Patent Application
20110214989
The present invention is directed to known ceramic sensor elements, in particular sensor elements which operate based on electrolytic properties of certain solids, specifically the capacity of these solids to conduct certain ions. Sensor elements of this type are in particular utilized in motor vehicles. As an important example of ceramic sensor elements in motor vehicles, reference may be made to those that are used to determine the composition of an air-fuel mixture, these also being called “lambda sensors” and playing an important role in the reduction of pollutants in exhaust gases, both in spark ignition and in compression ignition engines. However, the present invention is also applicable to other types of ceramic sensor elements, for example to particle sensors or similar types of sensors having solid electrolytes, in particular in exhaust gas sensor systems. Without limiting the scope of protection, the present invention is described below using the example of lambda sensors, although, as recited above, other types of sensor elements may also be manufactured.
The air/fuel ratio known as “lambda” (A) is generally used in combustion engineering to designate the ratio between a mass of air actually available and the mass of air theoretically needed for combustion (i.e., the stoichiometric mass of air). In this process, the air/fuel ratio is measured with the aid of one or more sensor elements generally at one or more or places in the exhaust system of an internal combustion engine. Accordingly, “rich” gas mixtures (in other words, gas mixtures having an excess of fuel) have an air/fuel ratio λ<1, whereas “lean” gas mixtures (in other words, gas mixtures having a deficit of fuel) have an air/fuel ratio λ>1. In addition to motor vehicles, these and similar sensor elements are also used in other branches of engineering (in particular combustion engineering), for example in aviation engineering or in the regulation of burners, for example in heating plants or power stations.
Lambda sensors are known from the related art in numerous different specific embodiments. A first specific embodiment is represented by the so-called discrete-level sensor, the operating principle of which is based on measurement of an electrochemical difference in potential between a reference gas and the gas mixture to be analyzed. The reference electrode and the measuring electrode are connected together by the solid electrolyte. Owing to their good oxygen ion-conductive characteristics, generally speaking zirconium dioxide (e.g., yttrium-stabilized zirconium dioxide, YSZ) or similar ceramics are used for the solid electrolyte. As an alternative or in addition to discrete-level sensors, so-called pumping cells are also used, in which an electrical pumping voltage is applied to two electrodes joined together by the solid electrolyte and the pumping current is measured by the pumping cell. The described sensing principles of discrete-level cells and pumping cells may also be advantageously utilized in combination in so-called multi-cell devices, in particular in broadband lambda sensors. Various exemplary embodiments of lambda sensors that may also be modified according to the present invention within the scope of the present invention are discussed in “Sensoren im Kraftfahrzeug” (Sensors in motor vehicles), 2nd edition, Robert Bosch GmbH, April 2007, pp. 154-159.
With such sensor elements, however, for example sensor elements of the type discussed above, the selection of the material, and in particular the selection of the material for the solid electrolyte, in many cases represents a compromise. For example, on the one hand severe demands are placed on the electrolytic properties of the material for the solid electrolyte. The material in particular needs to have a high conductivity, for example, for ions of the gas component that is to be detected. Such an ionic conductivity is generally achieved with the aid of lattice voids, for example oxide ion voids in a metal oxide lattice. In order to create these oxide ion voids, generally speaking doping is used. In this process, a doping material is added to the metal oxide of the matrix material of a solid electrolyte, and occupies lattice sites of the metal oxide, but creates oxygen voids as a result of having a different valency from the metal of the metal oxide of the matrix material. These oxygen voids may, for example, create conductivity for oxygen ions. A typical example of this kind of doping involves zirconium dioxide which is doped with an oxide of a lower-valency metal. Usually, yttrium oxide is used for this doping. Thus yttrium-stabilized zirconium dioxide is most usually used in lambda sensors as the material for the solid electrolyte.
