Note: Descriptions are shown in the official language in which they were submitted.
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Background of the Invention
Electr.cal discharge devices, such as igniter plugs,
particularly those intended for use in aircraft engines of the
jet or internal combustion types, are sub~ect to a number of
environmental extremes, extremes of temperature being probably
the severest factor to which an igniter plug and its components
are subjected. If the plug is of the high-voltage type,
generally about 15,000 to 25,000 volts are required to cause an
electrical discharge across the spark gap, which high voltages
further aggravate the hostility of the environment to which the
plug is exposed. Moreover, due to difficulties in properly
insulating high voltage systems, flashover problems between
ad~acent components, in the electrical cable and in the
ignition system itself are frequently encountered.
Consequently, low voltage, i.e., from about 1,000 to
S,OQ0 volts, ignition systems have been developed, which systems
require the use of a low voltage or shunted gap-type igniter
plug, which is supplied with low voltage, high energy from a
capacitor discharge system. In a shunt-type plug, the gap between
the center electrode and the outer shell or ground electrode is
bridged by a semi-conducting ceramic material which when pulses
at low voltage allows the stored energy from the capacitor to
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discharge to ground across the igniter plug tip. Thus the
ceramic tip which comprises the bridge member must possess
certain physical and electrical characteristics to function
properly in the environmental extremes created in modern
engines and, in particular, jet aircraft engines.
In general, shunted ceramic tips are of two varieties,
a surface treated insulator and the homogenous type wherein
the entire insulator is a semi-conductor. An example of the
former is described in U.S. Patent No. 2,953,704, wherein an
aluminum oxide ceramic insulator is coated with a sintered
mixture of cuprous oxide and ferric oxide. An example of the
latter is described in U.S. Patent No. 3,558,959, wherein a
semi-conducting ceramic body is formed of bonded particles
of silicon carbide.
In the continuing search for improved materials to
resist heat, thermal shock, spark erosion, and in general the
hostile environment of modern engines, insulators comprised
essentially of beryllium oxide have been found to be superior
to more commonly used aluminum oxide insulators, both with
respect to improved thermal conductivity and resistance to
thermal shock. However, semi-conducting coatings developed
for aluminum oxide based ceramics have proven unsatisfactory
when applied to beryllium oxide ceramics. The present invention
relates to an electrically semi-conducting coating composition
suitable for coating beryllium oxide electrical insulators,
the coating composition consisting essentially of a mixture
of from 25 to 90 percent by weight of lanthanum oxide, from
5 to 75 percent by weight of cuprous oxide and from 0 to
25 percent by weight of ferric oxide.
In a further aspect the present invention relates to
a beryllium oxide ceramic insulator body having an electrically
semi-conducting coating formed thereon and bonded thereto, the
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coating comprising a sintered mixture of lanthanum oxide and
cuprous oxide.
In a still further aspect the invention is used in
a shunt-type igniter plug comprising an outer metal shell, a
ground electrode integral with said shell, a central electrode
having a firing tip, said central electrode mounted in an in-
sulator disposed within said shell, the firing tip of said cen-
tral electrode being in opposed spaced relation to the ground
electrode forming a spark gap therebetween, and provides improved
electrically semi-conducting means adjacent said spark gap and
in electrical contact with the opposed electrodes. The improved
electrically semi-conducting means comprises a beryllium oxide
ceramic body disposed about the central electrode. The ceramic
body has formed thereon and bonded thereto an electrically
semi-conducting coating, the coating comprising a sintered mix-
ture of lanthanum oxide and cuprous oxide. The coating is
in electrical contact with said opposed electrodes and forming
a bridge across said spark gap.
The features of the invention will become apparent
from the following description with reference to the accompanying
drawings which are illustrative only of preferred embodiments of
the invention.
Description of the Drawings
Figure 1 is a partially schematic view, in longitudinal
cross section, of the lower tip portion of an igniter plug em-
bodying the invention; and
Figure 2 is a view similar to Figure 1, showing a
differently configured lower tip portion of an igniter plug
embodying the invention.
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Description of the Invention
With reference to Figure 1, a low-voltage igniter
plug 10 comprises a hollow, cylindrical body shell 11 which is
generally made of a nickel-steel alloy. The lower extremity of
shell 11 is formed as a radially inwardly directed flange 12.
