DE102009029415A1 - Sensor element with multipart diffusion barrier - Google Patents

Abstract

A sensor element (110) for determining at least one property of a gas in a measurement gas space (122) is proposed, in particular for determining a proportion of a gas component in the measurement gas space (122). The sensor element (110) comprises at least one first electrode (112), at least one second electrode (114) and at least one solid electrolyte (116) connecting the first electrode (112) and the second electrode (114). The first electrode (112) can be supplied with gas from the measurement gas space (122) via a gas access path (124). The gas access path (124) has at least one diffusion barrier (130) which has at least one first diffusion barrier area (138) and at least one second diffusion barrier area (142). The second diffusion barrier area (142) is configured with more pores than the first diffusion barrier area (138).

Description

State of the art

The invention is based on known sensor elements for determining at least one property of a gas in a measuring gas chamber. In principle, this can be any property of the gas, for example a physical and / or chemical property. In particular, the invention will be described below with reference to sensor elements by means of which a proportion of at least one gas component in the gas can be determined, that is, for example, a partial pressure and / or a concentration of this gas component, in particular of oxygen. The sensor element can in particular be used to measure an air ratio λ in an exhaust gas of an internal combustion engine. Such sensor elements are also referred to as lambda probes and are for example in Robert Bosch GmbH: Sensors in the motor vehicle, issue 2007, pages 154-159 described. Reference may be made by way of example to the statements made there, and the illustrated sensor elements may also be modified according to the invention within the scope of the present invention. In particular, the sensor elements may comprise one or more solid electrolytes, in particular ceramic solid electrolytes, for example yttrium-stabilized zirconium dioxide.

For the measurement of a gas composition in the diesel exhaust gas, for example, limit current lean-load probes are required as individual probes, which are characterized by a unique lean characteristic curve. A prerequisite for this is a shielding of a pump cell anode from the exhaust gas and the associated property of the probe to exclude anodic oxidation reactions of fuel gas components. In this regard, for example, on WO 2008/080698 to get expelled.

In particular, compared to lambda probes for measuring exhaust gas compositions of gasoline engines, lambda probes for measurements in diesel exhaust gas must meet additional requirements, which result from the special mode of operation, the operating conditions and the evaluation mechanisms. In this regard, however, arise in conventional lambda sensors numerous technical challenges. A first technical challenge consists of manufacturing spreads, which are due in particular to a comparatively complex design. For example, there are variations in the limiting currents and in the so-called k values, which characterize the ratio of a Knudsen portion to a gas phase portion in the diffusion of the gas entering the probe (see, for example, in this regard WO 2008/080698 ). Due to manufacturing-related scattering of the measuring current, ie the limiting current I _p , which is set via a diffusion barrier in the ceramic sensor element as a measure of λ, currently after production of the ceramic sensor elements, the limiting current must be adjusted either on the sensor element or by subsequent trimming of electronic components on the TKU characteristic become. This adjustment can be dispensed with if the probe is recalibrated during overrun, ie under defined conditions. However, this usually only succeeds if the limiting current deviates by a maximum of 20% from the nominal value. Taking account of age-related variations, this means a limit current spread of less than 10% when new. This is the maximum permissible measure for production-related scattering. To ensure this, a design is needed which is not susceptible to manufacturing tolerances, especially in the production of a diffusion path.

Another challenge is the adjustment of the required k values, the mean value shift (MWV) and the dynamic pressure dependence (DDA) for the required measurement accuracy. The dynamic pressure dependence is the time and / or frequency dependence of the probe signal in a total change in pressure to understand. As a result of dynamic pressure pulses, fluctuations in the pump current Ip occur in the case of broadband lambda probes (dynamic pressure dependence, DDA). The mean value of the pumping current is increased compared with the static mean pressure measured at the same mean pressure. To describe this phenomenon, the term mean value shift (MWV) was introduced. In order to be able to meet the required measuring accuracies, in particular in diesel exhaust gas, it is necessary to reduce the static pressure dependence, ie. H. the k value, whereby the influence on other functional quantities, such as the mean value shift and / or the dynamic pressure dependence, should remain low. Functionally, however, as the k-value decreases, other magnitudes influencing the accuracy of the probe, such as the mean value shift, increase. The realization of small k values, in particular for diesel applications, or the targeted adjustment of k values over a large range, for example generally for special operating conditions, especially in the gasoline engine, in front of a turbocharger or the like, without the values for the mean value shift or dynamic pressure dependence exceeds certain limits adapted to accuracy requires the development of a new, innovative diffusion barrier design.

A third challenge is in particular a sooting and / or poisoning resistance of the sensor elements. The mocking and / or poisoning mechanism and the soot and / or poisoning susceptibility in the diesel exhaust are different due to the exhaust gas composition, the exhaust gas temperature, the pressure and similar sizes different from the corresponding mechanisms or vulnerabilities in the exhaust gas of a gasoline engine. The design of the sensor elements must take this fact into account and be adapted accordingly both with respect to the sooting in the diffusion path and with respect to the poisoning in the electrode design and / or in the electrode cavity design.

Disclosure of the invention

Accordingly, a sensor element is proposed for determining at least one property of a gas in a measurement gas space, in particular a sensor element according to one or more of the sensor elements described above. In particular, the sensor element can be used to determine a proportion of a gas component in the measurement gas space. The sensor element comprises at least one first electrode, at least one second electrode and at least one solid electrolyte connecting the first electrode and the second electrode. The first electrode can be acted upon with gas from the measuring gas space via a gas inlet path. For example, the first electrode can be arranged in the interior of the sensor element, for example, in the interior of a layer structure. The Gaszutrittsweg may include, for example, one or more channels and / or holes through which a total of the gas and / or a gas component from the sample gas space can reach the first electrode, for example by flow and / or diffusion mechanisms.

The gas access path has at least one diffusion barrier. In the context of the present description, a diffusion barrier means an element by means of which, for example, a limiting current can be set by setting appropriate diffusion and / or flow mechanisms. In particular, the diffusion barrier, as explained in more detail below, comprise at least one porous material. The diffusion barrier is intended to be set up overall in order to at least limit a simple inflow of gas to the first electrode and thus can act, for example, or primarily as a flow barrier, so that downstream open channels can function as diffusion resistance at all and can not be overcome by throughflow.

