Abstract:
A gas sensor, whose purpose is to determine a physical property of a measuring gas, e.g., to determine the concentration of a gas component or the temperature of an exhaust gas. The gas sensor includes a sensor element arranged in a metal housing which is sealed by at least one sealing element arranged in a metal receptacle. The metal receptacle is affixed to the housing. The sealing element surrounds the sensor element in a centered position along its longitudinal extension L or on its half facing the measuring gas.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a gas sensor. 
   BACKGROUND INFORMATION 
   A gas sensor of this type is discussed, for example, in German Published Patent Application No. 197 07 456. The gas sensor includes a metal housing in which two molded ceramic parts are positioned axially one behind the other, which include openings for receiving a sensor element. Between the molded ceramic parts is an intermediate space, in which a glass seal that surrounds the sensor element is provided. The molded ceramic part is sealed in the housing by a sealing ring. 
   The gas sensor discussed in German Published Patent Application No. 197 07 456 provides that when a seal is introduced into a molded ceramic part, the danger of cracking is high due to temperature fluctuations during operation of the gas sensor. In addition, the sealing between the molded ceramic part and the housing requires complex manufacturing technology. 
   German Published Patent Application No. 197 51 424 discusses a gas sensor including a metal housing in which a molded ceramic part including a recess for receiving a sensor element is arranged. The molded ceramic part encircles the sensor element in its center. The end of the sensor element facing the measuring gas includes one or more measuring elements, e.g., electrochemical cells. At the end of the sensor element facing away from the measuring gas, contact surfaces, which are connected electrically by contacting with conductor elements leading out of the gas sensor, are arranged on the sensor element. Between the molded ceramic part and the end of the sensor element facing the contacting, the sensor element is enclosed by a glass seal. The glass seal is positioned in a metal receptacle which is affixed to the housing by a welded connection. The metal receptacle, the end of the sensor element facing the contacting, and the contacting are surrounded by a metal sleeve, which in turn is connected to the housing by an additional welded connection. 
   To produce the sensor elements for such gas sensors, ceramic sheets imprinted with functional layers are laminated together and sintered. During sintering, the ceramic sheets shrink. In this process, slight warping of the sensor elements is often unavoidable. The recess of the molded ceramic part for receiving the sensor element is therefore dimensioned so that the sensor element has play in the recess. Since the sensor element is fixed only at its end facing the contacting by a positive material connection, the sensor element is able to vibrate in the recess due to the vibrations that occur in operation, which may result in damage to the sensor element. Furthermore, building the sensor element into the rigid molded ceramic part is complex, requires difficult production techniques, and may damage the sensor element. 
   SUMMARY 
   The gas sensor according to the present invention and the method according to the present invention for producing the gas sensor provide that a sensor element is sealed in a housing using at least one sealing element in a cost-effective manner involving simple production technology, the sensor element being unaffected by vibrations that occur during operation. To this end, the sealing element is placed in a metal receptacle, which in turn is affixed to the metal housing, and the sealing element encloses the sensor element centered along a longitudinal extension L or on the side facing the measuring gas. 
   The sensor element is held primarily by the sealing element in the metal receptacle (as well as to a lesser degree by the contacting). This eliminates the need for one molded ceramic part, so that the metal receptacle may be exposed directly to the measuring gas. 
   The sealing element may include glass or glass ceramic, and forms a positive material connection with the sensor element and the molded metal part. A glass seal or a glass ceramic seal is able to adapt itself to the shape of the sensor element. This makes it possible to hold even warped sensor elements securely. To prevent mechanical stresses under temperature fluctuations, the expansion coefficient of the sealing element and the expansion coefficient of the sensor element differ by no more than 10 percent. 
   The glass or the glass ceramic is introduced into the receptacle for example as a powder filling, in the form of a pre-pressed or fused glass pellet or in the form of a pre-pressed powder mixture in tablet form. The glass contains a glass-forming component; the glass ceramic includes a ceramic component and a glass-forming component, for example in the form of a ceramic powder and a glass-forming powder. During a subsequent heat treatment, the glass-forming component of the glass or of the glass ceramic melts and forms a positive material connection with the surrounding materials. 
