Abstract:
A seal for a sensor element of a gas sensor for determining the oxygen content in exhaust gases of internal combustion engines. The seal includes at least one sealing element that is inserted into a longitudinal bore of a housing and that includes a mixture of at least one ceramic compound and at least one fluoride compound.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a seal for a sensor element of a gas sensor. 
     BACKGROUND INFORMATION 
     A seal for a sensor element of a gas sensor is known, for example, from German Published Patent Application No. 195 32 090 A1, in which the sensor is mounted into a longitudinal bore of a housing by way of at least two sealing members and a deformable auxiliary seal arranged between the sealing members. The two sealing members are made of magnesium aluminum silicate (steatite), and the sealing member mounted between those sealing members is made of the hexagonal allotrope of boron nitride. 
     SUMMARY OF THE INVENTION 
     The seal according to the present invention is both gas-tight and impermeable to liquids, in particular to fuels, and moreover possesses very high temperature resistance. This is achieved by way of a mixture of at least one ceramic compound and at least one fluoride compound. In addition, the use of this mixture instead of a seal configuration made up of sealing elements of different chemical compositions yields simplified handling and assembly. 
     In a particularly advantageous manner, steatite, i.e. the combustion product of soapstone, having the approximate chemical formula 3MgO.4SiO 2 .H 2 O, in a mixture with a fluoride compound, is used as the ceramic compound. This ensures particularly high temperature stability. 
     In a further preferred embodiment, boron nitride is used as the ceramic compound, the hexagonal allotrope of BN being preferred. The hexagonal allotrope of boron nitride is very fine-grained and similar to its isostere graphite, a highly deformable compound, so that the tightness and flexibility of the sealing element are decisively improved. 
     Advantageously, a metallic fluoride, in particular a divalent or trivalent metallic fluoride, is used as the fluoride compound. The addition of a metal fluoride of this kind allows an increase in the coefficient of thermal expansion of the powder packet in temperature ranges from 500 to 1000 degrees. The coefficient of thermal expansion of the sealing element is thereby adapted to those of, for example, chromium steel or yttrium-stabilized zirconium dioxide (YSZ). 
     In a preferred embodiment, the quantitative concentration of the fluoride compound is 15 to 70 wt. %, in particular 20 to 30 wt. %, in terms of the total mass of the seal. By using a fluoride compound in the form of a powder having an average particle diameter α 50  of 0.5 to 10 μm, in particular 1 to 5 μm, the coefficient of thermal expansion is adapted particularly well to that of YSZ. As a result of the use of the metal fluoride, the coefficient of thermal expansion (CTE) of the powder packet which is used as the sealing element is, for example, 10 to 18×10 −6  Kelvin −1  in the temperature range from 500 to 1000 degrees C. The coefficient of thermal expansion of YSZ, in contrast, is 10×10 −6  Kelvin −1 , so that by appropriately varying the metal fluoride, the coefficient of thermal expansion can be adapted in such a way that no thermally induced stresses occur between the seal and the solid electrolyte body of the gas sensor. This makes it possible, in particular, for the powder packet of the seal to function in consistent and stable fashion even in hot gases and in continuous operation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The FIGURE shows a cross section through a gas sensor having a seal arrangement according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     The FIGURE shows a gas sensor  10 , for example, an electrochemical oxygen sensor, which possesses a metallic housing  12  that has threads  13  as mounting means for installation into a measured gas tube (not depicted). Housing  12  has a longitudinal bore  15  with a shoulder-shaped annular surface  16 . Located on shoulder-shaped annular surface  16  is, for example, a metallic sealing ring  18  on which a measured gas-side ceramic shaped element  21  rests. Measured gas-side ceramic shaped element  21  has a continuous measured gas-side opening  22  running in the direction of longitudinal bore  15 . Also arranged in longitudinal bore  15 , spaced away from measured gas-side ceramic shaped element  21 , is a connector-side ceramic shaped element  23 . Connector-side ceramic shaped element  23  has a centrally arranged and continuous connector-side opening  24 , also running in the direction of longitudinal bore  15 . Measured gas-side opening  22  of measured gas-side ceramic shaped element  21  and connector-side opening  24  of connector-side ceramic shaped element  23  run in alignment with one another. Located in openings  22 ,  24  is a plate-shaped sensor element  27  having a measured gas-side end section  28  and a connector-side end section  29 . 
     Measured gas-side end section  28  of sensor element  27  projects out from housing  12  and is surrounded by a protective tube  31  that is fastened to housing  12 . The protective tube has entrance and exit openings  32  for the gas to be measured. Connector-side end section  29  possesses connecting contacts  34  which also project out of housing  12 . Contact is made to connection contacts  34  by way of a contact plug (not depicted) equipped with connection cables. Connector-side end section  29  projecting out of housing  12  is surrounded by an encapsulation (not depicted) which protects end section  29  from environmental influences. 
     Located between measured gas-side ceramic shaped element  21  and connector-side ceramic shaped element  23  is a sealing element  37  consisting of a mixture consisting of a ceramic compound and a fluoride compound, for example boron nitride or steatite as the ceramic compound and calcium fluoride, magnesium fluoride, or strontium fluoride, aluminum fluoride, or yttrium fluoride, or another fluoride of the rare earths as the fluoride compound. If boron nitride is used, it is present in the form of its hexagonal allotrope. The concentration of the fluoride compound is 15 to 70 wt. %, in particular 20 to 30 wt. %, in terms of the total mass of the sealing element  37 . Connector-side ceramic shaped element  23  presses onto this sealing element  37 . The compressive force of connector-side ceramic shaped element  23  is applied by a metal sleeve  40 . Metal sleeve  40  has, for example, multiple uniformly distributed rearward-facing prongs  41  which engage into notches  42  shaped onto housing  12 . It is also possible, however, to weld metal sleeve  40  to housing  12 . Sealing element  37  consisting of the ceramic-fluoride mixture is preshaped into a ring, by sintering at a low temperature of, for example, 500 degrees, before installation into longitudinal bore  15  of housing  12 . The annular sealing element  37  formed in this manner is inserted, in accordance with the exemplary embodiment, into longitudinal bore  15  which already contains sensor element  27 . Connector-side ceramic shaped element  23  is then arranged above sealing element  37 . Metal sleeve  40  is then placed onto the connector-side ceramic shaped element. A force which acts via connector-side ceramic shaped element  23  on sealing element  37  is then exerted on metal housing  40  by way of a plunger. The prefabricated ring of sealing element  37  is thereby deformed in such a way that the material of sealing element  37  presses against sensor element  27  and housing  12 . 
     It has been found that the sealing effect is determined substantially by the nature and concentration of the metallic fluoride compound. 
     The fact that a force proceeding from metal sleeve  40  acts continuously on sealing element  37  is essential to achieving tightness with respect to gas and fuel over a wide temperature range. Because the CTE is modified by the metallic fluoride, the result of a corresponding mixture of the ceramic component with the corresponding fluoride component is that the compressive force proceeding from metal sleeve  40  acts on sealing element  37  even at higher temperatures. 
     Utilization of sealing element  37  according to the present invention is not limited to the sealing of planar sensor elements in metallic housings. It is entirely possible also to use a sealing element  37  of this kind to seal so-called finger probes. All that is then necessary for this application is to adapt the configuration of the prefabricated ring for sealing element  37  to the geometry of the longitudinal bore and of the contact surface between the housing and the finger-shaped sensor element.