Abstract:
A ceramic pin heating element having exterior heating conductors and integrated electrical connection surfaces, and method for making same. A first body is injection molded from a first injection-moldable ceramic composite compound having a first electrical resistance, and a second body is injection-molded from a second injection-moldable ceramic composite compound having a second electrical resistance about the first body so as to form a compound body. The compound body is then sintered. The resulting ceramic pin heating element is directly formed without additional steps for protecting (insulating) the element so as to prevent the exterior, complementary heating conductors from contacting the housing and/or terminal bolts. The danger of breakage of the pins at the mounting location and/or at the seal seat through variations in the thickness of contact and/or insulation coatings is minimized.

Description:
FIELD OF THE INVENTION 
     The present invention relates to ceramic pin heating elements, and in particular, to a ceramic pin heating element for an internal combustion engine, the pin heating element having integrated connector contacts. 
     BACKGROUND INFORMATION 
     Connection of ceramic pin heating elements having exterior heating element shafts in a metal housing with simultaneous sealing from the combustion chamber is complex. The installation position causes contact between the heating element shafts having different polarity and the grounded housing. Therefore, to avoid short circuits, one of the two heating element shafts must be electrically insulated from the housing in the area of the seal seat. Also, the other heating element shaft, having opposite polarity, must be electrically insulated from the connector bolt. The application of electrical insulation layers is currently implemented through separate vitrification steps or through application, for example by printing, and burning-in of electrically insulating layers. To avoid one-sided mechanical stresses of the installed pin heating element due to the applied insulation layers, the corresponding open connector shaft is also provided with a layer of the same thickness but with electrical conductive properties, since there might otherwise be a danger of breakage of the ceramic pin heating element in the area of the seal seat. Another possibility is for the ceramic pin heating element to be fully insulated from the housing through application of a coating. As with a potential-free A-probe, contact is made through a divided terminal bolt. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method for making a ceramic pin heating element having exterior conductors and integrated electrical connection surfaces. The method comprises injection molding a first body from a first injection-moldable ceramic composite compound having a first electrical resistance; injection molding a second injection-moldable ceramic composite compound having a second electrical resistance about the first body so as to form a compound body; and sintering the compound body. 
     The present invention also provides a ceramic pin heating element comprising a first body injection molded from a first injection-moldable ceramic composite compound, the first body having a first electrical resistance, and a second body injection molded from a second injection-moldable ceramic composite compound, the second body having a second electrical resistance. The first body and second body form a compound body, the compound body being sintered after injection molding of the first body and the second body. 
     A ceramic pin heating element according to an embodiment of the present invention has exterior heating conductors based on Si 3 N 4 /MSi 2 , where M is at least one of Mo, Nb, Ti and W, and has integrated electrical connection surfaces using multiple-component injection-molding technology. 
     An advantage of the method and heating element of the present invention is direct shaping of the pin heating element without additional steps for protecting (insulating) the element so as to prevent the exterior, complementary heating conductors from contacting the housing and/or terminal bolts. Another advantage according to the present invention is minimization of the danger of breakage of the pins at the mounting location and/or at the seal seat through variations in the thickness of contact and/or insulation coatings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A shows a schematic cross-sectional view of a ceramic pin heating element according to the present invention. 
     FIG. 1B shows a cross-section through line B—B of FIG.  1 A. 
     FIG. 1C shows a cross-section through line C—C of FIG.  1 A. 
     FIG. 1D shows a cross-section through line D—D of FIG.  1 A. 
     FIG. 2 shows a flow chart of a method according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1A, an exemplary ceramic pin heating element of the present invention includes a heating conductor  2 , a housing  12 , an insulator body  22  and a connector bolt  32 . Heating conductor  2  has a negative pole  3 , a positive pole  6  and a tip  9 , which is heated during operation of the heating element. Negative pole  3  has a lower section  4  disposed in housing  12 , and a generally semi-cylindrical upper section  5  projecting from housing  12 . Positive pole  6  has a lower section  7  disposed in housing  12 , and a generally semi-cylindrical upper section  8  projecting from housing  12 . Housing  12  forms a generally cylindrical, annular casing around lower sections  4  and  7  of negative pole  3  and positive pole  6 , respectively, and around portions of insulator  22 . Insulator  22  has a generally cylindrical stem section  24  encased by negative pole  3  and positive pole  6 , a curved section  26  disposed between housing  12  and a portion of lower section  4  of negative pole  3 , and a semi-disc shaped section  28  disposed between connection bolt  32  and lower section  4  of negative pole  3 . As embodied herein, connection bolt  32  is electrically connected to positive pole  6  and housing  12  is electrically connected to negative pole  3 . 
     Heating conductor  2  is made of an injection-moldable ceramic composite compound having a relatively high conductivity. Insulator  22  is made of an injection-moldable ceramic composite compound having a relatively low conductivity. Thus, one of the injection-moldable ceramic composite compounds having a relatively high conductivity is substantially conductive, and the other one of the injection-moldable ceramic composite compounds having a relatively low conductivity is substantially insulative. Housing  12  is preferably made of free-cutting steel. Alternatively, housing  12  may be made of other suitable metals or materials. Connector bolt  8  is also preferably also made of free-cutting steel, but may also be made of other suitable metals or materials. FIGS. 1B-1D show cross-sectional views through the heating element of FIG. 1A at various levels. 
     Referring now to FIG. 2, an exemplary method according to the present invention includes injection molding a first body from a first injection-moldable ceramic composite compound having a first electrical resistance, as shown in block  102 . As shown in block  104 , the next step of an exemplary method of the present invention is injection molding a second injection-moldable ceramic composite compound having a second electrical resistance about the first body so as to form a compound body. The first body may form heating element  2  or, alternatively, insulator body  22  of the ceramic pin heating element, while the second injection-moldable ceramic composite makes up, respectively, insulator body  22  or heating element  2 . Sintering the compound body is then performed, as shown in block  106 . 
     The ceramic pin heater is produced using suitable organic process aids. As embodied herein, suitable process aids include grafted polypropylene, such as POLYBOND 1001 made by Uniroyal Chemical company, in combination with cyclododecane and/or cyclododecanol. Other suitable organic process aids include combinations of polyolefin waxes, such as HOSTAMONT TPEK 583 made by the firm Ticone GmbH, or of polyoxymethylene, such as CATAMOLD made by the firm BASF AG. The suitable organic process aid(s) are added to composites Si 3 N 4 /MSi 2  (where M is at least one of Mo, Nb, W and Ti), the composites having differing specific electrical resistances following sintering. The composites are then formed into molded bodies through injection molding. The sintering is then performed, preferably as a two-step sintering process including a presintering and a primary sintering step. 
     A method in accordance with the present invention uses a preconditioned SI 3 N 4  powder with appropriate sintering additives such as Al 2 O 3 , Y 2 O 3 , and the like and an admixture of MSi 2  (M: Mo, Nb, W, Ti) in differing proportions. The admixture of MSi 2  is such that a component A, which is highly insulating following the sintering fire, and a very highly conductive component B, are created. 
     Injection-moldable compounds AS and BS are prepared from the preconditioned ceramic powder mixtures A and B with a suitable organic binder system which according to an exemplary embodiment of the present invention is composed of grafted polypropylene in combination with cyclododecane and/or cyclododecanol. 
     An insulation body made of compound AS is formed by injection molding. A conductive compound BS is injection molded around the insulation body AS, for example in the two-component injection molding procedure. 
     Debinding and presintering is performed under an inert gas at a pressure of 1 bar and a temperature of up to 900° C. A primary sintering process follows the forming process after debinding and presintering. Primary sintering occurs under a defined N 2  partial pressure with the N 2  partial pressure being not higher than 10 bar and the total sintering pressure being elevated through addition of an inert gas, such as, for example, Ar, to values up to 100 bar, the sintering gas being at a temperature between 1000° C. and the sintering temperature which is no higher than 1900° C. 
     Alternatively, primary sintering is performed under a defined N 2  partial pressure with the N 2  partial pressure varied with the temperature so that the partial pressure is within a range which is bounded by the following relationships and the total sintering pressure being elevated through addition of an inert gas, such as, for example, Ar, to values up to 100 bar: 
     
