DE102008054617A1 - Structured electrode for ceramic sensor elements - Google Patents

Abstract

The invention relates to a sensor element (110), in particular for detecting a property of a gas in a measuring gas space. The sensor element (110) comprises at least one solid electrolyte layer (116) with at least one ceramic solid electrolyte material (118) and at least one electrode (112) which directly or indirectly contacts the solid electrolyte layer (116). The electrode (112) comprises at least one ceramic material (122) and at least one metallic material (120). The electrode (112) has a lateral extent and, within the lateral extent, has a structuring (126, 140) extending parallel and / or perpendicular to the lateral extent. The patterning (126, 140) is arranged to increase an electrode area of the electrode (112).

Description

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 various fields of science and technology, the invention described below is not limited to a particular application. In particular, however, such sensor elements can be used in the field of exhaust gas analysis, for example in motor vehicles, stationary engines or incinerators. For example, such sensor elements can be used in automobiles to measure air-fuel gas mixture compositions. In particular, sensor elements of this type are used in so-called lambda sensors and play an essential role in the reduction of pollutants in exhaust gases, both in gasoline engines and in diesel technology. As an example of possible structures of such sensor elements can on Robert Bosch GmbH: "Sensors in the motor vehicle", June 2001, pages 112 to 117 to get expelled. However, the invention is also applicable in principle to other types of sensor elements which comprise solid electrolytes of the type described, for example, in addition to jump probes and / or broadband probes on particle sensors or similar types of sensors with solid electrolytes. Without limiting the scope of protection or other possible types of sensor elements and uses according to the invention, the invention will be explained below using the example of lambda probes, although, in the light of the above, other types of sensor elements can be manufactured.

ceramic Sensor elements based on the use of solid electrolyte materials are based, usually have at least one electrode, by means of derer the at least one solid electrolyte layer directly or indirectly, d. H. by direct contact or by interposition of a or more intermediate layers, is contacted. For example in lambda sensors, which are used as so-called "finger probes" usually single-layer, unstructured outer electrodes (OE) and internal electrodes (IE) in thin-film technology, for example, by vapor deposition or sputtering, produced by platinum. Also printed electrodes, such as produced by pad printing Grid electrodes in thin-film technology are known. For example, as electrode materials, metal-ceramic electrode materials (Cermet) used. For example, noble metal cermets can be used such as platinum cermets. An example for the ceramic component of such cermets is yttria-stabilized zirconia (YSZ).

Next Lambda probes of the type of finger probes are also other types of Lambda probes known, for example planar probes, which in ceramic film technology are constructed. This usually flat unstructured Screen printed inner electrodes and outer electrodes applied with platinum YSZ cermet in thick film technology.

Farther For example, the sensor electrodes described above may be replaced by a Selection of sintering parameters, protective layer properties or by additional processes such as impregnation or affects an electrical activation in their properties or be adjusted.

The Electrode structures in known sensor elements, such as Sensor elements of the type described above, however, make in their manufacture technically a challenge. That's the way it is, especially in front of you the background of sharply increased precious metal prices, in particular required for precious metals like platinum, powerful electrodes with the lowest possible amounts of precious metal used to create. This defines the performance usually by the electrode activity, d. H. the catalytic Activity, and the maximum possible metabolic rate per unit area of an electrode, for example one Outer electrode.

requirement for a large substance turnover at the electrodes However, the largest possible number of catalytic active centers within a unit volume unit the electrode and a good electron and / or ion conductivity of the entire electrode system, for example in a Nernst cell.

Disclosure of the invention

It Therefore, starting from the problem described above, a sensor element and a method for producing a sensor element proposed which powerful electrodes at yet provide low demand for precious metals.

A basic idea of the present invention is the recognition that there is a direct correlation of the efficiency of the electrodes with the available three-phase boundary between electron-conductive electrode material, ion-conductive solid electrolyte material and the surrounding gas medium. Furthermore, there is a correlation between the efficiency of the electrode and the number of times for the Gas inlet mainly responsible pores, responsible for ionic conduction solid electrolyte material (for example, YSZ) and the precious metal of the electrode, in particular platinum, which is responsible for the decomposition and oxidation catalysis. Pores, solid electrolyte material and electrode metal form triple points as active centers of the electrode or of a catalyst. The maximization of electrode efficiency and electrode activity in sensor elements, with simultaneously reduced amounts of noble metals, is achieved by enlarging these three-phase boundaries and by increasing the active centers through large surfaces with finely divided porosity, finely divided precious metal and solid electrolyte material with good gas access.

Farther should the electrode system through the cell or cells of the sensor element through, for example, a Nernst cell of the sensor element, be well conductive and therefore have a low internal resistance. If a low internal resistance, for example, even at low Temperatures, in particular temperatures of less than 350 ° C, can be achieved, the use of the electrode system in one not actively heated sensor ceramics are considered, for example, in cost-effective two-wheel sensor applications.