In contrast to conventional diffusion barriers, it is proposed in the sensor element according to the invention to design the diffusion barrier at least in two parts. Thus, the diffusion barrier has at least one first diffusion barrier region and at least one second diffusion barrier region. These at least two diffusion barrier regions may preferably adjoin one another directly, but may also be arranged at a distance from one another in the gas inlet path.

The sensor element may in particular be configured such that gas from the measurement gas space first passes through the second diffusion barrier region or a first part of the second diffusion barrier region, then through the first diffusion barrier region and then again through the second diffusion barrier region or a second part of the second diffusion barrier region. The arrangement may be configured such that the gas passes through the second diffusion barrier region perpendicularly, for example through and / or diffused, in particular perpendicular to a layer structure of a layer structure of the sensor element, and laterally through the first diffusion barrier region, for example parallel to the layer structure. For example, the sensor element may be configured such that first a vertical flow and / or a vertical gas diffusion through a first part of the second diffusion barrier region, followed by a lateral flow and / or gas diffusion through the first diffusion barrier region, followed by a renewed vertical flow and or a renewed vertical diffusion through the second diffusion barrier region or a second part of the second diffusion barrier region.

In particular, the diffusion barrier may be designed such that the first diffusion barrier region contributes to a majority to a diffusion resistance of the diffusion barrier, wherein the second diffusion barrier region acts as a flow barrier and / or homogenizer at the beginning and / or end of the diffusion barrier.

In particular, the arrangement of the first diffusion barrier region and the second diffusion barrier region can take place in the direction of the gas access path, so that the second diffusion barrier region or parts of the second diffusion barrier region act as flow barrier and / or as homogenizer and the first diffusion barrier region acts as the actual diffusion barrier. In this case, for example, a first part of the second diffusion barrier region may be arranged at the beginning of the diffusion barrier, which has, for example, a porous structure as a flow barrier and which has the least share of the total diffusion resistance. This may be followed by the first diffusion barrier region, which may comprise, for example, an open structure and which may comprise a first diffusion barrier region Main contribution to the diffusion resistance of the diffusion barrier can cause. In turn, a second part of the second diffusion barrier region, which can act as a homogenizer and thus bring about a homogeneous electrode connection to the open structure of the first diffusion barrier region, can adjoin it. For example, the second part of the second diffusion barrier region may have a smaller porous structure than the first part of the second diffusion barrier region. The second part of the second diffusion barrier region can also be wholly or partially identical to the first part, for example in that the second diffusion barrier region is designed as a layer through which multiple flows.

If an electrode connection cavity is provided after the second part of the second diffusion barrier region which acts as a homogenizer, which however can also be omitted, so that a direct connection to the electrode can take place, then the diffusion resistance of this homogenizer should generally be selected such that a preferably homogeneous application of the first Electrode with gas. In particular, the first electrode and / or the electrode connection cavity can cover a region of the first diffusion barrier whose gas concentration gradient, eg. B. oxygen concentration gradient, is to be homogenized by the homogenizer, for example in the longitudinal direction of the channels.

In this case, the gas access path should be designed such that the second diffusion barrier region is located upstream of and / or downstream of the first diffusion barrier region. This means that, for example, the gas on the way from the measuring gas space to the first electrode first has to pass through the second diffusion barrier region in order to reach the first diffusion barrier region (upstream) and / or the gas passes the first diffusion barrier region on the way from the measuring gas chamber to the first electrode to then pass through the second diffusion barrier region to the first electrode. A combination of the options mentioned is also possible, so that, for example, the gas first passes through a first part of the second diffusion barrier region before the gas then enters the first diffusion barrier region in order finally to reach the first electrode through a second part of the second diffusion barrier region.

The second diffusion barrier region is intended to have a finer pored design than the first diffusion barrier region. This generally means that in the second diffusion barrier region the proportion of Knudsen diffusion should be higher than in the first diffusion barrier region, whereas in the first diffusion barrier region the fraction of gas phase diffusion should be greater than in the second diffusion barrier region. The term porosity does not necessarily mean that the first diffusion barrier region is also completely configured with a porous material. As will be explained in more detail below, the first diffusion barrier region may be basically porous (but more porous than the second diffusion barrier region) or, alternatively, also open pore and / or fully open, but still confining diffusion. The second diffusion barrier region, which has a fine-pored configuration, thus acts at least predominantly as a flow restriction, whereas the first diffusion barrier region at least predominantly acts as a diffusion barrier. Thus, a diffusion barrier is proposed which functionally represents a flow barrier diffusion barrier combination.

As described above, the second diffusion barrier region may in particular comprise a porous material having at least one of the following properties: a pore size between 0.03 μm and 3 μm; a porosity between 5% and 60%, in particular a porosity less than 40%. The second diffusion barrier region may in particular have a thickness, ie an extent parallel to the local direction of the gas access path, between 10 μm and 200 μm, in particular between 20 μm and 100 μm.

The first diffusion barrier region may in particular be a plurality, i. H. For example, include 2, 3, 4 or more, extending along the Gaszutrittswegs channels. These channels preferably have a height of 5 to 50 micrometers, more preferably 10 to 25 micrometers, a width of 50 to 500 micrometers, more preferably 100 to 150 micrometers, and a length of 500 to 5000 micrometers, more preferably 1200 to 3000 micrometers. In this case, the gas inlet path can be designed in such a way that gas from the measuring gas chamber enters these channels on the way to the first electrode substantially perpendicular to the channels. For example, the Gaszufrittsweg be designed for this purpose angled. However, a slight deviation from a vertical entry is also possible in principle, for example a deviation at an angle of not less than 45 °.

