Ceramic heating elements

Abstract

New ceramic heating elements are provided that have a recessed portion for receiving an electrical lead. Such ceramic heating elements can provide a reduced cross-sectional dimension across element regions that interface with electrical lead(s) as well as a more secure engagement of an electrical lead. Heating elements can be highly useful in a variety of application, including e.g. for fuel ignition for gas cooking appliances as well as vehicular glow plugs.

Description

The present application claims the benefit of U.S. provisional application No. 61/009,381 filed Dec. 29, 2007, which is incorporated by reference herein in its entirety.

BACKGROUND

1. Field of the Invention

In one aspect, ceramic heating elements are provided that have a recessed portion for receiving an electrical lead. Such ceramic heating elements can more secure engagement of the heating element with an electrical lead. In a further aspect, ceramic heating elements are provided that have a conductive zone of substantially equal or increasing cross-section along a length of the element. The present heating elements are useful in a variety of application, including e.g. for fuel ignition for gas cooking appliances as well as vehicular glow plugs that have strict space constraints.

2. Background

Ceramic materials have enjoyed great success as heating elements (includes igniters) in e.g. gas-fired furnaces, stoves and clothes dryers. Ceramic heating element production includes 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. See, for instance, U.S. Patent Publication 2006/0131295 and U.S. Pat. Nos. 6,028,292; 5,801,361; 5,405,237; and 5,191,508.

Typical igniters have been generally rectangular-shaped elements with a highly resistive “hot zone” at the heating element tip with one or more conductive “cold zones” providing to the hot zone from the opposing heating element end. One currently available igniter, the Mini-Heating element, available from 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 (“MoSi₂”), and silicon carbide (“SiC”).

Since these heating elements are resistively heated, each of its ends must be electrically connected to a conductive lead, typically a copper wire lead. Ceramic heating elements have been connected to electrical contact by direct welding or brazing to wire or by brazing to an intermediate metal lead frame which is then welded or brazed to wire. See U.S. Pat. Nos. 7,241,975 and 6,933,471.

For heating elements that have cylindrical or other non-rectangular cross-section configurations, such attachment of electrical contacts can result in an increase in the diameter of the insulating section (where the electrical leads interface with the heating element). Such increased dimensions can be problematic for a number of applications, such as appliances or automotive environments where tight specifications may exists for the outer dimensions of the heating element block of the heating element. Additionally, separation of the electrical lead from the heating element can result in device failure.

It thus would be desirable to have new heating element systems. It would be particularly desirable to have new heating elements that have cylindrical or other non-rectangular cross-sectional configurations and that have comparatively narrow cross-sectional dimensions across regions that interface with electrical contacts. It would be further desirable to have new heating elements that have secure engagement of an electrical lead to the heating element.

SUMMARY

In one aspect, ceramic heating elements are provided that have a recessed portion for receiving an electrical lead. Such ceramic heating elements can provide a reduced cross-sectional dimension across element regions that interface with electrical lead(s) as well as a more secure engagement of the lead(s) to the heating device. Consequently, heating elements can be highly useful in a variety of applications, including e.g. for fuel ignition for gas cooking appliances as well as vehicular glow plugs.

In a preferred aspect, a heating element may comprise at least one recess (e.g. hole) that can receive an electrical lead, where the recess is positioned at a bottom face of the heating element, although the recess also suitably may be situated in other regions of a heating element, such as a side portion of an element.

In certain aspects, the recess may be tapered, e.g. inwardly tapered (decreasing cross-sectional area), which can further secure an engagement of an electrical lead with the heating element.

Preferably, a conductive zone (i.e. region of relatively low resistivity) of the heating element forms at least a portion of the wall surface of the recess. As a consequence, power from an electrical lead nested within the recess can flow through the heating element via such conductive zone.

In a further aspect, ceramic heating elements are provided that have a conductive zone of substantially equal or increasing cross-section from a proximal end of the heating element along the element length. In particular, the cross-sectional dimension of the conductive zone that forms at least a portion of the wall surface of the recess for receiving an electrical lead will have a cross-sectional dimension at a portion that contacts the recess that is substantially equal to or greater than the cross-section dimension of that same conductive zone further along that conductive zone's length.

It has been found that such conductive zone configurations can avoid undesired warpage upon sintering of the heating element.