Accordingly a concept for the production of a sensor element is proposed, in which, for example by using printing methods, by means of a minimized amount of electrode material by means of a horizontal and / or vertical structuring, increased efficiency is achieved. The invention is in the following essentially with reference to platinum YSZ cermet electrodes. Also However, other embodiments are possible in principle For example, embodiments with precious metals other than metallic Component and / or other types of ceramics than ceramic Component. For example, electrode systems can be made of precious metals such as palladium (Pd) and / or ruthenium (Ru) and / or iridium (Ir) and / or Rhodium (Rh) and from metal mixtures or metal alloys, z. As Pt / Pd alloys and / or Pt / Au alloys. The transfer The embodiments of such electrode systems is easy implemented.

It Accordingly, according to the invention is a sensor element proposed, which in particular for detecting a property a gas can be used in a measuring gas space. For the possible Embodiments and uses of the sensor element may in particular to the above description. So can the sensor element For example, as a lambda probe, for example as unicellular and / or multicellular lambda probe, designed to be a concentration and / or a partial pressure of oxygen in a sample gas space determine, for example, in an exhaust gas of an internal combustion engine. However, other embodiments are also possible in principle For example, embodiments for measuring a concentration and / or a partial pressure of one or more other types of gas component in the gas and / or for detecting other parameters of the gas, for example for detecting particle contamination or the like.

The Sensor element comprises at least one solid electrolyte layer at least one ceramic solid electrolyte material. Let it go basically any solid electrolyte materials and / or use any geometry of the solid electrolyte layer. Especially preferred is the use of oxygen ion-conductive Solid electrolyte materials, in particular of yttrium-stabilized Zirconium dioxide (YSZ). Basically, however, are others ceramic solid electrolyte materials used.

The Sensor element further comprises at least one of the solid electrolyte layer directly or indirectly contacting electrode. Under an electrode is basically an element to understand on which an installation or an expansion of ions into or out of the solid electrolyte layer can be done and which a corresponding electrically conductive contact provides. Under a direct or indirect contact is an embodiment to understand in which the electrode directly or with the interposition of at least one intermediate layer is in contact with the solid electrolyte layer.

The electrode comprises at least one ceramic material and at least one metallic material. In this respect, the electrode can be designed in particular as a cermet electrode. In this case as well, a solid electrolyte material can basically be used for the ceramic material, preferably a solid electrolyte material with a conductivity for the same type of ion as the solid electrolyte material of the solid electrolyte layer. For example, again yttrium-stabilized zirconia can be used for the ceramic material of the electrode. However, other materials can be used as phase-stabilizing additives to zirconia, alternatively or additionally, be used, such as other oxides, in particular metal oxides, for example, Ce ₂ O _3, MgO, Sc ₂ O _3, CaO or mixtures of the above and / or other oxides.

In principle, any types of metals, metal mixtures or metal alloys can be used for the metallic material. Particularly preferred is the use of one or more of the following materials: platinum, palladium, rhodium, gold. Also, mixtures of said and / or other metals are conceivable and alloys of said and / or other metals such as platinum-palladium alloys and / or platinum-gold alloys. However, other embodiments are possible in principle.

The Sensor element can one or more of said solid electrolyte layers and one or more of said electrodes. Also different embodiments of electrodes within a Sensor element are conceivable, for example, at least one inventive Configuration of the electrode, combined with at least one State of the art corresponding embodiment of another electrode. in principle For example, sensor elements of the planar probe type can be produced, For example, simple jump probes, broadband probes or the like Types of Planar Probes. Also known from the prior art Abutments for finger probes are basically can be produced and used according to the invention.

The at least one electrode has a lateral extent. Under a lateral extension is to be understood as an extension which essentially parallel to the expansion of the solid electrolyte layer extends. For example, this lateral extent may be limited for example in the form of separate electrode geometries, for example rectangular, round, oval or polygonal electrode geometries. Such electrode geometries can, for example, by structured order processes, for example printing processes, be generated.

According to the invention proposed the electrode within the lateral extent with a parallel and / or perpendicular to the lateral extent to provide extending structuring. Under a structuring is to understand a configuration of the electrode in general in which at least one physical and / or chemical Property of the electrode in the mentioned direction, ie parallel and / or perpendicular to the lateral extent. In particular, this physical and / or chemical property may be to a thickness variation, a conductivity variation, a density variation, a variation of porosity a geometric variation or other types of variations which act at least one physical and / or chemical property.

These Structuring should be arranged to an electrode surface to increase the size of the electrode. Under an electrode surface is generally an area to understand, which the provides three-phase boundary, ie a transition between the metallic material for providing a electrical conductivity, a ceramic material for providing ionic conductivity and a surface for gas access. In other Words should be compared by the proposed structuring to non-structured, conventional electrodes the number the triple points are increased.

Especially it is preferred if the structuring is a lateral structuring So, a structuring parallel to the lateral extent the electrode. The lateral structuring can in particular a Thickness variation of a layer thickness of the electrode include, wherein the thickness variation comprises at least two local maxima of the layer thickness. For example, it may be a succession of mountains and valleys act in the layer thickness of the electrode. Furthermore, for example, a Pyramidal structure with a plurality of pyramids, a wave structure with a plurality of troughs and wave crests, one Tip structure with a plurality of peaks, d. H. surveys with a pointed end, or a combination of said structures or other structures. The majority of local Surveys can be regular or irregular be arranged. Furthermore, alternatively or additionally, structuring also includes one or more of the following structuring comprising: a roughening in the electrode surface; a line pattern in the electrode surface, in particular an irregular one Line pattern, but also regular line pattern conceivable are; a grid in the electrode surface, in particular a dot pattern, especially an irregular one Dot pattern; a hole pattern, in particular an irregular one Lace pattern, but also regular hole pattern possible are.

