Abstract:
A sensor, in particular for determining the oxygen content in exhaust gases of internal combustion engines, is proposed, as well as a method for its manufacture. The sensor includes a receptacle, arranged in a longitudinal bore ( 16 ) of a metal housing ( 10 ), for a sensing element ( 12 ), in which receptacle the sensing element ( 12 ) is received in gas-tight fashion via a sensing element seal, the sensing element seal being a glass seal ( 57 ). The receptacle has a measured-gas-side ceramic shaped element ( 20 ) and a connector-side ceramic shaped element ( 27 ), which are arranged axially one behind the other. A cavity ( 55 ) into which the glass seal ( 57 ) is pressed while hot is configured between the two ceramic shaped element ( 20, 27 ).

Description:
BACKGROUND INFORMATION  
         [0001]    The invention proceeds from a sensor according to the species defined in the principal claim. A sensor of this kind is known from U.S. Pat. No. 5,467,636, in which a planar sensing element is immobilized in gas-tight fashion, by way of a sealing element, in a passthrough of an exhaust-gas-side lower ceramic shaped element. The exhaust-gas-side ceramic shaped element has on the end surface facing away from the exhaust gas a recess which surrounds the passthrough and into which a glass seal is introduced. A further ceramic shaped element, which is joined via a metal solder join to the housing, sits on the glass seal. The glass seal encloses the sensing element inside the recess, and constitutes a gas-tight join between ceramic shaped element and sensing element at this point.  
         SUMMARY OF THE INVENTION  
         [0002]    The sensor according to the present invention, having the characterizing features of the principal claim, has the advantage that a mechanically stable and gas-tight join is possible between the planar sensing element and both ceramic shaped elements.  
           [0003]    The hermetic seal of the sensing element thereby achieved is vibration-proof, so that while the sensor is being used in the motor vehicle, the sensing element can be immobilized over the utilization period in mechanically stable and hermetic fashion. The method according to the present invention makes it possible for gas-tight immobilization of the sensing element to be attained efficiently.  
           [0004]    The features set forth in the dependent claims make possible developments of and improvements to the sensor according to the invention and the method for its manufacture. A particularly mechanically stable and gas-tight join between the sensing element and the ceramic shaped elements is achieved if the glass seal covers the sensing element over as large an area as possible, but does not penetrate appreciably into the front region which is subject to high thermal stress when the sensor is later operated. The arrangement of a powdered additional seal on the measured-gas site in front of the glass seal prevents the molten glass from penetrating, during the melting process, into the front region of the sensing element that is subject to high thermal stress. It is advantageous for the manufacturing process that the two ceramic shaped elements are configured, on the end faces which face toward one another, in the form of a die and punch, and act accordingly on one another. This makes possible compression of the glass seal, and of the powdered additional seal that is optionally used, utilizing the geometry of the ceramic shaped elements. The presence of a gap between die and punch has the advantage that the glass seal can escape into the gap upon compression. This makes it possible to work with a high compressive force. At the same time, it prevents the two end faces of the ceramic shaped elements from striking one another. In addition, a further glass seal can be inserted into the annular gap between the ceramic shaped elements, or an annular metal foil or plate can be set in place, thus resulting in a positive join between the two ceramic elements. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]    Three exemplary embodiments of the invention are depicted in the drawings and explained in more detail in the description below. In the drawings:  
         [0006]    [0006]FIG. 1 shows a sectioned depiction through a sensor according to the invention;  
         [0007]    [0007]FIG. 2 shows a first exemplary embodiment of a sensing element seal for the sensing element in the uninstalled state, with an apparatus for manufacturing the seal;  
         [0008]    [0008]FIG. 3 shows a second exemplary embodiment of a sensing element seal in the uninstalled state; and  
         [0009]    [0009]FIG. 4 shows a third exemplary embodiment of a sensing element seal in the uninstalled state. 