Preferred heating elements of the invention have an outer or substantially U-shaped or L-shaped electrical path, i.e. where the electrical path extends from (i) an outer conductive zone to (ii) an hot or ignition zone and then through (iii) a second outer conductive zone. Such an outer or U-shaped or L-shaped electrical path is different than and distinguished from a co-axial path that contains an interior first conductive zone that is encased by an outer conductive zone.

Particularly preferred heating elements of the invention may have cylindrical or other non-rectangular cross-section configurations. In a preferred aspect, preferred heating elements of the invention have a rounded cross-sectional shape along at least a portion of the heating element length (e.g., the length extending from where an electrical lead is affixed to the heating element to a resistive hot zone). More particularly, preferred heating elements may have a substantially oval, circular or other rounded cross-sectional shape for at least a portion of the heating element length, e.g. at least about 10 percent, 40 percent, 60 percent, 80 percent or 90 percent of the heating element length, or the entire heating element length. A substantially circular cross-sectional shape that provides a rod-shaped heating element is particularly preferred. The invention also provides heating elements that have non-rounded or non-circular cross-sectional shapes for at least a portion of the heating element length.

Preferred heating elements comprise multiple regions of differing electrical resistivity, i.e. preferred ceramic heating elements may comprise a first conductive zone, a resistive hot zone, and a second conductive zone, all in electrical sequence. Heating elements of the invention may have a variety of electrical configurations. As discussed, in preferred systems, the heating element may have a substantially U-shaped electrical path, e.g. where opposing conductive zones are separated by an interposed hot or ignition zone.

Ceramic heating elements of the invention can be employed at a wide variety of nominal voltages, including nominal voltages of 6, 8, 10, 12, 24, 120, 220, 230 and 240 volts.

As mentioned, the heating elements of the invention are useful for ignition in a variety of devices and heating systems. More particularly, heating systems are provided that comprise a sintered ceramic heating element as described herein. Specific heating systems include appliances such as gas cooking units, heating units for commercial and residential buildings. Vehicular (e.g. automotive, watercraft) glow plugs also provided that comprise a sintered ceramic heating element as described herein.

Other aspects of the invention are disclosed infra.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 through 4 show schematically preferred heating element systems;

FIG. 5 shows in a cut-away view a further preferred heating element; and

FIG. 6 (which includes FIGS. 6A-6C) shows plan views of a further preferred heating element. FIG. 6B is a view sliced along line B-B of FIG. 6A, and FIG. 6C is a view sliced along line C-C of FIG. 6A.

DETAILED DESCRIPTION

As discussed above, in one aspect, ceramic heating element systems are provided that include new configurations for mating of electrical lead components. In a further aspect, ceramic heating element systems are provided that include conductive region(s) that can provide notable benefits, including reduced warpage upon sintering. Preferred ceramic heating elements of the invention having a substantially outer or U-shaped or L-shaped electrical path.

Referring now to the drawings, FIGS. 1 through 4 show in a schematic cut-away view a preferred heating element 10 where conductive zones 12A and 12B mate with interposed hot (ignition) zone 14 to thereby form an electrical pathway. As can be seen, outer conductive zones 12A and 12B together with interposed hot (ignition) zone form a substantially U-shaped or L-shaped electrical pathway that traverses an outer or perimeter portion of the heating element 10.

Conductive zone 12A defines in part recess 16 that engages with electrical lead 18 during use of element 10. In preferred systems, heating element 10 may be encased with a metal fixture 20 and affixed therethrough, e.g. via a metal braze 22. The interior region 24 encased by conductive zones 12A, 12B and ignition zone 14 may be void or may have an insulative (heat sink) composition.

It also may be preferred to include an exterior insulative layer 25 on heating element portions that contact metal fixture 20. Such an exterior insulative layer may be suitably formed by dip coating or other application of an insulative ceramic composition.

FIG. 5 shows in a partial cut-away view a particularly preferred heating element where conductive zone 12A forms a portion of walls 16A of recess 16 that receives an electrical lead. In this preferred configuration, conductive zone 12A and 12B mate with interposed hot (ignition) zone 14 to thereby form an electrical pathway. The heating element also includes central insulator region 24 with outer insulator 25A that encases at least a portion of the first conductive zone 12A as well as insulator 25B that encases at least a portion of conductive zone 12B.

