Abstract:
A plate-shaped sensor element is proposed, in particular for determining the oxygen level in exhaust gases of internal combustion engines. The sensor element has at least one measuring cell with an oxygen-ion-conducting solid electrolyte and a heating element, the measuring cell and the heating element being connected with an electrical insulation layer. The material of the insulation layer is made of at least one crystalline, non-metallic material and at least one glass-forming material, a glazing filled with the crystalline, non-metallic material being formed when the sensor element is sintered.

Description:
BACKGROUND INFORMATION 
     German Patent Application No. 43 42 731 describes a gas sensor with a tubular finger-shapes sensor element in which one of the printed conductors running on the outside of the tubular sensor element is covered by an electrically insulating layer formed by a mixture of a crystalline, non-metallic material and a glass-forming material, a glazing, filled with the crystalline non-metallic material being formed upon heating. 
     Furthermore, German Patent Application No. 29 07 032 corresponding to U.S. Pat. No. 4,294,679), for example, describes a planar sensor .a element for determining the oxygen level in gases, in which a measuring cell is connected to a resistance heating element via an Al 2 O 3  insulating layer. The ceramic heater insulation made of Al 2 O 3  is electrically insulating and is used porously sintered to compensate for the different sinter contractions and different thermal expansion coefficients of Al 2 O 3  and the adjacent ZrO 2  solid electrolyte layer. This, however, has the disadvantage that gaseous and liquid components diffuse from the exhaust gas into the reference atmosphere through the porous insulation layer and thus affect the measuring signal. In addition, components of the exhaust. 
     SUMMARY OF THE INVENTION 
     The gas sensor according to the present invention has the advantage that the insulation layer is gas-tight and has a good electrical insulation capability, good adhesion to the solid electrolyte ceramic, and good heat conductivity. The good adhesion results, in particular, from the fact that shrinkage of the insulation layer material due to sintering is approximately equal to that of the solid electrolyte ceramic material. The compression stresses arising in the insulation layer due to the different thermal expansion coefficients of the insulation layer and the solid electrolyte foil are reduced in part by the plastic deformation due to the softening characteristics of the glass phase and uniformly distributed over the boundary surface with the solid electrolyte ceramic. Thus local stress concentrations that might cause cracks are fully avoided. The glass materials used have an initial softening temperature that is lower than the 1250° C sintering temperature. The powder mixture used in the process for manufacturing the sensor element has proved to be particularly well-suited. The paste produced with the powder mixture is particularly well-suited for screen printing of the gas-tight insulation layers. 
     The particular the properties regarding gas-tightness and heat conductivity are achieved if Al 2 O 3  with a particle size of d 50 &lt;0.40 μm is used as the crystalline, non-metallic material. Gas-tightness of the insulation layer is further improved when a particle size distribution of d 90 &lt;1 μm is set. With this particle size and particle size distribution, a gas tightness 2 to 4 times greater than is achievable with conventional ceramic layers can be achieved. d 50  denotes the average particle size referred to the mass; d 90  denotes the particle size with 90% of the mass being finer or the same. By suitable selection of particle size and particle size distribution of materials B and C in the following table, the sintering temperature can be reduced from 1600° C. to 1250° C. The melting point of the glass-forming material, with which a glazing filled with a crystalline, non-metallic material, for example, Al 2 O 3 , is formed, is the limit for the sintering temperature. An insulation layer that is particularly well-suited for heater insulation is achieved with a proportion of 60 wt. % of crystalline non-metallic material to 40 wt. % of glass-forming material in the raw material mixture. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a cross-section through an exhaust-gas-side part of a sensor element. 
     FIG. 2 shows an enlarged view of a layer system of the sensor element illustrated in FIG.  1 . 
    
