Patent Application
20170030857
The present invention relates to a sensor element for acquiring at least one property of a measurement gas in a measurement gas compartment, in particular for acquiring a portion of a gas component in the measurement gas or a temperature of the measurement gas, which includes a bearer element and at least one solid electrolyte layer, and a method related thereto.
From the existing art, a large number of sensor elements and methods for acquiring at least one property of a measurement gas in a measurement gas compartment are known. These can be in principle any physical and/or chemical properties of the measurement gas, and one or more properties can be acquired. In the following, the present invention is described in particular with reference to a qualitative and/or quantitative acquisition of a portion of a gas component of the measurement gas, in particular with reference to an acquisition of an oxygen portion in the measurement gas portion. The oxygen portion can for example be acquired in the form of a partial pressure and/or in the form of a percentage. Alternatively or in addition, however, other properties of the measurement gas can also be acquired, such as the temperature.
From the existing art, in particular ceramic sensor elements are known that are based on the use of electrolytic properties of particular solid bodies, i.e. ion-conducting properties of these solid bodies. In particular, these solid bodies can be ceramic solid electrolytes, such as zirconium dioxide (ZrO2), in particular yttrium-stabilized zirconium dioxide (YSZ) and scandium-doped zirconium dioxide (ScSZ), which can contain small added amounts of aluminum oxide (Al2O3) and/or silicon oxide (SiO2)
For example, such sensor elements can be fashioned as so-called lambda sensors or as nitrogen oxide sensors, as known for example from K. Reif, Deitsche, K.-H., et al., Kraftfahrtechnisches Taschenbuch, Springer Vieweg, Wiesbaden (2014), pp. 1338-1347. Using broadband lambda sensors, in particular planar broadband lambda sensors, for example the oxygen concentration in the exhaust gas can be determined within a wide range, and in this way the air-fuel ratio in the combustion chamber can be inferred. The air number λ (lambda) describes this air-fuel ratio. Nitrogen oxide sensors determine both the nitrogen oxide concentrations and also the oxygen concentration in the exhaust gas.
Despite the advantages of the sensor elements known from the existing art, and methods for producing them, there is still potential for improvement here. Thus, the sensor element is standardly produced using the so-called ceramic thick-layer technique. This technique permits only large minimum dimensions, both with regard to the structural widths of standardly at least 30 μm and also given layer thicknesses of standardly more than 10 μm. For this reason, microelectrochemical elements have been developed. Here, a solid body electrolyte in the form of a thin layer is used. The deposition of YSZ as a thin film for gas sensor elements or microfuel cells (μ-SOFC) is nowadays done on silicon nitride (Si3N4) or SiO2. Both can also be used non-stoichiometrically, and are amorphous. Here, Si3N4 or SiO2 is used as an electrical insulator that separates the solid body electrolytes from silicon. Silicon has good electrical conductivity at high temperatures. The growth of YSZ on these scaffoldings or templates is polycrystalline, and, depending on the growth conditions or deposition parameters, is for example granular or columnar. Granular growth results in a low ionic conductivity, and column growth results in a mechanically unstable membrane, because the YSZ is often “exposed” so that gas access can be ensured to the two surfaces of the YSZ layer given tensile-stress mechanical loading, and results in poor ionic conductivity perpendicular to the columns.
Therefore, a sensor element for acquiring at least one property of a measurement gas in a measurement gas compartment, and a method for the production thereof, are proposed, that at least largely avoid the disadvantages of known sensor elements and methods, and permit the production of a mechanically robust thin membrane having very good ion conductivity.
A sensor element for acquiring at least one property of a measurement gas in a measurement gas compartment, in particular for acquiring a portion of a gas component in the measurement gas or a temperature of the measurement gas, includes a bearer element and at least one solid electrolyte layer, the solid electrolyte layer being situated on the bearer element, the solid electrolyte layer being at least partly epitaxially fashioned, the bearer element having at least one opening, so that the solid electrolyte layer has at least one membrane segment.