As depicted in FIG. 5, in a preferred configuration, recess 16 contacts conductive zone 12A whereby walls 16A of the recess are formed by conductive zone 12A. In other less preferred embodiments, the entire surface of the walls that define recess 16 are part of the conductive zone 12A.

In the depicted preferred configuration, by only a portion of the walls that define recess 16 being part of the conductive zone 12A, that conductive zone 12A can have a substantially equal or increasing cross-section along a length of the element. Thus, as shown in FIG. 5, the cross-section dimension of conductive zone 12A at the element proximal end (as shown by dimension a) is substantially the same as or less than the cross-sectional dimension of that conductive zone for the substantial portion of that zone's length (length y as depicted in FIG. 5), e.g. for at least about 50, 60, 70, 80, 90, 95 or even 100 percent of that zone's length y as shown in FIG. 5. Thus, as shown in FIG. 5, dimension a will be the about the same as or less than the depicted dimensions a′ or a″. As referred to herein, references to the first conductive zone having “substantially the same” or “about the same” (or other similar phrase) cross-section along its length means that the cross-section dimension (such as a relative to a′ and a″ as shown in FIG. 5) does not vary by more than 5, 10 or 20 percent. In certain aspects, references to the cross-section dimension of the conductive zone does not include the interface of that conductive zone and the mating ignition zone.

As discussed above, this configuration of the first conductive zone cross-sectional dimension has provided notable benefits, including reduced undesired warpage upon sintering of the heating element.

FIG. 5 also shows a preferred configuration of recess 16, where recess 16 inwardly tapers, i.e. recess 16 has a decreased cross-section along its length. Such a tapered configuration can provide more secure engagement of an electrical lead nested within the recess.

In use, an electrical lead is nested within recess 16 and provides power through the depicted electrical pathway (see pathway as shown by arrows in FIG. 5) that extends from conductive zone 12A to hot (ignition) zone 14. Interior ceramic insulator 24 can provide further mechanical strength to the heating element.

FIGS. 6A through 6C show a further heating element 10 in a preferred configuration where only a portion of recess 16 contacts conductive zone 12A. That is, only a portion of the surface that defines recess 16 is a component of conductive zone 12A. In the system depicted in FIG. 6, the balance of the walls defining recess 16 is a component of insulator region 24. In exemplary preferred systems, up to about 20, 30, 40, 50, 60, 70, 80 or 90 percent of the surface area of the walls of recess 16 may be a component of the component of conductive zone 12A, with the balance of the surface area of the walls of recess 16A being a component of the central or interior insulator region 24.

As with the element of FIG. 5, in use, an electrical lead is nested within recess 16 and provides power through the depicted electrical pathway from conductive zone 12A, through hot (ignition) zone 14 and then through conductive zone 12B to provide a substantially U-shaped or L-shaped outer electrical pathway.

As shown in FIGS. 5 and 6A, in preferred systems, the length of first conductive zone 12A that contacts recess 16 is greater than the length of second conductive zone 12B that is on the distal side of resistive (ignition) zone 14. For example, in certain preferred configurations, the length of second conductive zone 12B (shown as y′ in FIG. 5) is no more than 90, 80, 70, 60, 50, 40, 30, 20 or even 10 percent the length of the first conductive zone 12A (length y is FIG. 5). In certain preferred configurations, second conductive zone 12B does not contain or contact recess 16, as shown in FIGS. 5, 6A and 6B.

As discussed above, and exemplified in FIGS. 5 and 6, preferably, at least a substantial portion of the heating element length has a rounded cross-sectional shape along at least a portion of the heating element length, such as length a shown in FIG. 6A. FIGS. 5 and 6 depict a particularly preferred configuration where heating element 10 has a substantially circular cross-sectional shape for about the entire length of the heating element to provide a rod-shaped heating element. However, as discussed above, preferred systems also include those where only a portion of the heating element has a rounded cross-sectional shape, such as where up to about 10, 20, 30, 40, 50, 60, 70 80 or 90 percent of the heating element length, e.g. where in such designs, the balance of the heating element length may have a profile with exterior edges.