At the same time, however, in conventional oxygen sensors the solid electrolyte, in addition to its function as the electrolyte, also has the function of a carrier material. This however places high demands on this material with regard to mechanical stability and resistance to thermal shock. Since the mechanical strength and load-bearing capacity of conventional solid electrolyte materials diminishes, however, with increasing doping and the resultant rising ionic conductivity, these two objectives are in conflict.
A further technical challenge related to the problems described above is the elimination of leakage currents. Since because of the requirement for mechanical stability doping cannot be increased without limitation, and since nevertheless a stipulated ionic conductivity must be achieved for the sensor elements to operate, in many cases the sensor elements are operated at elevated temperatures. In order to make this possible, as an example, heating elements are used. However, it is then possible under certain conditions, in particular at higher temperatures, for the ionic conductivity to cause harmful leakage currents to occur in the sensor element, for example between the electrodes of the sensor element and the heating element. For this reason, generally speaking, both the heating element and also the electrodes are provided with complex insulation in the areas not needed for the operation of the sensor element. Where layered structures are used, generally speaking feedthroughs to tracks located on lower layers are also provided with complex insulation in this manner. Despite the high level of complexity involved in making insulating layers, generally speaking, rejects occur during manufacture of the sensor elements, as a result of high leakage currents.
Consequently, a sensor element is proposed for determining at least one property of a gas in a measuring gas chamber, which at least to a large extent avoids the set of problems described above and at least to a large extent resolves the conflict of objectives described. The sensor element may in particular be set up to demonstrate the presence of a component of a gas mixture, for example to demonstrate the presence of oxygen. The sensor element may in particular be set up in order to determine the composition of the exhaust gas in the exhaust system of an internal combustion engine. In this connection reference may be made, for example, to the specific embodiments known from related art and described above, which may be modified according to the present invention. Other embodiments and/or applications, however, are also possible in principle, for example detection of the presence of other types of gas components and/or use as a particle counter.
A fundamental aspect of the exemplary embodiments and/or exemplary methods of the present invention is to separate the functionality of mechanical stabilization and carrier function from that of ionic conductivity, so that these may be optimized in isolation from each other. By way of this separation into the functions of mechanical stability and ionic conductivity, optimization may thus be achieved with regard on the one hand to the ionic conductivity desired at certain points and undesired at other points, and on the other hand to the mechanical carrier function.
The proposed sensor element accordingly includes at least one cell having at least one first electrode, at least one second electrode, and at least one solid electrolyte which joins the first electrode and the second electrode together and is made of a solid electrolyte material. Here, the sensor element may be a single-cell or a multiple-cell sensor element; once again reference is made to the description above. The sensor element may thus, for example, include a simple discrete-level cell or a simple single-cell broadband sensor structure or may also be of a more complex design, for example in line with the known broadband sensors having an additional reference electrode, as described above. Here, one, several, or all cells of the sensor element may be of a configuration according to the exemplary embodiments and/or exemplary methods of the present invention, a cell in this case being understood as a combination of at least two of the electrodes present and at least one solid electrolyte joining these electrodes together. Furthermore, electrodes may generally also be of a multi-part design.
Furthermore, the sensor element includes at least one carrier element made of a carrier material. This carrier material has a lower ionic conductivity than the solid electrolyte material. For example, at ambient temperature and/or at temperatures between ambient and 300° C. and/or at temperatures between 300° C. and 600° C. and/or at temperatures between 600° C. and 1000° C., the carrier material may have an ionic conductivity lower by at least a factor of 2, which may be by at least a factor of 10 and in particular may be by at least a factor of 100 than the solid electrolyte material.
The carrier element is situated in order to perform a carrier function, i.e., in order to mechanically stabilize the at least one cell. Here, “mechanical stabilization” is to be understood as a function in which the cell, in particular the material of the cell's solid electrolyte, is relieved during the occurrence of the mechanical stresses that are habitual in the operation of the sensor element, for example bending stresses, tensile stresses or compressive stresses or combinations of such stresses and/or of other types of stress. Specifically, this relief may occur in such a manner that the cell and the carrier element together form a cantilevered element, which may also be combined with further components of the sensor element. The mechanical stabilization may, for example, be achieved by having the carrier element partially or completely surrounding the cell, in particular the solid electrolyte or the solid electrolyte material. This may be achieved, for example, by having the carrier element form an open or closed frame, into which the cell and/or its solid electrolyte is/are partially or completely inserted. This frame may include a single insertion opening or several insertion openings, into which the cell or the solid electrolyte may be inserted. Various specific embodiments are explained in greater detail below.