The second diffusion barrier region may for example be formed at least partially as a flow barrier layer and / or comprise at least one such flow barrier layer. In particular, this flow barrier layer can have a lateral extension which is substantially parallel to a longitudinal extension of the channels. The flow barrier layer can in this case, in particular, at least the channels partially cover. The Gaszutrittsweg may be configured such that gas from the measurement gas space on the way to the first electrode first passes through the flow barrier layer at least once, then passes through the channels and then optionally passes through the flow barrier layer at least one more time. Also, an embodiment in which the gas from the measuring gas chamber on the way to the first electrode enters the channels immediately, without first pass through the flow barrier layer, and then to penetrate after exiting the channels, the flow barrier layer to reach the first electrode is conceivable.

The sensor element may in particular have a layer structure, with several layer planes. The first diffusion barrier region and the second diffusion barrier region can be arranged in different, preferably adjacent, layer planes. For example, the first diffusion barrier region may be wholly or partly printed on the second diffusion barrier region or vice versa. For example, the flow barrier layer of the second diffusion barrier region can be printed on the channels of the first diffusion barrier region. The gas access path can moreover have an electrode cavity of the first electrode arranged in a further layer plane, which is preferably different from the layer planes mentioned. Under an electrode cavity is an adjacent to the electrode open or filled with a gas-permeable, porous medium cavity to understand, which forms a gas reservoir in front of the electrode. The electrode cavity should in this case be downstream of the diffusion barrier.

Furthermore, the gas access path may have a connection cavity arranged in a further layer plane, which may also be identical to the layer plane of the electrode cavity. This connection cavity is preceded by the diffusion barrier and, preferably without diffusion limitation and / or flow limitation, is connected to the measurement gas space. For this purpose, for example, a gas inlet bore can be provided perpendicular to the layer planes and / or parallel to the layer planes, for example on an end face of the sensor element. As already explained above, the second diffusion barrier region, in particular a part of the second diffusion barrier region downstream of the first diffusion barrier region, may preferably have a greater diffusion resistance than a gas diffusion resistance of the attachment cavity.

The sensor element according to one or more of the embodiments described above has a multiplicity of advantages over known sensor elements. A new diffusion barrier concept is proposed, which corresponds to the requirements of future generations of sensor elements both in terms of manufacturability and with regard to the setting of static and / or dynamic functional measured variables as well as the increased requirements for sooting resistance and / or poisoning resistance. The proposed flow barrier diffusion barrier combination, combined with optimized electrode cavity connections and connections to the exhaust gas, can take these demands into account. The main diffusion resistance, ie the larger proportion of the total diffusion resistance of the diffusion barrier, should drop off at the first diffusion barrier region. This can, as described above, for example, by channels, in particular by open channels, can be realized. In contrast, the second diffusion barrier region can in particular be configured as a finely porous layer (flow barrier layer), for example with pore sizes between 0.03 μm and 3 μm and / or between 5% and 60% porosity, and can, for example, cover the channels over a large area. The design of this flow barrier layer in the diffusion path realizes both a large inlet area for the gas and a minimal diffusion path length. As a result, this flow barrier layer mainly acts as a flow barrier, that is to say as a damping element, with a small proportion of the total diffusion resistance that can be adjusted by the layer thickness. The access of gas to the diffusion path is realized, for example, by openings in a gastight covering layer covering this flow barrier diffusion barrier combination. These openings allow, for example, on the one hand via a connection cavity and in particular a gas inlet hole (for example, laterally, frontally and / or drilled) the exhaust side connection and on the other hand via the optional electrode cavity, the connection to the first electrode. The advantages of the configuration of the sensor element according to the invention are, in particular, an improved manufacturability, a possibility of setting the k value and an increased sooting resistance and / or poisoning resistance.

Overall, the proposed design takes into account the increased demands on a new generation of limit current sensors or sensor element generation for diesel and gasoline engines. For diesel applications, in particular, small k values can be set, or for other applications, k values can be set in a wide range. Small diffusion resistances via a finely porous medium (Knudsendiffusion) are ensured by the proposed design reliably reproducible, due to a large inlet area, a small diffusion path perpendicular to the Pressure direction of the flow barrier. In combination with an open channel structure and the higher diffusion resistance (gas phase diffusion) small k-values can be realized. These flow and diffusion resistances, which can be set independently of each other with respect to the k value and the diffusion resistance, allow adjustment and / or adjustment of the static and dynamic variables over a wide range due to the special arrangement of the flow-diffusion barrier combination. Furthermore, there are small mean value shifts and / or dynamic pressure dependencies at low k values. For example, the upstream second diffusion barrier region can have a k value of greater than 0.2 bar or more. The downstream first diffusion barrier region may, for example, have a k value of less than 0.2 bar. With a small total k value, the flow load of the electrode connection cavity in the region of the transition to the diffusion barrier can be homogenized and the volume correspondingly minimized by the flow barrier in the form of the second diffusion barrier region with the diffusion resistance adapted to a covered surface, for example downstream from the main diffusion resistor in the form of the first diffusion barrier region become. Thus, in the realization of small k values, the increase in DDA / MWV can be attenuated because the electrode void volume correlates directly with the dynamic quantities. In addition to this effect on the electrode cavity volume, the flow barrier acts as a damping element upstream of the main diffusion resistance and optionally also downstream Knudsen diffusion resistance (flow barrier). If an electrode connection cavity is provided after the second part of the second diffusion barrier region which acts as a homogenizer, which however can also be omitted, so that a direct connection to the electrode can take place, then the diffusion resistance of this homogenizer should generally be selected such that a preferably homogeneous application of the first Electrode with gas. In particular, the first electrode and / or the electrode connection cavity can cover a region of the first diffusion barrier whose gas concentration gradient, eg. B. oxygen concentration gradient, is to be homogenized by the homogenizer, for example in the longitudinal direction of the channels.

Further advantages arise with regard to the resistance to sooting. An upstream, finely porous flow barrier in the form of the second diffusion barrier region, for example with a porosity of less than 40%, acts like an upstream filter. The proposed design offers the possibility to design the entrance area over a large area and thus to achieve a small flow density and by the realization of flow gradients in the filter the influence of the sooting, d. H. the clogging of the filter pores, to minimize the characteristic. Through flow density gradients in the upstream filter, parallel conduction paths can be created which, to some degree of sooting, i. H. By adding pores in the input area, these soot can compensate without affecting over the life of the sensor element, the functional variables beyond the allowable tolerance addition.