    
     DETAILED DESCRIPTION 
     Plate-shaped sensor element  10  illustrated in FIGS. 1 and 2 has an electrochemical measuring cell  12  and a heating element  14 . Measuring cell  12  has, for example, a first solid electrolyte foil  21  with a large surface  22  on the measured gas side and a large surface  23  on the reference gas side, as well as a second solid electrolyte foil  25  with a reference channel  26  integrated therein. On large surface  22  on the measured gas side there is a measuring electrode  31  with a printed conductor  32  and a first terminal contact  33 . On large surface  23  on the reference gas side of first solid electrolyte foil  21 , there is a reference electrode  35  with a printed conductor  36 . Furthermore, a through-plating  38  is provided in first solid electrolyte foil  21 , through which printed conductor  36  of reference electrode  35  is guided to large surface  22  on the measured gas side. In addition first terminal contact  33 , a second terminal contact  39 , connected to through-plating  38  and thus forming the contact point for reference electrode  35 , is also located on large surface  22 . Measuring electrode  31  is covered with a porous protective layer  28 . 
     Heating element  14  has, for example, a support foil  41  with an outer large surface  43  and an inner large surface  43 ′, which, in this embodiment is composed of the material of the two solid electrolyte foils  21 ,  25 . An outer insulation layer  42  is applied to inner large surface  43 ′ of support foil  41 . A resistance heater  44  with a wave-form heating conductor  45  and two terminal conductors  46  is located on outer insulation layer  42 . Outer insulation layer  42  and support foil  41  have two heater through-platings  48  each flush to one another, which run from the two terminal conductors  46  to outer large surface  43  of support foil  41 . Two heater terminal contacts  49  are arranged on outer large surface  43  of support foil  41 , which are connected to heater through-platings  48 . 
     An inner insulation layer  50  is on resistance heater  44 . The large surface of inner insulation layer  50  is connected to the large surface of the second solid electrolyte foil  25 . Thus heating element  14  is thermally connected to measuring cell  12  via inner insulation layer  50 . 
     The two solid electrolyte foils  21  and  25  and support foil  41  are composed of ZrO 2 , partially stabilized with 5 mol. % Y 2 O 3 , for example. Electrodes  31 ,  35 , printed conductors  32 ,  36 , through-platings  38  and terminal contacts  33 ,  39  are made of platinum cermet, for example. In this embodiment, a platinum cermet is also used as the material for the resistance heater, the ohmic resistance of leads  46  being selected to be less than that of heating conductor  45 . 
     A screen printing paste with the following composition is used for producing insulating layers  42  and  50 : 
     50 wt. % powder mixture 
     40 wt. % organic solvent 
     5 wt. % organic plasticizer 
     5 wt. % organic binder. 
     The composition may vary as follows: 
     Powder mixture: 20 to 70 wt. % 
     Solvent: 20 to 70 wt. % 
     Plasticizer: 1 to 15 wt. % 
     Binder: 1 to 15 wt. %. 
     Hexanol can be used as the solvent, for example, phthalate as the plasticizer and polyvinylbutyral as the binder, for example. 
     The raw material components are homogenized in appropriate mixing units such as ball mills or three-roller mills, so that a paste suitable for screen printing is obtained. 
     The powder mixture contains Al 2 O 3  (alumina), for example with a specific sintering activity and a glass-forming material, such as an alkaline earth silicate glass. Ba—Al silicate can be used, for example, as an alkaline earth silicate glass. Barium can be replaced with strontium up to 30 atomic %. 
     The alkaline earth silicate glass can be introduced as a pre-melted glass frit or as a glass-phase raw material mixture. The material mixture may contain electrically conducting impurities up to 1 wt. %. This concerns, in particular, Na 2 O, K 2 O, Fe 2 O 3 , TiO 2 , Cu 2 O, or other semiconducting oxides. The level of electrically conducting impurities in commercially available raw materials is usually less than 0.2 wt. %. 
     Alumina is selected so that, at a sintering temperature needed for forming a glazing filled with alumina when the powder mixture is sintered, alumina alone has a sintering activity resulting in a relative sintering density of at least 95%. This is the case of aluminas B and C in the table below. The table shows the actual sintering density ρ S  in g/cm 3  and the relative sintering density ρ S /ρ th  in % for three different aluminas A, B and C. 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
               
               
                 Parameter 
                 Alumina A 
                 Alumina B 
                 Alumina C 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Particle 
                   
                   
                   
               
               
                 Size: 
               
               
                 d 50  (μm) 
                 0.45 
                 0.34 
                 0.20- 
               
               
                   
                   
                   
                 0.30 
               
               
                 d 90  (μm 
                 1.6- 
                 0.50 
                 0.30- 
               
               
                   
                 2.6 
                   
                 0.40 
               
               
                 Sintering 
               
               
                 Activity 
               
               
                 t s  (° C.) for 
                 1490 
                 1330 
                 1280 
               
               
                 ρ s  = 3.80 
               
               
                 g/cm 3   
               
               
                 ρ s /ρ th  = 95% 
                 3.43 
                 3.90 
                 3.96 
               
               
                 ρ s  (g/cm 3 ) 
               
               
                 for t s  = 1400° C. 
               
