Abstract:
A planar, layered sensor element for detecting a physical property of a gas to be analyzed is provided. The sensor element has at least one inner, first solid-electrolyte layer which is situated between two outer solid-electrolyte layers, a second solid-electrolyte layer being one of the outer solid-electrolyte layers. The inner, first solid-electrolyte layer and the second solid-electrolyte layer contain zirconium oxide stabilized with yttrium oxide. The inner, first solid-electrolyte layer has a higher yttrium-oxide content than the second solid-electrolyte layer, the yttrium-oxide content being based on the zirconium oxide.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a planar, layered gas sensor element.  
       BACKGROUND INFORMATION  
       [0002]     Planar, layered sensor elements are discussed, for example, in  Automotive Electronics Handbook,  2 nd  Ed., Ronald K. Jurgen, McGraw-Hill, 1999. A distinction is made amongst, inter alia, voltage-jump lambda sensors, wide-range lambda sensors, and limiting-current sensors. The sensor elements have a plurality of solid electrolyte foils or films, to which (and between which) different layers, e.g., electrodes or porous layers, are applied. In addition, voids are introduced into (or between) the solid-electrolyte foils.  
         [0003]     The solid-electrolyte foils are made up of zirconium oxide (ZrO 2 ) stabilized with yttrium oxide (Y 2 O 3 ), along with small additions of aluminum oxide (Al 2 O 3 ) and/or silicon oxide (SiO 2 ). The level of yttrium oxide is usually 4 to 5 mole percent.  
         [0004]     In this context, it is disadvantageous that such solid-electrolyte foils have a low tensile strength, and that cracks can occur in such solid-electrolyte foils, due to mechanical loading or stress caused by temperature differences.  
         [0005]     Published German patent document DE 198 57 470 discloses that a foil binder layer positioned between two solid-electrolyte foils can be provided with an yttrium-oxide content of 16 mole percent.  
       SUMMARY OF THE INVENTION  
       [0006]     The planar, layered sensor element according to the present invention is a sensor element having solid-electrolyte layers made of zirconium oxide stabilized with yttrium oxide, which sensor element has a high tensile strength and may resist high mechanical loads and stresses occurring due to temperature differences.  
         [0007]     The sensor element according to the present invention includes a first solid-electrolyte layer positioned on the inside of the sensor element, which first solid-electrolyte layer has a higher yttrium-oxide level than a second solid-electrolyte layer positioned on the outside. As used in this specification, the level of yttrium oxide is the level of yttrium oxide in mole percent, based on the zirconium oxide, as long as nothing else is mentioned. Since the externally situated, solid-electrolyte layers are particularly subjected to high mechanical loadings and stresses, the susceptibility to cracking of the outer solid-electrolyte layer is advantageously reduced by selecting a low yttrium-oxide level for the outer solid-electrolyte layer. However, the first inner solid-electrolyte layer has a higher yttrium-oxide level, which means that the conductivity of the first solid-electrolyte layer with regard to oxygen ions is improved. This improves the measuring performance of an electrochemical cell, which is formed by two electrodes and the first solid-electrolyte layer region situated between the two electrodes.  
         [0008]     The first and the second solid-electrolyte layers may have a level of zirconium oxide of at least 85 mole percent, e.g., 90 mole percent. The first solid-electrolyte layer may have an yttrium-oxide level which is at least 1 mole percent (e.g., 2 mole percent) greater than the yttrium-oxide level of the second solid-electrolyte layer.  
         [0009]     An excellent strength of the sensor element, in addition to an improved measuring performance of the sensor element, may be achieved by providing a first solid-electrolyte layer that has 4 to 7 mole percent yttrium oxide, and providing a second solid-electrolyte layer has 3 to 4 mole percent yttrium oxide (in each instance, based on the zirconium oxide).  
         [0010]     The second solid-electrolyte layer may be formed by a layer applied to the outer surface of the sensor element, using thick-film technology. This layer is used, for example, to cover an electrode and/or electrode lead situated on a surface of the sensor element. The second solid-electrolyte layer may cover the outside surface of the sensor element completely or substantially completely.  
