CA1064248A

CA1064248A - Silicon carbide resistance igniter

Info

Publication number: CA1064248A
Application number: CA222,622A
Authority: CA
Inventors: John I. Fredriksson; Samuel H. Coes
Original assignee: Norton Co
Current assignee: Saint Gobain Abrasives Inc
Priority date: 1974-04-23
Filing date: 1975-03-20
Publication date: 1979-10-16
Also published as: JPS579203B2; US3875477A; JPS50146897A

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 at 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

10~4;~48 CKGROUND OF THE INVENTION
The invention relate~ to igniterJ for fuel burning devices Juch as domestic and industrial liquid fuel and gaJ
burning appliances. More particularly, the invention relate~
to ceramic resistance igniters for gas burning appliance~
such as kitchen ranges, furnaces, clothe~-dryers and th~ like.
The concept of non-pilot light ignit-r~ ha~ beon known for years. The earlier type of ignitor was th~ incan-descent wire device such as an olectrically heatod platinum wire coil. These are fragile and, in md~t application~, require a step-down transformer. Ceramic reJistance ignitorJ
made their appearance in about 1937. U. S. Patent 2,089,394 de~cribes a total electrical ignition system in which a ceramic resistance igniter composed of "Durhy Material" is utilized to ignite a fluid fuel systom. Durhy i~ a den~e ~intered silicon carbide impregnated with silicon. A U-~haped ~ ;~
ceramic igniter is disclosed in U. S. Patent 2,095,253 wh-re the igniter i~ composed of sintered and Jilicon impregnated silicon carbido. Thi~ igniter element is formed by fir~t preforming 120 grit (142 microns) and finer ~ilicon carbide -~
, material, into rods of ~uitable length, which are then fired ' ~-to pre~int~r the silicon carbide. The rods aro then cut into the de~ired length and ~lotted to form a U-~haped lement which is sub~equently imprognated with ~ilicon metal. Another baJic type of ~ilicon carbide igniter i~ that deJcribod in U. S. patent 3,052,814. Thi~ i~ a Jparkplug type igniter a oppo~od to the pure reJistance typo mentioned above and i~
compo~ed of ~ilicon nitride bonded with ~ilicon carbide. Still ~-anothor silicon carbide igniter device i~ de~cribed in U. S.
Patent 3,282,324 aJ part of a compl-to ignition and hoat in~oction -- 1 -- :;

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Slli~On Cc1~2~ ylinc3~r h.lvi~g a !il)ir~l cut ~hich provides a rel~tively ~ l p(~rcfntage of the hot area which radiates directly to the ~nvironment.
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 10 physically reinforce ceramic resistance igniters by such ! ~ -approaches as that described in U. S. Patents 3,372,305 and 3,467,812. 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 spiral configuration is cut in the sintered silicon 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 i9`:
described in U. S. Patent 3,454,345. 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/sïlicon oxynitride mixture is one manufactured and sold by Norton Company, Worcester, Massachusetts, ana its foreign affiliates - -under the trademark CRYSTOLON 63. ;`~
Despite the substantial amount of activity in the ceramic resistance igniter field, the igniters enjoying most 30 widespread use today, for most applications, are still of the ~-.
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~ t~;)r~, In v~,w ~,1 th~ r l)t ~ /3y el-i~is and th~? .~llt ~,f v~rio~s S~llV.~S whieh S~l~)w t~at pi]~t lights consune fr(7m 1() to 15 perc~nt of the to~ s consumed in this cGuntry~ thele i5 obviously a cornpeliing n~ed for an igniter to rcplace the presently used pilot light.
It is, therefore, a principal object of the present invention to provide a ceramic resistance igniter for liquid and gas fuel burning devices which is free of the foregoing deficiencies, and which is physically rugged, heats rapidly, survives hundreds of thousands of heating cycles, is simple electrically and structurally, has low susceptibility to premature burn out, and radiates primarily to the environment .
SUMMARY OF THE INVENTION
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, 0 to 4 percent by weight of silica, 0 to 0.25 percent by weight of iron or iron-based compound~, a maximum of 100 parts per mi,llion of boron and a minor amount, generally not in -~
excess of 0.25 percent, of miscellaneous impurities. The composition also contains a very small ton 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 SiC is necessary to raise the high temperature (e.g.
1000C) resistivity of the igniter to a level on the order of .06 to .26 ohm centimeters. The boron content is preferably kept b~ ow 50 ppm to maintain reasonably low resistivity at low and high temperature, the low resist1vity at room temperature being particularly important from the standpoint of heat up time.
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Thus, in accordance with the present teachings, a ceramic resistance igniter is provided which has a composition which consists essentially of from 95 to 99.9% by weight of silicon carbide, 0.05 to 0.50% by weight of aluminum, 0 to 4%
by weight of silicon oxide, 0 to 0.25% by weight of iron or compounds thereof, a maximum of lO0 parts per million of boron and up to 0.25% by weight of miscellaneous impurities. The composition is sintered and exposed to a nitrogen atmosphere at a temperature from 1500C to 2000C for 15 to 180 minutes.
- lO In accordance with a more specific teaching, a sintered ceramic resistance igniter is provided which a com-position which consists essentially of from 95 to 99.9% by weight of polycrystalline alpha silicon carbide, 0 to 4% by weight of silicon oxide, 0 to lO0 parts per million of boron, 0.05 to 0.5% by weight of aluminum, the igniter has a density of 2.60 to 2.70 gms/cc, has a resistivity at room temperature of from 0.10 to 1.70 ohm centimeters and at 1000C of from 0.06 to 0.26 ohm cen~imeters and has at least 50% of the surface area of the hot zone of the igniter radiating directly to the enviro-ment.

