Note: Descriptions are shown in the official language in which they were submitted.
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BACXGROUND OF THE I~VE~TION
The invention relates to igniters for fuel ~urning
devices such as domestic and industrial liquid ruel and gas
burning appliances. More particularly, the invention relates
to ceramic resistance igniters for gas burning applia~ces
such as kitchen ranges, furnaces, clothes-dryers and the like.
The concept of non-pilot light igniters has been
known for years. The earlier type of igniter was the incan-
descent wire device such as an electrically heated platinum
wire coil. These are fragile and, in most applications,
require a step-down transformer. Ceramic resistance igniters
made their appearance in about 1937. U.S. Patent 2,0~q, 3Q4 to Mc~be~ of
August 10, 1937 describes a total electrical ï.gni.tion system in which a
ceramic resistance igniter composed of "Durhy Material" is
utilized to ignite a fluid fuel system. Durhy is a dense
sintered silicon carbide impregnated with sllicon. ~ U-shaped
ceramic igniter is disclosed in U.S. Patent 2,095,253 to ~eyroth of Oct. 12/37
where the igniter is composed of sin~ered and silicon imprégnated
silicon carbide. This igniter element is formed by first
preforming'120 grit (142 microns) and iner silicon carbide
material, into rods o~ suitable length, which are then fired
to presinter the silicon carbide. The rods are then cut
into the desired length and slotted to form a U-shaped ele~ent
which is subsequently impregnated with silicon metal. Another
ha~ic type of silicon carbide igniter is that described in U.S. ~atent 3,052,814
to Edwards et al of Sept. 4/62. This is a sparkplug type igniter as
opposed to the pure resistance type mentioned above and is
composed of silicon nitride bonded ~ith silicon carbide. Still
another silicon carbide igniter device is described in U. S.
Patent 3,282,324 to Romanelli of Nov. 1/66 as part o~ a complete ignition
and heat injection
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system. In this case the silicon carbide is a sintered
silicon carbide cylinder having a spiral cut which provides
a relatively small percentage of the hot area which radiates
directly to the environment.
By nature of their use, resistance igniters must
be small in dimension, particularly in terms of their cross-
section and overall length. Because of these physical
parameter restrictions, prior art silicon carbide igniters
are very fragile. As a result, attempts have been made to
physically reinforce ceramic resistance igniters by such
approaches as that described in U.S. Patents 3,372,305 to
Mikulec of Mar. 5/68 and 3,467,812 to Terrell of Sept. 16/69.
Both of these igniters have a spiral configuration which is
fabricated of a sintered tube of silicon carbide which is
made as dense as possible. The sp ~al configuration is cut
in the sintered silico~ carbide tube, which is then supported
by an aluminum oxide rod which passes through the opening
of the spiral igniter body.
Still another type of resistance igniter is described
in U.S. Patent 3,454,345 to Dyre of July 8/69. This igniter
is composed of a sintered mixture of silicon carbide and
silicon oxynitride wherein the silicon oxynitride functions
as a bond for a relatively coarse 10F silicon carbide, i.e~,
a mixture of particles of 1340 microns and finer in size with
10 percent by weight of silicon oxynitride. This silicon
carbide/silicon oxynitride mixture is one manufactured and
sold by Norton Company, Worcester, Massachusetts, and its
foreign affiliates under the trademark CRYSTOLON 63.
Despite the substantial amount of activity in the
ceramic resistance igniter field, the igniters enjoying most
widespread use today, for most applications, are still of the
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pilot light type. In yie~ of the current energy crisi~ and the result of :-
various surYey~ which ~ho~ that pilot ligfit~ cons.~me.from la to 15 percent of
the total gas consumed in this country, there is ob~iously a compelling need
for an igniter to replace.tfie presently-used pïlot light~
It ïs-, therefore, a principal object of the present in-
vention to provide a ceramïc resïstance i`gniter for liquïd and gas fuel burning
devices whi.ch.ïs free of the foregoing dèfïcïencies, and wfiich is physically
rugged, heats~ rapi`dly, survïves hundreds of thousands of heating cycles, is
simple electrically and structurally, has low susceptibility to premature burn
out, and radiates primarily to the enviroment.
Thus, in accordance with the present teachings, a mono-
li.thic ceramic resistance igniter is provided which has a flat elongated
configuration essentially rectangular in cross-section and includes terminal
connecting means. at one end. A hot zone extend therefrom which is comprised
of at least one leg having a hairpin shape where the end of the leg opposite
the terminal connecting ends has a greater cross-section than the cross-
secti.on of the indi.vidual elements which ma~e up the hairpin shaped leg and
has at least 50% of the surface area of the hot zone radiating directly to the
environment.