The solid electrolyte layer can have a thickness of from 40 nm to 5 μm, which may be 50 nm to 3 μm, and more particularly 200 nm to 2 μm, for example 1 μm. Correspondingly, the solid electrolyte layer can be realized significantly thinner in comparison to conventional ceramic sensor elements. In this way, the sensor element of the present invention can be produced smaller as a whole, and can also be used in spatially constricted locations. Between the solid electrolyte layer and the bearer element, in some sections at least one insulating layer can be situated, the insulating layer being produced of at least one electrically insulating material, in particular Si3N4 and/or SiO2. Between the solid electrolyte layer and the bearer element there can be situated at least one intermediate layer, the intermediate layer being fashioned epitaxially and so as to be oxidic, electrically conductive, and gas-permeable, or the intermediate layer being fashioned epitaxially and so as to be oxidic and electrically insulating. The solid electrolyte layer can be produced from at least one material that includes zirconium dioxide stabilized with yttrium oxide. A portion of the yttrium oxide can have a gradient on the zirconium dioxide, which may be perpendicular to a layer plane of the solid electrolyte layer. Thus, the zirconium dioxide is stabilized with yttrium oxide in order to improve the material strength and to set, or adapt, the ionic conductivity. Via the gradient, a kind of transition can be created from the ionically conductive material of the solid electrolyte layer to the material of the electrodes, which is electrically conductive but has an increased electrolyte resistance. The solid electrolyte layer can have, in the membrane segment, an opening on a side facing the bearer element. On an upper side, facing away from the bearer element, of the solid electrolyte layer, and on a lower side, facing the bearer element, of the solid electrolyte layer, in each case there can be situated an electrode. The electrodes may be porous. The porosity of the electrodes is selected such that on the one hand the measurement gas, or the ions of the measurement gas, such as oxygen ions, move through the electrodes to the solid electrolyte layer, and on the other hand a contiguous electrically conductive structure of the electrode has to be present. The electrodes can be electrically contactable from the same side. In other words, an electrical contacting of the electrodes can take place from one and the same direction. The membrane segment can be fashioned to separate two different measurement gas compartments from one another. The electrodes can in this way be oriented toward two different measurement gas compartments, or can be exposed to these compartments.
The method for producing a sensor element for acquiring at least one property of a measurement gas in a measurement gas compartment, in particular for acquiring a portion of a gas component in the measurement gas or a temperature of the measurement gas, includes the provision of a bearer element, the application of at least one solid electrolyte layer onto the bearer element in such a way that the solid electrolyte layer is epitaxially fashioned, and the partial removal of the bearer element in order to form at least one opening in such a way that the solid electrolyte layer has at least one membrane segment.
Here, the solid electrolyte layer can be applied onto the bearer element using pulsed laser deposition, chemical vapor deposition, or sputtering. The solid electrolyte layer can be applied onto the bearer element at a temperature from 600° C. to 1000° C. and a pressure of not more than 0.05 mbar.
The partial removal of the bearer element can be accomplished by trenching or etching. The solid electrolyte layer can be applied onto the bearer element in a thickness of from 40 nm to 5 μm, which may be 50 nm to 3 μm, particularly 200 nm to 2 μm, e.g. in a thickness of 1 μm. Between the solid electrolyte layer and the bearer element, in some segments at least one insulating layer can be situated, the insulating layer being made of at least one electrically insulating material, in particular Si3N4 and/or SiO2. Between the solid electrolyte layer and the bearer element, there can be situated at least one intermediate layer, the intermediate layer being formed epitaxially and so as to be oxidic, electrically conductive and gas-permeable, or the intermediate layer can be fashioned epitaxially and so as to be oxidic and electrically insulating. The solid electrolyte layer can be produced from at least one material that includes zirconium dioxide stabilized with yttrium oxide. The solid electrolyte layer can be applied onto the bearer element in such a way that a portion of the yttrium oxide on the zirconium dioxide has a gradient, which may be perpendicular to a layer plane of the solid electrolyte layer. The solid electrolyte layer can be partly removed in the membrane segment in such a way that an opening is fashioned on a side facing the bearer element.
In the context of the present invention, a solid electrolyte is to be understood as a body or object having electrolyte properties, i.e. ion-conducting properties.
This also includes the raw material of a solid electrolyte, and thus its formation as a so-called green part or brown part, which does not become a solid electrolyte until after a sintering. In particular, the solid electrolyte can be fashioned as a solid electrolyte layer or from a plurality of solid electrolyte layers. In the context of the present invention, a layer is to be understood as a unified mass having a flat extension of a certain height that is situated above, underneath, or between other elements. A layer is thus a three-dimensional body whose measurements in two dimensions, which represent the surface formation of the layer, are significantly larger than a measurement in the third dimension, representing the height of the layer. Correspondingly, in the context of the present invention a layer plane is to be understood as a plane of the layer that represents the surface extension. Thus, in the context of the present invention an orientation perpendicular to the layer plane is an orientation perpendicular to the surface extension of the layer, and an orientation parallel to the layer plane is an orientation parallel to the surface extension of the layer.
In the context of the present invention, an electrode is to be understood in general as an element that is capable of contacting the solid electrolyte in such a way that a current can be maintained through the solid electrolyte and the electrode. Correspondingly, the electrode can include an element at which the ions can be built into the solid electrolyte and/or removed from the solid electrolyte. Typically, the electrodes include a noble metal electrode, which can be applied on the solid electrolyte for example as a metal-ceramic electrode, or can be connected to the solid electrolyte in some other way. Typical electrode materials are platinum-cermet electrodes. Other noble metals, such as gold or palladium, can however also be used in principle.