Furthermore, the proposed design results in increased poisoning resistance. A homogeneous electrode load and thus an improved poisoning resistance are ensured by a homogeneous flow load of the electrode cavity, which is also referred to as the electrode connection cavity. This is ensured by virtue of the fact that the first diffusion barrier region which acts as a flow barrier is located downstream of the first diffusion barrier region acting as the actual diffusion barrier, whose diffusion resistance of the electrode cavity geometry and / or in particular to the diffusion resistance of the channel ends covered by the electrode and / or the electrode connection cavity can be adjusted laterally , Another prerequisite here may be the central arrangement of the first electrode above the gas outlet that can basically be implemented by the design.

Further advantages arise with respect to the limiting current scattering and / or the k-value scattering, which in many cases is a prerequisite for thrust balance. The improved manufacturability of the gas diffusion path and thus the lower scattering of the limiting current and the k value is ensured by the proposed new arrangement of the components of the diffusion barrier. The minimization of the influence of smudges and / or overlaps which occur in conventional sensor elements, in particular in the edge regions of the printed layers on the scattering of the boundary currents and / or the k-values by the optional, perpendicular to the printing direction flow through the printed components of the diffusion barrier and / or other components of the gas access path, in particular the connection elements achieved.

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:

1 a sectional view through an embodiment of a sensor element according to the invention;

2 a perspective detailed representation of a diffusion barrier of the sensor element according to 1 ; and

3 a perspective view of a second embodiment of a sensor element according to the invention.

embodiments

In the 1 and 2 is a first embodiment of a sensor element according to the invention 110 shown in different illustrations. It is an embodiment of a broadband lambda probe. For further details of such broadband lambda probes and the operation of such lambda probes, for example, to the above-mentioned prior art or, for example, the DE 10 2006 062 060 A1 to get expelled. It is a marginal flow Magersonde with a unique characteristic. The sensor element 110 has a layer structure, with a first electrode 112 , a second electrode 114 and one of these two electrodes 112 . 114 connecting solid electrolyte 116 For example, yttria-stabilized zirconia. The two electrodes 112 . 114 are arranged in the illustrated embodiment in the same layer plane. However, other arrangements are possible in principle, for example, arrangement with superimposed electrodes 112 . 114 in which the electrodes 112 . 114 for example on opposite sides of the solid electrolyte 116 are arranged.

Furthermore, the sensor element 110 in the illustrated embodiment optionally one on the surface of the sensor element 110 arranged reducing electrode 118 on. This reducing electrode 118 serves to reduce and optionally adjustability of an internal resistance of the first electrode 112 , the second electrode 114 and the solid electrolyte 116 comprehensive pumping cell. The operation of the reducing electrode is, for example, in DE 10 2006 062 060 A1 to which also regarding further possible details of the sensor element 110 can be referenced.

The second electrode 114 is in the illustrated embodiment in a reference air channel 120 arranged. This reference air channel 120 For example, the second electrode 114 connect to a reference air space, such as an engine compartment of a motor vehicle. The concept of the "reference air channel" 120 is wide and can include, for example, an exhaust duct. For example, at the second electrode 114 expanded gas via the reference air duct 120 escape. In this case, the reference air channel 120 an exhaust duct. Other configurations are possible in principle, for example embodiments in which the second electrode 114 directly or indirectly with a sample gas chamber 122 communicates.

The first electrode 112 is via a gas access route 124 with gas from the sample gas chamber 122 acted upon. This gas access route 124 includes in the illustrated embodiment, one in the layer plane of the electrodes 112 . 114 arranged connection cavity 126 , a section 128 , a section 128 with a diffusion barrier 130 and one also in the layer plane of the electrodes 112 . 114 arranged electrode cavity 132 , Not shown in 1 is a connection of the connection cavity 126 to the sample gas chamber 122 which can be realized for example by a frontal bore, a channel in the layer planes and / or a gas inlet bore perpendicular to the layer planes. The diffusion path through which the gas travels through the gas inlet path 124 has to cover is in 1 symbolic with the arrow 134 designated.

The diffusion barrier 130 is in the layer structure below the layer plane with the electrodes 112 . 114 arranged and from this layer plane by a cover layer 136 separated. As in particular from the perspective detailed representation in 2 is apparent, is the diffusion barrier 130 designed according to the invention in two parts and has a first diffusion barrier region 138 in the form of parallel to the diffusion path 134 running channels 140 on, as well as an overlying second diffusion barrier region 142 in the form of a fine-pored flow barrier layer 144 , While these two area 138 . 142 conceptually to a diffusion barrier 130 are summarized, actually acts the second diffusion barrier region 142 actually as a flow barrier, whereas the actual diffusion barrier through the first diffusion barrier region 138 is formed. The flow barrier layer 144 covers the channels 140 completely in the illustrated embodiment, so that gas on the Gaszutrittsweg 124 first the flow barrier layer 144 has to go through, then into the channels 140 enters, through this in the direction of the electrode cavity 132 flows and then a second time through the flow barrier layer 144 into the electrode cavity 132 must enter.

The first diffusion barrier area 138 can be configured in various ways and can, for example, the channels 140 in the form of porous filled, slightly porous filled, partially filled and / or structured channels. This first diffusion barrier area 138 represents the main diffusion resistance and serves, for example, for setting small k values in the range, for example, between 0.01 bar <k <0.3 bar. Depending on the filling, however, it is also possible to set adjusted k values, for example in the range between 0.01 bar <k <5 bar.