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~ i("~t,~ s i~ r~ ,y o~ ic,l~al ine~ds w~ h l~sult in ~ it,r h~lvil~(3 a controllc~3 dcnsity of f rom ~bollt 2 . 6() t o 2 . 70 ~3r~rns E~(:r cubi c c~-nt imeter. 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 igniter is ideally suited as a fuel : igniter for such devices as gas clothes dryers, in that the ~i 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 20 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 square inch. Finally, the present , ',` . .
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Jniter has an inh~r~nt ability to with~tand at lea~t 200,000 heat-up and cool-down cycle~. This i8 unexpectod in view of the relatively low density of the igniter, but it i~ believed that this re~ults from a combination of chemical compo~ition, proces~ing conditions involved in the fabrication of Qaid igniter, and the high percentage of the heating area which radiates directly to the environment. By the expreJsion "area which radiates directly to the environment" we mean hot a~ea that doe~ not "see" other hot areaJ. Thus the inqide surface of a cylindrical heating element would "see" other hot portionJ of the inside surface (or a hot ~upport element) and would not be considered aq "radiating directly to the ; environment". The "hot" area of the igniter of Figure 1 is the surface of that part of the element of smaller crosJ- -section, that i8, the portion of 8a, 8b, lOa, and lOb of minimum cross-section. In Figure2, about 55~ of the surface of the "hot" area is "outside" surface. To keep the outside ~urface above 50%, the thickness of the igniter should not be greater than twice the width of the legs. From the design 20 of Figure 3, the outside area will always be greater than 50~. -The present igniter is monolithic and self-~upporting, needinq no supporting device such as that required for the succeJqful utilization 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 ignit~rs as set forth above. The most deJirable ~ configuration i~ that of a leg having a hairpin qhape - including terminal connecting end~, becauJe ~his shape presents at least 50 percent of the Jurface area of the hot '`
zone of the igniter to the surrounding environment. With a , - 5 - ;~

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le~s tendency for hot spot~ to develop. Thi~ characteri~tic, plu8 the relatively large cross-section, minimizes 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 TH~ 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. 1.
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 F~gures 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 com~leted by the approximately ;3 20 centrally located slot 12 which traverses from the end of the ,. 3 igniter opposite the terminal connecting ends towards said i end~ but stopping substantially short thereof; and a ~lot 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 lOa and lOb for each leg. In the slots 14 and 16 at the

