EP2106543A1

EP2106543A1 - Sensor element with suppressed rich gas reaction

Info

Publication number: EP2106543A1
Application number: EP07822677A
Authority: EP
Inventors: Henrico Runge; Holger Reinshagen; Lothar Diehl; Thomas Seiler
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2006-12-29
Filing date: 2007-11-16
Publication date: 2009-10-07
Also published as: US20100126883A1; JP5328669B2; WO2008080698A1; DE102006062056A1; JP2010515033A

Abstract

Proposed is a sensor element (110) for determining at least one physical property of a gas mixture. The sensor element (110) has at least one first electrode (116) and at least one second electrode (118) and at least one solid electrolyte (112) connecting the at least two electrodes (116, 118). The at least one first electrode (116) is connected to at least one gas space (122) and/or a reference space (330) via at least one diffusion resistance element (314). The at least one second electrode (118) is connected to at least one gas space (122) via at least one flow resistance element (310). The at least one flow resistance element (310) and the at least one diffusion resistance element (314) are designed such that the limiting current of the at least one first electrode (116) is less than the limiting current of the at least one second electrode (118).

Description

description

title

Sensor element with suppressed fat gas reaction

State of the art

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

With the so-called air ratio "lambda" (λ), the ratio between an actually offered air mass and a theoretically required (ie stoichiometric) air mass is generally referred to in combustion technology, whereby the air ratio is usually determined by means of one or more sensor elements Correspondingly, "rich" gas mixtures (ie gas mixtures with a fuel surplus) have an air ratio λ <1, whereas "lean" gas mixtures (ie gas mixtures with a fuel deficiency) have an air ratio λ> 1 In addition to automotive technology, such and similar sensor elements are also used in other areas of technology (in particular combustion technology), for example in aviation technology or in the control of burners, for example 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 gas exposed reference electrode and a measuring gas exposed to the measured measuring electrode is based. Reference electrode and measuring electrode are connected to one another via the solid electrolyte, wherein doped zirconium dioxide (eg yttrium-stabilized ZrC ^) or similar ceramics are generally used as the solid electrolyte due to its oxygen-ion-conducting properties. 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 to control the gas mixture composition by the jump point λ = 1 active. Various exemplary embodiments of such jump probes, which are also referred to as "Nernst cells", are described, for example, in DE 10 2004 035 826 A1, DE 199 38 416 A1 and DE 10 2005 027 225 A1.

Alternatively or in addition to jump probes, so-called "pump cells" are used, in which an electrical "pumping voltage" is applied to two electrodes connected via the solid electrolyte, the "pumping current" being measured by the pump cell In the case of pump cells, generally both electrodes are connected to the gas mixture to be measured, whereby one of the two electrodes is exposed directly to the gas mixture to be measured (usually via a permeable protective layer) can not get directly to this electrode, but must first penetrate a so-called "diffusion barrier" to get into a cavity adjacent to this second electrode. In this case, the diffusion barrier used is usually a porous ceramic structure with specifically adjustable pore radii. If lean exhaust gas passes through this diffusion barrier into the cavity, oxygen molecules are electrochemically reduced to oxygen ions by means of the pumping voltage at the second, negative electrode, are transported through the solid electrolyte to the first, positive electrode and released there again as free oxygen. The sensor elements are usually operated in the so-called limiting current operation, that is, in an operation in which the pumping voltage is selected such that the oxygen entering through the diffusion barrier is completely pumped to the counterelectrode. In this operation, the pumping current is approximately proportional to the partial pressure of the oxygen in the exhaust gas mixture, so that such sensors Sorelemente often be referred to as Proportionalsensoren. In contrast to jump sensors, such proportional sensors can be used as so-called broadband sensors over a comparatively wide range for the air ratio lambda. Such broadband probes are described, for example, in DE 38 09 154 C1 and in DE 199 38 416 A1.

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 Such a construction is described, for example, in EP 0 678 740 B1, in which the oxygen partial pressure in the cavity described above adjoins the second electrode by means of a Nernst cell measured and the pumping voltage adjusted 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, however, have various problems. 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 600-700 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. Similar effects also play a role for other oxygen-supplying redox systems present in the gas mixture, for example C (VCO) The current is thus in the range of rich mixtures (fat pump current) due to 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 (eg, 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.

Disclosure of the invention

The invention is based on the above-described findings that the Fettpumpstrom and the pumping current in the region of slightly lean exhaust gases is essentially determined by the supply of hydrogen and / or other reducing gases in the region of the anode of a pump cell. Accordingly, a basic idea of the present invention is to shield the anode from hydrogen and / or other reducing gases without compromising lean operation.

