Abstract:
An electrically stable radio frequency interference suppressing glass-type resistor seal composition forming a gas-tight seal within the center bore of a ceramic insulator is disclosed that contains borosilicate glass, barium borate glass, mullite, zirconia, carbon black, bentonite, sucrose, lithium carbonate and small, approximately equal amounts of antimony and silicon.

Description:
This invention is an improved metal-glass seal resistor composition for a resistor spark plug. More specifically, this invention is an improvement to the seal composition disclosed in U.S. Pat. No. 4,112,330 issued Sept. 5, 1978 and assigned to the assignee of this application. 
     Resistor spark plugs employ a glassy, relatively high resistance seal material between the terminal screw and the center electrode. During spark plug manufacture, such a seal composition is added as a particulate mixing to the center bore of an insulator body onto the upper end of a previously placed center electrode. A metal terminal screw is then placed in the bore of the insulator so that its lower end rests on top of the particulate composition. The assembly is then fired in a furnace at a relatively high temperature to fuse the glass and soften the material so that the terminal screw can be pushed down into the fused composition. 
     Thus, the firing of the composition produces a fused glassy mass that provides a gas-tight seal in the interior of the spark plug insulator body between the center electrode and the terminal screw. The composition contains metal particles which during the firing operation fuse and provide a bond between the metal conductors and the resistive seal composition. 
     Metallized glass seal resistor compositions such as those disclosed in U.S. Pat. No. 4,112,330 serve a useful function in the operation of a spark plug in an automobile engine. The high resistance material within the spark plug suppresses the generation of radio frequency electromagnetic radiation which would interfere with radio reception in nearby vehicles. In performing this important function, it is important that the original particulate mixture fuse upon firing to form a mass that has a predictably high level of resistance and that such level of resistance not change appreciably during prolonged usage of the spark plug in engine operation. The glass metal compositions disclosed in the above-identified U.S. patent have functioned well in millions of spark plugs in automotive engines. However, with the design of more sophisticated automotive ignition systems, it is desired that resistor seal formulations be available which will predictably fire to form resistors with a more narrow resistance range. Furthermore, it is desired that the formed seals maintain resistor values close to their as-fired condition throughout prolonged engine operation. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with my invention, these and other objects are obtained utilizing a particulate metal- and glass-containing composition consisting essentially by weight of 24 to 33 percent glass, 18 to 25 percent mullite, 36 to 49 percent zirconia, 1.2 to 1.6 percent carbon black, 0 to 2.0 percent bentonite, 0.6 to 0.8 percent sucrose, 0.8 to 1.2 percent lithium carbonate, 0.5 to 1.3 percent antimony and 0.5 to 1.3 percent silicon. The glass content of this resistor composition is suitably composed of 25 to 50 percent borosilicate glass and about 50 to 75 percent barium borate glass. It is preferred that the content by weight of the particulate metal constituents, antimony and silicon, be about equal. It is also preferred that the particulate size range of a mixture of all these constituents be in the range -28 +100 mesh. 
     An advantage of the above-described mixture is that it forms a seal in which the electrical resistance is suitably high and not particularly sensitive to firing time or temperature. It may suitably be fired at a temperature in the range of about 1650° F. to 1725° F. in the manufacture of the spark plug. The mixture softens upon fusion of the glass and antimony constituents and forms a seal having a resistance at room temperature in the range of about 4,000 to 7,000 ohms. Furthermore, the resistance drop of the fused resistive material due to aging in a spark plug in an operating engine is low. 
     Other objects and advantages of my invention become more apparent from a detailed description thereof which follows in which reference will be had to the drawing. The drawing shows a spark plug partially broken away to show its internal construction and the location of the subject resistor seal. 
    
    
     DETAILED DESCRIPTION 
     Referring to the drawing, the spark plug 10 comprises a conventional outer metal shell 12 having a ground electrode 14 welded to the lower end thereof. Positioned within the metal shell 12 and secured in the conventional manner is the insulator 16. The ceramic insulator 16 may be of a high alumina base material such as covered by U.S. Pat. No. 2,760,875, issued to Karl Schwartzwalder and Helen Blair Barlett. The insulator 16 is formed with a center bore having a lower portion 18 of relatively small diameter and an upper portion 20 of larger diameter which are connected by the insulator center bore ledge 22. Positioned in the lower portion 18 of the insulator center bore is the conventional nickel center electrode 24. The center electrode 24 is preferably nickel although other metals which can be coated with antimony and silicon may be used. The center electrode 24 has an enlarged head 26 at the upper end thereof which rests on the inner insulator center bore ledge 22 and a lower end 28 thereof projecting beyond the lower tip of the insulator 16. Positioned in the upper portion 20 of the insulator center bore is a terminal screw 30. The resistor element or seal 32 of this invention, which will be hereinafter fully described, is positioned in the insulator center bore 20 and is bonded to the center electrode head 26, to the terminal screw 30 and to the inner walls of the ceramic insulator. The center bore ends of the center electrode 26 and the terminal screw 30 have a metal coating 29 thereon which will be hereinafter also fully described. The end of the terminal screw 30 is formed with an unroughened surface 31 for purposes described fully hereinafter. 
     The resistor seal composition of this invention is a modification of the formulations disclosed in columns 2 and 5 of U.S. Pat. No. 4,112,330. Those patented formulations were developed to provide RFI suppression in spark plugs as well as a relatively stable resistance value. Millions of spark plugs employing seal formulations described and claimed in that patent have been made and used in automobiles. However, the subject formulation provides dramatic improvements over the U.S. Pat. No. 4,112,330 patent formulations both in ease of spark plug manufacture and in spark plug performance. 
     Following is a preferred formulation for the subject spark plug resistor glass seal: 
     
