Docket No. 62750-PCT (E-4182) Express Mail Label No. EV754866614US

CERAMIC IGNITERS

The present applicationclaims the benefit of U.S. provisional application number 60/650,396, filed February 5, 2005, which is incorporated herein by reference in its entirety.

BACKGROUND

1. Field of the Invention

In one aspect, the invention provides new methods for manufacture ceramic resistive igniter elements that include substantially pressureless sintering of the formed green igniter element. Igniter elements also are provided obtainable from fabrication methods of the invention are provided.

2. Background.

Ceramic materials have enjoyed great success as igniters in e.g. gas-fired furnaces, stoves and clothes dryers. Ceramic igniter 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. Patents 6,582,629; 6,278,087; 6,028,292; 5,801,361; 5,786,565; 5,405,237; and 5,191,508.

Typical igniters have been generally rectangular-shaped elements with a highly resistive "hot zone" at the igniter tip with one or more conductive "cold zones" providing to the hot zone from the opposing igniter end. One currently available igniter, the Mini-Igniter™, 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 ("MoSi ₂ "), and silicon carbide ("SiC").

Igniter fabrication methods have included batch-type processing where a die is loaded with ceramic compositions of at least two different resistivities. The formed

green element is then densified (sintered) at elevated temperature and pressure. See the above-mentioned patents. See also U.S. Patent 6,184,497.

While such fabrication methods can be effective to produce ceramic igniters, the protocols can present inherent limitations with respect to output and cost efficiencies.

It thus would be desirable to have new ignition systems. It would be particularly desirable to have new methods for producing ceramic resistive elements. It also would be desirable to have more efficient production methods.

SUMMARY OF THE INVENTION

New methods are now provided for producing ceramic igniter elements which includes sintering a ceramic igniter element in the absence of substantially elevated pressures. Such pressureless sintering fabrication can provide enhanced output and cost efficiencies relative to prior approaches.

Preferred methods of the invention include forming a ceramic igniter element that comprises a sintering aid and then hardening the formed element at elevated temperatures such as in excess of 1400 ⁰ C, more typically in excess of 1600 ⁰ C such as at least 1700 ⁰ C or 1800 ⁰ C. Preferably, the sintering is conducted under an inert atmosphere, e.g. in an atmosphere of an inert gas such as argon or nitrogen. The hardening can be conducted in the absence of substantially elevated pressures, e.g. a pressure of no more than 1, 2 or 3 atmospheres, more typically a pressure of no more than 1 or 2 atmospheres.

Preferably, the hardening treatment provides a ceramic element that is at least 95 percent dense, more preferably a ceramic element that is at least 96, 97, 98 or 99 percent dense. The hardening process which includes the noted elevated temperatures is conducted for a time sufficient to achieve such densities, which may be several hours or more.

As mentioned, sintering occurs in the presence of one or more sintering aid materials which are typically admixed with a ceramic composition (e.g. one or more ceramic powders) that is employed to form a ceramic element.

It has been found that use of one or more sintering aids can facilitate densification of a ceramic composition even in the substantial absence of elevated pressures during a sintering process.

A variety sintering aid materials may be suitably employed to form ceramic elements in accordance with the invention. Preferred sintering aid materials include rare earth oxides, such as yttria (yttrium oxide), a gadolinium material (e.g. a gadolinium oxide or Gd ₂ O ₃ ), a europium material (e.g. a europium oxide or Eu ₂ O ₃ ), a ytterbium material (e.g. a ytterbium oxide or Yb ₂ O ₃ ), or a lanthanum material (e.g. lanthanum or La ₂ O ₃ ).

Particular ceramic compositions and method of forming the green ceramic element may be utilized to facilitate producing a dense ceramic element in the absence of substantially elevated pressures.

More specifically, preferred ceramic compositions employed to form a ceramic element may be at least substantially free or completely free of silicon carbide, or other carbide material. As referred to herein, a ceramic composition is at least substantially free of silicon carbide or other carbide material if it contains less than 5 weight percent of silicon carbide or other carbide material based on total weight of the ceramic composition, more typically less than about 4, 3, 2, 1 or 0.5 weight percent based on total weight of the ceramic composition.

Preferred ceramic compositions employed to form a ceramic element through the low pressure densification processes of the present invention may advantageously comprise alumina (Al ₂ O ₃ ) and/or aluminum nitride (AlN).

- A -

For sintering a ceramic element that comprises alumina, preferably sintering of the element is conducted in an atmosphere that is at least substantially free of nitrogen (e.g. less than 5 volume % nitrogen based on total atmosphere), or more preferably at least essentially free of nitrogen (e.g. less than 2 or 1 volume % nitrogen based on total atmosphere), or more preferably completely free of nitrogen. For instance, sintering may be conducted in an Argon atmosphere.

For sintering a ceramic element that comprises AlN, preferably sintering of the element is conducted in an atmosphere that contains at least some nitrogen, e.g. at least about 5 volume percent of nitrogen (i.e. at least 5 volume % nitrogen based on total atmosphere), or higher levels such as at least about 10 volume percent of nitrogen (i.e. at least 10 volume. % nitrogen based on total atmosphere).