The flow barrier layer 144 of the second diffusion barrier region 142 On the other hand, it represents a finely porous flow barrier layer. It is a region with a high k value, for example k> 0.3 bar. This flow barrier layer 144 covers the channels over a large area, which creates a damping and a filtering effect. Furthermore, this flow barrier layer effects 144 an inflow density gradient. It also ensures a homogeneous flow load and / or diffusion load of the electrode cavity 132 and / or the connection cavity 126 , It makes a small contribution to the total diffusion resistance and thus to the total limiting current I _p and to the total k value. About the thickness and / or the porosity of this flow barrier layer 144 it is possible to influence dynamic and / or static quantities, for example the quantities of MWV, DDA and k-value described above. In this way, for example, an optimum can be set for a given k value and a mean value shift with regard to the probe accuracy.

In contrast to conventional probe geometries in which, for example, the connection cavity 126 , the diffusion barrier 130 and the electrode cavity 132 are produced by overlapping printing layers, which are responsible for variations of the limiting current I _p and the k value, have in the inventive design of the sensor element 110 and in particular the multi-part diffusion barrier 130 Cavity smearing due to inaccuracies in the pressure has no influence on the diffusion resistance and thus not on functional variables, such as I _p and the k value. In addition, there is an increased sooting resistance through the porous flow barrier layer 144 with a large inlet area, ie a low inflow density, and sufficiently parallel flow paths and / or diffusion paths across the channels 140 , by implementing Einstromdichtegradienten over the entrance surface. The electrode cavity 132 and the surface of the first electrode 112 in particular centrally above one another and above the diffusion barrier 130 be arranged, for example, the first electrode 112 centrally above the electrode cavity 132 , which also acts as an electrode connection cavity, may be arranged.

Overall, the sensor element described has 110 , which exemplifies in the 1 and 2 is shown and on the multipart diffusion barrier 130 has, compared to conventional sensor elements 110 a number of benefits and additional options. Thus, conventional broadband probes generally follow the concept that the diffusion path is formed parallel to the printing direction and the diffusion elements are flowed through laterally by the gas in succession. For this purpose, it is necessary to connect the inflow and / or outflow regions of porous diffusion barriers through cavities to the gas path, for example a drilled gas access hole. Both diffusion barriers and cavities are printed several times in thick film technology on a foil backing with a security overlap side by side. Unavoidable smudges and leakage of the printed layers cause considerable scattering in the overlap of the diffusion elements, for example cavities and diffusion barriers. Since the diffusion type in the cavity (gas phase diffusion dominates) and the diffusion barrier (Knudder diffusion and gas phase diffusion, depending on the pore size distribution in the ratio 0.1 to 5) is very different, this results in high values for the scattering of the total diffusion resistance and thus in the limiting current I _P and k- Value. For this reason, the limit current at the new sensor element generally has to be adjusted for all series sensor elements.

That in the 1 and 2 presented new concept of the diffusion barrier 130 on the other hand, it is provided that the diffusion path is basically perpendicular to the printing direction by the elements of the diffusion barrier which are produced one above the other, for example by screen printing in this case 130 must be done. In particular, the gas passes through the diffusion barrier layer 144 essentially perpendicular to the channels 140 of the first diffusion barrier region 138 to get. As a result, the influence of smearing on the edges of the printed layers and the lateral diffusion barrier cavity connections to the diffusion barrier 130 eliminates and minimizes the production-related dispersion with respect to the limiting current and / or the k value. The remaining scatter, caused by a non-uniform layer thickness across the laminate, may be further eliminated, for example, by a thrust balance strategy. A connection to the as the actual diffusion resistance of the diffusion barrier 130 acting channels 140 of the first diffusion barrier region 138 takes place, for example, by pressure layers, in particular the at least one flow barrier layer 144 , In this way, a connection of the first diffusion barrier region 138 , which provides the actual diffusion resistance, to the cavities 126 and or 132 be guaranteed. The flow barrier layer 144 is traversed vertically. The channels 140 , That is, the region in which the main diffusion resistance drops, experienced perpendicular to the pressure layer of the flow barrier layer 144 a flat, running perpendicular to the direction of pressure indiffusion. I _p and k value are not affected by smearing of all the pressure resistance layers determining the diffusion resistance.

The electrode cavity 132 can also be designed in several parts and can, as in 1 indicated below the actual electrode cavity 132 an electrode connection cavity 146 which is adjacent to the flow barrier layer 144 is connected. Before the gas is perpendicular to the electrode connection cavity 146 flows in and is homogeneous in the electrode cavity 132 distribute the gas diffuses in the channels 140 which provide the main diffusion resistance. The length of the diffusion path in the channels 140 and thus the size of the diffusion resistance can in particular by the distance of the openings in the cover layer 136 and thus be determined only by a well-defined process step. The channels 140 may in particular have a length between 0.5 and 5 mm, in particular of 1.5 mm. The openings in the cover layer 136 are placed so that the outgoing channel ends of the channels 140 make no contribution to the diffusion resistance. The influence of smearing at the ends of the channels 140 to the limiting current and / or the k value can be eliminated.

Remaining differences in the opening geometries, manufacturing technology causes, for example, by leaking and / or smearing when printing the cover layer 136 , In addition, have only a marginal impact on the size of the diffusion resistance, since the length of the diffusion path is greater by a factor of 100 than the expected differences in the opening geometries. The proposed design thereby meets the requirements for a low recalculation of the threshold currents to eliminate sensor element matching, which causes high costs, in favor of implementing an adjustment via a thrust balance strategy in diesel applications.