         ______________________________________                PercentageMaterial             By Weight______________________________________Barium borate glass  19.7Borosilicate glass   8.9Zirconia, ZrO.sub.2  42.6Mullite (-40+100 particle size)                21.8Bentonite            1.7Carbon black (Thermax)                1.4Sucrose (10-X)       0.7Lithium carbonate, Li.sub.2 CO.sub.3                1.0Antimony, Sb         1.1Silicon, Si          1.1______________________________________ 
    
     In the above formulation, the barium borate glass preferably is a composition containing 75 weight percent B 2  O 3  and 25 weight percent BaO. However, such glasses containing different proportions of barium oxide and boron oxide are suitable, as disclosed in U.S. Pat. No. 4,112,330. The preferred borosilicate glass is a composition containing, in weight percent, SiO 2  --65 percent, B 2  O 3  --23 percent, Al 2  O 3  --5 percent, and PbO--0.5 percent. 
     The zirconia is a semiconductor material that contributes to the stabilization of the resistance of the plug. The mullite serves as a filler, the bentonite clay serves as a binder and the lithium carbonate serves as a flux. The composition is initially employed as a powder mixture, preferably in the size range of -28+100 mesh. During the manufacture of the plug as described above, the mixture is heated to a temperature in the range of about 1650° F. to 1725° F. At such temperatures, the glass fuses as does the antimony. The fused glass, together with the other components of the mixture, serves to form a gas impervious seal (32 in the drawing). The zirconia, carbon black and metal levels cooperate to provide suitable electrical resistance. During the fusion of the glass, antimony and silicon coat and bond to the metal terminal and electrode of the spark plug (see 29 in the drawing). The above compositional ranges are critical to obtain the desired benefits of this invention. 
     Several 1,000-piece spark plug manufacturing runs have been undertaken and compared to a preferred prior art composition within ranges specified in U.S. Pat. No. 4,112,330. During manufacture of the spark plugs, batches of spark plugs are conveyed through a furnace maintained at temperatures in the range of 1650° F. to 1725° F. for the purpose of fusing constituents of the seal composition. In large scale day-to-day continuous operations, it is observed that the temperature in the furnace and the residence time of the spark plugs in the furnace vary considerably. The electrical resistance of spark plugs employing seal formulations of the U.S. Pat. No. 4,112,330 could vary over the range from 2,500 to 12,000 ohms. Furthermore, in usage in automotive engines, the resistance of such spark plugs would decrease by as much as 10 percent or more. It is now desired to reduce the sensitivity of the seal material to furnace operation and residence time as well as the relative instability of the resistance of the material during engine operation. The subject seals achieve these goals remarkably well. 
     The several batches of spark plugs of the above composition consistently had initial resistances following firing of the seal composition in the range of 4,000 to 7,000 ohms. This narrow range of resistance was observed despite wide differences in firing temperature and time at temperature. Thus, the new formulation of this invention reduces the necessity of tight controls on manufacturing operations and provides an improved plug that is more acceptable in automotive vehicles having newer electronic ignition systems. Furthermore, the plugs using the subject seal compositions display much lower reduction in electrical resistance in engine operations. In 100 hours of continuous engine test operation, the average reduction in resistance was much lower than that obtained with the preferred composition of the patented seal formulation. 
     The subject seal formulations also provided an additional benefit in spark plug manufacturing in that they are more fluid immediately following the firing temperature so that when the terminal posts 30 are pushed into the softened seal mixture 32, there is reduced scrap due to bending of the hot, soft terminal post. 
     While my invention has been described in terms of a specific embodiment thereof, it will be appreciated that other compositions could be readily adapted by one skilled in the art. Accordingly, the scope of my invention is to be considered limited only by the following claims.