The inner edge surface 13 of flange 12 is cylindrical and coaxial
with the shell 11, which surface 13 of flange 12 constitutes a
ground electrode. A central electrode 14 extends longitudinally
within the shell 11 and is coaxial therewith, the firing tip 15
of the
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electrode 14 terminating in spaced relationship with the surface
13 thus forming an annular spark gap 16. When the plug 10 is
installed in an engine, the flanged portion 12 is in direct
communication with the combustion chamber of the engine and is
grounded to the engine by contact between the shell 11 and the
engine block. The upper end of central electrode 14 is seated
in an annular insulating member 17 disposed within the shell 11
which insulates the central electrode 14 from the shell 11. The
insulating member 17 may be formed of any suitable insulating
material such as porcelain, borosilicate glass, aluminum oxide
ceramic, beryllium oxide ceramic or the like. Another insulating
member 18, formed principally of beryllium oxide, is annularly
disposed abou~ the lower end of the central electrode 14, and
extends from the lower end of insulating member 17 to the upper
radially inwardly directed surface of th.e flange 12. The lower
periphery and the lower face of the insulating member 18 is
coated ~ith a semi-conducting layer 19, said layer 19 being in
intimate contact with the lower end of the central electrode 14
and the upper surface of the radially inwardly directed flange
12, whereby current flow can occur across the semi-conducting
coating 19 upon the application of low voltage, the current flow
resulting in ionization of the spark gap 16, thus enabling a
high energy, low voltage spark to discharge across the gap 16
and between the firing tip 15 of the central electrode 14 and
the surface 13 of flange 12.
With reference to Figure 2, a low voltage igniter plug
20 comprises a hollow, cylindrical body shell 21. The lower
extremity of body shell 21 is formed as a radially inwardly
directed flange 22, the inner edge surface 23 of which is
cylindrical and coaxial with the shell 21 which surface 23 o~
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flange 22 constitutes a ground electrode. A central electrode
24 extends longitudinally within the shell 21 and coaxial there-
with, the lower end of electrode 24 terminating in an annular,
outwardly directed flange 25, said flange 25 having a lesser
diameter than the diameter of radially inwardly directed flange
22 of shell 21, said flanges defining between them an annular
spark gap 26. The upper end of central electrode 24 is seated in
an annular insulating member 27 disposed within body shell 21,
which insulates the central electrode 24 from the shell 21. The
insulating member 27 may be formed of any suitable insulating
material, such as porcelain, borosilicate glass, aluminum oxide
ceramic, beryllium oxide ceramic or the like. Another insulsting
member 28 formed principally of beryllium oxide is annularly
disposed about the lower end of the central electrode 24 and
extends from the lower end of insulating member 27 to both the -
upper surface of flange 22 and the upper surface of flange 25,
forming a bridge across the spark gap 26 formed between flange
25 and surface 23 of flange 22. The lower face of the insulating
member 28 i8 coated with a semi-conducting layer 29, said layer
29 being in intimate contact with the upper surfaces o~ flanges
22 and 25, whereby current flow can occur along the semi-conduct-
ing coating 29 upon the application of low voltage, the current
flow resulting in ionization of the spark gap 26, thus enabling
a high energy, low voltage spark to discharge across the gap 26
and between flange 25 of central electrode 24 and surface 23 of
flange 22.
As described hereinabove, the electrically semi-
conducting layer (designated at 19 in Figure 1 and at 29 in
Figure 2~ which shunts the electrodes of the spark gap is in the
form of a film-like coating formed on the beryllium oxide ceramic
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insulator member. The semi-conducting layer is comprised of a
fired mixture of lanthanum oxide (LazO3) and copper oxide, pre-
ferably cuprous oxide (Cu20). The said mixture may also contain
iron oxide, preferably ferric oxide (Fe2O9). The firing of the
mixture of oxides is carried out at such temperature and for such
length of time that at least one of the oxides is sintered, i.e.
heated to the point of incipient fusion. The liquid vehicle in
which the oxides are preferably applied to the beryllium oxide
substrate is driven-off or volatilized and the resulting electri-
cally fiemi-conducting layer is compacted into a low resistance,
smooth-surfaced coating layer which is firmly bonded to the
surface of the beryllium oxide substrate.
In order to function satisfactorily in electrical
discharge devices, such as low voltage igniter plugs, an
electrically semi-conducting layer shunting the electrodes of such
plugs must have a relati~ely smooth and unblemished outer surface
confronting the gap; a relatively low resistivity both initially
and during the life of the plug under the varying temperatures
and other service conditions encountered during use; and must be
well-bonded to the ceramic refractory substrate. Unless the
semi-conducting layer is smooth and substantially free from imper-
fections, it will not function uniformly over the area of the
spark gap. Unless such layer is of relatively low resistivity,
i.e. from about 10,000 to about 500,000 ohms, preferably from
about 10,000 to about 100,000 ohms, it will not sufficiently
strongly ionize the gap between the electrodes prior to spark
discharge. Unless the layer is well-bonded to the ceramic sub-
strate, it will flake or spall-off the substrate under prolonged,
arduoùs service conditions.