The structures mentioned can be produced in various ways. Thus, for example, a method for producing a sensor element, in particular a sensor element according to one or more of the described embodiments, is proposed, in which the structuring is already generated during the application of the electrode. Thus, for example, a structured application technique of an electrode paste can be used by means of which the electrode paste is applied in a structured manner, for example directly or indirectly to the solid electrolyte layer. In particular, these can be structured printing techniques, for example printing techniques, with which one or more of the above-described lateral structuring is realized. Thus, for example, tampon printing techniques, screen printing techniques or similar printing techniques can be used. Particularly preferred are mask techniques in which an electrode paste with a pattern and / or grid is applied structured. Alternatively or additionally, however, the lateral structuring can also be produced in a subsequent structuring technique, in which the structuring is produced after application of the electrode paste. In particular, this structuring can be produced, for example, by means of a brush technique and / or a plucking technique, by means of which, for example, one or more of the mentioned structurings can be produced. However, other types of production of the mentioned structures or combinations of production methods are conceivable in principle.

alternative or in addition to the lateral structuring, which extending parallel to the lateral extent of the electrode, Vertical structuring is also possible, ie structuring perpendicular to the lateral extent of the electrode. This is how structuring can be done For example, a gradient in at least one physical and / or chemical property of the electrode perpendicular to the lateral Extension include. In particular, it may be a porosity gradient and / or a material gradient and / or a density gradient act. Alternatively or additionally, the structuring can also a gradient in a concentration of the metallic material comprise, in particular one or more of the above-mentioned metallic Materials.

to Generation of the gradient in the direction perpendicular to the lateral Expansion of the electrode can be fundamentally different Use methods which generate such a gradient or changes are suitable. In particular, as For example, an electrode paste with a pore former can be used. Such pore-forming agents are known from the prior art and For example, materials included in sintering in the Go over the gas phase and leave a cavity. Typical examples of pore formers are: glassy carbon, flame black, waxy hydrocarbon compounds, polymer beads or combinations of said and / or other types of pore-forming agents.

So For example, the electrode paste can be equipped with a pore former be compared to a surrounding material of the electrode paste, for example, an organic component of the electrode paste, has different density. Due to the density differences between the pore former and the other elements of the paste migrates the pore builder in the applied electrode paste, leaving a Concentration gradient of the pore-forming agent within the not yet dried layer of the electrode paste is formed. For example may be the pore former with a lower density than the rest Materials or at least one material component of the electrode paste be equipped, for example, a binder component of the electrode paste. It is particularly preferred if the pore-forming agent is designed in this way is that it migrates away from the solid electrolyte layer, so that the concentration of the pore-forming agent in the direction away from the solid electrolyte layer increases. After sintering then forms due to the concentration gradient of the pore-forming agent a gradient in a pore density or porosity in the electrode. In this way, for example, at one of the solid electrolyte layer pioneering side of the electrode a increased surface to be created.

One Another aspect of the present invention, which alternatively or in addition to the embodiments described above can be used, is an aspect of an indirect contact the solid electrolyte layer through the electrode by means of one or several intermediate layers. The idea of these intermediate layers can basically be combined with the structuring described above the electrode in one or more of the embodiments described above be used. Alternatively, however, use is complete Unstructured electrodes conceivable. Basically for the possible embodiments of such sensor elements For example, to the above description of possible embodiments of Sensor elements are referenced with structured electrodes, wherein Such sensor elements then the aspect of at least one Intermediate layer must be supplemented.

The proposed sensor element according to this aspect The invention again comprises at least one solid electrolyte layer with at least one ceramic solid electrolyte material and at least an electrode contacting the solid electrolyte layer indirectly with at least one ceramic material and at least one metallic one Material. The contacting is indirect in the sense that between the electrode and the solid electrolyte layer at least an intermediate layer is introduced. This intermediate layer has According to the invention, a higher ionic conductivity and / or higher electron conductivity as the solid electrolyte layer.

This aspect of the invention aims to keep the total resistance of the electrode system as small as possible, as is advantageous, for example, for the low-temperature behavior of exhaust gas probes. For this purpose, the at least one contact resistance of the electrode and the solid electrolyte layer is reduced. For example, density and good ion-conducting solid electrolyte materials can also be used for the intermediate layer. For example, the same Festelekt rolytmaterialien, which are also used for the solid electrolyte layer, use alone or in combination with other materials in the at least one intermediate layer. However, other materials, such as ceramic materials, are in principle applicable. It is particularly preferred if both the solid electrolyte material and the intermediate layer each comprise zirconium dioxide, for example yttrium-stabilized zirconium oxide, wherein the intermediate layer has a higher doping, in particular a higher yttrium doping and / or a higher doping with yttrium oxide, than that Solid electrolyte material of the solid electrolyte layer.