In the context of the present invention, a heating element is to be understood as an element that is used to heat the solid electrolyte and the electrodes at least to their functional temperature, and which may be to their operating temperature. The functional temperature is the temperature starting at which the solid electrode becomes conductive for ions; it is approximately 350° C. To be distinguished from this is the operating temperature, which is the temperature at which the sensor element is standardly operated, and which is higher than the functional temperature. The operating temperature can be for example from 700° C. to 950° C. The heating element can include a heating region and at least one conducting path. In the context of the present invention, a heating region is to be understood as the region of the heating element that, in the layer construction, overlaps with an electrode along a direction perpendicular to the surface of the sensor element. Standardly, the heating region heats up more strongly during operation than does the conducting path, so that these can be distinguished. The different heating can for example be realized in that the heating region has a higher electrical resistance than does the conducting path. The heating region and/or the conducting path are for example fashioned as electrical resistance paths, and are heated through application of an electrical voltage. The heating element can for example be produced from a platinum cermet or a platinum layer.
In the context of the present invention, an epitaxial formation is to be understood as a formation in which at least one crystallographic orientation of the applied material corresponds to a crystallographic orientation of the bearer element.
A basic idea of the present invention is the production of an epitaxial YSZ layer on a silicon layer, subsequently to be removed, as a solid electrolyte for use for oxygen sensors or a microfuel cell, in which the yttrium oxide content is less than 10 mol % and the layer thickness is typically from 50 nm to 3 μm. For this purpose, YSZ is deposited on a crystalline silicon surface, for example using laser beam vaporization, with low background O2 partial pressure, i.e. less than 0.05 mbar, and a high substrate temperature, i.e. greater than 600° C. Given suitable deposition parameters, the yttrium-stabilized zirconium dioxide grows epitaxially on the silicon. The silicon is subsequently selectively removed at least at some locations under the YSZ layer, for example through trenching or etching using caustic potash.
Through selective removal of the silicon at particular locations, the growth of epitaxial YSZ directly on the silicon surface is achieved, with subsequent exposure of the YSZ layer.
In another specific embodiment, SiO2 and/or Si3N4 can be applied as insulating layer at locations of the sensor at which the ion-conductive properties of YSZ are not required or not desired.
In a further specific embodiment, the yttrium-stabilized zirconium dioxide layer can be sealed or reinforced after exposure through an insulating layer, for example made of silicon dioxide (SiO2) or silicon nitride (Si3N4). This is advantageous in particular when ion conductivity parallel to the layer is necessary.
In addition, epitaxially yttrium-stabilized zirconium dioxide can be grown on silicon with one or more epitaxial, oxidic, electrically conductive, ion-conducting intermediate layers, for example made of La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF), Sm0.5Sr0.5CoO3 (SSC), La1-xSrxMnO3 (LSMO), and these intermediate layer(s) can be contacted by metal electrodes. The intermediate layers can be gas-permeable, alternatively or in addition to their ion-conducting property.
In still another specific embodiment, the epitaxial yttrium-stabilized zirconium dioxide is grown on silicon with one or more epitaxial, oxidic, electrically insulating intermediate layers. In this way, a better electrical insulation against the silicon is brought about.
In still another specific embodiment, the growth of the yttrium-stabilized zirconium dioxide can be controlled with a gradient in the yttrium dioxide concentration of the YSZ layer perpendicular to the layer (plane) for better lattice matching of the yttrium-stabilized zirconium dioxide and of the silicon. This is advantageous if the optimal YSZ composition for ion conductivity and for epitaxial growth on silicon is different.
The epitaxial yttrium-stabilized zirconium dioxide can be applied onto the silicon by pulsed laser deposition or sputtering at low oxygen pressure, i.e. a pressure of less than 0.05 mbar. In particular, here a modification of the oxygen pressure during the deposition process is advantageous. In order to prevent the oxidation of the silicon substrate, a first yttrium-stabilized zirconium dioxide layer should be grown on the silicon with a very low oxygen pressure, or oxygen partial pressure, i.e. less than 10−5 mbar. After formation of this first YSZ layer on silicon, the oxygen pressure can be increased in order to achieve the required ion-conductive and/or structural properties of the yttrium-stabilized zirconium dioxide.
In yet another specific embodiment, after exposure of the YSZ layer through the removal of a partial region of the YSZ layer, in particular the partial region formerly adjoining the silicon substrate, for example either a YSZ layer having a different composition or yttrium-stabilized zirconium dioxide with diffused-in silicon can be removed.