Specifically, the solid electrolyte material, as described above, may be and/or include a ceramic solid electrolyte material. Also the carrier material may include a ceramic material, which offers the special advantage that use may be made of a shared production process for the ceramic materials. The carrier material, as described above, has a lower ionic conductivity than the solid electrolyte material. The carrier material may also have a lower electronic conductivity than the solid electrolyte material. The solid electrolyte material may be an electronic insulator. The carrier material may especially be electronically insulating. The carrier material may also be ionically insulating. Thus the carrier material may, for example, include at least one ceramic insulating material, where “ceramic insulating material” may be understood as a material which acts as an insulator with regard both to ionic conductivity and to electronic conductivity. Thus the ceramic insulating material may, for example, include an aluminum oxide, in particular Al2O3. Alternatively or additionally, however, other types of insulating materials, in particular ceramic insulating materials, may also be used.
The carrier material may be designed to act as an insulator with regard to ionic conductivity, in other words may, itself, have no ionic conductivity. Fundamentally, however, the carrier material itself may also demonstrate ion-conducting properties, which, however, are less marked than the ion-conducting properties of the solid electrolyte material. Thus the carrier material may, for example, contain at least one second solid electrolyte material, having a lower ionic conductivity than the solid electrolyte material. This is, technically, comparatively simple to accomplish, for example by using at least partially identical matrix materials, for example metal oxides, for the material of the solid electrolyte and the second solid electrolyte material of the carrier material. For example, zirconium dioxide may be used both for the solid electrolyte material and also for the second solid electrolyte material. This offers the advantage that the manufacturing process for the sensor element may be designed to be more reliable, since the solid electrolyte material and the second solid electrolyte material may essentially be compatible, for example with regard to thermal expansion. The solid electrolyte material and the second solid electrolyte material may then, for example, be different with regard to doping, which will have a significant impact on the ionic conductivity. Thus, for example, the second solid electrolyte material of the carrier material may receive a considerably lesser degree of doping than the material of the solid electrolyte. In that process, for example, the same combination of doping materials may be used for the solid electrolyte material and the second solid electrolyte material, or different types of doping materials may be used, in order to create the different levels of ionic conductivity in the solid electrolyte material and the second solid electrolyte material. For example, yttrium may be used as the doping material for the solid electrolyte material and/or the second solid electrolyte material, for example in the form of yttrium oxide. For example, Y2O3 may be used. Alternatively or additionally, other doping materials may also be used, which may include oxides of a divalent and/or trivalent element, in particular a metal.
For the solid electrolyte material and also if necessary, although this is a less preferred solution, for the second solid electrolyte material, one or more of the following doping materials, for example, may be used: scandium, in particular Sc2O3, erbium, ytterbium, yttrium, calcium, lanthanum, gadolinium, europium or dysprosium. The second solid electrolyte material may, however, which may be used in a completely undoped form. For example, a zirconium oxide, for example zirconium dioxide, may be used for the solid electrolyte material, together with, as an example, one or more of the doping materials listed. The carrier material may then, for example, be zirconium dioxide that is undoped and/or has only been doped to a much lesser degree.
The use of scandium, in particular in the form of Sc2O3, is in particular may be used as the doping material for the solid electrolyte material, since scandium-doped matrix materials, in particular scandium-doped zirconium dioxide, have a high ionic conductivity, higher for example than yttrium-doped zirconium dioxide. Various embodiments are conceivable.