Accordingly, a sensor element for determining at least one physical property of a gas mixture in at least one gas space is proposed which has at least one first electrode and at least one second electrode and at least one the at least one first electrode and the at least one second electrode connecting solid electrolyte. In particular, this sensor element can be operated such that the at least one first electrode is operated as anode and the at least one second electrode as cathode. Between these at least two electrodes, a pumping voltage is applied, which is preferably between 100 mV and 1.0 V, more preferably between 300 mV and 800 mV and optimally in the range between 600 mV and 700 mV. In this case, a pumping current can then be measured by the sensor element.

The at least one first electrode is connected via at least one diffusion resistance element to the at least one surrounding gas space (for example, one of the sensor cells). surrounding gas space), in which the gas mixture composition is to be determined, and / or a reference space associated with known gas mixture composition. The at least one second electrode is connected to the at least one gas space via at least one flow resistance element. The at least one flow resistance element and the at least one diffusion resistance element are configured such that the limiting current of the at least one first electrode is smaller than the limiting current of the at least one second electrode. In this case, limit currents are preferably set in which a ratio <1/100, in particular <1/1000, is present. The limiting current of the at least one first electrode is preferably 1 to 20 microamps, particularly preferably 10 microamps, and the

Limit current of at least one second electrode at 500 microamps to 3 milliamps, more preferably at 1.5 milliamperes. The limiting current of an electrode is defined as the saturation pumping current, d. H. the maximum pumping current, which can be achieved by increasing the pumping voltage between the at least two electrodes. This limiting current can be defined, for example, for oxygen and oxygen ion transport through the solid electrolyte as the current which is achieved when all of the oxygen molecules which reach the cathode-operated electrode are completely transported through the solid electrolyte to the anode. Usually, the sensor element is operated with this limiting current, i. H. In this mode, the pump current is approximately proportional to the gas molecule concentration, and the reverse current of the opposite electrode, which was previously operated as an anode, becomes experimentally polarized by reverse polarity determined, so that now the former anode is operated as a cathode.

The setting of the condition for the limiting current ratio can be fulfilled in particular in that the at least one diffusion resistance element has a greater diffusion resistance than the at least one flow resistance element. The diffusion resistance is the resistance which an element opposes to a concentration difference Δc between both sides of the element of length 1 and thus hinders a diffusion (current j): D-

The diffusion coefficient D is composed (inverse additive) of the diffusion coefficients for the gas phase diffusion and for the Knudsend diffusion, which both have different temperature dependencies. The temperature dependence of the flow thus depends on the proportions of the individual types of diffusion. Typically, the flux changes by about 4% when the temperature changes by 100 ^° C. For setting a desired limiting current, it is thus possible to influence the geometry of the resistance elements (cross section, length) or also the material properties and the temperature.

For example, the same diffusion medium (eg a porous material) can be used for this embodiment of the diffusion resistors for the at least one diffusion resistance element and the at least one flow resistance element, but in different layer thicknesses, such that a higher layer thickness occurs, for example, in front of the at least one first electrode is used as before the at least one second electrode. Alternatively or additionally, an adjustment of the surface of the resistance elements can take place. The limiting current increases at least approximately proportionally with the cross-sectional area available for the diffusion, and inversely proportionally with the length or layer thickness of the resistance elements.

In addition, however, the at least one flow resistance element preferably has a greater flow resistance than the at least one diffusion resistance limiter element. In this case, the flow resistance is defined as that resistance which an element opposes to a pressure difference between both sides of the element and thus prevents a flow between both sides of the element. The flow resistance can be adjusted, for example, by increasing or decreasing a pore size of a porous medium, and / or by varying a channel cross-section, a channel geometry or a channel length. The advantageous relationship described above between the limiting currents causes the above-described shielding effect of the at least one first electrode against reducing gases, such as hydrogen. It is particularly favorable if this shielding is effected by the at least one diffusion resistance element having a diffusion channel which connects the at least one first electrode to the at least one gas space and / or the at least one reference space. This diffusion channel should preferably have a large length, ie a length which is large compared to the mean free path of the gas molecules at the corresponding operating temperature of the sensor element (for example 700- 800 ⁰ C). In this way, the difference between gas phase diffusion and flow resistance can be maximally utilized in order to bring about a shielding of the at least one first electrode. If gas molecules in the diffusion channel (although of course also several diffusion channels can be used) have no other collision partners except the walls of the diffusion channel, transport would only occur via Knudent diffusion with the same behavior for flow and diffusion. By virtue of the configuration as a diffusion channel, on the other hand, there is only a slight diffusion transport of fatty gas to the at least one first electrode (usually the anode) and thus only a small flow of fat pumping. Advantageously, the at least one diffusion channel is provided with a height in the range between 2 L to 25 L and a width in a range of 2 L to 25 L and a length in the range of between 0.5 mm and 20 mm. Here, L is the mean free path of the molecules of the gas mixture at an operating pressure of the sensor element, which is usually in the range of normal pressure. This dimensioning of the at least one diffusion channel has proved to be particularly favorable in order to prevent the diffusion of fatty gas to at least one first electrode.