It has been found that an electrically semi-conducting
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coating material possessed of the above-mentioned desirable
properties and suitable for coating predominantly beryllium oxide
(i.e. from about 75% to 99% by weight BeO~ based ceramic insulator
bodies comprises a sintered mixture of from 25% to 90% by weight
lanthanum oxide (La203), from 5% to 75% by weight cuprous oxide
(Cu20) and from 0% to 25% by weight ferric oxide (Fe20,). A
preferred semi-conducting coating comprises a sintered mixture
of from 65% to 90% by weight lanthanum oxide (La203), from 10%
to 30% by weight cuprous oxide (Cu2O) and from 0% to 5% by weight
ferric oxide (Fe20~).
The above ranges of weight percentages of the said
oxides were determined by preparing about two hundred fifty
mixtures of lanthanum, cuprous and ferric oxides of broadly
varying compositions ranging from 0% to 100% by weight lanthanum
oxlde, from 0% to 100% by weight cuprous oxide and from 0% to
100% by weight ferric oxide and coating each composition on
individual beryllium oxide discs as will be more fully described
hereinafter. The coated discs were fired at temperatures ranging
from 2150F to 2450F and the resistance of the fired coatings
was measured. The resistance values were plotted on ternary
diagrams and correlated with the respective metal oxide com-
position which yielded the respective resistance value. Metal
oxide compositions in the above ranges were, from analyses of
the ternary diagrams, found to produce coatings having acceptable
low resistance properties and, a fortiori, would be suitable ln
the preparation of electrically semi-conducting coatings
particularly compatible with beryllium oxide ceramic substrates.
The coating is preferably prepared, generally speaking,
by grinding the dry oxides to a fine powder and slurrying the
same with a liquid media comprising water, a wetting agent and
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a glycol. The oxide slurry is painted on the beryllium oxide
substrate by brushing or spraying to a thickness of between about
O.OOS to 0.010 inches (5 to lO mils). The coated substrate is
then fired in a kiln or the like at a temperature of from about
2150F to 2450F until a smooth coated surface is obse{ved, the
liquid carrier, of course, being volatilized. As the beryllium
oxide is somewhat porous, the coating will to some extent pene-
trate the pores, thus enhancing the adherence of the coating to
the substrate.
The composition of preferred embodiments of electrically
semi-conducting coatings on beryllium oxide ceramic substrates in
accordance with the invention and a preferred manner of forming
electrically semi-conducting coatings on beryllium oxide ceramic
substrates are set forth in the following example, which example
is intended solely for the purpose of illustration and is not to
be construed as in any way limiting the scope of the invention.
EXAMPLE 1
A series of one-gram samples of lanthanum oxide (La20
Kerr-McGee, Code 528), cuprous oxide (Cu20~ Fisher C-477, Lot
723251) and ferric oxide (Fe203, Columbia 347 Grade) having the
weight percentages indicated in Table l are prepared, each con-
stituent of each sample being weighed, on a Mettler H15 (trade
mark) analytical balance, to the nearest milligram. The dry oxides
constituting each sample are ground together in a mortar and
pestle and are placed in individual plastic vials. To each vial
is added the following.
Glycerol................................... 3 drops
Tergitol ~4 (trade mark for a wetting
agent manufactured by Union Carbide Corp.) . 2 drops
Distilled Water............................ 12 drops
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Note: The "drops" in which the volumes of glycerol, Tergitol #4
(trade mark) and water are given are drops from a standard
-analytical burette.
The dry oxides are thoroughly mixed with the liquid
additives to form a uniform slurry or suspension of the oxides
in the liquit carrier. Each slurried sample is painted on a
separate disc of beryllium oxide, each disc measuring approxi-
mately 0.5 inches in diameter by 0.070 inches thick and having
a 8eO content of about 99% by weight. To assure uniform appli-
cation of each sample on each disc the following procedure isused.
Pressure sensitive masks or stencils are made by
punching 0.25 inch diameter holes in masking tape, cutting off a
section containing one hole and pressing the tape section onto a
disc, positioning the hole in the center of the disc. Each
sample is then painted, to a thickness of about 5 mils, over the
hole in the tape section and when the mask is peeled away a 0.25
inch diameter sample is centered and coated on each disc.
