Low-voltage type igniter plug having semi-conductor structure for use in jet and other internal combustion engines

A low-voltage type igniter plug having semi-conductor structure for use in jet and other internal combustion engines comprising; the semi-conducting body mounted adjacent a spark-gap in electrically contact with both center and ground electrodes, and essentially consisting of silicon carbide particles less than 5 microns in average diameter, and alumina particles less than 1 micron in average diameter, the weight percentages of the silicon carbide particles ranging from 65 to 80, the weight percentages of the alumina particles ranging from 20 to 35, the silicon carbide particles and the alumina particles being mixed with an addition of suitable binder means, and hot pressed at the temperature above 1800 degrees Celsius, and at the pressure above 200 Kg/cm.sup.2.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a low-voltage type igniter plug having 
semi-conductor structure for use in jet and other internal combustion 
engines in which the semi-conductor structure is particularly improved. 
In jet engine igniters, an electrically semi-conducting material is mounted 
within a spark gap between firing-tip of a center electrode and a ground 
electrode. The semi-conducting material allows for limited current flow to 
occur along the surface of the semi-conducting material upon application 
of a low 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 the 
specification of U.S Pat. No. 3,558,959 filed Apr. 24, 1968 as 
continuation-in-part, and patented Jan. 26, 1971. 
According to the publication of U.S. Pat. No. 3,558,959, a ceramic 
semi-conductor body is hot-pressed with silicon carbIde (SiC) and alumina 
(A1.sub.2 0.sub.3) as essential components which is found to be adequate 
under severe service condition, in particular high combustion zone 
temperatures and fuel wetted condition encountered in many those day 
engines. 
In recent years, however, it is demanded for the igniter plug to normally 
function under a high pressure such as, for example, 20Kg/cm.sup.2 for the 
purpose of safety. 
Under such circumstances, there is possibility that no small amount of 
erosion will occur even in the semiconductor body carried by U.S. Pat. No. 
3,558,959. 
Therefore, it is an object of this invention to provide a low-voltage type 
igniter plug having semi-conductor structure improved to have 
significantly long service lives when assembled to provide a 
semi-conductor surface along which a high energy spark discharge occurs in 
a low voltage under the high pressure. 
According to the present invention, there is provided a low-voltage type 
igniter plug having semi-conductor structure comprising; a center 
electrode having a firing-tip mounted within a tubular insulator which in 
turn are placed within an interior of a metallic shell; a ground electrode 
being electrically contact with the metallic shell and in spaced, a 
spark-gap relationship with the firingtip of the center electrode; an 
electrically semi-conducting body, surface of which is mounted adjacent 
the spark-gap in electrically contact with both the center and ground 
electrodes; the improvement in which the semi-conducting body essentially 
consisting of silicon carbide particles less than 5 microns in average 
diameter, and alumina particles less than 1 micron in average diameter, 
the weight percent of the silicon carbide particles ranging from 65 to 80 
inclusive, the weight percent of the alumina particles ranging from 20 to 
35 inclusive, the silicon carbide particles and the alumina particles 
being mixed with an addition of suitable amount of binder means, and 
sintered by means of hot press at the temperature above 1800 degrees 
Celsius inclusive, and at the pressure above 200Kg/cm.sup.2 inclusive. 
Thus providing a tough-structured conductor body of nearly 1 in theoretic 
density in which particles are aligned in well-ordered manner with small 
number of defects, enabling to decrease the amounts of erosion even when 
the semi-conducting body is exposed to a spark discharge under the high 
pressure. 
Other object and advantages will be apparent with reference to the 
following specification, attendant claims, and drawings.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to FIG. 1 which sectionally shows a leading portion of an 
igniter plug 100, a metallic shell designated by numeral 1 has a leading 
portion 11 as seen lower part of the drawing. The leading portion 11 has a 
tapered surface 11a at its inner wall to act as a ground electrode, the 
leading end of which is terminated at an annular end 12 of 6.4 mm in 
diameter. Within the metallic shell 1, is a center electrode 2 coaxially 
placed, the leading end of which terminates in a radially enlarged head 21 
of 4.0 mm in diameter to form an annular spark gap 10 with an inner wall 
of the annular end 12 somewhat extended from the metallic shell 1. Upper 
part of the center electrode 2 is seated in a tubular insulator 4 disposed 
within a space 30 between the center electrode 2 and metallic shell 1. The 
dimensional relationship between annular end 12 and head 21 is such as to 
provide a compact igniter plug. 
In the meanwhile, an electrically semi-conducting body 3 is generally 
formed into annular-shape, and extends annularly around the lower 
extremity of the insulator 4 to the tapered surface 11a of the metallic 
shell 1. 
In this instance, the lower corner of the body 3 is beveled to form a 
frustoconical surface 3a in general, so that the frustoconical surface 3a 
is brought into engagement with the tapered surface 11a at the time of 
assemble. 
Both the tapered surface 11a and the head 21 of the center electrode 2 are 
in electrical contact with a lower end surface 31 of the body 3, so that 
the current flow along the lower end surface 31 of the body 3 ionizes 
adjacent air, and enables a high-energy low voltage spark 2 Kilo volt for 
example) to occur. 
