High voltage ceramic igniter

Details High voltage ceramic igniter High voltage ceramic igniter

1997-04-03

1998-10-13

Marcantoni, Paul

Compositions

Electrically conductive or emissive compositions

Carbide containing

219270, 219553, 501 89, 501 91, 501 92, 501 93, 501 96, F23Q 722

Patent

active

058207899

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

Ceramic materials have enjoyed great success as igniters in gas fired furnaces, stoves and clothes dryers. Ceramic igniter production requires constricting an electrical circuit through a ceramic component, a portion of which is highly resistive and rises in temperature when electrified by a wire lead. One conventional igniter, the Mini-Igniter.TM., available from the Norton Company of Milford, N.H., is designed for 8 volt though 48 volt applications and has a composition comprising aluminum nitride ("AlN"), molybdenum disilicide ("MoSi.sub.2 "), and silicon carbide ("SiC"). As the attractiveness of the Mini-Igniter.TM. has grown, so has the number of applications requiring small igniters with rated voltages exceeding the conventional 24 volts. However, when used in such applications, the 24V Mini-Igniter.TM. is subject to temperature runaway and so requires a transformer in the control system to step down from conventional line voltage (i.e., 120 volts). Accordingly, there is a need for small, higher voltage igniters designed for either 120 or 230 line voltage applications which do not require an expensive transformer but still possess the following requirements set by the appliance and heating industries to anticipate variation in line voltage:


______________________________________ Time to design temperature <5 sec Minimum temperature at 85% of design voltage 1100.degree. C. Design temperature at 100% of design voltage 1350.degree. C. Maximum temperature at 110% of design voltage 1500.degree. C. Hot-zone Length <1.5" Power (W) 65-100. ______________________________________
Because the amperage used for these high voltage applications will likely be comparable to that used in 24 volt applications (i.e., about 1.0 amp), the increased voltage will likely be realized by increasing the resistance of the igniter.
The resistance of any body is generally governed by the equation Rs=Ry.times.L/A, igniters is already about 1.2 inches, it can not be increased significantly without reducing its commercial attractiveness. Similarly, the cross-sectional area of the smaller igniter, between about 0.0010 and 0.0025 square inches, will probably not be decreased for manufacturing reasons. Therefore, it appears that the desired increase in the resistance of the small, high voltage igniters will be realized by increasing its resistivity.
Because the Mini-Igniter.TM. is comprised of one highly resistive material (AlN), one moderately resistive material (SiC), and one highly conductive material (MoSi.sub.2), one obvious avenue for increasing the igniter's resistivity is to reduce its MoSi.sub.2 and SiC contents while adding AlN. However, one trial composition (containing about 76 volume percent ("v/o" or "vol %") AlN, 9 v/o MoSi.sub.2, and 15 v/o SiC) was found to be unsatisfactory in that it not only was slow to reach the design temperature (due to low MoSi.sub.2 levels), it also possessed a significant negative temperature coefficient of resistivity ("NTCR") and so was subject to temperature runaway above about only 1350.degree. C. A NTCR means that as the temperature of the igniter increases, its resistance decreases. This decrease makes the igniter hotter than it would be if the resistance was constant. If the NTCR is too extreme, the igniter is slow and cool at 85% and unstable at 110% of the rated voltage. Indeed, such an igniter may exhibit runaway at less than the 110% rating, in which case the amperage and temperature continue to rise even at a constant voltage until failure (burnout) occurs. Rather, it is preferable for the igniters to possess a positive temperature coefficient of resistance ("PTCR") or a moderate NTCR. Whereas a ceramic having a PTCR increases in resistivity when its temperature is increased from 1000.degree. C. to 1400.degree. C., a ceramic having a moderate NTCR decreases in resistivity by less than 25% when its temperature is increased from 1000.degree. C. to 1400.degree. C. Either a PTCR or a moderate NTCR would allow for a more gradual

REFERENCES:
patent: 5514630 (1996-05-01), Willkens et al.

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