The proposed sensor element in one or more of the embodiments described above, as well as the proposed method for the production of sensor elements have a multiplicity of advantages over known sensor elements and known methods. For example, in many sensor elements, the available electrode area, for example, the electrode area for the outer electrodes (OE) and the inner electrodes (IE), is limited by the dimensioning and the structure of the sensor elements. Typical available OE areas move in planar jump probes, for example, between 0.04 and 0.1 cm ² . In this case, for example, rectangular or oval surfaces can be used, which can be printed by means of a Pt-YSZ cermet electrode paste directly onto a solid electrolyte layer, for example in the form of a ceramic carrier film. The composition of the electrode pastes, for example the proportion of platinum, YSZ and optionally further components, in particular organic components such as binders, solvents or the like, can be at least co-determined for application of the electrodes by printing technology requirements. The minimum usable particle size of possible starting materials, such as raw material powders, and thus the fineness of the resulting cermet can be limited by cost and safety aspects. For example, the grain size of the solids used may be predetermined not to fall below a certain limit, although this would be helpful in maximizing the number of active sites. For example, a typical Pt-YSZ cermet electrode paste has, for example, a composition of 60 to 70% by weight of platinum, 10 to 25% by weight of YSZ and 10 to 20% by weight of organic fractions having a grain size d90> 15 μm for the incorporated metal and ceramic powder.

Around the number of active sites with less platinum use too can maximize according to the first one described above Aspect of structuring the electrode the electrode surface according to the invention by a horizontal (i.e. parallel to the lateral extent) and / or vertical (i.e., perpendicular to the lateral extent) structuring enlarged become.

• – Abgasraum
• – Äußere Elektrode (OE)
• – Zwischenschicht
• – Festelektrolytschicht
• – Zwischenschicht
• – Innenelektrode (IE)
• – Referenzluft.

By contrast, in the further aspect of the additional at least one intermediate layer, an electrical and / or ionic conductivity of the electrode system is specifically improved. The total resistance of the electrode system results from the sum of the volume resistances through the solid electrolyte layer and the two electrode layers (bulk) as well as the contact resistances between the electrodes and the solid electrolyte layer or the ceramic substrate (interface). For example, in the case of a lambda probe in the finger probe design, such a design can be structured as follows:

• - exhaust gas space
• - Outer electrode (OE)
• - intermediate layer
• - Solid electrolyte layer
• - intermediate layer
• - inner electrode (IE)
• - Reference air.

Also However, other layer structures are basically possible and to look at it analogously. To the total resistance of the electrode system keep as small as possible, for example, to improve of the low-temperature behavior described above, can be by means of the proposed, good ion-conducting and dense interlayers greatly reduce the contact resistance.

For example Intermediate layers of YSZ can be used, which also have a Have a share of metallic material. For example may again be one or more of the metallic ones described above Materials are used, such as platinum. Preferably is the concentration of metallic materials in the interlayer but lower than in the electrode.

Farther may, alternatively or additionally, the porosity the intermediate layer are kept lower than the porosity the electrode. For example, the porosity of at least one intermediate layer between the porosity the electrode, such as the platinum cermet electrode, and the porosity lie the solid electrolyte layer.

For example, the solid electrolyte layers can have 3 to 7 mol%, for example 5.5 mol%, of YSZ, these molar amounts generally being based on the molar content of yttrium oxide in the zirconium oxide. For example, 5.5 mol% of Y ₂ O _{3 may be present} in the ZrO _{2 of} the solid electrolyte, whereas, for example, 9.0 mol% of Y ₂ O ₃ in the intermediate layer and, for example, 11.0 mol% in the cermet. example example, a solid electrolyte support of tetragonal stabilized ZrO ₂ (TPZ) with 3.0 mol% Y ₂ O ₃ in ZrO ₂ or partially stabilized 5.5 mol% Y ₂ O ₃ in ZrO ₂ can be used. For example, 5.5 mol% to 8.0 mol% of Y ₂ O ₃ can be used for the intermediate layer, in particular in cubic, fully stabilized form. For example, 8.0 mol% to 11 mol% Y ₂ O ₃ can be used for the cermet of the electrodes.

The at least one intermediate layer can improve the bonding between the electrode and the solid electrolyte layer, for example the ceramic substrate of the solid electrolyte layer, and reduce the contact resistance. The at least one intermediate layer can likewise be structured, for example according to one or more of the structuring methods described above. Thus, structuring can take place parallel to the lateral extent of the electrode and / or a structuring perpendicular to this lateral extent of the electrode. The structuring techniques described above can be fully referenced in this connection. Thus, for example, at the same time structuring of the at least one intermediate layer and structuring of the at least one electrode may be present. The intermediate layer can be designed, for example, non-porous. For example, the intermediate layer may have a proportion of 5 to 11 mol% Y ₂ O ₃ in ZrO ₂ , in particular a proportion between 7 and 10 mol% Y ₂ O ₃ and particularly preferably a fraction of about 8 mol% Y ₂ O ₃ in ZrO ₂ .

These higher doping of the intermediate layer compared to the Although solid electrolyte layer usually causes a lower one mechanical strength of the at least one intermediate layer in comparison to the solid electrolyte material of the solid electrolyte layer. This can however, especially if the intermediate layer is a given one Thickness does not exceed, be tolerated, as with rising Doping, although the strength decreases, the ionic conductivity, however increases. In this way, the electrical and / or ionic connection be improved between solid electrolyte layer and electrode and Thus, the contact resistance can be reduced.