The semi-conducting body 3 is manufactured as follows: 
First step: Taking silicon carbide powder ranging from 65% to 80% by 
weight, and alumina ranging from 20% to 35% by weight, and mixing these 
two components of the powders in a tumbling mill for three hours with an 
addition of binder means such as magnesia (0.3% by weight), calcium oxide 
(0.5% by weight), silicate dioxide (1.9% by weight), and further adding a 
suitable amount of distilled water, and polyvinyl alcohol (0.5% by weight) 
as an organic binder. 
Second step: these powders mixed as above, are rolled after desiccated to 
obtain the powder particles of around 450 microns which contains silicon 
carbide particles of less than 5 microns in average diameter, and alumina 
particles of less than 1 micron in average diameter. Then, the powders are 
pressed by means of steel mould under the pressure of 2000 Kg/cm.sup.2. 
Third step: the moulded powders is forced into carbon die to be sintered by 
means of hot press under the following condition: 
(1) Heating the moulded powders at the rate of 20 degrees Celsius per 
minute, and press them at the pressure of 150.about.250 Kg/cm.sup.2 when 
reached to 1200 degrees Celsius. 
(2) Holding the above pressure for half hours within the temperature 
ranging from 1700 to 1900 degrees Celsius. 
(3) Gradually cooling the moulded powders, and take the sintered powders of 
the mould when cooled below 1400 degrees Celsius. 
Forth step: the sintered Powders taken out of the mould, is adequately 
ground into the annular electrically semi-conducting body 3 dimensionally 
adapted to be incorporated into the igniter plug 100. 
Now, the igniter plug 100 is connected to a capacitor-discharge type 
exciter (not shown) capable of 4 joules, and operated under a pressurized 
atmosphere of 25 Kg/cm.sup.2 to experimentally measure erosion rate of the 
body 3. 
The erosion of the body 3 is expressed by the weight loss caused from the 
spark discharge of 1000 cycles as a unit. 
FIG. 2 shows how the erosion rate (gram) changes depending upon changes of 
the mixing ratio of silicon carbide particles and alumina particles with 
the former and latter particles as being in turn 2.0 and 0.4 microns in 
average diameter. 
The temperature and pressure are taken as 1850 degrees Celsius, and 250 
Kg/cm.sup.2 at the time of sintering. 
As a result, significantly reduced amount of erosion has found when the 
weight percentages of the silicon carbide particles ranges from 65 to 80, 
while that of the alumina particles being from 20 to 35 as apparently seen 
in FIG. 2. 
Further, FIG. 3 shows how the erosion rate (gram) changes depending upon 
changes of the average diameter the silicon carbide particles, and alumina 
particles with the weight percentages of the former and latter particles 
as being, in turn, 65 and 35. 
The temperature and pressure are taken as 1850 degrees Celsius, and 250 
Kg/cm.sup.2 at the time of sintering in the same manner as mentioned 
above. 
As a result, drastically reduced amount of erosion has found when the 
average diameter of the silicon carbide particles is less than 5 microns, 
while that of the alumina particles being less than 1 micron as readily 
seen in FIG. 3. 
FIG. 4 shows how the erosion rate gram changes depending upon changes of 
the temperature and pressure at the time of sintering with the weight 
percentages of the silicon carbide particles and alumina particles as 65% 
and 35% in turn. 
In this instance, the silicon carbide particles and alumina particles are 
in-turn taken as 2 microns and 0.4 microns in average diameter 
Under these conditions, the amount of erosion (gram) changes depending upon 
the pressure (Kg/cm.sup.2) as designated by curve (A) at the constant 
temperature 1850 degrees Celsius, and at the same time, changing depending 
upon the temperature as designated by curve (B) at the constant pressure 
250 Kg/cm . 
By paying attention to the curves (A) and (B) of FIG. 4, it is found that 
the amount of erosion drastically reduces to less than 0.001 (gram) when 
the semi-conducting body 3 is sintered at the temperature of more than 
1800 degrees Celsius and at the pressure of more than 200 Kg/cm.sup.2. 
FIG. 5 shows a modified igniter plug according to the invention, in which 
the head 21 of a center electrode 2 has an axially reduced dimension, and 
a metallic shell 1 terminates in a circular flange 1f surrounding the head 
21. 
The electrically semi-conducting body 3 extends annularly around the lower 
extremity of the insulator 4 to an inner side of the flange 1f of the 
metallic shell 1. 
Both the flange 1f and the head 21 of the center electrode 2 are in 
electrical contact with the lower end surface 31 of the body 3, so that 
the current flow along the lower end surface 31 of the body 3 ionizes 
adjacent air, and enables a high-energy low voltage spark to occur. 
It noted that a suitable combination of the binder components may be 
selected among magnesia, calcium oxide, silicate dioxide, proper amount of 
distilled water, and polyvinyl alcohol. 
It is further appreciated that a firing-tip of the center electrode may be 
made of tungsten based alloy, or platinum-Indium based alloy to impart 
corrosion resistant properties to the firing tip. 
The ground electrode integrally extended from the metallic shell may be 
made of nickel-chromium-based alloy to impart corrosion resistance to the 
ground electrode. 
It will be apparent that various changes and modifications can be made from 
the specific details of the invention as set forth herein without 
departing from the spirit and scope of the attached claims.