Furthermore, in the 1 and 2 shown design of the sensor element 110 be easily adapted to optimize dynamic and / or static measurements. For example, for the accuracy of the measurement in diesel exhaust a smaller k-value is required, for example, k <0.3 bar, with respect to the entire diffusion barrier 130 , This k value is smaller than the k value usually realized in conventional lambda probes, which is typically above 0.4 bar. By means of the design according to the invention, the diffusion path becomes 134 designed such that the proportion of gas phase diffusion in the total diffusion resistance against Knudsendiffusion can be increased accordingly. Furthermore, it may be essential that the design be designed for a small k value such that the mean shift is not increased to negate the benefit of improved accuracy at low k values. Due to the trend that the mean value shift increases with the reduction of the k value, an optimum for both values must be found and specifically set for a maximum of probe accuracy when operating in the diesel exhaust line. Due to the inventive design of the diffusion barrier 130 with the second diffusion barrier region acting as a flow barrier 142 and the first diffusion barrier region acting as the actual diffusion resistance 138 with its open or open-pored channels 140 exactly this optimization can be achieved. The gas access route 124 is designed such that the main diffusion resistance at the open or open-pore channels 140 of the first diffusion barrier region 138 drops. The second diffusion barrier region 142 is due to the large-area flow barrier layer 144 realized, which is designed to be finely porous. The gas inlet is through the openings in the dense covering layer 136 realized. Through the open or open-pored channels 140 For example, the main diffusion resistance is mainly carried by gas-phase diffusion, and the required k-values between 0.01 bar and 0.3 bar will vary depending on the patterning, width and height of the channels 140 achieved for use in diesel engine. The diffusion resistance of the open channels 140 is the second diffusion barrier region 142 in the form of the flow barrier layer 144 upstream and / or downstream. This second diffusion barrier region 142 can be realized as a layer of fine porous material and can be the channels 140 cover a large area. The gas access to this second diffusion barrier region 142 is, as shown above, through openings in the cover layer 136 realized, which in the figures generally with the reference numerals 148 . 150 are designated. Since the diffusion resistance in the porous Strömungsbarrierenschicht 144 is many times larger than in the channels 140 , the exhaust gas diffuses vertically through this, for example, formed as a pressure layer Strömungsbarrierenschicht 144 , which has, for example, a thickness of 20 to 100 microns, in the channels 144 ,

The design can be designed so that the diffusion resistance of the porous diffusion barrier layer 144 due to the vertical flow through this layer and the small layer thickness (path length) compared to the resistance in the channels 140 order of magnitude smaller. The design thus ensures that the resulting total limiting current and the resulting total k value limited only by this acting as a flow barrier diffusion barrier layer 144 being affected. The k value and the limiting current of the channels 140 determine the k-value or the limiting current of the sensor element 110 , Furthermore, the porous flow barrier layer 144 have a dampening effect that does not cause the DDA and the MWV to increase oversized. This finely porous flow barrier in the form of the flow barrier layer 144 provides in the exit area of the diffusion barrier 130 Overall, also for the fact that also the rear electrode cavity 132 and / or its electrode connection cavity 146 is charged homogeneously. Will the acting as a flow barrier second diffusion barrier region 142 , For example, the flow barrier layer 144 with respect to the diffusion resistance designed so that already at the entire interface to this flow barrier layer 144 the electrode cavity 132 and / or the electrode connection cavity 146 flows through homogeneously, so can a homogeneous electrode load of the first electrode 112 be realized with minimized connection cavity height. The reduction of the volume of the electrode cavity 132 however, a technical minimum is a basic requirement for low DDA or MWV values. The proposed design with the special arrangement of diffusion barriers and flow barriers thus offers for the first time the possibility of combining small k values, homogeneous electrode load with minimized electrode cavity height and a moderate MWV / DDA. Acts the flow barrier layer 144 Completely homogenizing over the entire connection surface, the electrode cavity height can optionally be reduced to zero. Acts the flow barrier layer 144 homogenization only to a limited extent due to high porosity and / or too small layer thickness, so homogenization should be achieved by increasing the electrode cavity height.

The electrode cavity 146 may for example have a height between 20 and 200 microns.

The flow barrier layer 144 may also only the first diffusion barrier region 138 , which may act as a diffusion barrier layer, be downstream or merely upstream. In particular, this may be in connection with the configuration of the first diffusion barrier region 138 in the form of the channels 140 will be realized. In this way, the flow barrier layer can, for example, act or be configured as a homogenization layer. In particular in this case and for the purpose of homogenization, the homogenization layer should have a lower porosity than the first diffusion barrier region 138 which is preferably in the form of the channels 140 is designed.

Depending on the layer thickness and porosity of the flow barrier layer 144 In this case, the proportion of the diffusion resistance of this layer to the total diffusion resistance and thus to the limiting current, the k value, the DDA and the MWV and the gas distribution, although not independently of one another, can thus be adjusted specifically for a high accuracy depending on the conditions of use. Another advantage is that in the proposed design concept for the targeted adjustment of these static and / or dynamic sizes layout adjustments are necessary only to a very limited extent. If, for example, the MWV is too high for the small k values to be set, the flow barrier layer can already be adapted by adjusting the layer thickness 144 and / or a reduction in the porosity of the material, the proportion of the diffusion resistance of the porous Strömungsbarrierenschicht 144 (Knudsendiffusion) are increased in the total diffusion resistance, which is accompanied by an increase in the k value and a reduction in MWV. Depending on the requirements and conditions of use, an optimum of probe accuracy can be achieved. In conjunction with the possibility of using the channel geometry and / or channel fills of the channels 140 additional influence on the diffusion resistance components and / or the diffusion properties and thus to take the dynamic and / or static variables, increases the possibility of optimization.

With the design of independently variable diffusion barriers 138 . 142 which act as a diffusion resistance or as a flow resistance, can therefore be varied relatively easily and widely over a wide range, especially in the case of high-temperature limit current probes, for example by channel structuring (eg via reproducible constrictions), the channel geometry ( for example via the inlet cross-section and / or the length), the channel filling (open, porous, open and porous) on the one hand and the material (porosity) and geometry (thickness, inlet surface) of the flow barrier layer 144 on the other hand can be interpreted accordingly.

The proposed design of the sensor element 110 is also distinguished by a significantly improved sooting and poisoning resistance compared to conventional sensor elements. So the entrance area is the combined flow / diffusion barrier 130 designed so that particles, depending on the size and depending on the thickness and porosity or composition of the flow barrier layer 144 , which acts as a flow barrier, there are impeded on further transport. The particle flow to the first electrode 112 down becomes due to the filtering action of the porous flow barrier layer 144 at least largely suppressed, while maintaining the small k value, since the through the second diffusion barrier region 142 formed flow barrier can be made very thin by the proposed design concept and thus the contribution of this Knudsen diffusion resistance to the total diffusion resistance can be kept low.

