Abstract:
A method for manufacturing a sensing element is provided, in particular for determining the oxygen content in exhaust gases of internal combustion engines, a composite construction having at least one ceramic paste (green film) present in film form being sintered to yield the sensing element, and sharp edges of the sensing element being blunted before sintering to increase the thermal shock resistance of the sensing element.

Description:
This application is a 371 of PCT/DE98/00525 filed Feb. 21, 1998. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a method for manufacturing a sensing element, in particular for determining the oxygen content in exhaust gases of internal combustion engines. 
     BACKGROUND INFORMATION 
     Sensing elements are configured, for example, as so-called planar sensing elements, which comprise a composite construction of individual layers arranged one above another in film form. The individual films of this composite construction are arranged one above another in defined fashion, resulting in various functional layers. The individual films of the composite construction are, for example, laid onto one another by means of a screen printing technique, as green films. The sensing elements usually have solid electrolyte films, electrode films, heating, conductor films, insulation films, and protective films. Conventional sensing films may also have substrate films with printed electrolyte layers, aluminum oxide substrate films with semiconductor sensors (TiO 2 , SrTiO 3 ). Instead of laying the green films onto one another, the latter, may also be obtained by individual printing steps. The arrangement of these different films one above another results in a laminated composite construction from which the sensing element is obtained by sintering. 
     During testing of the sintered sensing element, or during utilization thereof as intended, the individual layers of the sensing element are exposed to different temperatures. Because of these sudden temperature changes which occur with differing intensity, the sensing elements experience a temperature shock which leads to the occurrence of mechanical stresses in the surface region, in particular at the edges of the sensing element. In order to increase the temperature shock resistance of the sensing elements, U.S. Pat. No. 5,144,249 describes blunting the edges of the sensing element, i.e. equipping them with a chamfer. Chamfering is accomplished by way of a grinding operation sintering and after sectioning of the sensing elements. It is disadvantageous to subject completed sensing elements to a mechanical machining operation which is relatively complex and may lead to undesired damage to the sensing elements. 
     SUMMARY OF THE INVENTION 
     The method according to the present invention offers, in contrast, the advantage that blunting of the edges of the sensing element may be accomplished in a simple manner without the risk of impairing the sensing element. The edges of the sensing element are blunted prior to sintering, as a result, is possible to blunt the edge in any desired geometry using simple, non-chip-removing methods. In particular, blunting of the edges may be accomplished in a form deviating from a flat surface, for example in a convex or concave form, so that mechanical stresses which occur as a consequence of a temperature shock to the blunted edges cannot result in the creation of cracks. 
     In a preferred embodiment of the present invention, provision is made for the edges to be blunted by shaping, preferably by stamping of the film composite construction present in the green state. It is thereby possible, using a simple stamping tool, to shape the edges of the composite construction of green films in simple fashion, due to their soft consistency prior to sintering. By configuring a corresponding stamping tool, a blunting of the edges may be executed in any desired form. It is particularly advantageous if, once the stamping tools have been used, the stamping films laid in place are ones which allow shaping only of the edge region of the sensing element, and leave the other regions, in particular the planar regions of the sensing element, unmodified. In order to prevent adhesion of the green film composite construction of the sensing element in the stamping tool, the stamping film may be advantageously equipped with an anti-adhesion coating, in particular Teflon. 
     In a preferred embodiment, blunting of the edges is accomplished by use of a laser treatment. Use of the laser treatment makes it possible to advantageously accomplish noncontact blunting of the edges of the sensing element in the green state, so that any mechanical loads on the green film composite construction may be ruled out. It is possible to advantageously adjust the contour of the blunted edges of the sensing element by using a mask of an excimer laser. 
     By using the laser treatment, blunting of the edges may preferably be accomplished even before sectioning of the green films present in the composite construction, so that blunting of the edges may be accomplished very effectively. At the same time, the break points of the wafer, with the individual sensing elements may thereby be defined. 
     It is also preferred, in particular, if the laser treatment simultaneously accomplishes blunting of the edges and sectioning of the green film composite construction. By adjusting the laser output and the geometry of the laser beam, it is thus possible to accomplish edge blunting and sectioning in one operation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a sectioned depiction through a sensing element. 
     FIG. 2 shows a sectioned depiction through a panel of several sensing elements. 
     FIG. 3 shows geometrical structures of lasers. 
     FIG. 4 shows a use of a laser according to the present invention. 
     FIG. 5 shows a use of a stamping technique according to the present invention. 
     FIG. 6 shows a use of a stamping technique according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a sectioned depiction through a sensing element  10  which may serve, for example, to determine an oxygen content in exhaust gases of internal combustion engines in motor vehicles or of furnaces. Since the configuration and function of a sensing element  10  of this kind are commonly known, only the configuration that is important for explaining the present invention will be described below. The sensing element substantially has an elongated, flat-plate configuration that is composed of individual strata of various functional layers. As FIG. 1 illustrates, sensing element  10  possesses an electrochemical measurement cell  12  and a heating element  14 . Measurement cell  12  includes a first solid electrolyte film  16  and a second solid electrolyte film  18  which has an integrated reference gas conduit  20 . A measurement electrode  22  is associated with a measured-gas-side surface of electrolyte film  16 , and a reference electrode  24  is associated with the surface associated with reference gas conduit  20 . A porous cover layer  26  is arranged above measurement electrode  22 . 
     Heating element  14  has heating conductors  32  embedded in insulation layers  28  and  30 . Adjacent to insulation layer  30  is a further cover layer  34 . 
