in the cavity and through the second electrochemical Measuring cell (short-circuited Nernst cell or short-circuited fuel cell) is shielded.

In an arrangement in which the first electrochemical measuring cell is operated as a pumping cell, a structure in which the first electrode or the second electrode is arranged in the cavity is also possible. Either the installation electrode (eg the cathode), on which the measurement gas or the gas component to be measured is installed in the first solid electrolyte, can be arranged in the cavity, or the expansion electrode, to which the measurement gas or the detected gas component is removed from the first solid electrolyte, be arranged in the cavity. In the first case, the second electrochemical measuring cell is used to nachzuliefern measuring gas into the cavity, which is usually a slight partial negative pressure of the gas to be detected setting component in the cavity. In the second case, the second electrochemical measuring cell serves to dissipate excess measuring gas or the gas component to be detected out of the cavity, as a rule a slight partial overpressure of this gas component in the cavity. In the case in which the removal electrode is arranged in the cavity, the installation electrode is preferably separated from the measurement gas space via a diffusion barrier that delimits the current (see also below). In both cases described, the shielding of one of the two electrodes of the first electrochemical measuring cell serves in particular for the purpose of avoiding unwanted electrode reactions which could cause the characteristic to be unambiguous. If the installation electrode (eg the cathode) is arranged in the gas-tight cavity, it is for example at least largely prevented that unwanted decomposition reactions can take place at this electrode, for example the decomposition of water. If the removal electrode (for example the anode) is arranged in the cavity, it is at least largely prevented that fuel gases such as, for example, hydrogen or hydrocarbons can get there and can carry out undesired electrode reactions there. In both cases, therefore, the uniqueness of the pumping current characteristic is promoted. Which of the at least two electrodes of the first electrochemical measuring cell functions as a "mounting electrode" and which as an "expansion electrode" depends on the wiring of this first electrochemical measuring cell, as well as on the type of the first solid electrolyte and the type of gas to be detected. In the case of oxygen sensors, for example, the cathode, as the electrode connected to negative potential, acts as a mounting electrode, whereas the anode acts as an expansion electrode.

A Another possibility is in the gas-tight shielded Cavity to arrange at least one reference electrode, which is not identical is with the first electrode and the second electrode. This can be realized for example by a 5-electrode sensor element. This structure may, for example, in a multi-cell arrangement be used with a pumping cell and a Nernst cell, in which the reference electrode acts as a reference electrode of the Nernst cell. The Pump cell is in this case preferably by the first electrochemical Measuring cell formed. The Nernst cell, for example, by the first electrode and the reference electrode are formed, wherein the first electrode in turn preferably via a diffusion barrier is separated from the sample gas space. In this case, the Nernst cell measures for example, the electrochemical potential difference between the cavity and an electrode cavity between the first electrode and the diffusion barrier.

For the design of the gas-tight shielded cavity exist various possibilities. So this example be configured such that the first solid electrolyte and the second Solid electrolyte are at least partially identical to the component, wherein this example, by a common solid electrolyte layer can be realized. Also an embodiment in shape However, separate solid electrolyte membranes is conceivable.

Farther the at least one shielded cavity can be both inside be arranged of the sensor element as well as on a surface of the sensor element. The cavity can then be inside the layer structure be arranged, for example, between several solid electrolyte layers and / or other layer components of the layer structure, and can thus from the measuring gas space through at least one layer of the layer structure be separated. Alternatively or additionally, the cavity also arranged on a surface of the layer structure be, which assigns the sample gas space. The cavity can then open be arranged on this surface and of the sample gas space for example, separated by a gas-impermeable layer which, for example, only by the short-circuited Nernstzelle the second electrochemical measuring cell are interrupted can. However, other embodiments of the cavity are possible, such as making a cavity between two laminated solid electrolyte films. The cavity should be particular be carried out with the least possible volume, to ensure fast dynamics of the sensor element.

Brief description of the drawings

embodiments The invention is illustrated in the drawings and in the following Description explained in more detail. Show it

1 a prior art construction of a operated as a pump cell sensor element;

2 a pumping current characteristic of the in 1 illustrated sensor element;

3 a modification of a sensor operated as a pump cell sensor element according to the invention;

4 and 5 a comparison of the transport processes in mixed ladders ( 4 ) and a short-circuited Nernst cell according to the invention ( 5 ) for use in a sensor element according to the invention;

6 a pumping current characteristic of a sensor element according to the invention;

7 one too 3 alternative embodiment of a sensor element according to the invention with a arranged on the sensor surface cavity;

8th a further alternative embodiment of a sensor element according to the invention with internal electrodes;

9 a further embodiment of a sensor element according to the invention with external electrodes;

10 one too 9 alternative embodiment with a shorted Nernst cell in the form of a composite material;

11 an exemplary embodiment of a sensor element which can be operated as a jump cell according to the invention; and

12 an embodiment of a double-celled wideband cell operable according to the invention sensor element.