SUMMARY OP THE INYENTION
Compositionally the ceramic igniter of the present invention
consists of 95 to 99.9 percent by weight of alpha silicon carbide, 0.05 to 0.50
percent by weight of aluminum, O to 4 percent by weight silica, O to 0.25 per-
cent by weight of iron or iron-based compounds, a maximum of 100 parts per
million of boron and a minor amount, generally not in excess of 0.25 percent,
of miscellaneous impurities. The composition also contains a very small (on
the order of 500 ppm) amount of nitrogen which.is introduced into the silicon
carbide by a doping process which will be described in more detail subsequently.
The small amount of aluminum incorporated in the Si.C is necessary to raise the ~ :
high temperature (e.g. 10~0 C~ resistivity of the igni.ter to a level on the
order of .06 to .26 ohm centi`meters-. The boron content i`s preferably kept
below SO ppm to maintaïn reasonably lo~-res~sti~ity at low and high temperature,
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the lo~ resistiYity at room te~mperature heing particularly important from the
standpoint of heat up time.
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The i~niter shape is formed by conventional methods
which result in said igniter having a controlled density of
from about 2.60 to 2.70 grams per cubic centimeter. This
controlled density has the advantage of producing s silicon
carbide resistor with a higher resistivity than a more dense
silicon carbide, thus facilitating the formation of an igniter
with the required resistance, but with a relatively short
electrical path. The importance of this latter feature
relates to the fact that igniters generally are used in very
limited spaces, therefore, must be small in size. The high
resistivity of the controlled density igniters of the inven-
tion greatly facilitates this objective. As a result of the
composition, density, and the processing employed, the result-
ing silicon carbide ignlter is ideally suited as a fuel
igniter for such devices as gas clothes dryers, in that the
stringent requirements for such igniters are easily satisfied
by the igniters of the invention. To be acceptable for such
end uses, the igniter must have sufficient mechanical strength
to resist severe physical forces; the present igniter will
withstand a whipping type force of at least 125 g's. Such
an igniter must also be able to attain a temperature of about
1000C in less than 20 seconds while drawing a maximum of
6 amps at 132 volts, and in less than 60 seconds at an input
of 80 volts; the present igniter easily satisfies these
requirements by virtue of a room temperature resistivity of
0.10 to 1.70 ohm centimeters, and a resistivity at approximately
1000C of 0.06 to 0.26 ohm centimeter. Its overall physical
dimensions for gas fired clothes dryers and ranges is from
2.125 to 2.625 inches in length, with an effective cross- -
section of from 0.012 to 0.72 sauare inch. Finally, the present
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igniter has an inherent ability to wit~stand at le~s;t 2QO,QOQ heat-up and cool-
down cycles. This fs unexpected in vie~ of the relatively low dens~ty of the
igniter, 6ut it i`s ~elïeved that tfiis results-from a combination of chemical
compositïon, processïng conditions involved in the fafirïcation of said igniter,
and the high percentage of the heating area wfiich radïates dïrectly to the
enviroment. ~y the expressïon "area wfiîch raai`ates di`rectly to the enviro-
: ment" we mean hot area that does not "see" otfier fiot areas-. Thus the inside
surface of a cylïndrical heating element would "see" other hot portions of
the insïde surface (or a hot support element) and would not be considered as
la "radiating directly to the enviroment" with reference to the drawing wherein:
Fig. 1 shows one embodiment of the igniter of the present
invention;
Fig. 2 is a cross-section on the line 2-2 of Fig. l; and
Fig. 3 shows another form of the igniter of the present
invention.
The "hot" area of the igniter of Figure 1 is the surface
of that part of the element of smaller cross-section, that is, the portion of
; 8a, 8b, lOa, and lOb of minimum cross-section. In Figure 2, about 55% of the
surface of the "hot" area is "outside" surface. To keep the outside surface
above 50%, the thickness of the i8niter should not be greater than twice the
width of the lega. From the design of Figure 3, the outside area will always
be greater than 50%.
The present igniter is monolithic and self-supporting,
needing no supporting device such as that required for the successful utiliza-
tion of the silicon carbide igniter of U.S. patents 3,372,305 and 3,467,812.
This results from the relatively great thickness, i.e., cross-sectional area ;
of the present igniters as set forth above. The most desirable confirguration
is that of a leg haYing a hairpin shape including terminal connecting ends,
because this shape presents at least SQ percent of the surface area of the
3Q hot zone of the- igniter to the surrounding enviroment. With a
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high percentage of the heat area radiating outward, there is
less tendency for hot spots to develop. ~his characteristic,
plus the relatively large cross-section, minim-izes premature
burn out. It is even more desirable that the igniter be made
of two legs of hairpin configuration to maximize the igniter's
ability to quickly ignite a fuel exposed thereto.