3. terminal connecting ends thereof it i8 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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.umina based refractory cement. Thi9 i~ ~hown aJ Jmall d~b~
18 and 20. Larger qUantiti~J of refractory cement may be u~od if desired. Without the portion of cement Jo located in ~lot~
14 and 16 there i~ the danger of shorting out or breaking of the igniter should any force be exerted on the thermal connecting ends 4 and 6 ~o 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 cro~-section of their individual elements 8a, 8b, lOa and lOb.
This larger cross-sectional area of these endJ causeJ them to remain relatively cool and causes concentration of the hot zone of the igniter in those portions of the two leg~ in between these ends 22 and 24 and the terminal connecting end~
4 and 6. This 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 surface~ (tho~e parallël to the plane of the drawings) and the outer boundaries of the element would be considered as the applicable areas. The surfaces of the element defining the slots would not be 80 considered since they can radiate directly to their hot facing surfaces.
In a preferred form for gas dryers the present igniter is from 2.125 to 2.625 inches in length, with the i end 22 and 24 of the legs 8 and 10 each having an essentially rectangular cross-sectional area of from 0.020 to 0.039 ; square inch. Elements 8a, 8b, lOa and lOb of legs 8 and 10 :
each prefer bly have a cross-section of from 0.009 to 0.014 ~quare inch, the slots forming said elements are preforably . . ~
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! ~0'~ .(J~ 0 `~ $() I1)~ r~ y ~ ,ible vari~nts en th~ b;~ confi~r.~tion of the prc-sent igniter, on~ ch ~ing that shown in Figure 3 which has terminal --connecting ~nds 26 and 28 and a single hairpin shaped leg 30 comprised of elernents 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 99,9% ~y weight of a 50~O mixture of high purity 3.0 micron silicon carbide and 100F silicon carbide, and 0.05 to . 0 30% by weight of Al203. The preparation of the slip, and ?i ~
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 '~1 the igniter shown in Figure 1 or Figure 3. The length of the mold cavity is 12 inches although obviously said dimension ! -could ~e longer or shorter if desired. The green billet thus cast is allowed to stand in the mold for 10 or 15 , minutes after which it is removed and air dried for 8 to 16 -~ hours at 125 to 150C. To facilitate slicing of the billet into 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 Qua~er Oats Company. Other polymerizable organic material may also be used in place of the foregoing. The impregnation is carried out by immersion 1 of the green billet in the solution. The saturated billet i~ 30 is heat treat~ at about 95C for at least 12 hours after * A Trademark for a furfural~ehyde prepolymer dissolved in - furfuryl alcohol.
*~ A Trademark for phthalic anhydride.

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~ h t.ln~el-a~ ,l to ~ t I')()C and h~ld th~r~ for two hours. Th~? ~ is then .ll]owcd to cool.
The billet is ~liccd into igniter blan~s preferably about 0.135 inch in thickn~ss. 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 2000~C 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, preferably aluminum or an .j ~
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. The cement may be essentially any refractory, electrically insulating cement but ~ ~
the preferred cement is the high alumina type. Thé 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 performanca the igniter should be _ g _ , .~

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10~4Z48 ~emposed of from 97 to 99.9X by weight of polycry~tallino silicon carbide, 0.1 to 0.3X by weight of aluminum add-d a-` aluminum oxide in the original mixture, lo~ than 50 parts - per million of boron, and not more than 0.20X of mi-¢ellaneous impurities. It would al~o appear that an indeterminato amount of nitrogen must be introduced into the structure by ~ubjecting the initial green igniter- firJt bo a standard non-oxidizing type of firing ~tep at about 2200C or above, followed by firing in a nitrogen atmosphere at 1500 to 2000C. Attompt~
to combine these two ~teps into one fail to ffect the de~irod electrical properties in the final igniter. Thi~ i~ believod to bo due to the different rate~ of N2 diffu~ion into the SiC
crystals at the two different temperatures. When N2 i~
present during the initial high temperature firing (2200 to 2400C) it diffuse~ in sufficient quantitie~ into the body of the 8iC~80 that bulk SiC has a low re~istivity both at room and high temperatures thus providing too much current flow at the high temperature (over 6 amps at 132 volts). It i~
believed that when the igniter i~ fired in nitrogon at th-lower temperature (1500 to 2000C) a ~mall but ~ufficientamount of nitrogen diffuses into the surface of th- fino silicon carbide particles, which bridge the larger particles, to lower tho room temperature resistivity of the ignitor without significantly affecting the high temp-rtture resi~tivity. As a result this added N2 lowors the igniter rosponse;tim-, e.g., the time for the igniter to roach tho ~`
desir-d fuel ignition t-mperature.
Some prior art gas and liquid fuel igniters havo ~;
the inherent shortcoming of room temperature re~i~tivities that are too high, and elevated temporature resi-tiviti-~

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iO~4Z48 ~ at are too low for the mo~t ~ffective and efficiont operation. The igniter of the pre~ent invention is free of this problem having a preferred re~i~tivity at room temperature of from 0.15 to O.S 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 resistivitie~ 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 i9 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 undesirable increa~e in the room temperature resistivity caused by the ~-introduction of the aluminum, i.e., the nitrogen decreases ; the room temperature resi-stivty. The resulting igniter thus has a heretofore unknown combination of a relatively high elevated temperature resistivity and a low room temperature -`
resistivity.
m e oxygen content of the finished igniter i~
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 i~ not -- 11 -- ~ -'; ~ . ' , . ": ~' ' " " .
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10~4~48 .etrimental ~o long, a~ it i~ on the ~urface of the fired igniter and not between the SiC grain~ of the igniter where it would introduce a high re~i~tance. In -Qome ca~e~ it may be desirable to oxidize the igniters prior to sale or to apply an oxide coaring on the finished igniter; Yhe~e techniques are known in the art.
Where the expression '~percent" or "%" i~ used in the specification and claimq it iq intended to mean weight percent unleqs clearly states to have ~ome other meaning.