In 1 is a prior art sensor element 110 represented, which can be used in the construction shown as a broadband sensor element and, for example, in the limiting current operation, an oxygen concentration in a sample gas space 112 can measure. Such sensor elements 110 are often referred to as proportional sensors or LSPs (LSP: lambda probe proportional).

The sensor element 110 has a first electrode 114 on which inside the sensor element 110 in a cathode cavity 116 is arranged. The term "cathode cavity" is chosen here and below independently of how the first electrode 114 is electrically connected, so that in general could also be spoken of an "electrode cavity." Here is the cathode cavity 116 in this embodiment, annular around a gas inlet hole 118 arranged, through which gas from the sample gas space 112 in the cathode cavity 116 can get. The first electrode 116 is formed in several parts, each with the top and bottom of the cathode cavity 116 arranged, electrically interconnected sub-electrodes. The cathode cavity 116 is from the gas entry hole 118 through a diffusion barrier 120 shielded, which limits the gas access. This diffusion barrier 120 For example, it may be made of a porous, dense material, such as an alumina material.

On top of the sensor element 110 , so the the measuring gas space 112 facing side, is a second electrode 122 arranged. This second electrode 122 is also in radial arrangement around the gas inlet hole 118 designed. Between the first electrode 114 and the second electrode 122 there is a solid electrolyte 124 For example, a layer of yttria-stabilized zirconia, YSZ. This solid electrolyte 124 has oxygen ion-conducting properties, whereas the electron-conductive properties of this solid electrolyte material are negligible.

The first electrode 114 , the solid electrolyte 124 and the second electrode 122 together form an electrochemical measuring cell 126 , The second electrode 122 is from the sample gas chamber 112 through a protective layer 128 separated, which is gas permeable and an expansion of oxygen at the second electrode 122 allows, but which the second electrode 122 protects against contamination. For example, this protective layer 128 have a highly porous ceramic material, such as an Al ₂ O ₃ material.

The entire sensor element 110 according to 1 is produced in a layer structure and may preferably comprise a plurality of further layers, for example layers of insulating materials and / or further solid electrolyte layers. In addition, the sensor element has 110 a heating element 130 on, by means of which an optimum operating temperature can be adjusted. For example, this operating temperature can be adjusted to 500 to 800 ° to optimum oxygen ion-conducting properties of the solid electrolyte 124 adjust and / or the diffusion properties of the diffusion barrier 120 to set to a desired value.

In the in 1 shown typical broadband probe assembly is between the first electrode 114 and the second electrode 122 put a pumping voltage U _p . This pumping voltage U _p can be constant in time, but can also be designed variable in time. This is the first electrode 114 operated as a cathode and placed at ground potential, whereas the second electrode 122 is operated as an anode. The pumping current I _p , which between the first electrode 114 and second electrode 122 flows is measured. This current is due to the fact that at the cathode in the cathode cavity 116 existing oxygen in the form of oxygen ions in the solid electrolyte 124 is installed, to the anode 122 migrates and is released there again. Both electrodes 114 . 122 are thus the gas of the measuring room 112 exposed. Typically, the sensor element becomes 110 operated such that the pumping current I _p through the diffusion barrier 120 is limited, which is referred to as a limiting current operation. Typical pump voltages are in the range of 500 mV.

In 2 is a typical pumping current characteristic of a sensor element 110 according to the prior art, for example according to the arrangement in 1 represented. In this case, the pumping current I _{p is} plotted against the air ratio λ. Alternatively, the pumping current could also against the partial pressure of oxygen in the sample gas space 112 be applied.

The characteristic in 2 illustrates the non-uniqueness phenomenon already described at the beginning in the slightly lean and rich region. The lean region (λ> 1) is in 2 with the reference number 132 whereas the rich region (λ <1) in 2 with the reference number 134 is designated.

As in the solid curve in the lean area 132 can be seen, occurs in this area a positive pumping current, which has a clear relationship to the oxygen content of the exhaust gas. In the rich exhaust gas (area 134 However, it comes due to the decomposition of the water contained in the exhaust gas at the cathode also to a positive pumping current. Although the applied pumping voltage U _p is usually well below the decomposition voltage of the water, but since hydrogen is present in the exhaust gas, the water decomposition is energetically possible, because water is generated at the opposite electrode. The current is therefore in the rich range 134 limited by the hydrogen content (fat gases). Since this fat pumping current has the same direction as the lean pumping current, a conclusion of the pumping current I _p on the exhaust gas composition is generally no longer clearly possible due to the ambiguity of the characteristic curve. In addition, deviations from the linearity occur in the region near λ = 1, which further complicates assignment in the slightly lean region.