BRIEF DESCRIPTION OF THE DRAWINGS
; Fig. 1 is a longitudinal view of the largest surface
area of the igniter of the present invention.
Fig. 2 is a sectional view of the igniter of Fig. l.
Fig. 3 is a longitudinal view of the largest
surface area of another embodiment of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The preferred physical configuration of the instant
igniter is shown in Figures 1 and 2. Referring to Figure 1
the wing shaped elements 4 and 6 are terminal connecting ends,
Coextensive with the terminal connecting ends and with each
other are two hairpin shaped legs 8 and 10. The double
hairpin configuration is completed by the approximately
centrally located slot 12 which traverses from the end of the
igniter opposite the terminaI connecting ends towards said
ends but stopping substantially short thereof, and a slot in
each leg 8 and 10 identified as 14 and 16 respectively in
Figure 1. The electrical path begins at the terminal
connecting ends 4 and 6 and traverses the legs through a
substantial part of their length, forming two elements 8a, 8b
and 10a and 10b for each leg. In the slots 14 and 16 at the
terminal connecting ends thereof it is desirable, although
not absolutely necessary, to include a portion of an electri-
30 cally insulating cement such as a commercially available
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alumina based refractory cement. This is shown as small dabs
18 and 20. Larger quantities of refractory cement may be used
if desired. Without the portion of cement so located in slots
14 and 16 there is the danger of shorting out or breaking of
the igniter should any force be exerted on the thermal
connecting ends 4 and 6 so as to force said ends toward one
another. The ends or tips 22 and 24 of legs 8 and 10
respectively have a larger cross-section than the cross-
section of their individual elements 8a, 8b, lOa and lOb.
This larger cross-sectional area of these ends causes them
to remain relatively cool and causes concentration of the hot
zone of the igniter in tho~e portions of the two legs in
between these ends 22 and 24 and the terminal connecting ends
4 and 6. ~his configuration exposes, for direct radiation
to its environment, at least 50/~ of the total surface area
of the igniters hot zone. In calculating the area of hot
zone which radiates directly to the environment in Figs. 1
and 2 the upper and lower surfaces (those parallel to the
plane of the drawings) and the outer boundaries of the
element would be considered as the applicable areas. The
surfaces o the element defining the slots would not be so
concidered since they can radiate dlrectly to their hot
facing su~faces.
In a preferred form for gas dryers the present
igniter is from 2.125 to 2.625 inches in length, with the
end 22 and 24 of the legs 8 and lO each having an essentially ~`
rectangular cross-sectional area of from 0.020 to 0.03-9
square inch. Elements 8a, 8b, lOa and lOb of legs 8 and lO
each preferably have a cross-section of from 0.009 to 0.014
square inch, the slots forming said elements are preferably
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from 0.033 to 0.080 inch wide. There are many possible
variants on the basic configuration of the present igniter,
one such being that shown in Figure 3 which has terminal
connecting ends 26 and 28 and a single hairpin shaped leg
30 comprised of elements 30a and 30b, slot 32. Insulating
cement 34, is included between the terminal connecting ends
26 and 28. The end 36 has a slightly larger cross-sectional
- are than elements 30a and 30b of leg 30.
In one method of forming the present igniters a
casting slip is prepared having the preferred composition of
97 to 99,3O/o by weight of a 50% mixture of high purity 3.0
micron silicon carbide and 100F silicon carbide, and 0.05 to
0.30O~o by weight of Al203. The preparation of the slip, and
the casting thereof into plaster molds, is taught in U. S. :
Patent 2.964,823. The mold cavity has a cross-sectional
configuration and dimensions corresponding to the outline of
the igniter shown in Figure 1 or Figure 3. The length of the
mold cavity is 12 inches although obviously said dimens-ion
could be longer or shorter if desired~ The green billet
thus ca.~t is allowed to stand.in the mold for 10 or 15
minutes after which it is remo~ed and air dried.for 8 to 16
hours at 125 to 150C. To facilitate slicing of the billet
lnto igniter blanks, the billet is impregnated with a 25%
solution in isopropyl alcohol of a mixture of 100 parts by
weight of Fapreg P3 and 2 parts by weight of Activator , both
materials manufactured and sold by Quaker Oats Company. Other
polymerizable organic material may also be used in place of
the foregoLng. The impregnation is carried out by immersion
of the green bi.llet in the solution. The saturated billet
30 is heat treated at about 95C for at least 12 hours after
* A Trademark for a furfuralkehyde prepolymer dissolved in
furfuryl alcohol.
** A Trademark for phthalic anhydride.
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which temperature is raised to about 190C and held there for
two hours. The billet is then allowed to cool.