     Solid electrolyte films  16  and  18  and cover layer  34  are made, for example, of a stabilized zirconium oxide (ZrO 2 ). Electrodes  22  and  24  and heating conductors  32  are made, for example, of a platinum cermet. Insulation layers  28  and  30  are made, for example, of a mixture of aluminum oxide (Al 2 O 3 ) and glass-forming components. 
     The entire composite construction of individual layers possesses, when viewed in cross section, an approximately parallelepipedal configuration, at least edges  36  running in the longitudinal direction of the sensing element have a chamfer  38 . 
     Sensing element  10  is manufactured by successive lamination of the individual layers onto cover layer  34 , which at the same time constitutes a support. Definition of the layers may be accomplished by screen printing of a paste material which has the respective composition of the layer. After completion of this lamination, there results a composite construction of so-called green films of the individual layers, which possess a relatively soft consistency. The composite construction is a then subjected in a conventional manner to a sintering operation, sensing element  10  is created under the action of temperature and optional pressure. 
     According to the present invention, provision is now made, before sintering, for patterning chamfers  38  of edges  36 . Individual possibilities for achieving chamfers  38  will be discussed below. 
     FIG. 2 shows a portion of a panel of a plurality of sensing elements  10  present in the green state. In this, the individual layers of sensing elements  10  are laminated simultaneously for a plurality of sensing elements  10 , and the composite construction of green films for one sensing element  10  is then sectioned out. FIG. 2 shows portions of three sensing elements  10 . Parts identical to those in FIG. 1 are given identical reference characters, and will not be explained again. After lamination, cutting lines  40  are defined at which sectioning of sensing elements  10  is accomplished. Prior to sectioning of sensing elements  10 , a defined surface depression  42  may be introduced at cutting lines  40 . This surface depression  42  may be executed, for example, using an excimer laser  44  which has a specific mask. FIG. 3 shows, for example, two possible masks. According to the left-hand depiction, excimer laser  44  may possess a triangular mask so that surface depressions  42  are triangular in accordance with this depression. According to the exemplary embodiment depicted on the right in FIG. 3, the mask may also have delimiting surfaces extending in concave fashion. Other exemplary embodiments which exhibit mixed forms of planes running at various angles and/or concave and/or convex delimiting surfaces are also possible. 
     As FIG. 4 illustrates, excimer laser  44  is moved along the surface of the composite construction of green films. For this purpose, either excimer laser  44  may be movable, and/or the green films may be moved past excimer laser  44 . Surface depression  42  is patterned in terms of its depth and feed rate in accordance with the output setting of excimer laser  44 . 
     The patterning of surface depressions  42  yields blunted edges  36  with their chamfers  38 . Sensing elements  10  are then sectioned along cutting lines  40 , and are then subjected to the sintering operation. Sensing element  10  shown in cross section in FIG. 1 is then created. Because chamfers  38  are patterned while the films of sensing element  10  are in the green state, and because of the noncontact patterning with excimer laser  44 , sensing element  10  is not subjected to any mechanical stress, thus substantially ruling out damage. 
     Sectioning of the sensing elements may be accomplished via a further treatment with an excimer laser which has a corresponding mask. It is also possible, however, by selecting a mask and an output level for excimer laser  44 , to execute the surface depression and sectioning in one operation. 
     FIG. 5 indicates a further possibility for patterning chamfers  38 . In this, a sensing element  10  is acted upon by a stamping apparatus  45  after sectioning of the composite construction of green films. Stamping apparatus  45  possesses a contour  46  which allows shaping of edges  36  in such a way that the edges  36  then exhibit chamfers  38 . Depending on the shaping of contour  46 , chamfer  38  may also have a different contour as a result of stamping, for example planar and/or convex and/or concave sections. Contour  46  of stamping apparatus  45  may be created either by manufacturing a corresponding stamping apparatus  45 , or by laying a stamping film  48  into stamping apparatus  45 . Stamping film  48  is preferably equipped with an anti-adhesion coating, for example Teflon or titanium nitride. Since the green films still have a relatively soft consistency in the case of this shaping as well, chamfers  38  may easily be stamped in without causing impairment to the prefabricated sensing element  10 . 
     FIG. 6 shows a further exemplary embodiment in which stamping of a composite construction of sensing elements  10  may be accomplished. For this, stamping apparatus  45  possesses a stamping contour  50  which exhibits projections  52  corresponding to depressions  42 . Stamping contour  50  may also be equipped with an anti-adhesion coating. With the exemplary embodiment shown in FIG. 6, it is easy to stamp a plurality of sensing elements  10  in a multiple panel with one stamping step, subsequent sectioning occurring along cutting lines  40 . 
     Stamping apparatus  45  may advantageously have an upper die  54  and a lower die  56 , so that the upper and lower sides of sensing elements  10  may be stamped simultaneously in one process step. Because of the relatively soft consistency of the as-yet unsintered sensing elements  10 , surface depressions  42  may be stamped in with little energy expenditure, so that damage to the structure of sensing elements  10  may be excluded. 
     It is self-evident that when chamfers  38  are patterned either using excimer laser  44  or with stamping apparatus  45 , both sides of sensing element  10  are processed. For this purpose, either an apparatus acting in double-sided fashion may be provided, or the green film composite construction of sensing elements  10  is turned over. 
     In sum, it is clear that the configuration of chamfers  38  in various contours, which is desirable in order to increase the temperature shock resistance of sensing element  10 , may be effected using easily implemented actions. The outlay for tooling is relatively low, and the latter is subject essentially to no wear, so that long service lives may be expected. The additional use of consumable materials, for example as in the case of grinding of the sintered sensing element  10  defined in the existing art, is entirely eliminated.