In 3 on the other hand is a sensor element according to the invention 110 represented, which the in 2 illustrated problematic at least largely avoids. A basic idea of this structure is the first electrode 114 in a gas-tight cavity 138 to arrange. Otherwise, the structure of the electrochemical measuring cell corresponds 126 again largely according to the structure 1 , wherein this electrochemical measuring cell 126 but now a first electrochemical measuring cell 140 forms. The solid electrolyte 124 between the first electrode 114 and the second electrode 122 forms in this case a first solid electrolyte 142 , The second electrode 122 can directly the gas in the sample gas space 112 be exposed or may be similar to 1 , again a protective layer 128 have (in 3 Not shown).

In contrast to the structure according to 1 is the cathode cavity 116 , which in this example is identical to the gas-tight cavity 138 but not via a gas access hole 118 and a diffusion barrier 120 with the sample gas chamber 112 connected but completed by this.

In addition to the first electrochemical measuring cell 140 has the structure of the sensor element 110 according to 3 but still a second electrochemical measuring cell 144 on. This second electrochemical measuring cell 144 includes one on the solid electrolyte 124 , which now for the second electrochemical measuring cell 144 as a second solid electrolyte 146 acts, arranged third electrode 148 , which also the closed cavity 138 assigns, as well as a fourth electrode 150 which is on top of the second solid electrolyte 146 is arranged and thus the sample gas space 112 assigns. The fourth electrode 150 can from the sample gas space 112 additionally by a diffusion barrier 152 be separated, which may for example have similar properties as the diffusion barrier 120 in 1 ,

Furthermore, the third electrode 148 and the fourth electrode 150 through an electrical conductor 154 electrically connected to each other, said electrical conductor 154 can have such a low resistance that the two electrodes 148 . 150 are shorted. The second electrochemical measuring cell 144 thus forms a short-circuited Nernst cell or fuel cell. These terms are used synonymously here and below.

The wiring of the sensor element 110 For example, in turn similar to the structure in 1 respectively. In this case, in turn, the second electrode acts 122 as the anode of the sensor element 110 and lies in the gas of the sample gas space 112 (For example, an exhaust gas of an internal combustion engine) or is optionally of this by a protective layer 128 separated. The first electrode 114 is operated as a cathode and lies in the gas-tight cavity 138 , It is again applied a pumping voltage U _p of preferably about 500 mV, with which oxygen from the gas-tight cavity 138 almost completely pumped out (evacuation). The pumping voltage U _p may in turn be constant over time, but may again be configured variable in time. In addition, located in the gas-tight cavity 138 the third electrode 148 , which as the anode of the short-circuited fuel cell 144 serves. As the cathode of this short-circuited fuel cell 144 the fourth electrode acts 150 passing through the optional diffusion barrier 152 from the sample gas chamber 112 is disconnected.

As soon as oxygen from the exhaust gas to the cathode of the short-circuited fuel cell (fourth electrode 150 ), arises due to the partial pressure difference to the gas-tight cavity 138 (in which, for example, λ <1) according to the Nernst equation, an electric voltage between the fuel cell cathode (fourth electrode 150 ) and the fuel cell anode (third electrode 148 ). Because the fuel cell 144 shorted, the oxygen from the cathode (here the fourth electrode 150 ) to the anode (here the third electrode 148 ) in the closed cavity 138 pumped. There, the oxygen is directly at the cathode (here the first electrode 114 ) of the first electrochemical measuring cell 140 again reduced and to the anode of this first electrochemical measuring cell (here the second electrode 122 ), that is to the sample gas space 112 pumped out. The current through the first electrochemical measuring cell 140 is proportional to the oxygen concentration in the exhaust gas. The proportionality constant is determined by the diffusion barrier 152 above the cathode (fourth electrode 150 ) of the short-circuited fuel cell 144 certainly. Because the short-circuited fuel cell 144 preferably can operate in limit current operation, determined only the diffusion barrier 152 the transport, ie it usually does not differ from the in 1 dimensioned structure can be dimensioned. In order to keep the diffusion current sufficiently low, the diffusion barrier should 152 either very tight or partially covered by an insulating layer or the like. In addition, the electrodes, in particular the cathode of the first electrochemical measuring cell 140 (first electrode 114 ) be sufficiently large to the pumping ability of the first electrochemical measuring cell 140 to ensure even at high currents. Furthermore, the cavity needs 138 be performed with the least possible volume to ensure a fast dynamics. For example, the dimensioning of the electrodes, in particular the first electrode 114 , and the sizing of the cavity 138 , analogous to the dimensioning of the pumping electrodes 114 . 122 or the cathode cavity 116 in 1 be designed.