- The billet is sliced into igniter blanks preferably
about 0.135 inch in thickness. The slicing is best accomplished
with a diamond cut-off wheel. The three slots 12, 14 and 16
of Figure 1 are cut into the blanks, again with a diamond
cut-off wheel.
The green igniters are placed in a graphite holder
and fired at 2200 to 2450C in a reducing atmosphere for 1/4
to 4 hours. The fired igniters are subjected to a subsequent
firing, in nitrogen, at 1500 to 2000C for 15 to 18 minutes,
maintaining the nitrogen environment until the temperature
in the furnace has dropped to 800C.
The terminal connecting ends 4 and 6 in Figure 1
are then coated with a metal, prefe~ bly aluminum or an
aluminum alloy. This may be accomplished by any known method
such as dipping of the ends into molten metal or flame
spraying. The ends should also be sandblasted lightly prior
to applying the metal coating.
The final step in the fabrication of the present
igniter is the placing of the refractory, electrically
insulating cement, 18 and 20 in Figure 1. ~he cement may be
essentially any refractory, electrically insulating cement but
the preferred cement is the high alumina type. The quantity
of cement required, for the purposes stated above, is small
e.g. an amount of cement to fill the slots 14 and 16 of
; Figure 1, approximately 1/4 inch in from the far edge of the
terminal connecting ends. The slots may be filled further,
if desired.
For optimum performance the igniter should be
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composed of from 97 to 99.9% by weight of polycrystalline
- silicon carbide, 0.1 to 0.3% by weight of aluminum added as
aluminum oxide in the original mixture, less than 50 parts
per million of boron, and not more than 0.20% of miscellaneous
impurities. It would also appear that an indeterminate amount
of nitrogen must be introduced into the structure by subjecting
the initial green igniters first to a standard non-oxidizing
type of firing step at about 2200C or above, followed by
firing in a nitrogen atmosphere at lS00 to 2000~. Attempts
to combina these two steps into one fail to effect the desired
electrical properties in the final igniter. This is believed
to be due to the different rates of N2 diffusion into the SiC
crystals at the two different temperatures. When Nz is
present during ~he initial high temperature firing (2200 to
2400C) it diffuses in sufficient quantities into the body of
the SiC so that bulk SiC has a low resistivity both at room -
and high temperatures thus providing too much current flow at
the high temperature (over 6 amps at 132 volts~. It is
believed that when the igniter is fired in nitrogen at the
lower temperature (1500 to 2000C) a small but suEficient --
amount of nitrogen diffuses into the surface of the fine
silicon carbide particles, which bridge the larger particles,
to lower the room temperature resistivity of the igniter
without significantly affecting the high temperature
resistivity. As a result this added N2 lowers the igniter
response time, e.g., the time for the igniter to reach the
desired fuel ignition temperature.
Some prior art gas and liquid fuel igniters have
the inherent shortcoming of room temperature resistivities
that are too high, and elevated temperature resistivities
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that are too low for the most effective and efficient
- operation. The igniter of the present invention is free
of this problem having a preferred resistivity at room
temperature of from 0.15 to 0.5 ohm centimeter and at about
1000C of at least 0.1 ohm centimeter, resulting in a
response time at 80 volts of 10 to 60 seconds to attain
approximately 1000C.
- This unique set of resistivities results primarily
from the combination of the introduction of the prescribed
amount of aluminum into the crystal lattice of the silicon
carbide, and the post-firing nitrogen treatment which
introduces a relatively high percentage of nitrogen into the -~
crystal lattice of the finer silicon carbide grains. This
same treatment (it is believed) introduces only a very small
percentage of nitrogen into the crystal lattice of the larger
SiC crystals. The effect of the presence of aluminum is to
:
; increase the resistivity of the body, both at room temperature
and at elevated temperature; the latter is desirable but the
former is not. The nitrogen treatment subsequent to the
initial firing reverses or compensates for the undesi~able
increase in the room temperature resistivity caused by the
introduction of the ~luminum, i.e., the nitrogen decreases
the room temperature resistivty. The resulting igniter thus
has a heretofore unknown combination of a relatively high
elevated temperature resistivity and a low room temperature
resistivity.
The oxygen content of the finished igniter is
between about .04 and .1%. After use the oxygen content
will increase substantially due to surface oxidation of the
silicon carbide grains. This additional oxygen is not
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detrimental so long, as it is on the surface of the fired
igniter and not between the SiC grains of ~he igniter where
it would introduce a high resistance. In some cases it
may be desirable to oxidize the igniters prior to sale or
to apply an oxide coaring on the finished igniter; these
techniques are known in the art.
Where the expression "percent" or ''~0'' is used
in the specification and claims it is intended to mean
weight percent unless clearly states to have some other
meaning
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