In order to minimize the risk of fouling of the inlet area, which acts as a filter, the proposed design allows the entrance area of the flow barrier layer acting as a flow barrier 144 many times larger compared to conventional sensor elements 110 to design. The inflow density is thereby reduced by a factor of 3 to 15, and the susceptibility to erosion drastically reduced. In addition, the design creates an inflow density gradient (adjustable over area, thickness and porosity). As a result, the influence of a sooting, for example by a clogging of the pores of the flow barrier layer 144 , minimizes the diffusion barrier input region to the probe signal by first doping the regions of greatest current density and regions of initially lower current density over the lifetime of the sensor element 110 can compensate. Since these parallel conductive paths in the proposed design, the diffusion path of the main diffusion resistance of the underlying channels 140 extend only insignificantly, this has up to a certain degree of sooting no or only an insignificant effect on the characteristic of the sensor element 110 , Thus, despite the small k-values, the proposed design offers the possibility of having a large entrance area, inflow density gradients, and varying the thickness and / or porosity of the second diffusion barrier area 142 to decisively improve sooting resistance.

The improved poisoning resistance, ie the lower susceptibility of the electrode 112 to poisoning, is due to the homogeneous current density distribution at the functioning as a mounting electrode first electrode 112 secured. In contrast to the existing concepts with a lateral flow of diffusion resistors and / or electrode cavities and associated inhomogeneous electrode loads (current density distributions) can in the proposed design concept of the sensor element 110 the electrode 112 with its electrode surface perpendicular to the diffusion path 134 are arranged, for example via the electrode cavity arranged perpendicular to the diffusion direction 132 , Because also this electrode cavity 132 can be loaded homogeneously, which acts as a flow barrier layer Strömungsbarrierenschicht 144 at the gas outlet of the diffusion barrier 130 cause. It should be noted that this flow barrier layer acting as a flow barrier 144 at the second opening 150 towards the electrode cavity 132 identical or separate from the flow barrier layer 144 at the first opening 144 towards the connection cavity 126 can be configured. In the in the 1 and 2 illustrated embodiment, it is a common Strömungsbarrierenschicht 144 , However, other embodiments are possible in principle. For better manufacturability and / or process reliability, the use of a single flow barrier layer 144 prefers. The thickness, the porosity and the exit area of the flow barrier layer acting as a flow barrier 144 However, for this homogeneous load, it should be adapted to the cavity height, the electrode area and the total diffusion resistance.

For a homogeneous electrode load and a low MWV, it is optimal, especially if in the region of the second opening 150 at the electrode connection cavity 146 a flow barrier with adapted thickness and / or porosity is realized whose diffusion resistance is higher than that of the channels 140 in the area of the second opening 150 , This will over the entire area of the bonding surface of the electrode cavity 132 to the acting as a flow barrier second diffusion barrier region 142 ensures a homogeneous load, and the electrode cavity 132 can be minimized in height and thus by the volume of the electrode cavity 132 caused DDA and / or MWV.

Alternatively, flow inhomogeneities may occur at the bonding surface of the electrode cavity 132 by increasing the cavity height for a homogeneous electrode load, ie more homogeneous current density distribution, partially compensated, but this is usually realized at the expense of the dynamic probe properties.

The proposed design thus offers the possibility of increasing the poisoning resistance despite a minimum electrode cavity height and thus a low MWV, by virtue of a downstream diffusion barrier layer acting as a flow resistance 144 with adapted diffusion resistance as a covering layer over the first diffusion barrier region acting as an actual diffusion barrier 138 with low k-value (channels 140 ) a homogeneous electrode cavity load and homogeneous current density distribution of the first electrode 112 is guaranteed.

From the above, it can also be seen that the requirements for as Flow barrier acting first diffusion barrier region 138 in the entrance area of the first opening 148 and in the exit area of the second opening 150 the diffusion barrier 130 depending on the required design of the diffusion barrier design for setting the limiting current and / or the k value and for the design of the sensor element, in particular the pumping cell, may be different. In this way, for example, an adaptation to a different electrode geometry, to different current densities or the like can be ensured.

For example, the requirements for the second diffusion barrier region 142 in the area of the gas inlet at the opening 148 a smaller layer thickness of the flow barrier layer 144 require, a high porosity, a small pore size with low diffusion resistance of this part of the second diffusion barrier region 142 at the total diffusion resistance of the diffusion barrier 130 , As a result, a filter effect and the generation of flow density gradients in the entry region can guarantee the resistance to sooting at a low k value of the sensor element.

The embodiment of the second diffusion barrier region 142 which is the first diffusion barrier region 138 downstream, ie in the area of the second opening 150 , however, can be different. For example, the adaptation of the diffusion resistance of the second diffusion barrier region 142 which is the first diffusion barrier region 138 is downstream, so for example in the region of the second opening 150 , in the exit region for a homogeneous flow through the electrode connection cavity 146 and thus require a higher diffusion resistance for homogeneous electrode loading to ensure poisoning stability than in the entrance area at the first opening 148 , ie the first diffusion barrier region 138 upstream.

This consideration may also be in the sensor element according to the invention 110 be taken into account. The design offers the possibility of adaptation in that in making the flow barrier layer 144 For example, in screen printing, this flow barrier layer 144 for example, upstream and downstream of the first diffusion barrier region 138 can have different properties. For example, materials with different porosity can be printed upstream and downstream, or the number of printing steps upstream and downstream can be designed differently and thus via the height of the upstream or downstream flow barrier layer 144 different diffusion properties for the input and output range are set.

Alternatively, depending on the design of the diffusion resistance of the channels 140 and the flow barrier layer acting as a flow barrier 144 , the size of the openings 148 . 150 in the cover layer 136 for the cavity connections in the entrance area and outlet area of the flow barrier layer acting as a flow barrier 144 be adapted and thereby the diffusion resistances are designed differently. In order for a generally high diffusion resistance of acting as a flow barrier second diffusion barrier region 142 , in particular the flow barrier layer 144 To also create flow density gradients in the entrance area to ensure a homogeneous electrode cavity height, is the opening 148 in the entrance area preferably to be chosen correspondingly large in order to reduce the diffusion resistance at this point.