As described above, there are several possibilities for designing the carrier element in such a way that it may exercise its mechanical stabilizing function in relation to the at least one cell. Thus, for example the possibility exists to design the carrier element wholly or partly as a frame, with the possibility also for this frame to be either completely closed or alternatively partially open. Here, a frame is understood to be, for example, a flat element, possibly disk-shaped or plate-shaped, fundamentally of any desired external shape, such as round, polygonal or rectangular, in which at least one orifice, for which a through orifice has been made. This frame surrounds the at least one cell, in particular the solid electrolyte of this at least one cell, at least partially.
Alternatively or additionally, however, the carrier element may also completely or partially take the form of a carrier layer, which, for example, may be continuous and without orifices. The at least one cell may then be placed on the carrier layer of the carrier element either directly or indirectly, with such indirect placement involving interpolation of one or more intermediate layers. This placement may be done using normal layer-handling techniques. For example, board-printing techniques may be used here. The cells may be placed on one side or both sides of the carrier layer, with single-side placement being preferred for reasons of easier manufacturing.
The carrier layer may be of a cantilever design, conferring on the sensor element, and in particular the cell, cantilevered mechanical stability. The carrier layer may also have at least one orifice, which may be a plurality of orifices. These orifices may penetrate part-way or completely through the carrier layer. In this case, the solid electrolyte material of the at least one cell or, if several solid electrolyte materials are to be used, at least one of these solid electrolyte materials, may be placed at least partially in the at least one orifice. This may be done, for example, in the form of a lattice, with the carrier element having a plurality of orifices that all together form a lattice of holes or of orifices. In this case, the orifices may be disposed regularly or irregularly. The solid electrolyte material may then be placed in these orifices, so that the orifices, together with the solid electrolyte material placed in them, form “islands” which, together with at least two electrodes, may each form a part of a cell or a full cell. In this design, several of these “islands” may each be equipped with their own electrodes, or alternatively one or several shared electrodes may be provided to create the connection between these islands. Various other embodiments are conceivable.
The sensor element may be manufactured in layers, and may have at least two levels of layers. Levels of layers are to be understood as levels into which different materials are introduced. In this case, at least one electrical feedthrough may be provided, with the electrical feedthrough preferably penetrating through the carrier element. By contrast with habitual structures, in which these feedthroughs pass through solid electrolyte layers, in this proposed specific embodiment, the at least one feedthrough thus pass not through the solid electrolyte of the cell, but through the carrier material. Since this carrier material has lower ionic conductivity and may also lower electronic conductivity than the solid electrolyte material of the at least one cell, in this specific embodiment the effort for insulating the feedthroughs may be considerably simplified.
The proposed sensor element in one or more of the specific embodiments described above has considerable advantages by comparison with conventional sensor elements. Thus, for example, a material having a higher ionic conductivity than the surrounding carrier material, in particular a supporting ceramic material, may be introduced as the solid electrolyte material into the area of the actual cell. This makes it possible to optimize separately both the function of mechanical stability and that of ionic conductivity, since different materials may be used for the solid electrolyte material and the carrier material.
A further advantage is that the temperature needed for the operation of the sensor element may be reduced. This results in particular from the fact that the ionic conductivity of the at least one cell required to take measurements may be improved by way of optimization of the solid electrolyte material and may be achieved at considerably lower temperatures. The ion-conductive solid electrolyte material may demonstrate adequate functionality even at these lower temperatures. Consequently, a heating element, which may optionally be provided, may thus be completely omitted. This is advantageous in particular for cost-effective sensor element applications, for example in the field of sensor elements for motorcycles, in which only the heat given off by the exhaust gas may be used to heat up the sensor element and is sufficient to make the sensor element functional.