Brief description of the drawings

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

It demonstrate:

1A and 1B An embodiment of a prior art sensor element in different representations;

2A and 2 B a first embodiment of a sensor element according to the invention with an electrode having a pyramidal structure;

3A and 3B A second embodiment of a sensor element according to the invention with an electrode having a tip structure;

4 A third embodiment of a sensor element according to the invention with an electrode having a wave structure and an intermediate layer;

5 A fourth embodiment of a sensor element according to the invention with an electrode having a wave structure, a porosity gradient and an intermediate layer.

embodiments the invention

In the 1A and 1B is an example of a conventional sensor element 110 shown schematically. Shown is only one electrode at a time 112 of the sensor element 110 and a supply line 114 to this electrode 112 (only in 1A visible). It shows 1A the electrode 112 and the sensor element 110 in plan view, whereas 1B a sectional view of the sensor element 110 from the side shows.

The electrode 112 is on a solid electrolyte layer 116 applied, which is a solid electrolyte material 118 includes. For example, the solid electrolyte material may be an yttria-stabilized zirconia (YSZ), typically employing a 5.5 mol% YSZ. On this solid electrolyte layer 116 , which may be formed, for example, as a film layer, the electrode 112 applied. The electrode 112 For example, it is designed as a platinum YSZ cermet electrode and generally has a porous structure. Typically, for the preparation of this electrode 112 Electrode pastes used, which account for 60 to 70 wt .-% of metallic material 120 , in particular platinum, a proportion of 10 to 25 wt .-% of ceramic material 122 and organic constituents in a proportion of between 10 and 20% by weight, for example binder materials and / or solvents. The grain size for the incorporated powder of the metallic material 120 and the ceramic material 122 is typically d90> 15 μm. Typically, the electrodes have 112 a lateral extent parallel to a surface 124 and thus a lateral extent of the solid electrolyte layer 116 on which in 1B is denoted by D and which may for example be in the range of a few 10 microns to a few mm. However, other electrode dimensions are possible in principle. The thickness of the electrode 112 , what a 1B is denoted by d, for example in the range between a few microns to a few 10 microns or even 100 microns or above.

Like from the 1A and 1B can be seen, is the electrode corresponding to the prior art 112 unstructured within their lateral dimensions, ie (apart from a possibly present porosity) substantially homogeneous in their physical and / or chemical properties. In particular, the number of pores is uniform across the thickness of the electrode 112 distributed, and the thickness d of the electrode 112 is about the lateral extent of the electrode 112 essentially constant.

In the 2A to 5 On the other hand, different exemplary embodiments of a configuration of a sensor element according to the invention are shown 110 shown. Again, this is the sensor element 110 an example of an electrode 112 shown. Of course, a plurality of electrodes may be provided, for example electrodes lying opposite one another, electrodes in the same layer plane, single-cell or multi-cell sensor structures or the like. Also, not necessarily a planar geometry and a layer structure must be used, but it can, for example, a finger-like structure with curved surfaces are used.

In the 2A and 2 B is, in analogous representation to the 1A and 1B , A first embodiment of a sensor element according to the invention 110 shown. In contrast to the 1A and 1B is the electrode 112 in the embodiment according to the 2A and 2 B structured and has a structuring 126 parallel to the lateral extent of the surface 124 the solid electrolyte layer 116 on. Should this surface 124 have a curved shape, for example in finger probes, so this parallelism refers to the local lateral extent of the solid electrolyte layer 116 , As can be seen from the figures, the structuring is 126 in this case, for example in the form of a pyramidal shape 128 designed, in which the thickness of the electrode 112 , where mutatis mutandis, a plurality of electrode layers in this electrode 112 may be contained between approximately 0 and a maximum thickness d _max . The flanks of the pyramids of the pyramidal form 128 can be configured flat or curved, in 2 B a curved configuration is shown. As in 2A is recognizable, follows the structuring 126 in this embodiment, for example, a line pattern 130 in which the peaks or valleys of the pyramidal structures 128 along lines, with parallel lines intersecting with other parallel lines at, for example, an approximately right angle or other angle.

At one in the 3A and 3B illustrated second embodiment, which is similar to the 2A and 2 B is designed, is the pyramid structure 128 through a lace structure 132 replaced. This lace structure 132 forms, as in the plan view according to 3A is recognizable, for example, a dot pattern 134 , Again, the electrode points 112 in their extension parallel to the lateral extent of the solid electrolyte layer 116 , for example to the surface 124 , a variation in their layer thickness, being in the valleys of this structuring 126 the layer thickness may approximately disappear or at least may decrease to a percentage of, for example, less than 10% compared to the maximum layer thickness d _max . At their tips, the lace structure 132 sharp points on, but rounding can be provided. Other types of structuring are possible in principle.

The embodiments in the 2A to 3B show only two possible forms of lateral structuring in an extension parallel to a lateral extent of the solid electrolyte layer 116 , Instead of the dot pattern or line pattern illustrated in the figures, which could also be referred to as honeycomb patterning with a honeycomb pattern, numerous other types of lateral structuring are conceivable in which a periodic or non-periodic thickness variation in the layer thickness of the electrode 112 is used.