       
         Upper limit: log  p ( N   2 )=7.1566· In ( T )−52.719 
       
     
     
       
         Lower limit: log  p ( N   2 )=9.8279· In ( T )−73.988. 
       
     
     Where T is the sintering temperature in ° C. and p(N 2 ) is the partial pressure of N 2  in bar. The sintering temperature is no higher than 1900° C. 
     The electrical contact surfaces may also be provided with a thin metal coating such as one based on Ni or the like. 
     In an exemplary embodiment of the present invention a composite powder A of 54 wt % Si 3 N 4 , 2.58 wt % Al 2 O 3 , 3.42 wt % Y 2 O 3 , and 40 wt % MoSi 2  and a composite powder B of 36 wt % Si 3 N 4 , 1.72 wt % Al 2 O 3 , 2.28 wt % Y 2 O 3 , and 60 wt % MoSi 2  are produced. The mean particle size of the Si 3 N 4  used is 0.7 μm, and that of the MoSi 2  used is 1.8 μm. From these composite powders, injection-moldable compounds AS and BS are produced. Injection-moldable powder compounds represent highly filled dispersions. 
     A binder system suitable for powder injection molding preferably meets the following requirements: (1) dispersion effect for avoidance of clumping of powder; (2) good flow quality of the compounds during injection molding; (3) adequate bonding of a second compound injected over a blank; (4) low formation of pyrolysis carbon during thermal debinding in inert gas atmosphere and in air, since carbon negatively influences the properties of the sintered molded body; and (5) rapid debinding without formation of defects. 
     The combination of grafted polypropylenes and cyclododecane and/or cyclododecanol in accordance with a method of the present invention, for example, represents a binder system of this type. The polar compounds grafted onto the polypropylene chain such as acrylic acid or maleic acid anhydride attach to the surfaces of the powder. The polypropylene POLYBOND 1001 used in the present exemplary embodiment is a homo-polypropylene grafted with 6% acrylic acid of the company Uniroyal Chemical. 
     For production of the compound BS from the conductive ceramic, 82 wt % of preconditioned powder mixture B is kneaded with 12 wt % POLYBOND 1001 and 6 wt % cyclododecane under protective gas at 180° C. and is granulated through cooling with the kneader running. 
     In this way, a compound AS is produced from the insulating ceramic powder A, the filler content of which is matched to that of the compound BS such that test bodies injection molded and debinded from both compounds under the same sintering conditions have the same sintering shrinkage. 
     First the conductive body is formed with the compound BS through two-component injection molding. After withdrawal or change of the pusher which forms the recesses for the insulation areas in the injection molding tool, the voids formed are injected with the compound AS. In this process, a bond develops between the insulating body and the conductive path. 
     After thermal debinding and sintering according to sintering conditions 2, the AS component has a specific resistance of 10 7  Ωcm and the BS component has a specific resistance of 6*10 −3  Ωcm.