If fat gas (that is reducible gases, such as H ₂ or hydrocarbons) from the exhaust gas of the sample gas space 112 to the cathode (fourth electrode 150 ) of the short-circuited fuel cell 144 passes, the electrical voltage between the fuel cell cathode drops 150 and fuel cell anode (third electrode 148 ) directly together, as both at the cathode 150 as well as at the anode 148 there is a vanishing oxygen concentration. This brings the subsequent delivery of oxygen in the gas-tight cavity 138 to stop, and the pumping current I _{p of} the first electrochemical measuring cell 140 falls almost completely to zero. The sensor element 110 thus has a unique pump current characteristic I _p (λ), which in 6 (in comparison to 2 ) is shown schematically. The sensor element 110 can thus also be used as a sensor element due to its unique characteristic 110 be used for lean systems. The sensor element 110 according to 3 Thus, in particular compared to proportional counters with air reference the advantage that the characteristic in the rich gas range goes back to zero completely and no residual currents remain. In addition, eliminates the pressure of a reference gas channel. The pumping voltage U _p can be chosen lower, which offers advantages in terms of a reduced cross-sensitivity to water in the exhaust gas. Furthermore, the cathode undergoes 114 the first electrochemical measuring cell 140 usually no gas exchange, that is, no double layer transfer, which is beneficial to the dynamics of the sensor element 110 can affect.

In the 4 and 5 should the functioning of the short-circuited fuel cell 144 in 3 as well as other proposed sensor elements 110 with closed cavity 138 modeled with their microscopic processes are explained. Here is the short-circuited fuel cell 144 in 5 is shown and is another working principle, namely the in 4 illustrated functional principle of a mixed conductor (Mixed Ionic-Electronic Conductor MIEC) 156 compared.

Analogous to 3 is in both figures ( 4 and 5 ) assumed that the closed cavity 138 below the element to be explained (MIEC material 156 in 4 or second electrochemical measuring cell 144 in 5 ), whereas the sample gas space 112 located above. It is assumed that in the sample gas chamber 112 the oxygen partial pressure p _O2 * sets, whereas in the closed cavity 138 the oxygen partial pressure p _O2 ** sets, which is smaller than the oxygen partial pressure p _O2 *.

A MIEC material 156 intrinsically has both ion-conducting, in particular oxygen-ion-conducting properties as well as electron-conducting properties, which typically move in the same order of magnitude or at most differ by about one order of magnitude. MIEC materials are known from the field of lambda probes already from other applications, such as DE 43 43 748 . US 5,543,025 or US 2004/0183055 A1 , For example, doped oxide ceramics, such as perovskite-based and / or fluorite-based oxide ceramics, zirconia-based or ceria-based oxide ceramics, or similar ceramics may be used as MIEC materials. In doing so, as shown by the reaction equations in 4 is shown on Sei th of the sample gas space 112 (Oxygen partial pressure high) oxygen molecules dissociated and into oxidation vacancies of the lattice of the MIEC material 156 (in 4 symbolically symbolized with V •• / O). Oxidions on regular lattice sites of the oxide ion sublattice are in 4 symbolically designated OX / O. The illustrated nomenclature is also referred to as Kröger-Vink nomenclature. For this reaction two electrons are needed.

The reverse reaction involving oxygen molecules from the MIEC material 156 removed and attached to the shielded cavity 138 is delivered in 4 below the MIEC material 156 symbolically represented. For both reactions to occur, there is an opposite transport of electrons (e ^- ) and oxygen ions (O ^2- ) by the MIEC material 156 required.

In contrast, in the case of the short-circuited fuel cell or Nernst cell 144 in 5 the electron-conducting functions and the oxygen-ion-conducting functions are separated and are perceived by different materials. The former function (electron conduction) is provided by the electrical conductor 154 adopted, the latter function (ionic conduction, especially oxygen ion conduction) from the solid electrolyte 124 . 146 , The during the installation of the oxygen on the side of the sample gas chamber 112 (high oxygen partial pressure) expiring installation reaction or on the side of the closed cavity 138 (Low oxygen partial pressure) running expansion reaction proceeds largely identical to 4 , In contrast, however, the same solid electrolyte material can be used, which, for example, also for the first electrochemical measuring cell 140 in 3 is used. For example, again yttrium-stabilized zirconia can be used. This causes, as in 3 shown, the first solid electrolyte 142 and the second solid electrolyte 146 also by one and the same solid electrolyte layer 124 can be realized. Thus, no introduction of new materials in the layer structure is required, which would require a comprehensive new material test and which the material and manufacturing costs of the sensor element 110 would increase. There are only known materials, such as YSZ and metals such as platinum or another metal used, which in any case in the sensor element 110 and therefore no further exploration or development is required.

Core of the short-circuited fuel cell 144 or short-circuited Nernst cell is that over the solid electrolyte 146 , which acts as a kind of membrane, although an oxygen exchange between the sample gas space 112 and the closed cavity 138 can be done, but usually a thermodynamic equilibrium between these spaces 112 . 138 can not adjust. In this way, in the cavity 138 also a slightly increased or decreased oxygen partial pressure compared to the environment 112 getting produced. Furthermore, impurities such as water or electrode poisons can not adhere to those in the cavity 138 arranged electrodes 148 . 114 penetrate. In this way, both the non-uniqueness problem (see above) and a poisoning or CSD problem (see especially the examples in the 11 and 12 ) is effectively reduced.

