Method of manufacturing a semiconductor body for a low voltage type spark plug

Abstract: A low-voltage type spark plug including; a centre electrode (2) having a firing-tip(21) mounted within a tubular insulator (4) which in turn is placed within a metallic shell (1); a ground electrode (11) in electrical contact with said metallic shell (1) and defining with said firing-tip (21) of the centre electrode (2), a spark-gap (10); a semi-conductor body (3), a surface (31) of which is mounted adjacent the spark-gap in electrical contact with both the centre and ground electrodes (2, 11), characterised in that said semi-conductor body (3) is formed from silicon carbide particles of average diameter less than 5 microns and alumina particles of average diameter less than 1 micron in a ratio by weight in the range 65:35 to 80:20 inclusive and in that the silicon carbide and the alumina particles have been mixed with a suitable amount of binder and hot press sintered at a temperature greater than or equal to 1800 degrees Celsius and pressure greater than or equal to 200 kgf/cm².

Description: This invention relates to a method of manufacturing a semiconductor body for a low-voltage type spark plug, particularly for use in jet and other internal combustion engines. In jet engine spark plugs, an electrically semi-conducting material is mounted within a spark gap between the firing-tip of a centre electrode and a ground electrode. The semi-conducting material allows limited current flow along the surface of the semi-conducting material upon application of a small voltage, the current flow causes the requisite ionization and enables a high energy spark discharge with the low applied voltage. Various semi-conducting materials have heretofore been introduced, and extensively used in igniters fired by low-voltage, high-energy ignition systems. One example of the semi-conducting materials was disclosed in U.S. Patent No. 3,558,959. According to this document a ceramic semi-conductor body is hot-pressed with silicon carbide (SiC) and alumina (Al₂O₃) as essential components which is found to be adequate under severe service conditions, in particular high combustion zone temperatures and wet fuel conditions encountered in many engines. In recent years, however, it is required that the spark plug functions normally under a pressure as high as, for example, 20Kgf/cm² for safety purposes. Under such conditions there is a possibility that erosion will occur even in the semiconductor body made according to U.S. Patent No. 3,558,959. Therefore, it is an aim of this invention to provide a low-voltage type spark plug having an improved semiconductor structure with a significantly extended service life when assembled to provide a semiconductor surface along which a high energy spark discharges occurs at a low voltage while under high pressure. According to the present invention, there is provided a method of manufacturing a semiconductor body for a low-voltage type spark plug, the method comprising the steps of: forming said semiconductor body from silicon carbide particles and alumina particles in a ratio by weight in the range 65:35 to 80:20 inclusive, mixed with a suitable amount of binder; and hot press sintering said body at a temperature in the range of from 1700°C to 1900°C and at a pressure greater than or equal to 200 kgf/cm²; characterised in that said binder comprises a mixture of magnesia, calcium oxide and silicon dioxide; and in that said silicon carbide particles are of average diameter less than 5 microns and said alumina particles are of average diameter less than 1 micron. The invention provides a tough-structured semi-conductor body of nearly theoretic density in which the particles are aligned in well-ordered manner with a small number of defects, decreasing the amount of erosion when the semiconducting body is exposed to spark discharges under high pressure. The invention will be further described hereinafter with reference to the following description of exemplary embodiments and the accompanying drawings, in which: Fig. 1 is a partially schematic longitudinal cross section of the firing tip of a spark plug according to the invention; Fig. 2 is a graph showing how the amount of erosion varies according to the ratio of alumina to silicon carbide; Fig. 3 is a graph showing how the amount of erosion varies according to the diameters of the alumina and silicon carbide particles; Fig. 4 is a graph showing how the amount of erosion varies according to the sintering temperature and pressure; and Fig. 5 is a view similar to Fig. 1 of a modified spark plug according to the present invention. Fig. 1 shows, in section, the lower portion of spark plug 100. The metallic shell 1 has a lower portion 11 which has a tapered surface 11a on its inner wall and acts as a ground electrode, the lower end of which is terminated by annular end 12 6.4 mm in diameter. A centre electrode 2 is situated concentrically within the metallic shell 1, its lower end terminating in a enlarged head 21, 4.0 mm in diameter, forming an annular spark gap 10 with the inner wall of the annular end 12 of the metallic shell 1. The upper part of the centre electrode 2 is seated in a tubular insulator 4 disposed within a space 30 between the centre electrode 2 and metallic shell 1. A generally annular semi-conductor body 3 is fitted between the lower end of the insulator 4 and the tapered surface 11a of the metallic shell 1. The lower outside corner of the body 3 is beveled to form generally frustoconical surface 3a, so that the frustoconical surface 3a engages with the tapered surface 11a during assembly. Both the tapered surface 11a and the head 21 of the centre electrode 2 are in electrical contact with the lower end surface 31 of the body 3, so that current flow along the lower end surface 31 of the body 3 ionizes the adjacent air, and enables occurrence of high-energy low voltage spark (2 Kilo Volt for example) The semi-conducting body 3 is manufactured as follows: First step: silicon carbide powder and alumina in a ratio of between 65:35 and 80:20 by weight, are mixed in a tumble mill for three hours with a binder means such as magnesia (0.3% by weight), calcium oxide (0.5% by weight), silicate dioxide (1.9% by weight), and a suitable amount of distilled water, and polyvinyl alcohol (0.5 % by weight) as an organic binder. Second step: powders mixed as above are rolled after desiccation to obtain powder particles of about 450 microns containing silicon carbide particles of less than 5 microns average diameter and alumina particles of less than 1 micron average diameter. Then, the powders are pressed in a steel mould under a pressure of 2000 Kgf/cm². Third step: the moulded powders are forced into a carbon die to be hot press sintered as follows: The powders are (1) Heated at 20 degrees Celsius per minute, pressed at 150 to 250 Kgf/cm² when 1200 degrees Celsius is reached; (2) Held at the above pressure for half an hour at a temperature in the range 1700 to 1900 degrees Celsius; (3) Gradually cooled and taken out of the mould when they have cooled below 1400 degrees Celsius. Fourth step: the sintered powders are appropriately ground to form the annular semi-conductor body 3 to be incorporated into the igniter plug 100. The igniter plug 100 is connected to a capacitor-discharge type exciter (not shown) capable of providing 4 joules, and operated under a pressurized atmosphere of 25Kgf/cm² to measure the erosion rate of the body 3. The erosion of the body 3 is expressed by the weight loss caused by 1000 spark discharge cycles. Fig. 2 shows how the erosion rate in gram/1000 cycles varies according to the ratio of silicon carbide particles and alumina particles of 2.0 and 0.4 microns average diameter respectively. The temperature and pressure during sintering were 1850 degrees Celsius and 250 Kgf/cm². A significant reduction in the amount of erosion is found when the ratio by weight of silicon carbide particles to alumina particles is in the range 65:35 to 80:20, as clearly seen in Fig. 2. Further, Fig. 3 shows how the erosion rate (gram/1000 cycles) varies according to the average diameter of the silicon carbide alumina particles in a ratio of 65:35 by weight. The temperature and pressure during sintering were 1850 degrees Celsius, and 250 Kgf/cm² as above. A drastic reduction in the amount of erosion is found when the average diameter of the silicon carbide particles is less than 5 microns, and that of the alumina particles is less than 1 micron as is clearly seen in Fig. 2. Fig. 4 shows how the erosion rate grms/1000 cycles changes according to the temperature and pressure during sintering with the ratio by weight of silicon carbide to alumina particles being 65 to 35. In this instance, the silicon carbide particles and alumina particles are of 2 microns and 0.4 microns respectively average diameter. Under these conditions, the amount of erosion (gram/1000 cycles) changes with pressure at constant temperature 1850 degrees Celsius, (Kgf/cm₂) as shown by curve (A) and at the same time, changes with temperature at constant pressure 250 Kgf/cm² as shown by curve (B). As can be seen from curves (A) and (B) of Fig. 4 the amount of erosion drastically reduces to less than 0.001 (gram 1000 cycles) when the semi-conducting body 3 is sintered at a temperature of greater than 1800 degrees Celsius and a pressure of greater than 200Kgf/cm². Fig. 5 shows a modified igniter plug, in which the head 21 of the centre electrode 2 is axially shorter and the metallic shell 1 terminates in a circular flange 1f surrounding the head 21. The electrically semi-conducting body 3 is positioned between the lower end of the insulator 4 and the inner side of the flange 1f of the metallic shell 1. Both the flange 1f and the head 21 of the centre electrode 2 are in electrical contact with the lower end surface 31 of the body 3, so that current flow along the lower end surface 31 of the body 3 ionizes the adjacent air, and enables a high-energy low voltage spark to occur. It is noted that the binder components may be any suitable combination of magnesia, calcium oxide and silicate dioxide. An appropriate amount of distilled water and polyvinyl alcohol may be added. It is further appreciated that the firing-tip of the centre electrode may be made of a tungsten or platinum-Indium based alloy. The metallic shell may be made of a nickel-chromium-iron based alloy (such as "Inconel" TM).

