CA1075777A - Silicon carbide resistance igniter - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A monolithic ceramic resistance igniter of simple configuration is composed essentially of polycrystalline silicon carbide adapted for use in gas and liquid fuel burning systems. As a result of the combination, controlled density and large cross-sectional area, the igniter possesses an unusually high degree of physical ruggedness.. The igniter will attain a temperature of about 1000°C in well under 20 seconds drawing a maximum of 6 amps at 132 volts, with a room temperature resistivity of 0.10 to 1.70 ohm centimeters and a resistivity at about 1000°C of from 0.06 to 0.26 ohm centimeter. The igniter also has a physical construction such that a high percentage of its hot surface area radiates directly to the environment.

Description

7577~7 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 ,.

. . - ~ . ~ ~ . .

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 . .
- , .

` 1075'777 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, ~ ' ' ~ ' 107S77~
the lo~ resistiYity at room te~mperature heing particularly important from the standpoint of heat up time.

lQ

3a -3a-~075777 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 . .- . . - . , .
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 ,~,p _5_ . ~ . , 1075'777 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 :, ' , ~"

.

- - . : - .

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 . .

~(~7S777 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.

. .

1(~75777 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 :

. . ~
. . ~

.
., , - :
.

~ ~075777 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 . . - ., , :.

~07577~
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 , .

.

~.07577'7 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 `
.

. ' ' '' ' ` ' .~ .

~ - 12 - ~

- . .. , . . ~

Claims (2)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A monolithic ceramic resistance igniter having a flat elongated configuration essentially rectangular in cross-section, including terminal connecting means at one end, a hot zone extending therefrom comprised of at least one leg having a hairpin shape, where the end of said leg opposite the terminal connecting ends has a greater cross-section than the cross-section of the individual elements making up said-hairpin shaped leg, and having at least 50% of the surface area of said hot zone radiating directly to the environment.

2. The monolithic ceramic resistance igniter of Claim 1 comprised of polycrystalline silicon carbide and consisting of two interconnected hairpin shaped legs, the overall length of said igniter being from 2.125 to 2.625 inches, the ends of said legs opposite the terminal connecting ends having a cross-sectional area of from 0.013 to 0.049 square inch, the elements of said hairpin shaped legs having a cross-section of from 0.006 to 0.018 square inch, and the width of the slots separating said elements being from 0.012 to 0.080 inch.