Also, while a rounded cross-sectional shape is preferred for many applications, preferred heating elements of the invention also may have a non-rounded or non-circular cross-sectional shape for at least a portion of the heating element length, e.g. where up to or at least about 10, 20, 30, 40, 50, 60, 70 80 or 90 percent of the heating element length (as exemplified by heating element length a in FIG. 6A) has a cross-sectional shape that is non-rounded or non-circular, or where the entire heating element length (as a heating element length is exemplified by length a in FIG. 6A) has a cross-sectional shape that is non-rounded or non-circular.

Dimensions of heating elements of the invention may vary widely and may be selected based on intended use of the heating element. For instance, the length of a preferred heating element (length a in FIG. 6A) suitably may be from about 0.5 to about 5 cm, more preferably from about 1 about 3 cm, and the heating element maximum cross-sectional width may suitably be from about (width b in FIG. 6A) suitably may be from about 0.2 to about 3 cm.

Similarly, the lengths of the conductive and hot zone regions also may suitably vary. Preferably, the length of first conductive zone (length c in FIG. 6A and y in FIG. 5) of a heating element depicted in FIG. 6A may be from 0.2 cm to 2, 3, 4, or 5 more cm. More typical lengths of the first conductive zone will be from about 0.5 to about 5 cm. The height of a hot zone (length d in FIG. 6A) may be from about 0.1 to about 2, 3, 4 or 5 cm, with a total hot zone electrical path length (shown as the dashed line in FIG. 6A) of about 0.5 to 5 or more cm, with a total hot zone path length of up to about 0.5 to 1, 2 or 3 cm generally preferred.

In preferred systems, the hot or resistive zone of a heating element of the invention will heat to a maximum temperature of less than about 1450° C. at nominal voltage; and a maximum temperature of less than about 1550° C. at high-end line voltages that are about 110 percent of nominal voltage; and a maximum temperature of less than about 1350° C. at low-end line voltages that are about 85 percent of nominal voltage.

A variety of compositions may be employed to form a heating element of the invention. Generally preferred hot zone compositions comprise at least three components of 1) conductive material; 2) semiconductive material; and 3) insulating material. Conductive (cold) and insulative (heat sink) regions may be comprised of the same components, but with the components present in differing proportions. Typical conductive materials include e.g. molybdenum disilicide, tungsten disilicide, nitrides such as titanium nitride, and carbides such as titanium carbide. Typical semiconductors include carbides such as silicon carbide (doped and undoped) and boron carbide. Typical insulating materials include metal oxides such as alumina or a nitride such as AlN and/or Si₃N₄.

As referred to herein, the term electrically insulating material indicates a material having a room temperature resistivity of at least about 10¹⁰ ohms-cm. The electrically insulating material component of heating elements of the invention may be comprised solely or primarily of one or more metal nitrides and/or metal oxides, or alternatively, the insulating component may contain materials in addition to the metal oxide(s) or metal nitride(s). For instance, the insulating material component may additionally contain a nitride such as aluminum nitride (AlN), silicon nitride, or boron nitride; a rare earth oxide (e.g. yttria); or a rare earth oxynitride. A preferred added material of the insulating component is aluminum nitride (AlN).

As referred to herein, a semiconductor ceramic (or “semiconductor”) is a ceramic having a room temperature resistivity of between about 10 and 10⁸ ohm-cm. If the semiconductive component is present as more than about 45 v/o of a hot zone composition (when the conductive ceramic is in the range of about 6-10 v/o), the resultant composition becomes too conductive for high voltage applications (due to lack of insulator). Conversely, if the semiconductor material is present as less than about 10 v/o (when the conductive ceramic is in the range of about 6-10 v/o), the resultant composition becomes too resistive (due to too much insulator). Again, at higher levels of conductor, more resistive mixes of the insulator and semiconductor fractions are needed to achieve the desired voltage. Typically, the semiconductor is a carbide from the group consisting of silicon carbide (doped and undoped), and boron carbide. Silicon carbide is generally preferred.

As referred to herein, a conductive material is one which has a room temperature resistivity of less than about 10⁻² ohm-cm. If the conductive component is present in an amount of more than 35 v/o of the hot zone composition, the resultant ceramic of the hot zone composition, the resultant ceramic can become too conductive. Typically, the conductor is selected from the group consisting of molybdenum disilicide, tungsten disilicide, and nitrides such as titanium nitride, and carbides such as titanium carbide. Molybdenum disilicide is generally preferred.