Such lateral or horizontal structuring can be used, for example, in an outer electrode (OE) and / or in an inner electrode (IE). As shown above, however, further embodiments are possible, for example embodiments in which not all electrodes are configured in a structured manner. The structuring 126 For example, it can be created with recesses, holes, or patterns that fit into the electrode 112 can be introduced. The maximum layer thickness d _max can be, for example, between 5 μm and 100 μm, for example between 10 μm and 50 μm, and in particular about 30 μm.

The structuring 126 can already do this while generating the electrode 112 into the electrode 112 be introduced. Thus, for example, a structuring by a printing screen, by means of which, for example, the above-described electrode paste on the solid electrolyte layer 116 can be applied. The printing screen may also include corresponding patterns, such as grids, dot patterns, or the like. For grid structures, it is also recommended to use an irregular grain grid, for example, to avoid moiré effects. Are periodic or approximately periodic rasters or patterns ver For example, the periodicity may be, for example, a few 10 μm to a few 100 μm, for example 30 μm to 50 μm. In the case of grids and grids, it is possible according to the invention to reduce areal coverage and thus consumption of electrode paste, for example of platinum paste.

Alternatively or in addition to an introduction of the structuring 126 when applying the electrode paste structuring can also be introduced later. For example, a structuring 126 by subsequent roughening, imprinting, grinding, brushing or a combination of said treatments and / or other treatments. This treatment can be carried out, for example, on a partially moist or already on a dried electrode layer in the green state after application of the paste, for example after printing.

Alternatively or additionally, structuring can also take place, for example, by using pore formers in a topmost printing layer or by using a very low density voiding agent which, after application, so to speak, floats in the electrode paste and thus a density gradient in the electrode 112 generated, which, for example, can be recognized only after the sintering fire. In that sense, in the 2A to 3B shown lateral structuring 126 also with a vertical structuring perpendicular to the surface 124 the solid electrolyte layer 116 be combined.

For the preparation of the lateral structuring 126 can also use a variety of techniques, which can also be used in combination. Thus, the variation of the layer thickness can be done for example by means of an embossing, wherein the electrode 112 is structured. Alternatively or additionally, the solid electrolyte layer may also be used 116 be structured. For example, a wave or pyramid structure can be created in this way. Alternatively or additionally, the variation of the layer thickness can also be produced by printing the electrode paste, followed by picking and / or unwinding with a correspondingly structured roller.

there For example, it is very difficult to make firmly fixed electrode pastes deploy. When not dried can be very tough and / or sticky electrode pastes, for example electrode pastes with long-chain binder systems, also directly after application, for example, imprinting, due to their tackiness, d. H. Tackiness, roughened on the surface. Various other techniques are possible.

Furthermore, it should be noted that in the 2A to 3B only one layer of the electrode 112 is shown. It is also possible to produce multilayer electrodes, wherein one, several or all of these layers can be configured in a structured manner. Furthermore, the electrodes can 112 also be provided with a protective layer not previously shown in the figures, for example, a porous, gas-permeable, ceramic protective layer which the electrode 112 For example, it can protect against dirt.

As indicated above, another aspect of the present invention is to further enhance the functionality of the electrode structures through the use of one or more intermediate layers. In this way, overall the electrical and ionic conductivity of the electrode system can be increased and the total resistance of the electrode system can be reduced. Such intermediate layers can also be combined with a lateral structuring. So shows 4 a first embodiment of a sensor element 110 , in one of the 2 B and 3B analog representation, in which this concept of the invention is realized. At first an electrode will be used 112 with a metallic material 120 and a ceramic material 122 used, which in turn optionally a lateral structuring 126 parallel to the surface 124 the solid electrolyte layer 116 can have. This lateral structuring 126 is in the illustrated embodiment as a wave structure 136 designed, for example, the wave structure 136 turn a line pattern 130 analogous to the embodiment according to FIG 2A can have. For the possible manufacturing processes of this electrode 112 as well as the possible configuration of the electrode paste can be largely referenced to the above description.

In contrast to the embodiments according to the 2A to 3B is the electrode 112 however, in the in 4 shown embodiment indirectly on the solid electrolyte layer 116 applied. Between the actual electrode 112 and the solid electrolyte layer 116 is an intermediate layer in the illustrated embodiment 138 brought in. Also this intermediate layer 138 can be a lateral structuring 126 have, for example, again in the form of a wave structure 136 , Also level intermediate layers 138 without structuring 126 are possible in principle. Furthermore, the idea is to use an intermediate layer 138 also usable without lateral or vertical structuring. For the optional structuring 126 the intermediate layer 138 can be largely referenced to the above description, so that this intermediate layer 138 For example, again in the form of honeycomb structures, pyramidal structures, wave structures, lace structures, line patterns, lattice structures or the like. For example First, the at least one intermediate layer 138 on the solid electrolyte layer 116 be applied, for example, be printed. The structuring can in turn already take place during the application and / or subsequently. Subsequently, the at least one electrode 112 on the intermediate layer 138 be applied, for example, be printed, in turn, optionally structuring, for example, during application and / or can be done later.