         [0011]     In an alternative exemplary embodiment of the present invention, the second solid-electrolyte layer is formed by a solid-electrolyte foil. A solid-electrolyte foil is a solid-electrolyte layer which is produced from a so-called green foil, using a sintering process. After sintering, such solid-electrolyte foils usually have a thickness of 200 to 500 μm, and, prior to sintering, i.e., as a blank foil, they are printed over with pastes, using thick-film technology. After the sintering, the pastes form functional layers, such as electrodes, protective layers, insulation layers, voids, or porous layers (when pore-forming materials are used).  
         [0012]     The two outer surfaces of the sensor element parallel to the large surface of the sensor element may be formed by a solid electrolyte having a composition, which gives the solid electrolyte a high mechanical strength and, consequently a high tensile strength, for example.  
         [0013]     In a third exemplary embodiment of the present invention, the sensor element has a further solid-electrolyte layer on at least one of its outer surfaces extending perpendicularly to the large surface of the sensor element, the composition of the further solid-electrolyte layer corresponding to the composition of the second solid-electrolyte layer.  
         [0014]     Such sensor elements often have a measuring region heated by a heater. The second solid-electrolyte layer may be provided on the side of the sensor element adjacent to the heater since, on this side of the sensor element, high stresses may occur in the outer solid-electrolyte layer due to the temperature gradients produced by the heater. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]      FIG. 1  shows a cross-sectional view of a first exemplary embodiment of a sensor element according to the present invention, the view extending perpendicularly to the longitudinal axis of the sensor element.  
         [0016]      FIG. 2  shows a cross-sectional view of a second exemplary embodiment of a sensor element according to the present invention, the view extending perpendicularly to the longitudinal axis of the sensor element.  
         [0017]      FIG. 3  shows a longitudinal cross-sectional view of a third exemplary embodiment of a sensor element according to the present invention.  
         [0018]      FIG. 4  shows a detailed portion of another exemplary embodiment of a sensor element according to the present invention.  
         [0019]      FIG. 5   a  shows a schematic illustration of yet another exemplary embodiment of a sensor element according to the present invention.  
         [0020]      FIG. 5   b  shows a schematic illustration of yet another exemplary embodiment of a sensor element according to the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0021]     In  FIG. 1 , a cross-section of a first exemplary embodiment of a sensor element  10  according to the present invention is shown. The sensor element, which is referred to as a voltage-jump lambda sensor, includes three solid-electrolyte foils, namely an inner solid-electrolyte layer  21   a,  a first outer solid-electrolyte layer  31   a,  and a second outer solid-electrolyte layer  32   a.  A first electrode  41 , which is covered by a porous protective layer  52 , is applied to the outer surface of first outer solid-electrolyte layer  31   a.  A second electrode  42  is provided on first outer solid-electrolyte layer  31   a,  opposite to first electrode  41 . Second electrode  42  is situated in a reference-gas chamber  51 , which is formed inside the inner solid-electrolyte layer  21   a.  A voltage is generated between first and second electrodes  41  and  42 , due to the different partial pressures of oxygen at first electrode  41  (gas to be analyzed) and at second electrode  42  (reference gas). A heater  61  is provided between the inner solid-electrolyte layer  21   a  and the second outer solid-electrolyte layer  32   a,  which heater  61  is separated from surrounding solid-electrolyte layers  21   a  and  32   a  by a heater insulation  62 .  
         [0022]     A cross-section of a second exemplary embodiment of the present invention is shown in  FIG. 2 . In this figure and in the following figures, identical elements are indicated by the same reference numerals throughout. Sensor element  10  according to  FIG. 2  is referred to as a wide-range lambda sensor and includes four solid-electrolyte foils, namely a first inner solid-electrolyte layer  21   b  and a second inner solid-electrolyte layer  22   b,  a further solid-electrolyte layer  35 , and a second outer solid-electrolyte layer  32   b.  Situated between the further solid-electrolyte layer  35  and the first inner solid-electrolyte layer  21   b  is an annular measuring-gas chamber  53 ; the measuring gas located outside of sensor element  10  may reach the chamber  53  by traveling through a gas-entrance orifice  55  extending through the further solid-electrolyte layer  35  and through a diffusion barrier  54 . A reference-gas chamber  51  is introduced into the second inner solid-electrolyte layer  22   b.    