   The metal receptacle may be affixed to the housing by a positive material connection, e.g., by laser welding. Furthermore, on the side of the gas sensor facing away from the measuring gas, there is a sleeve that surrounds a section of the sensor element and the contacting of the sensor element. The metal receptacle and the sleeve are affixed to the housing by a common positive material connection. Especially favorable here in regard to production technology is a welded connection, e.g., a circumferential weld produced by laser welding. 
   The metal receptacle may be cup-shaped, the bottom of the cup-shaped metal receptacle including a recess for receiving the sensor element. At its open end the metal receptacle includes a section extending outward perpendicular to the longitudinal axis of the metal receptacle, to which an additional sleeve-shaped section is connected, so that the metal receptacle includes a collar-like expansion. The outer sleeve-shaped section of this collar-like expansion is affixed to the housing by laser welding. 
   An especially simple and cost-effective manufacture of the gas sensor is possible if the space from the sensor element to the side wall of the cup-shaped metal receptacle is less than or equal to twice the height of the sensor element at least in some places. The height of the sensor element refers to the extension of the sensor element perpendicular to its large surface. 
   In a first further development of the present invention, the metal receptacle contains a first and a second sealing element. The two sealing elements contain glass or glass ceramic as their main component, and are placed one behind the other in the receptacle in the longitudinal direction of the sensor element. The glass-forming component of the first sealing element, which faces the measuring gas, has a higher melting point than the glass-forming component of the second sealing element, which faces away from the measuring gas. When the sensor element is installed, the composite of metal receptacle, first and second sealing element, and sensor element is heated to a temperature at which the glass-forming component of the second sealing element completely melts, while the glass-forming component of the first sealing element does not melt or does not do so completely. This configuration of the sealing elements results in the second sealing element forming a gas-tight, positive material connection to the sensor element and metal receptacle, and the first sealing element preventing the glass of the second sealing element from being able to flow out of the receptacle. The configuration may also provide for the second sealing element to be positioned between the first sealing element and a third sealing element, the third sealing element having a viscous consistency at the temperatures at which the gas sensor is used. This configuration of the sealing elements reduces the risk of the glass or the glass ceramic cracking and the risk of the sensor element breaking in the area of the transition from the glass or the glass ceramic to the air. 
   In a second further development of the present invention, the metal receptacle includes a first and a second sealing element, which are positioned in the receptacle, one behind the other in the longitudinal direction of the sensor element. The first sealing element, which faces the measuring gas, contains a sintered ceramic, and the second sealing element, which faces away from the measuring gas, contains a glass or a glass ceramic. The first sealing element prevents the glass or the glass ceramic of the second sealing element from flowing out of the ceramic receptacle during manufacturing of the gas sensor. In addition, a wafer of pressed ceramic powdered material may be provided between the first and second sealing element as a third sealing element. 
   To produce the gas sensor according to the present invention, the sensor element and a sealing element or a plurality of sealing elements are placed in the metal receptacle and subjected to a heat treatment, during which the glass-forming component of at least one sealing element melts, so that the sensor element is sealed in a gas-tight manner in the metal receptacle by the sealing element. The composite of metal receptacle, sealing element, and sensor element is subsequently placed in the housing and the metal receptacle is fixed in the housing. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a sectional view of a first exemplary embodiment of a gas sensor according to the present invention. 
       FIG. 2   a  shows a section of a metal receptacle of the first exemplary embodiment corresponding to cut line IIa-IIa in  FIG. 2   b.    
       FIG. 2   b  shows a top view of the metal receptacle according to  FIG. 2   a.    
       FIG. 3  shows a sectional view of a second exemplary embodiment of a gas sensor according to the present invention. 
       FIG. 4   a  shows a section of a metal receptacle corresponding to cut line IVa-IVa in  FIG. 4   b.    
       FIG. 4   b  shows a top view of the metal receptacle according to  FIG. 4   a.    
       FIG. 5  shows a sectional view of a first embodiment of the first exemplary embodiment of the gas sensor according to the present invention. 
       FIG. 6  shows a sectional view of a second embodiment of the first exemplary embodiment of the gas sensor according to the present invention. 
       FIG. 7  shows a sectional view of a third embodiment of the first exemplary embodiment of the gas sensor according to the present invention. 