Finally, the layer can also be grown on an SOI (silicon on insulator) wafer. For example, such an SOI wafer can be obtained by introducing oxygen ions into a silicon wafer that forms the bearer layer. Through ion implantation, it is possible to control the depth (e.g. a few 100 nm) of the region in which the oxygen ions are brought in. In order now to produce a “trenched” silicon dioxide layer, through a high-temperature step the crystal is “healed”; here the introduced oxygen reacts with the silicon (after implantation, mainly at interstitial sites), and forms an insulating layer of silicon dioxide. Alternatively, an insulating layer can be applied on an Si wafer, and subsequently another Si layer, e.g. an epitaxial Si layer, can be deposited thereon. The advantage of growing the layer of the sensor element on an SOI wafer is a possibility of the electrical contacting of the lower porous electrode via the silicon of the bearer layer; the layer is conductive at 500° C., and it is possible to increase the conductivity through an additional doping. This takes place as follows: first, the yttrium-stabilized zirconium dioxide is epitaxially deposited on the bearer layer, i.e. the upper silicon layer of the SOI wafer. Subsequently there takes place the application and the structuring of the upper electrode. A structuring of the yttrium-stabilized zirconium dioxide and of the bearer layer defines the electrical contact of the lower electrode. Subsequently, the YSZ layer is exposed as membrane through the Si/SiO2/Si. After this, the electrode material of the lower electrode is applied and structured.
The sensor elements produced according to the specific embodiments according to the present invention have a higher ionic conductivity with lower electronic conductivity and higher mechanical stability, compared to conventional microelectrochemical sensor elements having a polycrystalline YSZ layer, because significantly fewer crystal boundaries occur in the YSZ layer. In particular, the ion conductivity parallel to the layer is significantly greater than is the case for polycrystalline or columnar YSZ layers.
Further optional details and features of the present invention result from the following description of exemplary embodiments, shown schematically in the Figures.
The specific embodiments of the present invention described in the following are described fundamentally in the context of a description of the method for producing a sensor element 10 according to the present invention. Here it is to be noted that identical components have been provided with identical reference characters. A sensor element 10 according to the present invention can be used to detect physical and/or chemical properties of a measurement gas, it being possible to acquire one or more properties. In the following, the present invention is described in particular with reference to a qualitative and/or quantitative acquisition of a gas component of the measurement gas, in particular with reference to an acquisition of an oxygen portion in the measurement gas. The oxygen portion can be acquired for example in the form of a partial pressure and/or in the form of a percentage. In principle, however, other types of gas components can be acquired, such as nitrogen oxides, hydrocarbons, and/or hydrogen. Alternatively or in addition, however, other properties of the measurement gas can be acquired. The present invention can be used in particular in the area of motor vehicle technology, so that the measurement gas compartment can particular be an exhaust gas train of an internal combustion engine, and the measurement gas can in particular be an exhaust gas.
Sensor element 10 according to the present invention fundamentally has the design described in the following. Sensor element 10 includes a bearer element 12 and at least one solid electrolyte layer 14. Solid electrolyte layer 14 is situated on bearer element 12. Solid electrolyte layer 14 is fashioned at least partly epitaxially. Bearer element 12 has at least one opening 16, so that solid electrolyte layer 14 has at least one membrane segment 18. Membrane segment 18 is that segment of solid electrolyte layer 14 that is not immediately contacted to bearer element 12. Bearer element 12 can for example be made of silicon. Electrodes (not shown in more detail) are situated on solid electrolyte layer 14. The electrodes are situated on opposite sides of solid electrolyte layer 14. The electrodes are made porous.
Here, a direction perpendicular to the layer plane in
In the following, a method is described for producing a sensor element 10 according to a second specific embodiment of the present invention, on the basis of
Intermediate layer 22 is applied as is solid electrolyte layer 14 in the way described with regard to the first specific embodiment. Correspondingly, intermediate layer 22 is situated between solid electrolyte layer 14 and bearer element 12. Such an epitaxial and electrically insulating intermediate layer 22 can for example replace the above-described insulating layer 20. Here, solid electrolyte layer 14 is also epitaxially fashioned above intermediate layer 22 or thereon. An epitaxial and ion-conducting intermediate layer 22 is appropriately fashioned only in that region in which membrane segment 18 is fashioned, i.e. between solid electrolyte layer 14 and a porous electrode (not shown in more detail).
In order to arrive at finished sensor element 10, bearer layer 12 is then removed at least in some segments, analogous to
As is shown in
| Number | Date | Country | Kind |
|---|---|---|---|
| 102015214387.2 | Jul 2015 | DE | national |
The present application claims priority to and the benefit of German patent application no. 10 2015 214 387.2, which was filed in Germany on Jul. 29, 2015, the disclosure of which is incorporated herein by reference.