         [0023]     As shown in  FIG. 2 , on opposite lateral sides of the first inner solid-electrolyte layer  21   b,  first electrode  41  is deposited in the measuring-gas chamber  53  and second electrode  42  is deposited in the reference-gas chamber  51 . A third electrode  43  is provided on the outside surface of the further solid-electrolyte layer  35 . In the measuring-gas chamber  53 , a fourth electrode  44  is situated on further solid-electrolyte layer  35 , opposite to third electrode  43 . The outside surface of the further solid-electrolyte layer  35  and third electrode  43 , as well as a lead to third electrode  43  extending along the longitudinal axis of the sensor element on its exterior, are covered by a first outer solid-electrolyte layer  31   b.  The first outer solid-electrolyte layer  31   b  is porous, so that the gas to be analyzed may reach the third electrode  43 . The first outer solid-electrolyte layer  31   b  has an opening in the region of gas-entrance orifice  55 .  
         [0024]     A longitudinal cross-section of a third exemplary embodiment of the present invention is shown in  FIG. 3 . Sensor element  10  according to  FIG. 3  is a wide-range lambda sensor that differs from the exemplary embodiment according to  FIG. 2  in that the sensor element  10  of  FIG. 3  includes three solid-electrolyte foils, namely a first outer solid-electrolyte layer  31   c,  a first inner solid-electrolyte layer  21   c,  and a second inner solid-electrolyte layer  22   c.  Measuring-gas chamber  53  and reference-gas chamber  51  are provided in the layer plane between first outer solid-electrolyte layer  31   c  and first inner solid-electrolyte layer  21   c;  the reference-gas chamber  51  is filled with a porous material. In the alternative, the reference-gas chamber may beformed by the porous, second electrode and/or the porous lead to the second electrode. Third electrode  43  is situated on the outside of the first outer solid-electrolyte layer  31   c , and the second electrode  42  is situated in the reference-gas chamber  51 , on the first outer solid-electrolyte layer  31   c.  Electrodes  41  and  44  situated in the measuring-gas chamber  53 , on the first outer solid-electrolyte layer  31   c,  combine the functions of the first and fourth electrodes of the second exemplary embodiment shown in  FIG. 2 . Heater  61  and heater insulation  62  are situated between first inner and second inner solid-electrolyte layers  21   c,    22   c.  The outside of the second inner solid-electrolyte layer  22   c  is covered by a second outer solid-electrolyte layer  32   c,  which is applied to the second inner solid-electrolyte layer  22   c  prior to sintering, using screen printing.  
         [0025]     As shown in a detailed portion in  FIG. 4 , a fourth exemplary embodiment of a sensor element  10  according to the present invention has an inner solid-electrolyte layer  21   d,  to the surface of which an electrode or an electrode lead  40  is applied. The surface of the inner solid-electrolyte layer  21   d  and electrode/electrode lead  40  is covered by an outer solid-electrolyte layer  31   d,  which is applied using screen-printing technology.  
         [0026]      FIGS. 5   a  and  5   b  schematically show fifth and sixth exemplary embodiments of the sensor element according to the present invention. Sensor elements  10  shown in  FIGS. 5   a  and  5   b  both include an inner solid-electrolyte layer  21   e,  the two main surfaces of which are completely covered by a first outer solid-electrolyte layer  31   e  and a second outer solid-electrolyte layer  32   e.  In sensor element  10  shown in  FIG. 5   b,  the lateral surfaces of inner solid-electrolyte layer  21   e  are additionally covered by a further outer solid-electrolyte layer  33 . To this end, the entire sensor element is coated on all sides (after being diced up), using a dipping operation, and subsequently dried and sintered, with the gas-entrance orifice, the region of terminal contacts, and the porous protective layer being removed.  
         [0027]     In the exemplary embodiments of  FIGS. 1 through 6 , the outer solid-electrolyte layer has an yttrium-oxide content of 3 to 4 mole percent. However, the inner solid-electrolyte layer contains 4 to 7 mole percent yttrium oxide.  