At the same time, however, the conflict of objectives described above, in which merely using a higher level of doping of the solid electrolyte material results in a reduction in the stability of the solid electrolyte material, may be eliminated. Thus, although measuring capability may be achieved even at markedly lower temperatures with the aid of the higher level of doping, nevertheless an increased resistance to thermal shock and/or mechanical stresses may be achieved, since the carrier element may provide those requisite properties. Since in addition, the sensor element may be operated at lower temperatures, thermal shocks occur moreover to a lesser degree, because for example the sensor element may be operated in a temperature range in which a drop of water touching it could not yet result in a critical temperature drop and a consequent critical temperature shock. For example, the sensor element may be operated in a temperature range below 400° C.
The carrier element may, as described above, include, for example, an insulating ceramic material such as aluminum oxide. Owing to the insulating effect of the substrate ceramic material of the carrier element, insulation of the heating element, of the electrodes and of the feedthroughs, for example, normally placed by means of serigraphy, may be reduced or made much less complex. This may lead to a marked saving in costs by way of a significant reduction in the printing steps and an increase in quality.
For sensor elements such as for example an unheated motorcycle sensor, this may mean that a sensor element based on an insulating ceramic material, for example aluminum oxide, having improved solid electrolyte material, for example as an inlay, and at the same time, adequate stability, would be operational even at lower temperatures. By comparison with sensor elements and material systems used heretofore, for example zirconium oxide doped exclusively with Y2O3, on the one hand considerable simplifications in the structure of the sensor element may be achieved thereby and on the other hand savings and/or simplifications in operation may also be attained. For example, as explained above, a heating element may be completely eliminated and/or simpler heating elements or ones operating at lower temperatures may be used.
The structure of the sensor element may for example, apart from the modifications described above, essentially be similar to a known structure of sensor elements. For example, sensor elements may have an external electrode, which is exposed to the gas or gas mixture either directly or after the gas has passed through a protective layer. An internal electrode may then be provided in a layer situated further down, with the outer electrode and the inner electrode being linked by the at least one solid electrolyte. In addition at least one reference electrode may be provided, as is the case, for example, with multiple-cell broadband sensors, for example according to the related art cited above.
For example, an inlay made of ion-conductive solid electrolyte material may be introduced into the area of a cell formed by the reference electrode and the outer electrode. This inlay made of the solid electrolyte material may be placed in and/or on an ionically and may also electronically non-conductive ceramic substrate. This may be done, for example, as described above, by the carrier element's having, for example, an orifice, for example in the form of a recess and/or a stamping. The solid electrolyte material may then be introduced into this orifice, for example in the form of a continuous layer, for example as a continuous piece of a foil. The rest of the sensor element may then be constructed in the usual way, for example using a reference electrode lying inside and an outer electrode placed on the outside.
Alternatively or additionally, as described above, the carrier element may completely or partially take the form of a carrier layer. A piece of a foil of a solid electrolyte material may then be placed on this carrier layer, for example by means of laminating. One advantage of this structure is that the ion-conductive layer of the solid electrolyte material is situated above the carrier material, with the result that here the outer electrode and the reference electrode are situated on top of the carrier layer without a feedthrough. This allows the construction of the layers to be considerably simplified.
On the other hand, as an alternative or in addition, an improved mechanical anchoring and a greater stability of the sensor element may be achieved with the aid of the lattice structure described above, which may also be referred to as a network. Thus the solid electrolyte material, which may have high ion-conductive properties, may be placed into a network of orifices, which thereby take the form of filled bore holes. The filled area may then ensure a sufficiently high ionic conductivity. Also in this case, for example, a structure with at least one electrode lying inside and at least one electrode lying outside may be used.
In all of the described variants of the method it is also possible to make use of at least one reference air duct. For example, such a reference air duct may be in the form of a printed duct, with a reference electrode being linked to a reference air duct containing a porous material. Open reference air ducts are also conceivable.
By the use of an insulating carrier material, for example an insulating ceramic material, one, several, or all customarily used insulating layers may be eliminated. In so doing, it should be ensured that the conductivity of the carrier material, in particular the substrate ceramic material, is sufficiently low for all the earlier requirements with regard to leakage currents to be met. For example, this may be guaranteed with the use of an aluminum oxide foil of appropriate purity as the carrier material.