Claims: A method of manufacturing a semiconductor body for a low-voltage type spark plug (100), the method comprising the steps of: forming said semiconductor body (3) from silicon carbide particles and alumina particles in a ratio by weight in the range 65:35 to 80:20 inclusive, mixed with a suitable amount of binder; and hot press sintering said body at a temperature in the range of from 1700°C to 1900°C and at a pressure greater than or equal to 200 kgf/cm²; characterised in that said binder comprises a mixture of magnesia, calcium oxide and silicon dioxide; and in that said silicon carbide particles are of average diameter less than 5 microns and said alumina particles are of average diameter less than 1 micron. A method according to claim 1, wherein the binder includes polyvinyl alcohol, or distilled water, or both. A method according to claim 1 or 2, wherein the binder is formed from percentages by weight of magnesia, calcium oxide and silicon dioxide of approximately 0.3, 0.5 and 1.9 respectively and a percentage by weight of polyvinyl alcohol of approximately 0.5.

Inventors: AOKI NOBORU, IZUOKA YUKI, MATSUMURA SHUZO, SUZUKI TAKAHIRO, AOKI, NOBORU, IZUOKA, YUKI, MATSUMURA, SHUZO, SUZUKI, TAKAHIRO
Applicants: NGK SPARK PLUG CO, NGK SPARK PLUG CO., LTD
Application year: 1989
Main classification: H01T13
IPC codes: H01T13