In the 7 to 10 are further embodiments of sensor elements 110 shown in pump cell operation, which continue to exploit the inventive principle described above. In particular, these illustrations also show that it may be advantageous to use the first electrode 114 or the second electrode 122 the first electrochemical measuring cell 140 in one or more gas-tight cavities 138 to arrange.

In 7 is one too 3 illustrated alternative embodiment, in which all the electrodes are arranged on a surface of the layer structure, which the measuring gas space 112 assigns. This arrangement may be the structure of the sensor element 110 simplify.

This construction shows that the electrodes of the electrochemical measuring cells 140 . 144 not necessarily on opposite sides of the solid electrolyte 124 . 142 . 146 must be opposite, but that they can also be arranged in parallel in a plane with, for example underlying solid electrolyte.

The gastight cavity 138 in the embodiment of the sensor element 110 according to 7 is realized, for example, that on the surface of the solid electrolyte 124 a capsule 158 is placed, which this cavity 138 forms, such that the first electrode 114 (Cathode of the first electrochemical measuring cell 140 ) and the third electrode 148 (Anode of the second electrochemical measuring cell 144 or short-circuited fuel cell) side by side in the cavity 138 are included.

Otherwise, the wiring of the sensor element 110 according to 7 for example, analogous to the wiring in 3 respectively. Again, a heating element can also be used 130 be provided with two leads 160 , of which one supply line is a mass line 162 is. In this as well as in other embodiments, for example, the first electrode 114 (Cathode of the first electrochemical cell 140 ) also with the ground line 162 of the heating element 130 be connected, but other configurations are possible.

In the previous embodiments of the invention in the 3 and 7 was the gas-tight shielded cavity 138 each arranged such that the first electrode 114 (Cathode of the first electrochemical measuring cell 140 ) in this closed cavity 138 is included. That this is not absolutely necessary, but that embodiments are possible in which the second electrode 122 (Anode of the first electrochemical measuring cell 140 or removal electrode) in this cavity 138 is arranged, the following examples show in the 8th . 9 and 10 ,

It shows 8th an embodiment in which all the electrodes 114 . 122 . 148 . 150 inside the sensor element 110 are arranged. Again, there is a cathode cavity 116 above the first electrode 114 provided, which via a gas inlet hole 118 and a diffusion barrier 120 with the sample gas chamber 122 is in communication and can be acted upon from this with, for example, exhaust gas. The second electrode 122 the first electrochemical measuring cell 140 is in a closed cavity 138 arranged and thus effectively protected, for example against contamination and / or against fatty gas reactions.

Again, in this gas-tight sealed cavity 138 a third electrode 148 which is part of a short-circuited second electrochemical measuring cell 144 forms. This second electrochemical measuring cell, which in turn has a second solid electrolyte 144 and one outside the cavity 138 arranged fourth electrode 150 includes, is in 8th only symbolically represented. In this case, in the illustration shown, the two electrodes 148 . 150 and the electrical conductor 154 integrally formed. Alternatively, however, these electrodes could also again, for example analogously to the representation in 7 , as planar electrodes, with a conductor track arranged therebetween and / or via a mixed conductive material (MIEC material) can be realized connected. Also other embodiments (see for example below 10 ) are conceivable.

In contrast to the preceding embodiments is in 8th an embodiment shown in which the fourth electrode 150 the second electrochemical measuring cell 144 not in contact with the sample gas chamber 112 but one from this sample gas space 112 separated environment space 164 , For example, this environmental space may be 164 to act an engine compartment of a motor vehicle, which a substantial gas exchange with the exhaust system (measuring gas space 112 ) having. This example shows that, in generalization of the idea described above, the fourth electrode 150 the second electrochemical measuring cell 144 is generally associated with a gas space, which is, for example, the sample gas space 112 itself or around the surrounding space 164 , which from the measuring gas chamber 112 is separated, can act. The fourth electrode 150 and the surrounding space 164 can, for example, via a reference channel 166 in connection, which, as all the aforementioned cavities also, for example, may be completely or partially filled with a highly porous material. Such structures with reference channels 166 are already known from broadband probes with air reference, but now no direct coupling of the expansion electrode 122 to this air reference, but a coupling via the short-circuited Nernst cell 144 , In this way, for example, prevents harmful gases or electrode poisons from the surrounding space 164 to the second electrode 122 reach and influence them. Also, the penetration of grease gases from the ambient space 164 , For example, in the form of oil vapors or the like, is prevented, so that in turn undesirable electrode reactions at the second electrode 122 be suppressed.