Overall, the inventive design of a sensor element according to one of the above embodiments is distinguished from the prior art by extremely low fat pumping currents. An interpretation of the pumping current, even in the lean range, can be made down to very small values for λ. By the at least one diffusion resistance element in the region in front of the at least one first electrode, which shields this at least one first electrode from diffusion, -O-

is (with application of the pumping current against λ) specifically reduces the slope of the "fat branch".

At the same time, the design of the at least one diffusion resistance element with low flow resistance prevents the risk of overpressure in the region of the at least one first electrode (usually anode) due to lack of gas removal, since gas molecules which form on the at least one first electrode can flow off directly , A further advantage of the embodiment of the sensor element according to the invention is that a reference channel is not necessarily required, which would have to be laboriously shielded from the gas space. In this way, for example, requirements for a probe housing, which surrounds the at least one sensor element, decrease.

The sensor element according to the invention can be further developed by various advantageous embodiments. Thus, for example, if at least one diffusion channel is used according to the above description, this at least one diffusion channel at at least one point of discharge to the gas space and / or to the reference space have a widening. This expansion can be done for example by a reduction and / or a bore extension. In this way, it is possible, for example, in the exhaust gas tract, to prevent the at least one diffusion channel from being added by liquid or solid impurities, which would impair the functionality of the sensor element.

Another possibility is to provide at least one cavity communicating with the at least one first electrode. This cavity is advantageously connected via the at least one diffusion channel with the at least one gas space and / or the at least one reference space. For example, this at least one cavity may comprise a widening of the at least one diffusion channel. Alternatively or additionally, the at least one cavity can also include a reaction space directly adjacent to the at least one first electrode, which encloses, for example, the entire at least one first electrode on one side. This at least one cavity serves the purpose that, for example, Hydrogen or other reducing gases can react (for example, by water) before they reach the at least one first electrode and there influence the electrode potential. In this at least one cavity, for example, a catalyst could additionally be provided in order to accelerate this reaction of reducing gases.

The at least one flow resistance element advantageously has at least one porous element. Thus, this corresponds to at least one diffusion resistance element of the commonly used in broadband probes in front of the cathode "diffusion barrier", as described for example in Robert Bosch GmbH: "Sensors in the motor vehicle", 2001, page 116 et seq. Advantageously, this porous element of the at least one flow resistance element is designed as a porous, extremely dense layer, as is known from the prior art. Advantageously, while a static pressure dependence k is used, which is at least 1 bar for the use of gasoline-powered internal combustion engines, but preferably is higher (for example, 3-4 bar). For diesel vehicles, k values in the range> 0.1 bar, preferably> 0.3 bar, for example in the range k = 0.45 ± 012 bar are used. The static pressure dependence k designates the pressure at which both diffusion types (diffusion diffusion and gas phase diffusion) are present to the same extent. With higher k values, diffusion dominates.

The at least one diffusion resistance element in front of the at least one first electrode may also have a porous element, for example to prevent soiling of the at least one first electrode. In this sense, the above-described at least one diffusion channel is already a "porous" element with a single large pore, but the at least one porous element in the region of the at least one first electrode is preferably designed with a large pore, ie with a small k value To form the lowest possible flow resistance.

This embodiment of the sensor element makes it possible, in particular, to achieve extremely low sensitivity of the lean pump flow for fast overall pressure changes (dynamic pressure dependence, DDA). Only the extremely small fat pump Ström shows a high dynamic pressure dependence. About the static pressure dependence of the lean pumping current, which is greater than that of the Fettpumpstroms, preferably even signal components of these two streams can be separated.

A further advantageous embodiment of the sensor element is that the diffusion of reducing gases, such as hydrogen, to the at least one first electrode is suppressed by appropriate local adjustment of the temperature. Thus, in particular, the at least one first electrode can be operated at a lower operating temperature than the at least one second electrode. For this purpose, for example, at least one tempering element (for example a heating resistor, a pelletizing element or a similar tempering element) may be provided, which temperature controls the at least two electrodes or the associated resistance elements differently. By increasing the temperature, a flow through a resistance element can be hindered, whereas diffusion is favored.

For example, this can be done by choosing a planar structure in which the at least two electrodes lie in one plane and are tempered at different temperatures. For example, this different tempering can be brought about by using a heating element, wherein the mean distance between the at least one heating element and the at least one first electrode is greater, preferably at least 10%, particularly preferably at least 20%, than the mean distance of the at least one heating element to the at least one second electrode. In this case, the average distance can be understood, for example, as the distance of the surface centers or an edge distance. As a result of this asymmetric tempering, diffusion through the at least one flow resistance element is favored in the region of the at least one second electrode, whereas diffusion is suppressed in the region of the at least one first electrode operated at low temperature.