       FIG. 8  shows a sectional view of a fourth embodiment of the first exemplary embodiment of the gas sensor according to the present invention. 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows a section of a gas sensor  10  as a first exemplary embodiment of the present invention. Gas sensor  10  is used for example to determine the temperature or the oxygen content of a measuring gas, and may be built into a measuring opening of an exhaust line of a combustion engine (not shown). Gas sensor  10  exhibits a housing  21  including threading  23  and a hexagon  22  for this purpose. Housing  21  encloses a planar, elongated sensor element  20 , which is configured as a ceramic multi-layer system. On a first section  26 , which is exposed to the measuring gas, sensor element  20  includes measuring elements such as electrodes or heaters. First section  26  of sensor element  20  protrudes from housing  21  into a measuring gas chamber  28 , which is surrounded by a protection tube  24  affixed to housing  21 . Protection tube  24  includes openings (no reference numeral) which may allow the measuring gas to access first section  26  of sensor element  20 . 
   Contact points (not shown) are provided in a second section  27  of sensor element  20 , which is separated from the measuring gas, on the outer surfaces of sensor element  20 . The contact points are electrically connected to the measuring elements by connecting leads located in the composite of layers of sensor element  20 . The contact points are in electrical contact via a contacting device with conducting elements (not shown), through which the measuring elements are connected to an evaluation circuit provided outside of sensor element  20 . Second section  27  of sensor element  20  and the contacting device are surrounded by a sleeve  25 , which is affixed to housing  21 .  FIGS. 1 ,  3 , and  5  through  8  show a section of sleeve  25 . 
   A sealing element  32 , which is positioned in a metal receptacle  31  for sealing element  32 , is provided for sealing first section  26  from second section  27  of sensor element  20 . Metal receptacle  31  is represented in  FIGS. 2   a  and  2   b  as a single element. Sealing element  32  encloses a longitudinal section of sensor element  20 . This longitudinal section is provided in the middle of sensor element  20  (in reference to its longitudinal extension L), or on the half of sensor element  20  that faces the measuring gas. Sealing element  32  thus also functions as a holder for sensor element  20 , and prevents sensor element  20  from vibrating in housing  21 . 
   Sealing element  32  includes a glass or a glass ceramic and is introduced into metal receptacle  31  in the form of a glass powder or a mixture of a ceramic powder (ceramic component) and a glass-forming powder (glass-forming component). The glass powder or glass-forming powder is based chiefly on the oxides BaO, SrO, ZnO, B 2 O 3 , Al 2 O 3 , MgO, CaO, and/or SiO 2 . The ceramic powder includes, for example, statite, forsterite, Al 2 O 3 , Al 2 O 3 .MgO, or ZrO 2  stabilized with CaO, MgO, or Y 2 O 3 , or mixtures thereof. 
   The source material for sealing element  32  is introduced as a powder filling into metal receptacle  31  with sensor element  20  and mechanically compacted. Alternatively, the source material may be introduced together with sensor element  20  into receptacle  31  as a pre-pressed or fused glass pellet or as a pre-pressed powder mixture in tablet form, the glass pellet or pre-pressed powder mixture including a recess for receiving sensor element  20 . 
   In the subsequent thermal treatment of the pre-assembled composite of sensor element  20 , sealing element  32  and metal receptacle  31 , the glass-forming component of the glass or the glass ceramic melts, so that a gas-tight connection is formed between sensor element  20  and sealing element  32  and between metal receptacle  31  and sealing element  32 . In this process, partial or complete crystallization of the glass or of the glass-forming components is producible via deliberate temperature management, so that sealing element  32  exists after the temperature treatment as a partially or fully crystallized glass ceramic. 
   Metal receptacle  31  is cup-shaped. Bottom  35  of metal receptacle  31  includes in its center a recess  33  for sensor element  20 . Recess  33  is rectangular in shape, corresponding to the cross section of sensor element  20 . The space between sensor element  20  and receptacle  31  is sufficiently small in the area of recess  33  to prevent sealing element  32  from flowing out during the melting process. Provided at the open end of receptacle  31  is a collar  34 , which may be placed on housing  21 . Collar  34  encircles housing  21  on its side away from the measuring gas, and is encircled in turn by sleeve  25 . Sleeve  25  and collar  34  are affixed to housing  21  by a common circumferential weld. 
   Measuring gas chamber  28  is bounded by receptacle  31  and by protective tube  24 . Sensor element  20  is the only element in measuring gas chamber  28 . 