The discs are then fired in an electrically heated
resistance element-type kiln <Burrell, Model 90, trade mark) to
the temperatures indicated in Table 1. The samples are heated at
about 200F per hour to a temperature of about 800F to evaporate
the liquid carrier after which heating is continued at a rate of
about 400F per hour until the desired temperature is attained,
Orton (trade mark) pyrometric cones being used to indicate the
heat output and uniformity of firing. After firing, the discs are
cooled and observed for smoothness and uniformity o coating. The
resistance of the sintered coating is measured at 22C with a
Simpson 260 ~trade mark) Ohmeter, the resistance for each sample
at the respective firing temperature is tabulated in Table 1, in
Ohms x 104.
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As sllown by the data presented in Table 1, the semi-
conducting metal oxide coating compositions according to the
invention (Samples No. 1 through 22) each have satisfactory
resistanceswhen coated on a beryllium oxide insulator material.
It will be observed that, generally speaking, the resistance of
the coating increases with increasing firing temperature, and sig-
nificantly increases at a firing temperature of 2450F, as
exemplified by firing a number of said samples at that tempera-
ture, which temperature of 2450~F is believed to be about the
upper limit at which the coating can be fired and still retain
satisfactory semi-conducting properties.
Samples ~o. 25 through 28 demonstrate that prior art
semi-conducting coatings consisting of mixtures of cuprous oxide
and ferric oxide, as taught in the aforementioned U. S. Patent
No. 2,953,704, though satisfactory when coated on aluminum oxide
ceramics~ are unsatisfactory when coated, as here, on a beryllium
oxide ceramic, the coatings being electrically non-conducting in
the latter instances.
Samples No. 30 and 31 are illustrative of the unsuit-
ability of mixtures of lanthanum oxide and ferric oxide as coatingsprepared from mixtures of these oxides are also electrically non-
conducting.
Samples No. 23, 24 and 29 are indicative of the unsuit-
ability of coatings respectively consisting en~irely of lanthanum
oxide, cuprous oxide or ferric oxide; satisfactory coatings being
produced only when a mixture of lanthanum and cuprous oxides is
employed or a mixture of these oxides plus ferric oxide, the
presence of the latter believed to promote lower resistivity at
higher lanthanum oxide ratios and appears to improve somewhat the
adherence of the coating to the beryllium oxide substrate.
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It is to be further borne in mind that although the
invention has been described with particular reference to its
applicability to shunted gap-type igniter plugs, the inventive
concept may be used in a variety of electrical semi-conducting
applications, such as for example, in printed circuit patterns,
high temperature resistors, grounding shunts for various electrical
components and the like, which varying applications would be
readily apparent to one skilled in the art, as the crux of the
invention resides in providing an electrically semi-conducting
coating that is suitable for application to a beryllium oxide
ceramic insulating material. As regards the coating composition
itself, it is to be understood that the use of ferric oxide as a
constituent of the mixture is optional and is not critical to the
formation of a satisfactory semi-conducting coating, the essential
constituents of the coating being lanthanum oxide and cuprous
oxide. In addition, although the metal oxide coating of the
invention is particularly intended for application to beryllium
oxide ceramic substrates, it may also be applied to insulating
bodies composed of, for example, aluminum oxides.
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TABLE 1
Ssmple Composition Resistance, Ohms x 104 at 22C
No. Percent by Weight After Firing at Indicated Temperature
La2O~ CuzOFe2O32150F 2250F 2350F 2450F
1 25 75 0 20 20 10
2 30 70 0 30 40 15
3 45 55 0 30 10 30
4 50 50 0 25 15 15
0 10 20 20
7 55 40 5 6 5 5 50
8 60 35 5 6 5 5 100
9 65 35 0 10 20 50
6 6 10 100
11 65 25 10 5 7 4
12 65 15 20 3 4 20
13 65 10 25 3 2 4
14 70 25 5 8 10 15 50
3 3 5 100
16 75 25 0 lO 7 15
17 75 20 5 6 10 10 20
18 75 15 10 5 3 20
19 75 10 15 5 5 15 200
6 10 10 50
- 21 85 15 0 10 10 4
22 85 10 5 lQ 10 20 50
23 100 0 0~nfiniteInfiniteInfinite
24 0 100 Infinite Infinite Infinite
0 92 8 Infinite Infinite Infinite
26 85 15 Infinite Infinite Infinite
27 0 75 25 Infinite Infinite Infinite
28 0 65 35 Infinite Infinite Infinite
29 O 100 Infinite Infinite Infinite
0 20 Infinite Infinite Infinite
31 90 0 10 Infinite Infinite Infinite
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