The sensor element according to one of the embodiments described above can be produced in particular in a layer structure. For example, the at least one first electrode and the at least one second electrode on opposite Be arranged sides of the at least one solid electrolyte, wherein the at least one first electrode as the gas space facing electrode is configured (outer pumping electrode, APE) and wherein the at least one second electrode than the at least one gas chamber remote electrode (inner pumping electrode) configured is. In order for gas mixture to be able to pass from the at least one gas space to the at least one second electrode, then a corresponding channel, a bore or a gas access hole or a similar opening must be provided, as is the case for example with broadband probes according to the prior art (see the above-mentioned Quote) the case is.

Another possible embodiment consists in that the at least one first electrode and the at least one second electrode are in turn arranged on opposite sides of the at least one solid electrolyte, wherein, however, the at least one first electrode has an electrode facing away from the gas space (IPE) and wherein the at least a second electrode assigns an electrode (APE) facing the at least one gas space. This structure is thus an "inverse" structure over the aforementioned structure.

Another possibility is to arrange the at least one first electrode and the at least one second electrode on the same sides of the at least one solid electrolyte, wherein the at least one first electrode and the at least one second electrode each have at least one electrode facing the gas space.

Brief description of the drawings

Embodiments of the invention are illustrated in the drawings and explained in more detail in the following description. Show it:

Figure IA is a prior art sensor element;

FIG. 1B shows a pumping current of the sensor element according to FIG. 1A, plotted against the schematically linearized air ratio λ;

FIG. 2A shows a sensor element according to the invention in a first embodiment; FIG. 2B shows a pumping current as a function of the schematically linearized air ratio λ of FIG

Sensor element according to Figure 2A;

FIG. 3 shows an embodiment of the sensor element with a cavity; FIG. 4 shows an embodiment of the sensor element with a cavity and a connection to a reference channel; FIG. 5 A shows a sensor element with asymmetrical electrode heating in plan view; and FIG. 5B shows the sensor element according to FIG. 5A in cross section.

Embodiments of the invention

FIG. 1A shows a construction of a sensor element 110 corresponding to the prior art. Here, as in the following explanations, this sensor element 110 is designed as a pumping cell, ie as a broadband sensor. The sensor element 110 comprises a solid electrolyte 112, which is usually a zirconium oxide. Depending on the type of gases to be detected, however, it is also possible to use other ion-conducting solid electrolytes or mixtures of such solid electrolytes. In this case, the solid electrolyte 112 is part of a sensor body 114. On opposite sides of the solid electrolyte 112, first electrodes 116 and second electrodes 118 are arranged. In the present exemplary embodiment, the first electrode 116 is designed as an outer pumping electrode (APE), and the second electrode 118 as an inner pumping electrode (IPE). The first electrode 116 is separated by a porous protective layer 120 from the surrounding gas space 122, wherein the porous protective layer 120 is configured such that gases from the first electrode 116 through the porous Protective layer 120 can flow without significant flow resistance. The porous protective layer 120 essentially serves to protect the first electrode 116 against soiling.

In contrast, the second electrode 118 in the exemplary embodiment according to FIG. 1A is arranged in an internal measuring space 124. In order to get from the gas space 122 to the second electrode 118, gas mixture must pass through a gas inlet hole 126. From the gas access hole 126, the gas mixture then passes through diffusion and flow (ideally, only diffusion) through a porous element 128 (which is often referred to in the art as a "diffusion barrier," but actually serves as a flow barrier) into the measurement space 124.

In the usual operation of the sensor element 110 according to FIG. 1A, a so-called "pumping voltage" U _{p is} applied between the two electrodes 116 and 118, such that the second electrode 118 is operated as cathode (negative electrode) and the first electrode 116 as anode Oxygen ions (O ^2- ), which, driven by the electric field between the two electrodes 116, 118, migrate to the first electrode 116, form there at the second electrode 118. There, elemental oxygen forms again, which passes through as gas the porous protective layer 120 can flow away. [Üb Üb] The sensor element 110 designed as a broadband probe is usually made according to FIG

Figure IA operated in a pump voltage range at about 600 mV. This pumping voltage is sufficient to operate the sensor element in the limiting current operation, but the pumping voltage is simultaneously below the decomposition voltage of water, so that at the second electrode 118, no decomposition of water should occur.

