METALLIZED GLASS SEAL RESISTOR COMPOSITION

Abstract: An electrically stable radio-frequency-­ interference-suppressing glass-type resistor seal composition forming a gas-tight seal within a centre bore of a ceramic insulator is disclosed that contains by weight 24 to 33 percent of a mixture of borosilicate glass and barium borate 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 1.3 percent lithium carbonate and 0.5 to 1.3 percent of both antimony and silicon in approximately equal amounts.

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 US-A-4,112,330. Resistor spark-plugs employ a glassy, relatively high-resistance seal material between the terminal screw and the centre electrode. During spark-­ plug manufacture, such a seal composition is added as a particulate mixture to the centre bore of an insulator body onto the upper end of a previously placed centre 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 centre 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 US-A-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 should fuse upon firing to form a mass that has a predictably high level of resistance and that such a level of resistance should 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 the present invention, these and other objects are obtained by 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 of 0.147 mm to 0.589 mm (-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 899°C to 940°C (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 the 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 resistor seal of the present invention. 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 an insulator 16. The ceramic insulator 16 may be of a high alumina base material such as covered by US-A-2,760,875. The insulator 16 is formed with a centre bore having a lower portion 18 of relatively small diameter and an upper portion 20 of larger diameter which are connected by an insulator centre bore ledge 22. Positioned in the lower portion 18 of the insulator centre bore is a conventional nickel centre electrode 24. The centre electrode 24 is preferably nickel, although other metals which can be coated with antimony and silicon may be used. The centre electrode 24 has an enlarged head 26 at the upper end thereof which rests on the inner insulator centre 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 centre bore is a terminal screw 30. A resistor seal 32 according to this invention, which will be hereinafter fully described, is positioned in the insulator centre bore 20 and is bonded to the centre electrode head 26, to the terminal screw 30 and to the inner walls of the ceramic insulator. The centre bore ends of the centre 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 US-A-4,112,330. Those patented formulations were developed to provide radio-frequency-interference (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 U.S. patent have been made and used in automobiles. However, the formulation of the present invention provides dramatic, unexpected improvements over the formulations disclosed in US-A-4,112,330, both in ease of spark-plug manufacture and in spark-plug performance. Following is a preferred formulation for a spark-plug resistor glass seal according to the present invention: Material Percentage By Weight Barium borate glass 19.7 Borosilicate glass 8.9 Zirconia, ZrO₂ 42.6 Mullite (-40+100 particle size) 21.8 Bentonite 1.7 Carbon black (Thermax) 1.4 Sucrose (10-X) 0.7 Lithium carbonate, Li₂CO₃ 1.0 Antimony, Sb 1.1 Silicon, Si 1.1 In the above formulation, the barium borate glass preferably is a composition containing 75 weight percent B₂O₃ and 25 weight percent BaO. However, such glasses containing different proportions of barium oxide and boron oxide are suitable, as disclosed in US-A-4,112,330. The preferred borosilicate glass is a composition containing, in weight percent, SiO₂ -- 65 percent, B₂O₃ -- 23 percent, Al₂O₃ -- 5 percent, and PbO -- 0.5 percent. The zirconia is a semi-conductor material that contributes to the stabilization of the resistance of the spark-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 0.147 mm to 0.589 mm (-28 +100 mesh). During the manufacture of the spark-plug as described above, the mixture is heated to a temperature in the range of about 899°C to 940°C (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 co-operate to provide a seal of 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 US-A-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 899°C to 940°C (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 according to US-A-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 seals of the present invention achieve these goals remarkably well. The batches of spark-plugs produced with the above composition according to the invention 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 seal formulation of this invention reduces the necessity of tight time and temperature controls on manufacturing operations and provides an improved spark-plug that is more acceptable in automotive vehicles having newer electronic ignition systems. Furthermore, the spark-plugs using the seal compositions of the invention 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 seal formulation according to US-A-4,112,330. The seal formulations of the invention 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 produced due to bending of the hot, soft terminal posts.

Claims: 1. A metal-glass seal resistor composition (32) for use between a terminal member (30) and an electrode member (24) in a resistor spark-plug (10), which composition comprises glass, mullite, zirconia, carbon black, bentonite, sucrose, lithium carbonate, antimony and silicon, characterised in that the composition consists 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 the seal initially consists essentially by weight of about 25 to 50 percent borosilicate glass and about 50 to 75 percent barium borate glass, and said composition, on fusion in said spark-plug (10), provides a glass seal having an electrical resistance in the range of 4,000 to 7,000 ohms and improved resistance stability thereafter during spark-plug use. 2. A metal-glass seal resistor composition according to claim 1, in which the quantities of antimony and silicon present are substantially equal to one another. 3. A metal-glass seal resistor composition according to claim 1 or 2, in which the particle size range of the composition lies in the size range of 0.147mm to 0.589 mm.

Inventors: STIMSON, GRANT LARRY
Applicants: GENERAL MOTORS CORPORATION
Application year: 1988
Main classification: H01T13
IPC codes: H01T13