Patent Application
20180321182
The present invention relates to a sensor element having an extended service life and a method for manufacturing such a sensor element.
Sensor elements for exhaust gas sensors are already known from the related art. For example, DE 102006002111 A1 provides a sensor element for gas sensors for determining the concentration of particles in gas mixtures, in particular soot sensors, including at least one measuring system that is exposed to the gas to be determined, at least one heating element that is integrated into the sensor element, and at least one temperature measuring element that is integrated into the sensor element, the heating element being spatially situated within the sensor element between the measuring system and the temperature measuring element.
According to example embodiments of the present invention, partially embedding the guide structure in the base body in the direction perpendicular to the surface of the ceramic base body results in interlocking between the guide structure and the base body, and thus in a sustainably strong connection between the guide structure and the base body. If the sensor element is subjected to intense thermal, hydrothermal, and/or corrosive stress over its service life, the connection of the guide structure to the base body remains undiminished.
The guide structure being partially embedded in the direction perpendicular to the surface of the ceramic base body is understood here in particular to mean that only complete embedding is excluded, and that the guide structure is excluded from being situated solely on the unstructured surface of the base body. In particular, this is understood here to mean that, in the surface of the ceramic base body which otherwise has a macroscopic design, a microstructure is provided in which the guide structure is partially accommodated in the direction perpendicular to the surface of the ceramic base body.
The guide structure is an electrically conductive structure, i.e., in particular the guide structure is made of a material whose resistivity at room temperature is less than 0.5 ohm mm2/m.
Refinements of the present invention provide that there is a minimum level with which the guide structure penetrates into the ceramic base body, and that there is a minimum level with which the guide structure protrudes from the ceramic base body. In this regard, it can be provided that the guide structure penetrates, i.e., is embedded, with at least 10% of its height in the direction perpendicular to the surface. Additionally or alternatively, in this regard it can be provided that the guide structure penetrates, i.e., is embedded, by at most 90% in the direction perpendicular to the surface.
The guide structure can, for example, be embedded with up to one-half of its height in the base body, which can be understood in particular to mean a penetration between 30% and 70% of its height.
The sensor element can in particular be the sensor element of a particle sensor, which on its surface includes two comb-like, interlocking interdigital electrodes as a guide structure, which during proper use are essentially directly exposed to an exhaust gas.
Moreover, the present invention relates to a method for manufacturing a sensor element, in particular a sensor element according to the present invention. The method according to an example embodiment of the present invention provides for the manufacture of such a sensor element by sintering a ceramic precursor base body and a noble metal-containing precursor guide structure after the noble metal-containing precursor guide structure has been applied to the ceramic precursor base body and partially introduced into the precursor base body.
It is possible to carry out the application by imprinting. It is additionally or alternatively possible to carry out the introduction by pressing, for example during the imprinting. Alternatively, pressing can also be carried out subsequent to the imprinting, for example with the aid of a pressing device.
It is possible for the ceramic precursor base body to be made of an unsintered ceramic film, for example a ceramic film which contains aluminum oxide, yttrium-stabilized zirconium oxide (YSZ), cordierite, forsterite, or polycrystalline silicon, and additionally contains binder and solvent.
Furthermore, it can be provided that the ceramic precursor base body is made of the unsintered ceramic film as described above, on which in addition at least one insulating paste is flatly applied. In the process, the noble metal-containing precursor guide structure is applied to and partially introduced into the at least one insulating paste.
It is provided in particular that the noble metal-containing precursor guide structure has a higher viscosity, i.e., is harder, than the at least one insulating paste. This ensures that the noble metal-containing precursor guide structure can be partially introduced into the insulating paste with little effort and with high precision.
It can be provided that the ceramic precursor base body is made of the unsintered ceramic film as described above, on which in addition a second insulating paste and subsequently a first insulating paste are flatly applied in succession. The precursor guide structure is in turn applied to the insulating pastes. The precursor guide structure is preferably pressed, in particular partially pressed, into the outer, first insulating paste.
It can be provided that the first insulating paste and the second insulating paste are different with regard to their physical, chemical, and rheological properties. It can thus be advantageous when the second insulating paste, which comes to rest between the ceramic film and the first insulating paste, fulfills the function of an adhesive layer. For this purpose, it can be provided that the second insulating paste has a higher solvent content than the first insulating paste, so that partial solubilization of the ceramic film takes place. Additionally or alternatively, it can be provided that the second insulating paste has a higher content of fine-particle, and thus sinter-active, zirconium oxide and/or a higher content of coarse-particle aluminum oxide than the first insulating paste, which in turn has adhesion-improving effects.
It can also advantageously be provided that the first insulating paste is softer, i.e., has a lower viscosity, than the second insulating paste. This facilitates the in particular precise pressing of the precursor guide structure significantly.