The at least one intermediate layer 138 serves the low-resistance connection between the solid electrolyte layer 116 and the electrode 112 , As stated above, this can greatly reduce the total resistance of the electrode system. This is, for example, for a low-temperature behavior for sensor elements used in exhaust gas probes 110 of particular advantage. In particular, intermediate layers can be used 138 use, which have a high ionic conductivity and are dense. In particular, ceramic materials can be used for this purpose 122 which use a small amount of metallic materials 120 can have.

For example, in one embodiment, the solid electrolyte material 118 the solid electrolyte layer 116 be prepared from 5.5 mol% Y ₂ O ₃ -stabilized ZrO ₂ . The Y ₂ O ₃ content is typically selected such that it optimizes the phase stability and the mechanical strength of the solid electrolyte material 118 guaranteed. For example, proportions of 3.0 to 8.0 mol%, in particular of 5.0 to 6.0 mol%, and particularly preferably the said proportion of 5.5 mol% Y ₂ O _{3 can be} used. The additional intermediate layer 138 is then advantageously composed of 8.0 mol% Y ₂ O ₃ -stabilized ZrO ₂ together. Also, for example, ranges between 7.0 mol% and 11.0 mol% are possible. The Y ₂ O ₃ content in this intermediate layer 138 is preferably chosen to optimize electron conductivity and oxygen ion conductivity (O ₂ ion conductivity). However, other embodiments are possible in principle.

In 5 is another embodiment of a sensor element according to the invention 110 represented, in which also an intermediate layer 138 is used. Insofar, for example, the description of the 4 to get expelled. Again, this interlayer 138 for example, with a lateral structuring 126 Mistake. Furthermore, the idea of the lateral intermediate layer is 138 in 5 again with the idea of a laterally structured electrode shown in the preceding exemplary embodiments 112 combined. The lateral structuring 126 is for example in the form of a lace structure 132 educated. However, other structures are possible in principle. As shown above, the concepts of using an intermediate layer can be understood 138 and the concept of lateral structuring 126 the intermediate layer 138 and / or the electrode 112 but in principle realize independently.

Furthermore, in the embodiment according to 5 presented a further concept of the invention, which can be used independently of the concepts described above, which, however, in principle in combination with the illustrated concept of using at least one intermediate layer 138 and / or the concept of using a lateral structuring 126 the at least one electrode 112 and / or the at least one intermediate layer 138 can be realized. This concept includes a vertical or vertical structuring 140 (The terms vertical and vertical structuring are used synonymously in the present description), in which there is a variation of physical and / or chemical properties in the electrode 112 and / or in which at least one intermediate layer 138 and the at least one electrode 112 composite electrode complex in a direction perpendicular to the lateral extent of the solid electrolyte layer 116 , for example, the surface 124 , results. As an example of such vertical structuring 140 may be a concentration gradient in a proportion of the metallic material 120 , for example in a platinum portion, are used. Alternatively or additionally, a gradient in a porosity or pore density can also be used, wherein the pores in 5 symbolically with the reference number 142 are indicated. Alternatively or additionally, a variation of other physical and / or chemical parameters for vertical structuring may also be used 140 be used.

As shown above, this vertical structuring can be done 140 generate in different ways. For example, a vertical variation of the density of the pores 142 be produced by using a pore-forming agent, which is subjected in a still liquid or at least wet electrode paste due to density differences of a continuous or gradual separation. For example, this pore-forming agent may float in the electrode paste after application, so that the density of the pores 142 on the of the solid electrolyte layer 116 opposite side of the electrode 112 is higher than that of the solid electrolyte layer 116 assigning page. Alternatively or additionally, it is also possible to apply a plurality of layers of electrode materials, each with a different proportion of pore former, so that a gradient in the density of the pore 142 can be generated. Furthermore, the vertical structuring 140 also in a proportion of the metallic material 120 in the electrode complex. For example, as shown above, the at least one electrode 112 a higher proportion of metallic material 120 included, for example, platinum, as the at least one intermediate layer 138 ,

The vertical structuring 140 is right in 5 by the reference numbers 144 and 146 indicated. This is the reference number 144 a high proportion of metallic material 120 , for example platinum, and increasing or high porosity. The reference number 146 on the other hand, denotes a high proportion of ceramic material 122 , for example to YSZ. The vertical gradient in the concentration of the metallic material, the porosity and the proportion of YSZ can be gradual, continuous or stepwise. Various configurations are possible.

The by the vertical structuring 140 symbolic in 5 indicated gradient in one or more physical and / or chemical quantities can be about the the electrode 112 and the intermediate layer 138 extend across the entire electrode complex. Furthermore, this gradient can optionally also in the solid electrolyte layer 116 continue into it. Generally, the gradient may preferably include one or more of the following: porosity; a metal content and / or ceramic content; a grain size of solids; a molar content of a dopant, in particular a doping oxide, in particular Y ₂ O ₃ , in a matrix, for example, a ZrO ₂ matrix. Combinations of the mentioned and / or other gradients are possible. In this case, the gradient of a porosity should preferably run in such a way that the porosity of the solid electrolyte layer 116 towards the intermediate layer 138 and to the electrode 112 increases. The metal content and / or the molar content of the dopant should also preferably increase in this direction. The grain size and / or the ceramic content, however, should preferably increase in the opposite direction.