Furthermore, the sensor element 110 according to FIG. 1A has a heating element 136, which may be designed, for example, as a meander-shaped heating element (for example as a platinum heating element). This heating element, which is typically heated at a λ probe at approximately 780 ⁰ C, increases the ionic conductivity of the Festelektro- LYTEN 112 and thus ensures higher limit currents. FIG. 1B shows the pumping current I _p , which is measured in the arrangement according to FIG. 1A with a current measuring device 130. This pumping current I _p is shown schematically here as a function of the air ratio λ, where λ = 1 corresponds to the stoichiometric gas mixture composition in the gas space 122. Accordingly, the range for λ> 1, which is denoted symbolically by reference numeral 132 in FIG. 1B, is referred to as "lean" range, whereas the range λ <1 is referred to as "rich" range and in FIG. IB symbolically with reference number 134 is designated. As can be seen from the curve of the pumping current I _p in FIG. 1B, the sensor element 110 has a considerable current in the rich region 134 in accordance with the prior art shown in FIG. 1A. Because of this high (magnitude) slope of the pumping current in the rich region 134, an assignment to an air ratio λ is only possible with difficulty from the measurement of the pumping current I _p . As described above, this considerable pumping current in the rich region 134 is due, in particular, to reducing gases in the region of the first electrode 116 operated as an anode.

Another effect, which is not shown in FIG. 1B, is the effect described above, that even in the lean region 132 in the vicinity of λ = 1, a deviation of the pumping current from the proportionality to the air ratio λ occurs. This deviation is also due to the presence of reducing gases, such as hydrogen, in the region of the first electrode 116. In particular, higher pump currents are often measured in the slightly lean region than would correspond to the proportionality.

In contrast to the prior art according to FIG. 1A, FIG. 2A shows a sensor element 110 according to the invention.

The sensor element 110 according to FIG. 2A has, in principle, great similarity with the sensor element 110 according to the prior art in FIG. 1A. Again, a solid electrolyte 112 is provided, which is contacted by two opposing electrodes 116, 118. Analogously to FIG. 1A, in turn, the second electrode 118 is designed as an inner pumping electrode and is arranged in a measuring space 124. Gas mixture can pass from the surrounding gas space 122 via a gas inlet hole 126 into the measuring space 124. Between the gas inlet hole 126 and the measuring space 124 is a Flow resistance element 310 is arranged, which in turn, analogous to Figure IA, as a porous element 128 is configured and through which gas mixture can diffuse (symbolically indicated in Figure 2A by the arrow 322). Typical pore sizes are about 0.1 to 3.0 micrometers.

In that regard, the sensor element 110 according to the exemplary embodiment in FIG. 2A essentially corresponds to the prior art according to FIG. 1A. In contrast to the sensor element 110 according to FIG. 1A, however, the sensor element 110 in the exemplary embodiment according to FIG. 2A has considerable differences in the region of the first electrode 116, which in turn is configured as an outer pump electrode. Thus, in the exemplary embodiment according to the invention according to FIG. 2A, no porous protective layer 120 is provided, through which gas emerging at the first electrode 116 could escape directly into the gas space 122. Instead, the first electrode 116 is shielded from the gas space 122 by a cover element 312. Is thereby formed between the first electrode 116 and the solid electrolyte 112 and the lid member 312, a diffusion resistance element 314 in the form of a diffusion channel 316. This diffusion channel 316 leading from the first electrode 116 away and opens into the gas entry hole 126. At an operating temperature of about 1000 ⁰ C and a pressure of 1 bar, the mean free path of the gas molecules is typically about 0.25 microns. Accordingly, transverse dimensions (width, height) in the range of a few μm are preferably selected for the diffusion channel 316 and a length of a few mm. These dimensions cause oxygen, which forms at the first electrode 116, to flow from the first electrode 116 to the gas inlet hole 126 under low flow resistance (indicated by the thick arrow 318 in FIG. 2A). In contrast, due to the high diffusion resistance of the long diffusion channel 316, hydrogen from the gas space 122 can only be made more difficult to pass from the gas space 122 to the first electrode 116 and change or influence the electrode potential there. This diffusion movement is shown symbolically in FIG. 2A with the thin arrow 320.

Again, the sensor element 110 according to the embodiment of the invention in Figure 2A is operated with a pumping voltage U _p of typically about 600 mV, and the pumping current I _p is measured. Again, a heating element 136 is also present. - o¬

seen, by means of which the sensor element 110 is operated at an operating temperature of typically a few 100 ⁰ C to about 1000 ⁰ C.

In FIG. 2B, analogously to FIG. 1B, a pumping current (limiting current) is represented by the sensor element 110 according to FIG. 2A as a function of the air ratio λ. Hereby, it becomes clear that the rich load of the pumping current I _p is in its slope ahead of the second electrode 118 due to the use of the high diffusion resistance diffusion resistance element 314 in front of the first electrode 116 and the high flow resistance (but low diffusion resistance) flow resistance element 310 (see area 134). is substantially smaller than the pumping current in the lean region 132. This effect is mainly due to the shielding of the first electrode 116 from hydrogen and / or other reducing gases. Also, (not shown in Figure 2B) in the slightly lean region (ie, in the region 132 near λ = 1), the adulteration (eg, non-linearities) is greatly suppressed by the presence of hydrogen and / or other reducing gases. so that the sensor element 110 according to FIG. 2A can be inserted down to the region close to λ = 1.