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Claims

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

1. A ceramic resistance igniter having a composition consisting essentially of from 95 to 99.9% by weight of silicon carbide 0.05 to 0.50% by weight of aluminum, 0 to 4% by weight of silicon oxide, 0 to 0.25%
by weight of iron or compounds thereof, a maximum of 100 parts per million of boron, and up to 0.25% by weight of miscellaneous impurities, said composition having been sintered and then exposed to a nitrogen atmosphere at a temperature of from 1500°C to 2000°C for 15 to 180 minutes.

2. The ceramic resistance igniter of Claim 1 having electrical characteristics such that said igniter draws a maximum of 6 amps at 132 volts and has an impact resistance of at least 125 g's.

3. The ceramic resistance igniter of Claim 2 having a response time at 80 volts of 60 seconds or less to attain 1000°C and an operational life of at least 200,000 cycles.

4. A monolithic ceramic resistance igniter having a composition consisting essentially of from 95 to 99.9% by weight of polycrystalline silicon carbide, 0 to 4% by weight of silicon oxide, 0 to 0.25% by weight of iron or compounds thereof, 0 to 50 parts per million of boron, and up to 0.25%
by weight of miscellaneous impurities; said silicon carbide containing from 0.05 to 0.50% by weight of aluminum in the crystal lattice thereof and nitrogen being introduce into said crystal lattice by subjecting said composition to an atmosphere of nitrogen at a temperature of from 1500°C to 2000°C for 15 to 180 minutes.

5. The ceramic resistance igniter of Claim 4 having a room temperature resistivity of 0.10 to 1.70 ohms centimeter and a resistivity of 1000°C of 0.06 to 0.26 ohm centimeter.

6. A sintered ceramic resistance igniter having a composition consisting essentially of from 97 to 99.9% by weight of polycrystalline silicon carbide, 0.1 to 0.3% by weight of aluminum contained in the crystal lattice of said silicon carbide, 0 to 100 parts per million of boron, and from 0 to 0.2% by weight of miscellaneous impurities, said composition having been doped with nitrogen by heating at 1500°C to 2000°C for 15 to 180 minutes; said ceramic igniter having a room temperature resistivity of 0.15 to 0.5 ohm centimeter, a resistivity at 1800°F of at least 0.1 ohm centimeter, a response time of 10 to 60 seconds to attain 1000°C, an operational life of at least 200,000 cycles, an impact resistance of at least 125 g's and the further property that said igniter draws a maximum of 6 amps at 132 volts.

7. A sintered ceramic resistance igniter having a composition consisting essentially of from 95 to 99.9% by weight of polycrystalline alpha silicon carbide, 0 to 4% by weight of silicon oxide, 0 to 100 parts per million of boron, 0.05 to 0.5% by weight of aluminum, said composition having first been preformed and fired at 2250°C to 2450°C in an inert atmosphere followed by firing in a nitrogen atmosphere at from 1500°C to 2000°C for 15 to 180 minutes; said igniter having a density of 2.60 to 2.70 gms/cc, and having resistivity at room temperature of from 0.10 to 1.70 ohm centimeters and at 1000°C of from 0.06 to 0.26 ohm centimeter.

8. A sintered ceramic resistance igniter having a composition consisting essentially of from 95 to 99.9% by weight of polycrystalline alpha silicon carbide, 0 to 4% by weight of silicon oxide, 0 to 100 parts per million of boron, 0.05 to 0.5% by weight of aluminum, said igniter having a density of 2.60 to 2.70 gms/cc, having a resistivity at room temperature of from 0.10 to 1.70 ohm centimeters and at 1000°C of from 0.06 to 0.26 ohm centimeter, and having at least 50% of the surface area of the hot zone of the igniter radiating directly to the environment.

9. A ceramic resistance igniter having a composition consiting of from 95 to 99.9% by weight of silicon carbide, 0.05 to 0.5% by weight of aluminum, 0.04 to 0.1% by weight of oxygen, 0 to 4% by weight of silicon oxide, 0 to 0.25% by weight of iron or compounds thereof, a maximum of 100 parts per million of boron, said composition having been exposed to a nitrogen atmosphere at a temperature of from 1500°C to 2000°C for 15 to 180 minutes.

10. The ceramic resistance igniter of Claim 9 including terminal connecting ends, said ends having been treated with an aluminum alloy.