In general, preferred hot (resistive) zone compositions include (a) between about 50 and about 80 v/o of an electrically insulating material having a resistivity of at least about 10¹⁰ ohm-cm; (b) between about 5 and about 45 v/o of a semiconductive material having a resistivity of between about 10 and about 10⁸ ohm-cm; and (c) between about 5 and about 35 v/o of a metallic conductor having a resistivity of less than about 10⁻² ohm-cm. Preferably, the hot zone comprises 50-70 v/o electrically insulating ceramic, 10-45 v/o of the semiconductive ceramic, and 6-16 v/o of the conductive material. A specifically preferred hot zone composition for use in heating elements of the invention contains 10 v/o MoSi₂, 20 v/o SiC and balance AlN or Al₂O₃.

As discussed, heating elements of the invention contain a relatively low resistivity cold zone region in electrical connection with the hot (resistive) zone and which allows for attachment of wire leads to the heating element. Preferred cold zone regions include those that are comprised of e.g. AlN and/or Al₂O₃ or other insulating material; SiC or other semiconductor material; and MoSi₂ or other conductive material. However, cold zone regions will have a significantly higher percentage of the conductive and semiconductive materials (e.g., SiC and MoSi₂) than the hot zone. A preferred cold zone composition comprises about 15 to 65 v/o aluminum oxide, aluminum nitride or other insulator material; and about 20 to 70 v/o MoSi₂ and SiC or other conductive and semiconductive material in a volume ratio of from about 1:1 to about 1:3. For many applications, more preferably, the cold zone comprises about 15 to 50 v/o AlN and/or Al₂O₃, 15 to 30 v/o SiC and 30 to 70 v/o MoSi₂. For ease of manufacture, preferably the cold zone composition is formed of the same materials as the hot zone composition, with the relative amounts of semiconductive and conductive materials being greater.

A specifically preferred cold zone composition for use in heating elements of the invention contains 20 to 35 v/o MoSi₂, 45 to 60 v/o SiC and balance either AlN and/or Al₂O₃.

For any of the ceramic compositions (e.g. insulator, conductive material, semiconductor material, resistive material), the ceramic compositions may comprise one or more different ceramic materials (e.g. SiC, metal oxides such as Al₂O₃, nitrides such as AlN, Mo₂Si₂ and other Mo-containing materials, SiAlON, Ba-containing material, and the like). Alternatively, distinct ceramic compositions (i.e. distinct compositions that serve as insulator, conductor and resistive (ignition) zones in a single heating element) may comprise the same blend of ceramic materials (e.g. a binary, ternary or higher order blend of distinct ceramic materials), but where the relative amounts of those blend members differ, e.g. where one or more blend members differ by at least 5, 10, 20, 25 or 30 volume percent between the respective distinct ceramic compositions.

A heat sink or insulator may suitably mate with a conductive zone or a hot zone, or both. Preferably, a sintered insulator region has a resistivity of at least about 10¹⁴ ohm-cm at room temperature and a resistivity of at least 10⁴ ohm-cm at operational temperatures and has a strength of at least 150 MPa. Preferably, an insulator region has a resistivity at operational (ignition) temperatures that is at least 2 orders of magnitude greater than the resistivity of the hot zone region. Suitable insulator compositions comprise at least about 90 v/o of one or more aluminum nitride, alumina and boron nitride. A specifically preferred insulator composition of an heating element of the invention consists of 60 v/o AlN; 10 v/o Al₂O₃; and balance SiC. Another preferred heat sink (insulator) composition for use with an heating element of the invention contains 80 v/o AlN and 20 v/o sic.

For certain systems, it may be desirable to include a power booster or enhancement zone of intermediate resistance in the electrical circuit pathway between the most conductive portions of that pathway and the highly resistive (hot) regions of that pathway. Such booster zones of intermediate resistance are described in U.S. Patent application Publication 2002/0150851 to Willkens. Generally, booster zones will have a positive temperature coefficient of resistance (PTCR) and an intermediate resistance that will permit i) effective current flow to a hot zone, and ii) some resistance heating of the booster region during use of the igniter, although preferably the booster zone will not heat to as high temperatures as the hot zone during use of the heating element.