FIG. 3 shows a second exemplary embodiment of a sensor element 110 according to the invention, which in turn can be used as a broadband sensor lambda probe. The structure of the sensor element 110 essentially corresponds to the design of the sensor element 110 according to the exemplary embodiment in FIG. 2A, so that reference can be made to this figure for the function and composition of the individual elements. In contrast to the exemplary embodiment according to FIG. 2A, however, the sensor element 110 according to the exemplary embodiment in FIG. 3 has two substantial modifications which bring about improvements in functionality compared with the structure in FIG. 2A and which can be implemented individually or in combination. Thus, on the one hand directly above the first electrode 116, a rectangular cross-section cavity 324 is provided. This cavity 324 preferably has a height and a width, which are each considerably larger than the mean free path of the gas molecules. Again, at an operating temperature of for example 1000 ⁰ C and thus a mean free path of about 0.25 microns thus the height of the cavity 324 is preferably at some 10, more preferably at some 100 microns, right into the Range of approx. 1 mm. The width of the cavity 324, ie its horizontal extent, is preferably in the range of a few 100 microns to a few mm. Preferably, the cavity 324 extends over the entire electrode surface of the first electrode 116. Via the diffusion channel 316, the cavity 324 is in communication with the gas inlet hole 126. Again, the diffusion channel 316 is preferably provided with a length of more than 0.5 mm. As described above, cavity 324 serves to facilitate the removal of reducing gases (eg, hydrogen) before these gases can reach first electrode 116. As an alternative embodiment, the cavity 324 may also be "interposed" in the diffusion channel 316 such that the gas access hole 126 communicates with the cavity 324 through a first portion of the diffusion channel 316 which in turn passes through a second portion of the diffusion channel 316 with the first electrode In this way, the reaction of the reducing gases in the cavity 324 is spatially completely separated from the first electrode 116.

As a second modification, the sensor element 110 in the exemplary embodiment according to FIG. 3 has an expansion 328 at an opening 326 of the diffusion channel 316 into the gas inlet hole 126. This expansion 328 serves to avoid fouling of the diffusion channel 316 and clogging by solid or liquid contaminants in the gas mixture. In the exemplary embodiment according to FIG. 3, the widening 328 is designed in the form of a countersink. Even stepped expansions or other forms of expansion are conceivable.

For the operation of the sensor element 110 according to the exemplary embodiment in FIG. 3, a pump voltage is again applied between the electrodes 116, 118 analogously to FIG. 2A, the first electrode 116 preferably again being operated as the anode and the second electrode 118 as the cathode.

FIG. 4 shows a third exemplary embodiment of a sensor element 110 according to the invention. The sensor element 110 according to the embodiment shown in Figure 4 has similarities with the embodiment of Figure 3. Accordingly, with respect to the function and designation of the individual elements - -

be largely referenced to this embodiment. Once again, a cavity 324 is provided above the first electrode 116 (typically anode), which cavity may be designed in the same way as the cavity 324 in FIG. In turn, a diffusion channel 316 is also provided, via which oxygen can flow away from the first electrode 116 (reference numeral 318), which, however, substantially prevents hydrogen diffusion (reference numeral 320) to the first electrode 116.

In contrast to the exemplary embodiment in FIG. 3, however, the diffusion channel 316 does not open in the gas inlet hole 126 (and thus indirectly in the gas space 122), but in a reference space 330. This reference space 330, which may, for example, be an engine environment of an internal combustion engine, is separate from the gas space 122, so that gas mixture can not get into the reference space 330. In this way, it is ensured that hydrogen and / or other reducing gases can not reach the first electrode 116, since usually in the reference space (eg.

Ambient air) only vanishingly small amounts of such reducing gases are present. Accordingly, it would also be possible to completely dispense with the diffusion channel 316. Thus, the sensor element 110 according to the exemplary embodiment in FIG. 4 is similar to the structure of known jump probes, in which an electrode is typically exposed to a reference gas. In contrast to such jump probes, however, the sensor element 110 is operated as a broadband sensor in the exemplary embodiment according to FIG. 4, since a (generally constant) pump voltage (the contacts are not shown in FIG. 4) is applied between the two electrodes 116, 118 and Pumping current is measured.