QUOTES INCLUDE IN THE DESCRIPTION

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

Cited non-patent literature

• - Robert Bosch GmbH: "Sensors in the Motor Vehicle", June 2001, pages 112 to 117 [0001]

Claims (10)

Sensor element ( 110 ), in particular for detecting a property of a gas in a measuring gas space, comprising at least one solid electrolyte layer ( 116 ) with at least one ceramic solid electrolyte material ( 118 ) and at least one of the solid electrolyte layer ( 116 ) directly or indirectly contacting electrode ( 112 ) with at least one ceramic material ( 122 ) and at least one metallic material ( 120 ), wherein the electrode ( 112 ) has a lateral extent, wherein the electrode ( 112 ) within the lateral extent a structuring extending parallel and / or perpendicular to the lateral extent ( 126 . 140 ), the structuring ( 126 . 140 ) is arranged around an electrode surface of the electrode ( 112 ) to enlarge. Sensor element ( 110 ) according to the preceding claim, wherein the structuring ( 126 . 140 ) a lateral structuring ( 126 ), wherein the lateral structuring ( 126 ) a thickness variation of a layer thickness of the electrode ( 112 ), wherein the thickness variation comprises at least two local maxima in the layer thickness. Sensor element ( 110 ) according to the preceding claim, wherein the thickness variation comprises a plurality of local peaks having at least one of the following shapes: a pyramidal structure; a wave structure; a lace structure; a honeycomb structure. Sensor element ( 110 ) according to one of the preceding claims, wherein the structuring ( 126 . 140 ) at least one of the following structurings ( 126 . 140 ) comprises: a roughening in the electrode surface; a line pattern in the electrode surface, in particular an irregular line pattern; a grid in the electrode surface, in particular a dot pattern, in particular an irregular dot pattern; a hole pattern, in particular an irregular hole pattern. Sensor element ( 110 ) according to one of the preceding claims, wherein the structuring ( 126 . 140 ) a vertical structuring extending perpendicular to the lateral extent ( 140 ), the vertical structuring ( 140 ) comprises one or more of the gradients extending perpendicular to the lateral extent: a porosity gradient; a material gradient; a density gradient. Sensor element ( 110 ) according to one of the preceding claims, wherein the structuring ( 126 . 140 ) a vertical structuring extending perpendicular to the lateral extent ( 140 ), the vertical structuring ( 140 ) a gradient in a concentration of the metallic material ( 120 ), in particular in a concentration of one or more of the following metals: Pt; Pd; Rh; Au; Ru; Ir; a Pt / Pd alloy; a Pt / Au alloy. Sensor element ( 110 ), in particular according to one of the preceding claims, comprising at least one solid electrolyte layer ( 116 ) with at least one ceramic solid electrolyte material ( 118 ) and at least one of the solid electrolyte layer ( 116 ) indirectly contacting electrode ( 112 ) with at least one ceramic material ( 122 ) and at least one metallic material ( 120 ), wherein between the electrode ( 112 ) and the solid electrolyte layer ( 116 ) at least one intermediate layer ( 138 ), in particular an intermediate layer having a structuring ( 126 . 140 ), the intermediate layer ( 138 ) a higher ionic conductivity and / or a higher electronic conductivity than the solid electrolyte layer ( 116 ) having. Sensor element ( 110 ) according to the preceding claim, wherein the solid electrolyte material ( 118 ) and the intermediate layer ( 138 each comprise yttrium-doped zirconium oxide, in particular yttria-doped zirconium oxide, wherein the intermediate layer ( 138 ) has a higher yttrium doping, in particular a higher yttrium oxide doping, than the solid electrolyte material ( 118 ). Method for producing a sensor element ( 110 ), in particular a sensor element ( 110 ) according to one of the preceding claims, wherein at least one electrode ( 112 ) directly or indirectly on a solid electrolyte layer ( 116 ) with at least one ceramic solid electrolyte material ( 118 ) is applied, wherein the electrode ( 112 ) has a lateral extent, wherein a lateral structuring extending parallel to the lateral extent (FIG. 126 ) in the electrode ( 112 ), wherein the lateral structuring ( 126 ) is produced by means of at least one of the following structuring techniques: by means of a structured application technique, an electrode paste by means of which the electrode paste is applied in a structured manner, in particular a printing technique; by means of a masking technique with which an electrode paste is patterned with a pattern or grid applied; by means of a subsequent structuring technique with which, after the application of an electrode paste, the lateral structuring ( 126 ), in particular by means of a brushing technique and / or a plucking technique. Method for producing a sensor element ( 110 ), in particular a sensor element ( 110 ) according to one of the preceding, a sensor element ( 110 ), wherein at least one electrode ( 112 ) directly or indirectly to one Solid electrolyte layer ( 116 ) with at least one ceramic solid electrolyte material ( 118 ) is applied, wherein the electrode ( 112 ) has a lateral extent, wherein a vertically structuring extending perpendicular to the lateral extent ( 140 ) in the electrode ( 112 ), wherein an electrode paste is applied with a pore-forming agent, wherein the pore-forming agent has a different density compared to a surrounding material of the electrode paste, wherein due to the density differences, a concentration gradient of the pore-forming agent in the applied electrode paste, wherein due to the concentration gradient of the pore-forming agent a gradient in a pore density in the electrode ( 112 ) arises.