If employed in a heating element, preferred booster zone compositions may comprise the same materials as the conductive and hot zone region compositions, e.g. preferred booster zone compositions may comprise e.g. AlN and/or Al₂O₃, or other insulating material; SiC or other semiconductor material; and MoSi₂ or other conductive material. A booster zone composition typically will have a relative percentage of the conductive and semiconductive materials (e.g., SiC and MoSi₂) that is intermediate between the percentage of those materials in the hot and cold zone compositions. A preferred booster zone composition comprises about 60 to 70 v/o aluminum nitride, aluminum oxide, or other insulator material; and about 10 to 20 v/o MoSi₂ or other conductive material, and balance a semiconductive material such as SiC. A specifically preferred booster zone composition for use in igniters of the invention contains 14 v/o MoSi₂, 20 v/o SiC and balance v/o Al₂O₃. A specifically preferred booster zone composition for use in igniters of the invention contains 17 v/o MoSi₂, 20 v/o SiC and balance Al₂O₃. A further specifically preferred booster zone composition for use in igniters of the invention contains 14 v/o MoSi₂, 20 v/o SiC and balance v/o AlN. A still farther specifically preferred booster zone composition for use in igniters of the invention contains 17 v/o MoSi₂, 20 v/o SiC and balance AlN.

The processing of the ceramic component (i.e. green body and sintering conditions) and the preparation of the heating element from the densified ceramic can be done by conventional methods and as discussed above. See U.S. Pat. No. 5,786,565 to Wilkens and U.S. Pat. No. 5,191,508 to Axelson et al.

A preferred fabrication method includes use of injection molding techniques. Thus, for instance, a base element may be formed by injection introduction of a ceramic material having a first resistivity (e.g. ceramic material that can function as an insulator or heat sink region) into a mold element that defines a desired base shape such as a rod shape. The base element may be removed from such first mold and positioned in a second, distinct mold element and ceramic material having differing resistivity—e.g. a conductive ceramic material—can be injected into the second mold to provide conductive region(s) of the igniter element. In similar fashion, the base element may be removed from such second mold and positioned in a yet third, distinct mold element and ceramic material having differing resistivity—e.g. a resistive hot zone ceramic material—can be injected into the third mold to provide resistive hot or ignition region(s) of the igniter element.

Alternatively, rather than such use of a plurality of distinct mold elements, ceramic materials of differing resistivities may be sequentially advanced or injected into the same mold element. For instance, a predetermined volume of a first ceramic material (e.g. ceramic material that can function as an insulator or heat sink region) may be introduced into a mold element that defines a desired base shape and thereafter a second ceramic material of differing resistivity may be applied to the formed base.

Ceramic material may be advanced (injected) into a mold element as a fluid formulation that comprises one or more ceramic materials such as one or more ceramic powders.

For instance, a slurry or paste-like composition of ceramic powders may be prepared, such as a paste provided by admixing one or more ceramic powders with an aqueous solution or an aqueous solution that contains one or more miscible organic solvents such as alcohols and the like. A preferred ceramic slurry composition for extrusion may be prepared by admixing one or more ceramic powders such as MoSi₂, Al₂O₃, and/or AlN in a fluid composition of water optionally together with one or more organic solvents such as one or more aqueous-miscible organic solvents such as a cellulose ether solvent, an alcohol, and the like. The ceramic slurry also may contain other materials e.g. one or more organic plasticizer compounds optionally together with one or more polymeric binders.

A wide variety of shape-forming or inducing elements may be employed to form an igniter element, with the element of a configuration corresponding to desired shape of the formed igniter. For instance, to form a rod-shaped element, a ceramic powder paste may be injected into a cylindrical die element. To form a stilt-like or rectangular-shaped igniter element, a rectangular die may be employed.

After advancing ceramic material(s) into a mold element, the defined ceramic part suitably may be dried e.g. in excess of 50° C. or 60° C. for a time sufficient to remove any solvent (aqueous and/or organic) carrier.

Thereafter, the heating element may be further densified (e.g. to greater than 95, 96, 97, 98 or 99 percent) by thermal treatment such as in excess of 1500° C., 1600° C., 1700° C. or 1800° C. A single or multiple thermal treatments may be conducted as desired to achieve final densities.

Heating elements of the invention may be used in many applications, including gas phase fuel ignition applications such as furnaces and cooking appliances, baseboard heaters, boilers, and stove tops. In particular, an heating element of the invention may be used as an ignition source for stove top gas burners as well as gas furnaces.