In FIGS. 5A and 5B, a fourth exemplary embodiment of a sensor element 110 is shown schematically in a plan view (FIG. 5A) and in a sectional view in a side view (FIG. 5B). In this embodiment, an asymmetrical heating of the two electrodes 116, 118 by a heating element 136 is used, such that the first electrode 116 is operated at a lower operating temperature than the second electrode 118. This can be done in particular by asymmetric arrangement of the heating element 136, in such that a heating zone, which in Figure 5A and in Figure 5B symbolic with Reference numeral 510 extends to a greater extent on the second electrode 118 than on the first electrode 116th

The sensor element 110 according to the embodiment in FIGS. 5 A and 5 B is an example of an embodiment in which both electrodes 116, 118 are arranged on the same side of a solid electrolyte 112. Accordingly, the oxygen ion current near the surface passes through the solid electrolyte 112 in an approximately horizontal direction. It is thus, in contrast to the "stacked" or vertical structures according to Figures 2A, 3 and 4, in Figures 5A and 5B to a planar structure.

Again, a lid member 312 is provided which, however, covers both the first electrode 116 and the second electrode 118 in this embodiment. In the region of the first electrode 116, a diffusion resistance element 314 with a diffusion channel 316 is again provided analogously to, for example, the exemplary embodiment in FIG. 2A. For the dimensioning can be made to a large extent on Figure 2A. Via the diffusion channel 316, the first electrode 116 is in fluid communication with the gas space 122 so that oxygen can flow out (reference numeral 318), whereas diffusion of hydrogen to the first electrode 116 (reference numeral 320) through the diffusion channel 316 is difficult. Again, the diffusion channel 316 preferably has a length (ie, from the edge to the gas space 122 to the nearest edge of the first electrode 116) of preferably more than 0.5 mm and preferably less than 20 mm, on the one hand a high diffusion resistance and on the other hand to ensure a low flow resistance.

Through the cover element 312, a measuring space 124 is furthermore formed above the second electrode 118, which typically again has a height of at least a few tens, preferably a few 100 μm and up to a few mm. This measuring space 124 is closed to the gas space 122 by the flow resistance element 310 in the form of a porous element 128, analogous to the exemplary embodiment in FIG. 2A. The planar embodiment according to FIGS. 5A and 5B permits a particularly simple electrical contacting of the electrodes 116, 118 via electrode contacts 332, 334 on the surface of the solid electrolyte 112 without the need for plated-through holes. Typically, the first electrode 116 is again operated as an anode via the electrode contact 332, whereas the second electrode 118 is operated as a cathode via the electrode contact 334. The electrical circuit and exposure to the constant pumping voltage U _p is analogous to the exemplary embodiment in Figure 2A.

By means of the heating element 136, the sensor element 110 according to the embodiment in Figure 5 A and 5B is typically again operated at some 100 ⁰ C to about 1000 ⁰ C. In this way, as described above, the ionic conductivity of the solid electrolyte 112 is increased. In this case, the heating element 136 (which may in general also be a tempering element 336, that is, for example, also a cooling element) may be arranged asymmetrically with respect to the electrodes 116, 118. For example, this asymmetry can be brought about in that the outer edge of the first electrode in plan view (Figure 5A) by a distance D (see Figure 5B) protrudes beyond the heating element 136, which may be for example a few mm, whereas the second electrode 118 is completely horizontal is covered by the heating element 136. In this way, for example, in the region of the diffusion channel 316 (or additionally in the region of the first electrode 116), an operating temperature can be set that is, for example, about 20% (on the Kelvin temperature scale) lower than the operating temperature in the region of the second electrode 118, the measuring space 124 and / or the flow resistance element 310. In this way, since the diffusion of hydrogen (reference numeral 320) through the diffusion channel 316 increases with increasing temperature, this diffusion can be additionally suppressed, whereas diffusion through the porous element 128 of the Flow resistance element 310 is favored by the elevated temperature.

As a variant of the embodiment in FIG. 5B, the diffusion resistance element 316 could also have one or more channels (for example bored substantially perpendicularly by a laser) in the cover element 312, which connects the gas space 122 with the Connect space above the first electrode 116. An embodiment of the diffusion channel 316 in layer technology, for example as a lamellar diffusion channel with a plurality of adjacent channel planes, or a configuration by means of a plurality of adjacent, parallel diffusion tubes is also conceivable. Such parallel diffusion tubes can be realized for example by manufacturing methods in which a laser drilling method is used.

Claims

claims

1. Sensor element (110) for determining at least one physical property of a gas mixture in at least one gas space (122), in particular for determining an oxygen concentration in the exhaust gas of an internal combustion engine, with at least one first electrode (116) and at least one second electrode (118) and at least a solid electrolyte (112) connecting the at least one first electrode (116) and the at least one second electrode (118), the at least one first electrode (116) being connected to the at least one gas space (122) via at least one diffusion resistance element (314). or at least one reference space (330) is connected, wherein the at least one second electrode (118) via at least one flow resistance element (310) with the at least one gas space (122) is connected, characterized in that the at least one

Flow resistance element (310) and the at least one diffusion resistance element (314) are configured such that the limiting current of the at least one first electrode (116) is smaller than the limiting current of the at least one second electrode (118).