The pressing of the precursor guide structure into the precursor base body can always be assisted in that, prior to application of the precursor guide structure, the precursor base body undergoes structuring with structures into which the precursor guide structure is subsequently partially introduced. The structures can be microstructures, i.e., can have structure sizes that are smaller than 150 μm in one spatial direction or in two spatial directions.
When reference is made to viscosities within the scope of the present patent application, these have been ascertained with a rotational viscometer at a shear rate of 30/s and a temperature of 20° C. When reference is made to tan delta values within the scope of the present patent application, these loss factors have been ascertained at a shear stress of 500 Pa.
The present invention is explained in greater detail with reference to the figures.
In contrast, solid electrolyte layer 11b is produced with the aid of screen printing of a paste-like ceramic material on solid electrolyte layer 11a, for example. The same solid electrolyte material of which solid electrolyte layers 11a, 11c are made is preferably used as the ceramic component of the paste-like material.
In addition, the sensor element includes, for example, a plurality of electrically insulating ceramic layers 12a, 12b, 12c, 12d, 12e, and 12f. Layers 12a through 12f are likewise produced with the aid of screen printing of a paste-like ceramic material on solid electrolyte layers 11a, 11b, 11c, for example. Aluminum oxide, for example, is used as the ceramic component of the paste-like material, since it has an essentially constant, high electrical resistance over a long period of time, even under thermal cycling.
The integrated form of the planar ceramic body of sensor element 10 is produced by laminating together the ceramic films imprinted with solid electrolyte layer 11b, with functional layers, and with layers 12a through 12f, and subsequently sintering the laminated structure in a manner known per se.
Sensor element 10 also includes a ceramic heating element 40 which is designed in the form of an electrical resistance conductor track and used for heating sensor element 10 in particular to the temperature of the gas mixture to be determined, or burning off the soot particles that accumulate on the large surfaces of sensor element 10. The resistance conductor track is preferably made of a cermet material, preferably as a mixture of platinum or a platinum metal with ceramic portions, for example aluminum oxide. The resistance conductor track is also preferably designed in the form of a meander, and includes vias 42, 44 as well as electrical terminals 46, 48 at both ends. The heat output of heating element 40 can be appropriately regulated by applying a corresponding heating voltage to terminals 46, 48 of the resistance conductor track.
For example, two measuring electrodes 14, 16 that are preferably designed as interlocked interdigital electrodes are applied to a large surface of sensor element 10. The use of interdigital electrodes as measuring electrodes 14, 16 advantageously allows a particularly accurate determination of the electrical resistance or the electrical conductivity of the surface material present between measuring electrodes 14, 16. Contact areas 18, 20 are provided for contacting measuring electrodes 14, 16 in the area of an end of the sensor element facing away from the gas mixture. The supply line areas of electrodes 14, 16 are preferably shielded from the influences of a gas mixture surrounding sensor element 10 by a further electrically insulating ceramic layer 12f.
In addition, a porous layer, not illustrated for reasons of clarity, which shields measuring electrodes 14, 16 in their interlocked area from direct contact with the gas mixture to be determined can be provided on the large surface of sensor element 10 provided with measuring electrodes 14, 16. The layer thickness of the porous layer is preferably greater than the layer thickness of measuring electrodes 14, 16. The porous layer preferably has an open porous design, the pore size being selected in such a way that the particles to be determined in the gas mixture can diffuse into the pores of the porous layer. The pore size of the porous layer is preferably in a range of 2 μm to 10 μm. The porous layer is made of a ceramic material that is preferably similar to the material of layer 12a or corresponds to same, and that can be produced with the aid of screen printing. The porosity of the porous layer can be appropriately set by adding pore builders to the screen printing paste.
A voltage is applied to measuring electrodes 14, 16 during operation of sensor element 10. Since measuring electrodes 14, 16 are situated on the surface of electrically insulating layer 12a, this initially results in essentially no current flow between measuring electrodes 14, 16.
If a gas mixture flowing around sensor element 10 contains particles, in particular soot, these particles accumulate on the surface of sensor element 10. Due to the open-pore structure of the porous layer, the particles diffuse through the porous layer to the immediate proximity of measuring electrodes 14, 16. Since soot has a certain electrical conductivity, when there is sufficient loading of the surface of sensor element 10 or of the porous layer with soot, this results in an increasing current flow between measuring electrodes 14, 16, which correlates with the extent of the loading.
If a voltage is now applied to measuring electrodes 14, 16 and the electric current that occurs between measuring electrodes 14, 16 is ascertained, a conclusion can be drawn concerning the accumulated particle mass. The concentration of all particles in a gas mixture that influence the electrical conductivity of the ceramic material situated between measuring electrodes 14, 16 is detected with this measuring method.
A first example embodiment of a sensor element 10 according to the present invention is described below.