         [0028]     In the exemplary embodiment according to  FIG. 1 , the outer solid-electrolyte layers include first outer solid-electrolyte layer  31   a  and second outer solid-electrolyte layer  32   a;  in the exemplary embodiment according to  FIG. 2 , the outer solid-electrolyte layers include first outer solid-electrolyte layer  31   b  and second outer solid-electrolyte layer  32   b;  in the exemplary embodiment according to  FIG. 3 , the outer solid-electrolyte layers include first outer solid-electrolyte layer  31   c  and second outer solid-electrolyte layer  32   c;  in the exemplary embodiment according to  FIG. 4 , an outer solid-electrolyte layer  31   d  is included; in the exemplary embodiment according to  FIG. 5   a,  the outer solid-electrolyte layers include first outer solid-electrolyte layer  31   e  and second outer solid-electrolyte layer  32   e;  and in the exemplary embodiment according to  FIG. 5   b,  the outer solid-electrolyte layers include, in addition to first and the second outer solid-electrolyte layers  31   e  and  32 , a further outer solid-electrolyte layer  33 . In the exemplary embodiment according to  FIG. 1 , an inner solid-electrolyte layer  21   a  is provided; in the exemplary embodiment according to  FIG. 2 , the inner solid-electrolyte layers include first inner solid-electrolyte layer  21   b  and second inner solid-electrolyte layer  22   b;  in the exemplary embodiment according to  FIG. 3 , the inner solid-electrolyte layers include first inner solid-electrolyte layer  21   c  and second inner solid-electrolyte layer  22   c;  in the exemplary embodiment according to  FIG. 4 , an inner solid-electrolyte layer  21   d  is provided; and, in the exemplary embodiments according to  FIGS. 5   a  and  5   b,  an inner solid-electrolyte layer  21   e  is provided.  
         [0029]     In accordance with the present invention, an outer layer is also a solid-electrolyte layer, which is covered by a further layer, if this layer is not predominantly made out of a solid-electrolyte material, or if this layer only covers a small region of the outer surface of the outer solid-electrolyte layer. Thus, in the exemplary embodiment according to  FIG. 1 , first electrode  41 , which is covered, on its part, by porous protective layer  52 , is applied to first outer solid-electrolyte layer  31   a.  Porous protective layer  52  only covers a small region of the first outer solid-electrolyte layer  31   a.    
         [0030]     Described below are two examples of sensor elements which simultaneously achieve a reduction in the tendency to crack and improvement in the measuring performance, which sensor elements have the compositions of the inner and outer solid-electrolyte layers as specified below:  
       EXAMPLE 1  
       [0031]     The outer solid-electrolyte layer contains 3.5 mole percent yttrium oxide, and the inner solid-electrolyte layer contains 5.5 mole percent yttrium oxide.  
       EXAMPLE 2  
       [0032]     The outer solid-electrolyte layer contains 3 mole percent yttrium oxide, and the inner solid-electrolyte layer contains 6 mole percent yttrium oxide.  
         [0033]     If the sensor element is made up of a plurality of solid-electrolyte layers, then the yttrium-oxide content of the solid-electrolyte layers may be graded, so that the transition between adjacent solid-electrolyte layers is softened, i.e., the difference in the yttrium-oxide level of adjacent solid-electrolyte layers is reduced.  
         [0034]     In the exemplary embodiment according to  FIG. 2 , further solid-electrolyte layer  35  is situated between first inner solid-electrolyte layer  21   b  and first outer solid-electrolyte layer  31   b.  In order to soften the transition, further solid-electrolyte layer  35  has an yttrium-oxide content which is between the yttrium-oxide content of first inner solid-electrolyte layer  21   b  and the yttrium-oxide content of first outer solid-electrolyte layer  31   b.  Accordingly, first outer solid-electrolyte layer  31   b  in the second exemplary embodiment shown in  FIG. 2  contains 3 mole percent yttrium oxide, further solid-electrolyte layer  35  contains 5 mole percent yttrium oxide, and first inner solid-electrolyte layer  21   b  contains 7 mole percent yttrium oxide.