In 3 is a second embodiment of a sensor element according to the invention 110 shown. This embodiment corresponds initially in many parts in the embodiment in the 1 and 2 , so that reference can be made largely to the above description of these figures.

In contrast to the above embodiment shows 3 in that basically the first electrode 112 and the second electrode 114 can also be arranged in different layer planes of the layer structure. Thus, in the embodiment according to 3 the second electrode 114 , which may for example be designed in two parts, in a higher layer plane and on opposite sides of a solid electrolyte 116 or more solid electrolyte layers.

Furthermore, in the embodiment according to the 1 and 2 a gas access 152 on a front side 154 of the sensor element 110 arranged. Again, this is not mandatory. Thus, the embodiment in 3 that the gas access 152 also on a top 156 the layer structure of the sensor element 110 can be provided and with the diffusion barrier 130 or the optional connection cavity 126 for example, through a gas access hole 158 , which penetrates the layer structure and, for example, can be configured as a bore, can be connected.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• WO 2008/080698 [0002, 0003]
• DE 102006062060 A1 [0030, 0031]

Cited non-patent literature

• Robert Bosch GmbH: Sensors in the motor vehicle, edition 2007, pages 154-159 [0001]

Claims (16)

Sensor element ( 110 ) for determining at least one property of a gas in a sample gas space ( 122 ), in particular for determining a portion of a gas component in the sample gas space ( 122 ) comprising at least one first electrode ( 112 ), further comprising at least one second electrode ( 114 ) and at least one the first electrode ( 112 ) and the second electrode ( 114 ) connecting solid electrolyte ( 116 ), the first electrode ( 112 ) via a gas access path ( 124 ) with gas from the sample gas space ( 122 ) is acted upon, wherein the Gaszutrittsweg ( 124 ) at least one diffusion barrier ( 130 ), wherein the diffusion barrier ( 130 ) at least one first diffusion barrier region ( 138 ) and at least one second diffusion barrier region ( 142 ), wherein the second diffusion barrier region ( 142 ) is designed to be finer pored than the first diffusion barrier region ( 138 ). Sensor element ( 110 ) according to the preceding claim, wherein the second diffusion barrier region ( 142 ) the first diffusion barrier region ( 138 ) is upstream and / or downstream. Sensor element ( 110 ) according to one of the preceding claims, wherein gas from the sample gas space ( 122 ) first through the second diffusion barrier region ( 142 ), then through the first diffusion barrier region (FIG. 138 ) and then again through the second diffusion barrier region. Sensor element ( 110 ) according to the preceding claim, wherein the gas through the second diffusion barrier region ( 142 ) passes perpendicular to a layer structure and through the first diffusion barrier region ( 138 ) passes laterally to the layer structure. Sensor element ( 110 ) according to one of the preceding claims, wherein the first diffusion barrier region ( 138 ) to a predominant proportion to a diffusion resistance of the diffusion barrier ( 130 ), wherein the second diffusion barrier region ( 142 ) as a flow barrier and / or homogenizer at the beginning and / or end of the diffusion barrier ( 130 ) acts. Sensor element ( 110 ) according to one of the preceding claims, wherein the second diffusion barrier region ( 142 ) has a porous material having at least one of the following properties: a pore size of between 0.03 μm and 3 μm; a porosity between 5% and 60%, in particular a porosity less than 40%. Sensor element ( 110 ) according to one of the preceding claims, wherein the second diffusion barrier region ( 142 ) has a thickness between 10 .mu.m and 200 .mu.m, in particular a thickness between 20 .mu.m and 100 .mu.m. Sensor element ( 110 ) according to one of the preceding claims, wherein the first diffusion barrier region ( 138 ) a plurality of along the Gaszutrittswegs ( 124 ) extending channels ( 140 ). Sensor element ( 110 ) according to the preceding claim, wherein gas from the sample gas space ( 122 ) on the way to the first electrode ( 112 ) substantially perpendicular to the channels ( 140 ) into the channels ( 140 ) entry. Sensor element ( 110 ) according to one of the two preceding claims, wherein the second diffusion barrier region ( 142 ) at least partially as a flow barrier layer ( 144 ) and / or at least one flow barrier layer ( 144 ), wherein the flow barrier layer ( 144 ) has a lateral extent which is substantially parallel to a longitudinal extent of the channels ( 140 ). Sensor element ( 110 ) according to the preceding claim, wherein the flow barrier layer ( 144 ) the channels ( 140 ) at least partially covered. Sensor element ( 110 ) according to one of the two preceding claims, wherein the gas access path ( 124 ) is set up such that gas from the sample gas space ( 122 ) on the way to the first electrode ( 112 ) the flow barrier layer ( 144 ) first passes at least once, then the channels ( 140 ) and then the flow barrier layer ( 144 ) passes at least once more. Sensor element ( 110 ) according to one of the preceding claims, wherein the sensor element ( 110 ) has a layer structure, wherein the first diffusion barrier region ( 138 ) and the second diffusion barrier region ( 142 ) are arranged in different, preferably adjacent layer planes. Sensor element ( 110 ) according to the preceding claim, wherein the gas access path ( 124 ) further arranged in a further layer plane electrode cavity ( 132 ) of the first electrode ( 112 ), wherein the electrode cavity ( 132 ) the diffusion barrier ( 130 ) is downstream. Sensor element ( 110 ) according to one of the two preceding claims, wherein the gas access path ( 124 ) continue one in another Layer plane arranged connection cavity ( 126 ), wherein the connection cavity ( 126 ) the diffusion barrier ( 130 ) is upstream. Sensor element ( 110 ) according to the preceding claim, wherein the second diffusion barrier region ( 142 ), in particular a first diffusion barrier region ( 138 ) downstream part of the second diffusion barrier region ( 142 ) has a larger diffusion resistance than a gas diffusion resistance of the bonding cavity ( 126 ).

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