Measuring electrodes 14, 16 are partially embedded in base body 50, in the present case partially embedded in electrically insulating layer 12a, in the direction perpendicular to surface 51 of base body 50, and are thus interlocked, in a manner of speaking, with the base body, thus in the present case with electrically insulating layer 12a. Base surfaces 14a, 16a of measuring electrodes 14, 16 are thus in contact with base body 50, while lateral surfaces 14b, 16b of measuring electrodes 14, 16 are partially accommodated (up to one-half here) in base body 50, and partially protrude (by one-half here) from base body 50. Surfaces 14c, 16c of measuring electrodes 14, 16 pointing away from ceramic base body 50 are not in contact with base body 50.
In addition, an electrically non-conductive porous layer, not illustrated for reasons of clarity, which shields measuring electrodes 14, 16 in their interlocked area from direct contact with the gas mixture to be determined can be provided on the large surface of sensor element 10 provided with measuring electrodes 14, 16. The layer thickness of the porous layer is preferably greater than the layer thickness of measuring electrodes 14, 16. The porous layer preferably has an open porous design, the pore size being selected in such a way that the particles to be determined in the gas mixture can diffuse into the pores of the porous layer. The pore size of the porous layer is preferably in a range of 2 μm to 10 μm.
Guide structure 52, as described above, can be measuring electrodes 14, 16 of a particle sensor designed as interdigital electrodes. Alternatively, guide structure 52 can also be the resistance track of a temperature sensor and/or of an electrical heater. Of course, guide structure 52 can also be any other conductor track included by sensor element 10.
In a first modification of the first exemplary embodiment, instead of solid electrolyte layer 11a, a layer 11a′ made of some other material, for example polycrystalline silicon, aluminum oxide, forsterite, or cordierite, is present.
In a second modification of the first exemplary embodiment (see
A second exemplary embodiment differs from the first exemplary embodiment in that electrically insulating ceramic layer 12a is made up of two layers situated one above the other, namely, a second sublayer 12a2 and a first sublayer 12a1 situated on second sublayer 12a2. Guide structure 52 is embedded only in first sublayer 12a1. The second exemplary embodiment is illustrated in
First sublayer 12a1 differs from second sublayer 12a2 with regard to its chemical and physical properties. Thus, second sublayer 12a2 has a higher pore content than first sublayer 12a1. For example, in an example embodiment, second sublayer 12a2 has a pore content of 5 vol % to 15 vol %, while first sublayer 12a1 has a pore content of 2 vol % to 8 vol %. The pore content of second sublayer 12a2 can, for example, be approximately twice the pore content of first sublayer 12a1.
In addition, second sublayer 12a2 has a content of yttrium-stabilized zirconium dioxide (YSZ), for example 2-10 weight percent, which is greater than a content of zirconium dioxide stabilized with yttrium, Ce, or Sc (YSZ), which first sublayer 12a1 optionally contains. However, first sublayer 12a1 is preferably made of pure aluminum oxide.
It is also provided that the zirconium dioxide contained in second sublayer 12a2 has a grain size (d50) which is smaller than 1 μm, and which is smaller than the grain size (d50) of the zirconium oxide optionally contained in first sublayer 12a1.
It is also provided that the aluminum oxide contained in second sublayer 12a2 is α-aluminum oxide.
The aluminum oxide contained in second sublayer 12a2 has a comparatively large grain size. Thus, 2-5 weight percent of the aluminum oxide contained in second sublayer 12a2 can have a grain size (d50) of larger than 3 μm. In contrast, the proportion of such coarse-grain aluminum oxide, in particular the portion of aluminum oxide grains larger than 3 μm, in first sublayer 12a1 is less.
Guide structures 52 described in the exemplary embodiments are highly insulated compared to other electrically conductive structure elements, for example heaters and/or temperature measuring devices, of sensor element 10, which means that an electrical resistance that forms between guide structures 52 and the other electrically conductive structure elements is at least 1 megaohm at 25° C. and/or at least 10 kiloohms at 850° C.
A description of how a sensor element 10 can be manufactured according to the present invention is described below by way of example.
In a first example, as is apparent in
Unsintered ceramic film 111a is imprinted with a precursor guide structure 152, made up of two precursor measuring electrodes 114, 116, in a screen printing process in a second method step 202. Precursor guide structure 152 is applied in the form of a platinum-containing screen printing paste. The platinum-containing screen printing paste has a relatively high viscosity, and is imprinted with a high enough pressure that it is pressed partially, up to one-half in the example, into unsintered ceramic film 111a during the imprinting.
As an alternative to effectuating the pressing directly during the imprinting, the pressing can be carried out subsequent to the imprinting, for example with the aid of a separate pressing device. It is also possible to create structures, preferably microstructures, prior to the imprinting in the ceramic film 111a, and to press precursor guide structure 152 into these structures.