               
                 after 2 h in 
                 85.7 
                 97.5 
                 99.0 
               
               
                 air 
               
               
                 ρ s /ρ th1  (%) 
               
               
                   
               
             
          
         
       
     
     In addition to aluminas B or C, also Mg spinel, fosterite or a mixture of these substances can be used as crystalline non-metallic materials. It is also possible to add Mg spinel, fosterite or a mixture of these substances to powder mixtures with aluminas B or C. These crystalline, non-metallic materials must, however, have a sintering activity that results in a sintering density of at least 95%. 
     EXAMPLE 1 
     Composition of the powder mixture: 
     60 wt. % Alumina B or C (see Table), 40 wt. % Ba—Al silicate glass powder (53 wt. % BaO, 5 wt. % Al 2 O 3 , 42 wt. % SiO 2 , specific surface area 5 m 2 /g), 
     Insulation resistance&lt;1 MΩ. 
     The powder mixture is homogenized and ground in a ball mill with 90% Al 2 O 3  grinding balls. Then an aqueous slurry is added with 500 g raw material mixture made up of alumina and Ba—Al silicate glass, 500 ml distilled water and 25 ml 10% aqueous polyvinyl alcohol solution. The slurry is ground in a ball mill with 90% Al 2 O 3  grinding balls for 1.5 hours. 
     EXAMPLE 2 
     This example differs from the powder mixture in Example 1 by the fact that instead of 40% wt. % Ba—Al silicate glass powder, the following composition is selected: 
     38 wt. % Ba—Al silicate glass powder, 
     1 wt. % kaolin, 
     1 wt. % barium carbonate (BaCO 3 , chemically pure), 
     Insulation resistance &gt;1 MΩ. 
     EXAMPLE 3 
     The composition of the powder mixture differs from that of Example 1 by the fact that instead of the Ba—Al silicate glass powder the following components are used: 
     40 wt. % of a calcinate composed of: 
     11 wt. % kaolin, 34 wt. % quartz (99% SiO 2 ) 
     55 wt. % BaCo 3  (chemically pure). 
     The components are ground in a ball mill with 90% Al 2 O 3  for two hours and calcined as loose particles in corundum capsules in an oxidizing atmosphere at 1000° C. for two hours and then ground again as described above. 
     Insulation resistance &gt;1 MΩ. 
     EXAMPLE 4 
     The composition of the powder mixture differs from that of Example 1 and Example 3 in the following: 
     70 wt. % alumina and 30 wt. % calcinate, 
     Insulation resistance &gt;1 MΩ. 
     EXAMPLE 5 
     As in Example 4, but instead of alumina with: 70 wt. % partially stabilized ZrO 2  with 3.5 wt. % MgO (35% monocline), 
     Specific surface area: 7 m 2 /g 
     Insulation resistance &gt;60 kΩ. 
     EXAMPLE 6 
     As Example 3, but: 
     50 wt. % alumina, 
     50 wt. % calcinate, 
     Insulation resistance &gt;1 MΩ. 
     EXAMPLE 7 
     As Example 3, but: 
     85 wt. % alumina, 
     15 wt. % calcinate, 
     Insulation resistance &gt;500 kΩ. 
     EXAMPLE 8 
     The composition corresponds to that of Example 7, with alumina containing the following components: 
     99.3% Al 2 O 3 , 0.3% Na 2 O 
     Specific surface area: 2.5 m 2 /g, 
     Insulation resistance &gt;300 kΩ. 
     EXAMPLE 9 
     The composition corresponds to that of Example 3, but instead of alumina, with the following components: 
     60 wt. % Mg spinel powder (MgO·Al 2 O 3 ) with &lt;0.5 wt. % free MgO and &lt;0.1 wt. % Na 2 O 
     Specific surface area: 8 m 2 /g, 
     Insulation resistance &gt;1 MΩ. 
     For preparing the layer system for sensor element  10  shown in FIGS. 1 and 2, the prepared paste is initially applied to ceramic support foil  41  using screen printing. Thus resistance heater  44  is printed onto insulation layer  42  using screen printing and a conventional cermet paste. Through-platings  48 , previously removed from insulation layer  42  and applied to support foil  41 , are made at the same time. The inner insulation layer  50  is applied to resistance heater  44  also using screen printing techniques. The layer thicknesses of insulation layers  42 ,  50 , which must be present prior to sintering, are set using an appropriate number of screen printing steps and/or by an appropriate selection of the screen printing parameters and paste properties (viscosity, etc.). In the exemplary embodiment, outer insulation layer  42  has a thickness of 18 μm and inner insulation layer  50  also has a thickness of 18 μm after sintering. 
     Heater element  41  thus manufactured is now laminated together with measuring cell  12 , which is also manufactured using printing techniques, and co-sintered at approximately 1400° C. At the sintering temperature, the ceramic and metallic components of the layer system are sintered. Thus the gas-tight electrical insulation layers  42  and  50  are formed by fusing the glass-forming material and sintering the crystalline components.