In 9 is again an embodiment of a sensor element 110 shown, which in principle the embodiment in 8th corresponds, so that in turn the second electrode 122 in a shielded cavity 138 is arranged, whereas the first electrode 114 via a diffusion barrier 120 with the sample gas chamber 112 communicates. The shielded cavity 138 in turn is via a second electrochemical measuring cell 144 with a gas space in communication, wherein in the in 9 shown embodiment of this gas space in turn, analogous to the 3 and 7 , the measuring gas chamber 112 itself is. All electrodes 114 . 122 . 148 and 150 are on the measuring gas chamber 112 facing surface of the layer structure of the sensor element 110 arranged, for example similar to the structure in 7 However, as mentioned, unlike the 7 the second electrode 122 instead of the first electrode 114 in the shielded cavity 138 is arranged. The realization of the third electrode 148 , the electrical conductor 154 and the fourth electrode 150 For example, can be done by integrally forming these components, for example by a jointly printed platinum element. The sealing of the cavity 138 opposite the sample gas chamber 112 can be done, for example, that, for example, centrally printed on this together Platinum element, in the production of a sealing bar 168 is printed, for example, a sealing ridge of a solid electrolyte material or other sealing material. The cathode cavity 116 and the closed cavity 138 are by a common cover layer 170 covered and sealed at the top. Furthermore, the sensor element 110 still, analogous to the preceding figures, a heating element 130 which may, for example, comprise a heater foil with one or more insulations and heating elements, and which in 9 is shown only symbolically. The wiring of the sensor element 110 can, for example, analogous to the circuit according to 8th take place, so that, for example, in turn, the first electrode 114 is switched as a cathode (for example, to ground potential) and the second electrode 122 as an anode or removal electrode, wherein the lead of the first electrode 114 for example, also connected to the ground line of a heater electrode or at least partially identical.

In 10 is an embodiment of a sensor element according to the invention 110 shown, which initially largely according to the embodiment according to 9 equivalent. Insofar, for the functioning and the illustrated elements can be largely based on the description of 9 to get expelled.

A difference too 9 results in the realization of the second electrochemical measuring cell 144 , so the short-circuited fuel or Nernst cell. This is in the in 10 illustrated embodiment realized in that in the region of the closed cavity 138 instead of the cover layer 170 a composite material 172 is used. For example, this composite material 172 in the form of an electrode paste containing yttrium-stabilized zirconia and platinum. In this way, the takes over the cavity 138 facing side of this composite material 172 the function of the third electrode 148 , and the measuring gas chamber 112 facing side of the composite material 172 the function of the fourth electrode 150 , The function of the electrical conductor 154 is through the composite material 172 taken himself. However, this also simultaneously assumes the function of the second solid electrolyte 146 , It is important that the composite material 172 is designed gas-tight. In addition, as in the other embodiments of the present invention, the gas-tight cavity 138 preferably filled with a porous element, for example, again a highly porous paste.

As before, in this embodiment, in which a composite material 172 is used, the electrical functions of the electron line and the ion conduction to different components of the composite material 172 distributed. This is how the composite material looks like 172 a solid electrolyte portion, which takes over the ionic conduction, in particular the oxygen ion conduction, and a metal portion (for example, platinum), which performs the function of electron conduction.

The previous embodiments of the sensor elements 110 in the 3 . 7 . 8th and 9 are each preferably operated as broadband probes, as well as the partially shown wiring shows. The inventive principle of the electrode shielding by using a gas-tight sealed cavity 138 and a short-circuited Nernst cell 144 for connecting this cavity 138 However, it can also be transferred to sensor elements with potentiometric measuring methods (hopping cells) or combined multicellular measurement setups. This is exemplified by the example of 11 and 12 clarified.

So shows 11 a jump cell structure in which the second electrode 122 over a protective layer 128 with the sample gas chamber 112 while the first electrode 114 inside the layer structure of the sensor element 110 again in a gas-tight cavity 138 is arranged. This gas-tight cavity 138 stands, similar to the representation in 8th , via a second electrochemical measuring cell 144 , which in turn is configured as a short-circuited fuel or Nernst cell, and a reference channel 166 with an environment space 164 in conjunction, which serves as an air reference. In this case, the electrochemical potential difference between the first and second electrodes can be measured, as shown symbolically in FIG 11 is indicated. From this potential difference can on the oxygen concentration in the exhaust gas in the sample gas space 112 getting closed. Otherwise, for the construction of the sensor element 110 be largely referred to the above descriptions. The advantage of the structure according to 11 lies in that the first electrode 114 , which serves as an air reference electrode, through the short-circuited Nernst cell 144 is effectively shielded, so that, for example, impurities in the form of electrode poisons or fuel gases not to this electrode 114 can get there and cause no CSD effects there.

In 12 Finally, an inventive multi-cell structure of a sensor element is shown, which the Nernst cell principle of 11 combined with the pump cell principle of the preceding embodiments. Such a measuring principle is basically known from the literature and is for example in EP 0 678 740 A1 described.