In order to achieve a sufficiently high mechanical strength, instead of using pure aluminum oxide, in particular a substrate, for the carrier material, an additional possibility is also to use a variant in which the aluminum oxide has zirconium oxide added to it. Thus, for example, Al2O3 may have ZrO2 added to it up to just below the percolation limit. In this way, even better mechanical characteristics of the sensor element may be achieved.
As described above, in particular doped zirconium oxide may be used for the solid electrolyte material. The use of scandium or an oxide of scandium as the doping material is particularly preferred. However, as described above, in principle also other types of doping materials may alternatively or additionally be used. Fundamentally, therefore, as an example materials having a high oxygen conductivity may be used as solid electrolyte materials, where such materials are compatible with the carrier material. Since the carrier material itself now generally speaking only has to carry out mechanical functions, for example a partially stabilized zirconium oxide having a low level of doping may be used as this carrier material, such a material having markedly better mechanical characteristics than the substrate material used heretofore. Overall, in this way sensor elements may be manufactured which are markedly different from known sensor elements, both with regard to their electrical properties and thus their functionality as a sensor element and also with regard to their mechanical and/or thermo-mechanical properties and load-bearing capacity.
Exemplary embodiments of the invention are shown in the drawing and explained in greater detail in the following description.
The exemplary embodiments illustrate three different specific embodiments of sensor elements 110, which are set up in order to determine at least one property of a gas in a measuring gas chamber 112, for example a physical and/or chemical property. Without limitation of other possible applications and embodiments, the invention is described below with reference to lambda sensors which are set up to determine the composition of the exhaust gas in the exhaust system of an internal combustion engine, and thus in particular the oxygen content of the exhaust gas in this exhaust system. Furthermore, sensor elements 110 are shown only in a simple single-cell structure. As described above, however, more complex structures of sensor elements 110 are also possible, such as structures containing several cells. In this context, reference may be made to related art. The modification according to the present invention of such more complex sensor elements 110 will be completely clear to those skilled in the art.
On the side of solid electrolyte 118 opposite to first electrode 114, which forms the outer electrode, a second electrode 122 is situated. This second electrode 122 may be situated, for example, in the interior of a layered structure, so that further layers may be connected below second electrode 122. This is indicated in
Solid electrolyte material 120 may take the form of a material having a high conductivity for oxygen ions, which may be achieved by appropriate doping using scandium. In that way, a level of oxygen ion conductivity needed for the operation of sensor element 110 may for example be set as low as the temperatures that are customary in the exhaust system of an internal combustion engine, for example a motorcycle, without any need for an additional heating element. For example, this may cover temperatures in the range between ambient and 300° C., for example 100° C. to 200° C. Since an increased doping of solid electrolyte material 120 is in many cases associated with a lower mechanical stability of the material 120, according to the present invention sensor element 110 includes a carrier element 126. The two electrodes 114, 122 and solid electrolyte 118 joining together electrodes 114, 122 together form a cell 128, for example a discrete-level cell and/or a pumping cell. To this end, sensor element 110 may for example provide appropriate drive circuits which cause the cell to operate in that manner. Because of the high level of doping described above, this cell 128 is however usually of a lower mechanical stability than conventional cells used in habitual sensor elements 110. Carrier element 126 accordingly provides mechanical stabilization of this cell 128. To this end, carrier element 126 in the exemplary embodiment shown in
Carrier element 126 incorporates a carrier material 132. As recited above, this carrier material 132 in the exemplary embodiment shown in
Furthermore, electrode leads 134, 136 are apparent in
Once again sensor element 110 as shown in
Furthermore, sensor element 110 as in
In addition sensor element 110 includes once again at least one carrier element 126, which confers mechanical stability on the at least one cell 128. Unlike the exemplary embodiment shown in
The remaining structure of sensor element 110 may largely be like the structure as shown in
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2008-043-932.0 | Nov 2008 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP09/65268 | 11/17/2009 | WO | 00 | 5/18/2011 |