     
    
     DETAILED DESCRIPTION  
       [0010]    The sensor depicted in FIG. 1 is an electrochemical sensor for determining the oxygen content in exhaust gases of internal combustion engines. The sensor has a metal housing  10  in which a flat-plate sensing element  12 , having a measured-gas-side end section  13  and a connector-side end section  14 , is arranged. Housing  10  is configured with threads as attachment means for installation into an exhaust pipe (not depicted). Also arranged in housing  10  is a longitudinal bore  16  having, for example, a first shoulder-like annular surface  17  and a second shoulder-like annular surface  18 .  
         [0011]    Arranged in longitudinal bore  16  is a measured-gas-side ceramic shaped element  20  having a measured-gas-side passthrough  24 , and having a measured-gas-side end face  21  and a connector-side end face  22 . Measured-gas-side end face  21  is configured with a conically extending sealing seat  23  which sits on a metal sealing ring  25  that rests against second shoulder-like annular surface  18 . Arranged above measured-gas-side ceramic shaped element  20  is a connector-side ceramic shaped element  27  having a connector-side passthrough  30  and having a measured-gas-side end face  28  and a connector-side end face  29 .  
         [0012]    A disk spring  31  that is under mechanical preload, which presses via a tubular retaining cap  32  onto measured-gas-side ceramic shaped element  27  that projects out of housing  10 , rests on connector-side end face  29  of connector-side ceramic shaped element  27 ; retaining cap  32  engages via snap-lock tabs  34  into an annular groove  33  arranged on the outer side of housing  10 . The two ceramic shaped elements  20 ,  27  are preloaded in the axial direction via retaining cap  32  and disk spring  31 , so that measured-gas-side ceramic shaped element  20  presses with conical sealing seat  23  onto sealing ring  25 . A gas-tight sealing seat thus forms between housing  10  and ceramic shaped element  20 .  
         [0013]    Measured-gas-side end section  13  projecting out of the housing is, for example, surrounded at a distance by a double-walled protective tube  37  having gas inlet and gas outlet openings  38 . On connector-side end section  14 , sensing element  12  has contacts (not depicted further) which make contact with connector cables  42  via a contact plug  41 . Connector plug  41  includes, for example, two ceramic elements which are held together by a clamping piece  43 . Connector-side end section  14  of sensing element  12 , which projects out of connector-side ceramic shaped element  27 , is surrounded by a metal sleeve  45  which is welded in gas-tight fashion to housing  10  and has a tubular opening  47  in which a cable passthrough  48  is located for the passage of connector cable  42 .  
         [0014]    Measured-gas-side ceramic shaped element  20  has on connector-side end face  22  a punch-shaped extension  51  which surrounds measured-gas-side passthrough  24 . Connector-side ceramic shaped element  27  has on measured-gas-side end face  28  a recess  52  into which punch-shaped extension  51  penetrates with a radial gap  53 . A cavity  55 , which is filled with a glass seal  57 , is formed between the end face of punch-shaped extension  51  and the bottom of recess  53 . It is also possible to configure punch-shaped extension  51  on connector-side ceramic shaped element  27 , and recess  52  on measured-gas-side ceramic shaped element  20 .  
         [0015]    Glass seal  57  causes sensing element  16  to be hermetically sealed in ceramic shaped elements  20 ,  27 . The dimensions of punch-shaped extension  51  and of recess  52  are such that an annular gap  59  is formed between the mutually facing annular surfaces of measured-gas-side ceramic shaped element  20  and connector-side ceramic shaped element  27 . The purpose of annular gap  59  is to allow the fusible glass of glass seal  57  to escape via radial gap  53  into annular gap  59  upon compression.  
         [0016]    A fusible glass, for example a lithium aluminum silicate glass or a lithium barium aluminum silicate glass, is suitable as glass seal  57 . Additives which improve the flow characteristics of the molten glass can be added to the fusible glass.  
