Electric heating – Heating devices – Resistive element: igniter type
Reexamination Certificate
2000-01-25
2001-08-21
Jeffery, John A. (Department: 3742)
Electric heating
Heating devices
Resistive element: igniter type
C219S552000, C219S540000, C219S260000
Reexamination Certificate
active
06278087
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to ceramic igniters and improved methods for making the igniters.
2. Background
Ceramic materials have enjoyed great success as igniters in gas fired furnaces, stoves, clothes dryers, and other devices that require ignition of gaseous fuel. Ceramic igniter production requires constructing 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™, available from the Norton Igniter Products of Milford, N.H., is designed for 12 volt through 120 volt applications and has a composition comprising aluminum nitride (“AlN”), molybdenum disilicide (“MoSi
2
”), and silicon carbide (“SiC”).
U.S. Pat. No. 5,786,565 to Willkens et al. (the “565 patent”) discloses highly useful ceramic igniters that comprise a) a pair of electrically conductive portions, each portion having a first end, b) a resistive hot zone disposed between and in electrical connection with each of the first ends of the electrically conductive portions, the hot zone having an electrical path length of less than 0.5 cm, and c) an electrically non-conductive heat sink material contacting the hot zone.
A variety of performance properties are required of ceramic igniter systems, including high speed (low time to heat from room temperature to design temperature) and sufficient robustness to operate for extended periods without replacement. Many conventional igniters, however, do not consistently meet such requirements. It thus would be desirable to have new ceramic igniter systems.
SUMMARY OF THE INVENTION
We have now discovered new, highly useful ceramic igniters that can exhibit exceptional performance properties, including long operational lives.
It was surprisingly found that the ceramic igniters disclosed in the above discussed '565 patent sometimes fail due to “burnout” of the igniter hot zone region. As mentioned above, the '565 patent discloses an igniter having a relatively short hot zone electrical path length of less than 0.5 cm. Without being bound by theory, it is believed that during operation of such an igniter, the power density generated at high line voltage gives rise to a high temperature gradient. This high temperature gradient is believed to result in accelerated oxidation of a localized region of the igniter hot zone, which may result in premature failure of the device.
In contrast, igniters of the invention can provide a more diffuse power density throughout a hot zone region, thereby avoiding undesirable temperature gradients in isolated hot zone areas while providing tip heating.
More specifically, in one aspect of the invention, ceramic igniters are provided that comprise: a) a pair of electrically conductive portions, each portion having a first end; and b) a resistive hot zone disposed between and in electrical connection with each of the first ends of the electrically conductive portions, wherein the hot zone has an electrical path length of between 0.51 cm and 2 cm.
Preferred igniters of the invention have a hot zone electrical path length of between 0.6 cm and 1.5 cm, more preferably from 0.6 cm to about 1.2 cm, still more preferably from about 0.7 cm to 0.9 cm. As used herein, the term “electrical path length” designates the length of the shortest path taken by an electrical current through the igniter hot zone region when an electrical potential is applied to the conductive ends of the igniter.
It is believed that such hot zone lengths can effectively diffuse power density throughout the hot zone region, without producing isolated temperature gradients which can lead to premature igniter degradation and failure. Moreover, the electrical path length limits (up to about 2 cm) result in effective heating and short times to ignition temperature, without the need for excessive power input into the system.
We also have found that preferably the hot zone region has a non-linear geometry, e.g. a substantially U-shaped design, whereby the hot zone extends without interruption across the top igniter width and then along a portion of each side of the igniter length. It is believed such non-linear designs can more effectively diffuse or reduce the power density within the hot zone region, relative to a comparable system having a linear hot zone.
Igniters of the invention preferably also have an electrically non-conductive portion (heat sink) in contact with the hot zone region. In particular, the non-conductive portion is preferably interposed or inserted between the electrically conductive portions and in contact with the hot zone region.
We also have found that preferably the bridge height of the hot zone (width of the hot zone in a rectangular igniter, discussed further below) is preferably at least about 0.05 cm, more preferably at least about 0.06 cm. A hot zone bridge height of from 0.05 cm to 0.4 cm is generally preferred; and a hot zone bridge height of from 0.06 cm to about 0.3 cm is more preferred.
Preferably, hot zones of igniters of the invention will contain a sintered composition containing a conductive material and an insulating material, and typically will further contain a semiconductor material. Conductive or cold zone portions of igniters of the invention will contain a sintered composition of similar components, with relatively higher concentrations of conductive material.
Igniters of the invention can be suitably operate over a wide range of voltages, including nominal voltages of 6, 8, 12, 24 and 120.
Further provided arc new methods for producing igniters of the invention, which include manufacture of a plurality of igniters from a single billet material, enabling significantly more efficient igniter production. Preferred methods of the invention for forming a ceramic igniter comprise a) providing an electrically conductive ceramic body that comprises a plurality of affixed igniter elements; b) inserting into each element an electrically non-conductive material; and c) densifying the plurality of igniter elements.
Other aspects of the invention are disclosed infra.
REFERENCES:
patent: 3681737 (1972-08-01), Magnusson et al.
patent: 3974106 (1976-08-01), Richerson
patent: 5085804 (1992-02-01), Washburn
patent: 5786565 (1998-07-01), Willkens et al.
patent: 5820789 (1998-10-01), Willkens et al.
patent: 5892201 (1999-04-01), Croucher et al.
patent: 6002107 (1999-12-01), Willkens et al.
patent: 6028292 (2000-02-01), Willkens et al.
Hamel Scott M.
Olson James M.
Willkens Craig A.
Corless Peter F.
Crosby Mike W.
Edwards & Angell LLP
Jeffery John A.
Saint-Gobain Industrial Ceramics, Inc.
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