As discussed above, heating elements of the invention will be particularly useful where rapid ignition is beneficial or required, such as in ignition of a heating fuel (gas) for an instantaneous water heater and the like. Heating elements also may be employed as glow plug in a variety of vehicles (automotive, watercraft).

The following non-limiting examples are illustrative of the invention. All documents mentioned herein are incorporated herein by reference in their entirety.

EXAMPLE 1

Heating Element Fabrication

Powders of a resistive composition (20 vol % MoSi₂, 5 vol % SiC, 74 vol % Al₂O₃ and 1 vol % Gd₂O₃), a conductive composition (28 vol % MoSi₂, 7 vol % SiC , 64 vol % Al₂O₃ and 1 vol % Gd₂O₃) and an insulating composition (10 vol % MoSi₂, 89 vol % Al₂O₃ and 1 vol % Gd₂O₃) are mixed with 10-16 wt % organic binder (about 6-8 wt % vegetable shortening, 2-4 wt % polystyrene and 2-4 wt % polyethylene) to form three pastes with about 62-64 vol % solids loading. The three pastes are loaded into the barrels of a co-injection molder. A first shot filled a cavity that has an hour-glass shaped cross-section with the insulating paste forming the supporting base. The part is removed from the first cavity and placed in a second cavity. A second shot fills the bottom half of the volume bounded by the first shot and the cavity wall with the conductive paste. The part is removed from the second cavity and placed in a third cavity. A third shot filled the volume bounded by the first shot, second shot and the cavity wall with resistive paste forming a hair-pin shaped resistor separated by the insulator and connected to conductive legs and having the configuration shown in FIG. 5. The part is then thermally debindered in Ar or N₂ at 500° C. for 24 h to remove the remaining binder and densified to 95-97% of theoretical at 1750° C. in Argon at 1 atm pressure.

The invention has been described in detail with reference to particular embodiments thereof. However, it will be appreciated that those skilled in the art, upon consideration of this disclosure, may make modification and improvements within the spirit and scope of the invention.

Claims

1. A ceramic heating element comprising an outer electrical path and a recessed portion to receive an electrical lead.

2. The ceramic heating element of claim 1 wherein the conductive zone has a cross-section for a substantial length of the heating element that is approximately equal to or greater than the cross-section of the conductive zone at the base portion of the heating element.

3. The heating element of claim 1 wherein the heating element has a rounded cross-sectional shape for at least a portion of the heating element length.

4. The heating element of claim 1 wherein the heating element comprises multiple regions of differing electrical resistivity.

5. The heating element of claim 1 wherein the heating element comprises in electrical sequence, a first conductive zone, a resistive hot zone, and a second conductive zone.

6. The heating element of claim 1 wherein the heating element has a substantially constant width for at least a substantial portion of the heating element length.

7. The heating element of claim 1 wherein walls of the recessed portion comprise portions of a conductive zone and insulator zone.

8. The heating element of claim 7 wherein up to 10, 20, 30, 40, 50, 60, 70, 80 or 90 percent of the surface area of the recessed portion walls are formed by the conductive zone, with the balance of the surface area of the recessed portion walls being formed by the insulator zone.

9. The heating element of claim 1 wherein the recessed portion is inwardly tapered.

10. The heating element of claim 1 wherein the heating element comprises a first conductive zone and a second conductive zone with a more resistive ignition zone interposed therebetween, and the first conductive zone has a greater length than the second conductive zone.

11. The heating element of claim 10 wherein the length of second conductive zone is no more than 90, 80, 70, 60, 50, 40, 30, 20 or 10 percent the length of the first conductive zone.

12. The heating element of claim 10 wherein the heating element electrical path extends in sequence from the first conductive zone to the ignition zone and then to the second conductive zone.

13. The heating element of claim 1 wherein the first conductive zone but not the second conductive zone contacts the recessed portion.

14. A method of igniting gaseous fuel, comprising applying an electric current across a heating element of claim 1.

15. A method of claim 14 wherein the current has a nominal voltage of 6, 8,10,12, 24,120, 220, 230 or 240 volts.

16. A heating apparatus comprising a heating element of claim 1.

17. The apparatus of claim 16 wherein the apparatus is cooking unit that comprises an igniter element.

18. The apparatus of claim 16 wherein the apparatus is a glow plug.

19. The apparatus of claim 16 wherein the apparatus is a vehicular glow plug.