Sintering, which transforms precursor guide structure 152 and precursor base body 150 into finished sensor element 10, takes place in a third method step 203. The sintering can take place, for example, for several hours at a temperature above 1200° C.
Of finished sensor element 10, only the upper layers of the distal end section (facing the exhaust gas) are illustrated in the right portion of
In a second example (see
This unsintered ceramic film 111a is imprinted over its entire surface with an insulating paste 112a, for example in a screen printing process, in second substep 201b of first method step 201. Insulating paste 112a includes aluminum oxide powder, for example, and is made workable by adding a binder and a solvent, for example polyvinyl butyral and butyl carbitol, respectively.
Second method step 202 takes place as in the first example, with the condition that precursor guide structure 152 is imprinted on insulating paste 112a and pressed into same. For this purpose, it has proven to be advantageous when precursor guide structure 152, in the present case the platinum-containing screen printing paste, has a higher viscosity than insulating paste 112a. For example, the viscosity of insulating paste 112a may be in the range between 30 Pas and 100 Pas, while the viscosity of precursor guide structure 152 may be in the range between 100 Pas and 600 Pas.
The final sintering takes place in third method step 203 as described above.
A third example illustrated in
A second insulating paste 112a2 is initially imprinted on unsintered ceramic film 111a. First insulating paste 112a1 is subsequently imprinted on second insulating paste 112a2. First insulating paste 112a1 and second insulating paste 112a2 can be identical with regard to their composition and their physical and chemical properties, but in this example they differ as follows. Second insulating paste 112a2 has a lower content of ceramic powder (aluminum oxide here) than first insulating paste 112a1. Accordingly, second insulating paste 112a2 has a higher content of binder (polyvinyl butyral here) and of solvent (butyl carbitol here) than first insulating paste 112a1. Additionally, the viscosity of second insulating paste 112a2 is higher than the viscosity of first insulating paste 112a1.
In this example, the layer thicknesses with which first insulating paste 112a1 and second insulating paste 112a2 are applied are the same. In addition, the tan delta values of the two insulating pastes 112a1, 112a2 are the same in this example.
30-80 weight percent of second insulating paste 112a2 is made up of ceramic powder (aluminum oxide here). Its viscosity is 30 Pas-100 Pas. Its tan delta value is between 1.2 and 100. It is applied in a thickness of 8 μm-25 μm.
50-80 weight percent of first insulating paste 112a1 is made up of ceramic powder (aluminum oxide here). Its viscosity is 10 Pas-60 Pas. Its tan delta value is between 1.2 and 100. It is applied in a thickness of 8 μm-25 μm.
Second insulating layer 11a2 of sensor element 10 has the function of an adhesive layer which improves the adherence of first insulating layer 11a1 and guide structure 52. For this purpose, 2 to 10 weight percent of fine-particle (d50 less than 1 μm) zirconium dioxide stabilized with yttrium, cerium, or scandium as a sinter-active adhesion promoter is mixed with second insulating paste 112a2. In addition, for this purpose 2 to 5 weight percent of coarse-particle (d50 greater than 3 μm) α-aluminum oxide is mixed with second insulating paste 112a2.
Second method step 202 takes place as in the second example, with the condition that precursor guide structure 152 is imprinted on first insulating paste 112a1 and pressed into same. For this purpose, it has proven to be advantageous when precursor guide structure 152, in the present case the platinum-containing screen printing paste, has a higher viscosity than first insulating paste 112a1. For example, the viscosity of precursor guide structure 152 can be in the range between 100 Pas and 600 Pas. The noble metal (platinum here) content of the platinum-containing screen printing paste is 60 to 90 weight percent. Ethylcellulose as binder and terpineol as solvent are added to the platinum-containing screen printing paste. The tan delta value of the platinum-containing screen printing paste is between 0.7 and 1.3, and is less than the tan delta value of first insulating paste 112a1. The platinum-containing screen printing paste is applied with a thickness of 5 μm-15 μm.
The subsequent sintering in third method step 203 takes place as described above.
The applicant has carried out robustness tests with the sensor elements described in the exemplary embodiments, as described in detail in German Patent application DE 10 2015 206 995 A1. Tests were carried out in such a way that in particular the parameters of the tests were selected so that a high proportion of conventional sensor elements (see
In contrast, with sensor elements 10 according to the present invention, it was even possible to carry out the same tests multiple times in succession without damage occurring to sensor elements 10 according to the present invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2015 222 108.3 | Nov 2015 | DE | national |
The present application is the national stage of International Pat. App. No. PCT/EP2016/076527 filed Nov. 3, 2016, and claims priority under 35 U.S.C. § 119 to DE 10 2015 222 108.3, filed in the Federal Republic of Germany on Nov. 10, 2015, the content of each of which are incorporated herein by reference in their entireties.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2016/076527 | 11/3/2016 | WO | 00 |