Second sensor element (110) according to the preceding claim, characterized in that the at least one flow resistance element (310) and the at least one diffusion resistance element (314) are designed such that the at least one flow resistance element (310) has a greater flow resistance than that at least one diffusion resistance element (314).

3. Sensor element (110) according to one of the two preceding claims, characterized in that the at least one diffusion resistance element (314) has a greater diffusion resistance than the at least one flow resistance stand element (310).

4. Sensor element (110) according to one of the preceding claims, characterized in that the limiting current of the at least one first electrode (116) is less than 1/100 of the limiting current of the at least one second electrode (118) and preferably less than 1/1000 the limiting current of the at least one second electrode (118).

5. sensor element (110) according to one of the preceding claims, characterized in that the limiting current of the at least one first electrode (116) is 1 to 20 microamps, preferably 10 microamps, and that the Grenzström the at least one second electrode 500 microamps to 3 milliamps, preferably 1.5 milliamperes.

6. Sensor element (110) according to one of the preceding claims, characterized in that the at least one diffusion resistance element (314) has a diffusion channel (316), via which the at least one first electrode

(116) is connected to the at least one gas space (122) and / or the reference space (330).

7. sensor element (110) according to the preceding claim, wherein the at least one diffusion channel (316) has a channel with a height in a range of 2 L to

25 L, a width in a range of 2 L to 25 L and a length in the range of 0.5 mm to 20 mm, where L is the mean free path of the molecules of the gas mixture at an operating pressure and an operating temperature of the sensor element (110) is.

8. sensor element (110) according to one of the two preceding claims, characterized in that the at least one diffusion channel (316) at least one orifice (326) to the at least one gas space (122) and / or reference space (330) an expansion ( 328).

9. sensor element (110) according to one of the three preceding claims, characterized by at least one additional, with the at least one first Elekt- The at least one cavity (324) is connected via the at least one diffusion channel (316) to the at least one gas space (122) and / or the at least one reference space (330).

10. Sensor element (110) according to the preceding claim, characterized in that the at least one cavity (324) has at least one catalyst, in particular a nickel catalyst and / or a platinum catalyst.

11. Sensor element (110) according to one of the preceding claims, wherein the at least one diffusion resistance element (314) has at least one porous element.

12. Sensor element (110) according to one of the preceding claims, character- ized in that the at least one flow resistance element (310) has at least one porous element (128).

13. Sensor element (110) according to the preceding claim, characterized in that the at least one porous element (128) has a static pressure dependence k between 0.3 bar and 1.3 bar.

14. Sensor element (110) according to one of the preceding claims, characterized in that the at least one diffusion resistance element (314) has a reference channel, wherein the at least one reference channel, the at least one first electrode (116) with at least one of the at least one gas space

(122) connects separate reference space (330).

15. Sensor element (110) according to one of the preceding claims, characterized in that at least one tempering element (336) is provided, wherein the at least one tempering element (336) is configured to the at least one first electrode (116) at a lower operating temperature operate as the at least one second electrode (118).

16. Sensor element (110) according to one of the preceding claims, characterized by at least one of the following layer structures: a) the at least one first electrode (116) and the at least one second electrode (118) are on opposite sides of the at least one solid electrolyte ( 112), wherein the at least one first electrode (116) has an electrode facing the gas space (122) and wherein the at least one second electrode (118) has an electrode facing away from the at least one gas space (122); b) the at least one first electrode (116) and the at least one second electrode (118) are arranged on opposite sides of the at least one solid electrolyte (112), wherein the at least one first electrode (116) has an electrode facing away from the gas space (122) and wherein the at least one second electrode (118) has an electrode facing the at least one gas space (122); c) the at least one first electrode (116) and the at least one second electrode (118) are arranged on the same sides of the at least one solid electrolyte (112), wherein the at least one first electrode (116) and the at least one second electrode (1 18 ) each have at least one electrode facing the gas space (122).

17. A method for determining at least one physical property of a gas mixture using a sensor element (110) according to one of the preceding claims, characterized in that the at least one first elec- (116) is operated as an anode that the at least one second electrode

(118) is operated as a cathode and that a pumping voltage between 100 mV and 1.0 V, preferably between 300 mV and 800 mV and more preferably between 600 mV and 700 mV between the at least one first electrode (116) and the at least one second Electrode (118) is applied, wherein a pumping Ström by the sensor element (110) is measured.