Again, one is on the surface of the solid electrolyte 124 arranged second electrode (also referred to as external potential electrode, APE,) and one inside the Sensorele management 110 in a cathode cavity 116 arranged first electrode 114 intended. The term "cathode cavity" is chosen independently of this, as the first electrode 114 is actually connected, that is, whether it is used as a cathode or as an anode. The cathode cavity 116 is again, analogous to, for example, the construction in 7 , via a gas access hole 118 and a diffusion barrier 120 with the sample gas chamber 112 whereas the second electrode is connected 122 immediately above a protective layer 128 with the sample gas chamber 112 connected is. The first electrode becomes, according to their position inside the sensor element 110 , often referred to as "internal potential electrode or IPE".

Again, a gas-tight closed cavity is still 138 provided, which in this embodiment (which, however, is not necessary again) in turn inside the sensor element 110 is arranged. This gas-tight cavity 138 again stands over a second electrochemical measuring cell 144 an environment space 164 in turn, in turn, this connection is a reference channel 166 includes. In this regard, for example, on 11 to get expelled.

In contrast to 11 However, in this embodiment according to 12 the cathode cavity 116 not with the closed cavity 138 identical, so that the first electrode 114 outside the closed cavity 138 is arranged. Instead, in the enclosed cavity 138 a reference electrode 174 arranged.

The functional principle of in 12 arrangement shown, for example, be designed such that a potential difference between the reference electrode 174 and the first electrode 114 is measured. About this measured potential difference or reference voltage, a pump voltage source can be controlled or regulated, which is a pumping voltage between the first electrode 114 and the second electrode 122 certainly. In this way, the pumping current between the first electrode 114 and the second electrode 122 be controlled so that the air ratio λ in the interior of the cathode cavity 116 always assumes the value λ = 1, which also in the closed cavity 138 prevails, as this in turn via the second electrochemical measuring cell 144 (or short-circuited fuel or Nernst cell) with the air reference in the form of the ambient space 164 communicates. From the pumping voltage, which for the production of this equilibrium weight condition in the cathode cavity 116 between the first electrode 114 and the second electrode 112 can be created on the actually in the sample gas chamber 112 existing air ratio λ are closed. This principle is used in the form of broadband sensors in many commercially available sensor elements. Here, too, makes the interposition of a short-circuited Nernst cell 144 between the actual air reference 164 and the closed cavity 138 positively noticeable, since this second electrochemical measuring cell 144 the reference electrode 174 effectively shields against any type of contaminants and electrode poisons that affect the electrode potential of this reference electrode 174 could influence.

QUOTES INCLUDE IN THE DESCRIPTION

This list The documents listed by the applicant have been automated generated and is solely for better information recorded by the reader. The list is not part of the German Patent or utility model application. The DPMA takes over no liability for any errors or omissions.

Cited patent literature

• DE 102004035826 A1 [0003]
• - DE 19938416 A1 [0003, 0005]
• DE 102005027225 A1 [0003]
• - DE 3809154 C1 [0005]
• EP 0678740 B1 [0006]
• - DE 10219881 A1 [0016]
• - DE 4343748 [0056]
• US 5543025 [0056]
• US 2004/0183055 A1 [0056]
• EP 0678740 A1 [0075]

Claims (18)