     FIG. 3  shows a section of a gas sensor  10  as a second exemplary embodiment of the present invention. The second exemplary embodiment differs from the first exemplary embodiment according to  FIG. 1  in the configuration of receptacle  31 . Corresponding elements are designated in  FIG. 3  by the same reference numerals as in  FIG. 1 . In the second exemplary embodiment, the space between sensor element  20  and the wall of cup-shaped receptacle  31  corresponds approximately to the height of sensor element  20  (i.e., to the extension of sensor element  20  in the direction perpendicular to its large surface), but at most to twice the height of sensor element  20 . The shape of the wall of receptacle  31  largely corresponds to the form of sensor element  20 , i.e., the wall cross section is rectangular. The edges of the wall are rounded. 
   In contrast to the first exemplary embodiment, receptacle  31  does not include a collar  34 . An S-shaped metal connecting piece  36 , which is affixed to metal receptacle  31  and to housing  21  by a welded connection  41 , is provided to connect receptacle  31  to housing  21 . 
     FIGS. 4   a  and  4   b  show a receptacle  31 , which tightly encloses sensor element  20  as in the exemplary embodiment shown in  FIG. 3 , but which includes a collar  34  via which metal receptacle  31  is affixed to housing  21  via a welded connection as in the first exemplary embodiment. 
     FIGS. 5 through 8  show various exemplary embodiments of gas sensor  10 , which differ from the first exemplary embodiment in the configuration of the sealing elements. Corresponding elements are designated in  FIGS. 5 through 8  by the same reference numerals as in  FIG. 1 . 
   In the exemplary embodiment according to  FIG. 5 , metal receptacle  31  includes a first sealing element  321  and a second sealing element  322 , first sealing element  321  being on the side of metal receptacle  31  facing the measuring gas. The two sealing elements  321 ,  322  are made up primarily of a glass or a glass ceramic, the melting temperature of the glass of first sealing  321  element being above the temperature to which the composite of receptacle  31 , first and second sealing elements  321 ,  322  and sensor element  20  is heated in order to fuse the glass of second sealing element  322 . Thus, after production, second sealing element  322  is joined to housing  21  and sensor element  20  by a positive material connection, and seals sensor element  20  in housing  21  of gas sensor  10  with a gas-tight seal. The glass-forming component of first sealing element  321  is not completely melted on during production. This prevents the material of second sealing element  322  from being able to flow out of receptacle  31  during melting. 
   The exemplary embodiment shown in  FIG. 6  corresponds to the exemplary embodiment according to  FIG. 5  and also includes a third sealing element  323  that is located on the side of second sealing element  322  facing away from the measuring gas. Third sealing element  323  includes a glass or a glass ceramic, and has the property of taking on a viscous consistency at the temperatures to which gas sensor  10  is exposed when used as intended. 
   In the exemplary embodiment according to  FIG. 7 , receptacle  31  includes a first sealing element  331  and a second sealing element  332 , first sealing element  331  being on the side of receptacle  31  facing the measuring gas. First sealing element  331  is a sintered ceramic wafer including a recess for sensor element  20 ; as in the first and second exemplary embodiments, second sealing element  332  contains a glass or a glass ceramic. First sealing element  331  prevents the glass-forming component from flowing out of receptacle  31  during melting. In this exemplary embodiment, recess  33  for sensor element  20  has a broader configuration (the distance from the bottom of receptacle  31  to sensor element  20  corresponds for example to the height of sensor element  20 ), so that sensor element  20  may be introduced into recess  33  more easily. 
   The exemplary embodiment shown in  FIG. 8  corresponds to the exemplary embodiment according to  FIG. 7  and also contains a third sealing element  333  located between first and second sealing elements  331 ,  332 . Third sealing element  333  is a wafer of pressed ceramic powdered material which also prevents the material of second sealing element  332  from flowing out during melting. In an additional exemplary embodiment, third sealing element  333  may replace second sealing element  332 . 
   Additional exemplary embodiments of the present invention are produced by combining the exemplary embodiments according to  FIG. 5  or  6  with the exemplary embodiments according to  FIG. 7  or  8 . Here sealing elements from one exemplary embodiment may be replaced by sealing elements from a different exemplary embodiment, or sealing elements from a different exemplary embodiment may be added. 
   Additional exemplary embodiments of the present invention provide that the sealing elements of the exemplary embodiments according to  FIGS. 5 through 8 , with their geometry adapted appropriately, are inserted into the receptacle of the second exemplary embodiment according to  FIG. 3  or  4   a  and  4   b .