         [0017]    Powdered substances such as copper, aluminum, iron, brass, graphite, boron nitride, MoS 2 , or a mixture of these substances, can be used as additives for plastification of glass seal  57  during the joining process. Lithium carbonate, lithium soap, borax, or boric acid are used, for example, as fluxes for glass seal  57 . The addition of compensating fillers, such as aluminum nitride, silicon nitride, zirconium tungstate, or a mixture of these substances, is suitable for adjusting the thermal expansion. A further improvement in the join between glass seal  57  and the ceramic of ceramic shaped elements  20 ,  27  is achieved if a ceramic binder, such as aluminum phosphate or chromium phosphate, is added to glass seal  57 .  
         [0018]    In order to achieve large-area wetting of sensing element  12  with glass seal  57 , in the present exemplary embodiments the side surfaces of measured-gas-side passthrough  24  and of connector-side passthrough  30  of ceramic shaped elements  20 ,  27  are each configured, toward cavity  55 , with a conically extending enlargement  61  (FIGS. 2, 3, and  4 ).  
         [0019]    Three exemplary embodiments of the sensing element seal in the uninstalled state, in each case with an apparatus for manufacturing glass seal  57 , are evident from FIGS. 2, 3, and  4 .  
         [0020]    The apparatus has a support  70  acting as die, with a receptacle  71  and a stop  72 . Ceramic shaped elements  20  and  27  are positioned in receptacle  71  with sensing element  12  received in passthroughs  24 ,  30 . The axial position of sensing element  12  is defined in this context by stop  72 , sensing element  12  resting with measured-gas-side end section  13  on stop  72 . Measured-gas-side ceramic shaped element  20  is first inserted with sensing element  12  into receptacle  71 . A glass blank  63 , for example in the form of a glass pellet or glass film, is placed onto the end surface of punch-shaped extension  51 , glass blank  63  having an opening with which glass blank  63  is slid over sensing element  12 . Connector-side ceramic shaped element  27  is then placed onto glass blank  63 , so that connector-side end section  14  of sensing element  12  projects through passthrough  30 . In the arrangement described, a compressive force of, for example, 600 kg-force is applied onto connector-side ceramic shaped element  27  using a pressing punch  74 . Beforehand, however, glass blank  63  was heated, for example by a heating device housed in support  70 , to a temperature above the softening temperature of the fusible glass or glass ceramic being used. Upon compression, the fluid glass blank  63  deforms and is thereby pressed into conical enlargements  61  and into radial gap  53 . Fusible glass flowing out via radial gap  53  can escape into end-surface annular gap  53 .  
         [0021]    A second exemplary embodiment is depicted in FIG. 3. This exemplary embodiment differs from the exemplary embodiment of FIG. 1 in that a further annular glass blank  64  is inserted into annular gap  59 . Upon compression, the fluid further glass blank  64 , like glass blank  63 , deforms so that annular gap  59  is additionally sealed with a further glass seal.  
         [0022]    A further exemplary embodiment of a sensing element seal is evident from the arrangement in FIG. 4. Here a further blank  65 , precompressed and optionally presintered, is arranged on the measured-gas side below glass blank  63 . Materials with good plastic deformability, such as talc, kaolin, clay, bentonite, graphite, boron nitride, etc. are in principle particularly suitable as the material for further blank  65 . As punch  74  is applied during compression of the fluid glass blank  63 , blank  65  is simultaneously deformed into its powder constituents, thus resulting in a powdered additional seal. Before the fusible glass flows in, the powder penetrates into the gap of measured-gas-side passthrough  24  formed by conical enlargement  61 , so that the fusible glass is prevented from flowing to the measured-gas end of ceramic shaped element  20  that is subject to high thermal stress.  
         [0023]    The apparatuses depicted in FIGS. 3 and 4 correspond to the apparatus of FIG. 2. The method for manufacturing glass seal  57  according to FIG. 4 can be carried out in accordance with the method implemented using the apparatus in FIG. 2. It is also possible, however, first to deform further blank  65  into powder using a punch and press it into the gap between sensing element and measured-gas-side passthrough, and then to compress glass blank  63  using the procedure according to FIG. 2. A further embodiment of the sensing element seal according to FIG. 4, having a further fused glass seal in annular gap  59  as in the case of the exemplary embodiment in FIG. 3, is also possible.