Sensor element ( 110 ) for determining at least one physical property of a gas in at least one sample gas space ( 112 ), in particular for determining an oxygen concentration in the exhaust gas of an internal combustion engine, comprising at least one first electrode ( 114 ) and at least one second electrode ( 122 ) and at least one the first electrode ( 114 ) and the second electrode ( 122 ) connecting first solid electrolyte ( 142 ), wherein the sensor element ( 110 ) at least one third electrode ( 148 ) and a fourth, with a gas space ( 112 ; 164 ) related electrode ( 150 ) and at least one the third electrode ( 148 ) and the fourth electrode ( 150 ) connecting second solid electrolyte ( 146 ), wherein the sensor element ( 110 ) further at least one gas-tight cavity ( 138 ), wherein the third electrode ( 148 ) in the cavity ( 138 ), characterized in that the third electrode ( 148 ) and the fourth electrode ( 150 ) by at least one electrical conductor ( 154 ) are electrically connected together. Sensor element ( 110 ) according to the preceding claim, wherein the third electrode ( 148 ) and the fourth electrode ( 150 ) through the electrical conductor ( 154 ) are short-circuited, wherein the electrical conductor ( 154 ) has an electrical resistance which is at least an order of magnitude smaller than an electrical resistance of the second solid electrolyte ( 146 ). Sensor element ( 110 ) according to one of the preceding claims, wherein the gas space ( 112 ; 164 ) and the sample gas space ( 112 ) are identical. Sensor element ( 110 ) according to one of claims 1 or 2, wherein the gas space ( 112 ; 164 ) one from the sample gas space ( 112 ) separate environmental space ( 164 ). Sensor element ( 110 ) according to one of the preceding claims, wherein at least one, preferably exactly one, of the following electrodes in the cavity ( 138 ): the first electrode ( 114 ), the second electrode ( 122 ), at least one reference electrode ( 174 ). Sensor element ( 110 ) according to any one of the preceding claims, wherein the first solid electrolyte ( 142 ) and the second solid electrolyte ( 146 ) are at least partially component identical. Sensor element ( 110 ) according to one of the preceding claims, wherein the sensor element ( 110 ) has a layer structure, wherein the cavity ( 138 ) is arranged in the interior of the layer structure and from the sample gas space ( 112 ) is separated by at least one layer of the layer structure. Sensor element ( 110 ) according to one of claims 1 to 7, wherein the sensor element ( 110 ) a layer structure with a measuring gas space ( 112 ) facing surface, wherein the cavity ( 138 ) on the measuring gas space ( 112 ) facing surface and from the sample gas space ( 112 ) by a gas-impermeable layer, in particular a gas-impermeable capsule ( 158 ), is separate. Sensor element ( 110 ) according to one of the preceding claims, wherein the second solid electrolyte ( 146 ) and the electrical conductor ( 154 ) are at least partially identical to a component, wherein the second solid electrolyte ( 146 ) as ceramic-metal composite material ( 172 ) is configured. Sensor element ( 110 ) according to the preceding claim, wherein the ceramic-metal composite material ( 172 ) has a composite material with a proportion of yttrium-stabilized zirconium dioxide and a proportion of platinum. Sensor element ( 110 ) according to one of the preceding claims, wherein the electrical conductor ( 154 ) at least one of the second solid electrolyte ( 146 ) comprises penetrating or immediate conductive trace which the third electrode ( 148 ) and the fourth electrode ( 150 ) connects. Sensor element ( 110 ) according to one of the preceding claims, wherein an electrode ( 114 . 122 ) from the first electrode ( 114 ) and the second electrode ( 122 ) existing electrode group in the cavity ( 138 ) is arranged. Sensor element according to the preceding claim, wherein the other electrode ( 114 . 122 ) from the first electrode ( 114 ) and the second electrode ( 122 ) existing electrode group with the sample gas space ( 112 ) via a first diffusion barrier ( 120 ) connected is. Sensor element ( 110 ) according to any one of the preceding claims, wherein the fourth electrode ( 150 ) with the gas space ( 112 ; 164 ) via at least one second diffusion barrier ( 152 ) connected is. Sensor element ( 110 ) according to one of the preceding claims, further comprising at least one heating element ( 130 ) with at least two supply lines ( 160 ), at least one of the supply lines ( 160 ) a ground line ( 162 ), wherein the first electrode ( 114 ) with the ground line ( 162 ) connected is. Method for determining at least one physical property of a gas in at least one sample gas space ( 112 ), in particular for determining an oxygen concentration in the exhaust gas of an internal combustion engine, wherein a sensor element ( 110 ) according to any one of the preceding claims, wherein between the first electrode ( 114 ) and the second electrode ( 122 Wherein a pump current (I _p) is measured wherein from the pump current (I _p) on the physical property, especially the oxygen concentration, is closed) a pump voltage (U _p) is applied. Method for determining at least one physical property of a gas in at least one measuring gas space ( 112 ), in particular for determining an oxygen concentration in the exhaust gas of an internal combustion engine, wherein a sensor element ( 110 ) is used according to one of the preceding claims, wherein the gas space ( 112 ; 164 ) one from the sample gas space ( 112 ) separate environmental space ( 164 ), wherein a potential difference between the first electrode ( 114 ) and the second electrode ( 122 ) and wherein from the potential difference on the physical property, in particular the oxygen concentration, is concluded. Method for determining at least one physical property of a gas in at least one measuring gas space ( 112 ), in particular for determining an oxygen concentration in the exhaust gas of an internal combustion engine, wherein a sensor element ( 110 ) is used according to one of the preceding claims, wherein the sensor element ( 110 ) at least one in the cavity ( 138 ) arranged reference electrode ( 174 ), wherein the first electrode ( 114 ) in one of the cavities ( 138 ) separate electrode cavity ( 116 ), wherein the electrode cavity ( 116 ) with the sample gas space ( 112 ) via at least one diffusion barrier ( 120 ), wherein a potential difference between the reference electrode ( 174 ) and the first electrode ( 114 ), wherein a pumping voltage between the first electrode ( 114 ) and the second electrode ( 122 ) is applied, wherein the pumping voltage is regulated to a target pumping voltage, such that the potential difference between the first electrode ( 114 ) and the reference electrode ( 116 ) assumes a setpoint value, wherein the desired pumping voltage is used to deduce the physical property.