It also may be preferred to form the ceramic elements through an injection molding process. As typically referred to herein, the term "injection molded," "injection molding" or other similar term indicates the general process where a material (here a ceramic or pre-ceramic material) is injected or otherwise advanced typically under pressure into a mold in the desired shape of the ceramic element followed by cooling and subsequent removal of the solidified element that retains a replica of the mold.

In injection molding formation of igniter elements of the invention, a ceramic material (such as a ceramic powder mixture, dispersion or other formulation) or a pre- ceramic material or composition may be advanced into a mold element.

In suitable fabrication methods, an integral igniter element having regions of differing resistivities (e.g., conductive region(s), insulator or heat sink region and higher resistive "hot" zone(s)) may be formed by sequential injection molding of ceramic or pre-ceramic materials having differing resistivities.

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 hase 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.

In preferred aspects of the invention, at least three portions of an igniter element are injection molded in single fabrication sequence to produce a ceramic component, a so-called "multiple shot" injection molding process where in the same fabrication sequence where multiple portions of an igniter element having different resistivity values (e.g. hot or highly resistive portion, cold or conductive portion, and insulator or heat sink portion). In at least certain embodiments, a single fabrication sequence includes sequential injection molding applications of a ceramic material without removal of the element from the element-forming area and/or without deposition of ceramic material to an element member by a process other than injection molding.

For instance, in one aspect, a first insulator (heat sink) portion can be injection molded, around that insulator portion conductive leg portions then can be injection molded in a second step, and in a third step a resistive hot or ignition zone can be applied by injection molding to the body containing insulator and resistive zones.

In another embodiment, methods for producing a resistive igniter re provided, which include injection molding one or more portions of a ceramic element, wherein the ceramic element comprises three or more regions of differing resistivity.

Fabrication methods of the invention may include additional processes for addition of ceramic material to produce the formed ceramic element. For instance, one or more ceramic layers may be applied to a formed element such as by dip coating, spray coating and the like of a ceramic composition slurry.

Preferred ceramic elements obtainable by methods of the invention comprise a first conductive zone, a resistive hot zone, and a second conductive zone, all in electrical sequence. Preferably, during use of the device electrical power can be applied to the first or the second conductive zones through use of an electrical lead (but typically not both conductive zones).

Particularly preferred igniters of the invention of the invention will have a rounded cross-sectional shape along at least a portion of the igniter length (e.g., the length extending from where an electrical lead is affixed to the igniter to a resistive hot zone). More particularly, preferred igniters may have a substantially oval, circular or other rounded cross-sectional shape for at least a portion of the igniter length, e.g. at least about 10 percent, 40 percent, 60 percent, 80 percent, 90 percent of the igniter length, or the entire igniter length. A substantially circular cross-sectional shape that provides a rod-shaped igniter element is particularly preferred. Such rod configurations offer higher Section Moduli and hence can enhance the mechanical integrity of the igniter.

Ceramic igniters 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.

The igniters 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 igniter element as described herein. Specific heating systems include gas cooking units, heating units for commercial and residential buildings, including water heaters.

Other aspects of the invention are disclosed infra.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. IA and IB show top and bottom views respectively of an igniter of the invention;

FIG. 2 A shows a cut-away view along line 2A-2A of FIG. IA; and

FIG 2B shows a cut-away view along line 2B-2B of FIG. IA.

DETAILED DESCRIPTION OF THE INVENTION

As discussed above, new methods are now provided for producing ceramic igniter elements that include hardening (densifiying) of a formed green ceramic element in the absence of substantially elevated pressures.

In accordance with the invention, sintering occurs in the presence of one or more sintering aid materials which are typically admixed with a ceramic composition (e.g. one or more ceramic powders) that is employed to form a ceramic element.

The one or more sintering aid materials are preferably employed in relatively low amounts, e.g. less than about 10, 8 or 5 weight percent relative to the weight of a ceramic composition in which the one or more sintering aid materials are utilized. More typically, one or more sintering aid materials are utilized in a ceramic composition at less than about 4 weight percent relative to the weight of a ceramic composition in which the one or more sintering aid materials are utilized, such as up to about 1, 2 or 3 weight percent relative to the weight of a ceramic composition in which the one or more sintering aid materials are utilized. One or more sintering aids are suitably employed in an amount of at least about 0.1, 0.25 or 0.5 weight percent relative to the weight of a ceramic composition in which the one or more sintering aid materials are utilized.

As discussed above, ceramic elements may be preferably formed by injection molding techniques. Thus, for instance and as discussed above, 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 resistivitities 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 MoSi ₂ , Al ₂ O ₃ , 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 5O ⁰ C or 6O ⁰ C for a time sufficient to remove any solvent (aqueous and/or organic) carrier.

The examples which follow describe preferred injection molding processes to form an igniter element.

Referring now to the drawings, FIGS. IA and IB shows a suitable igniter element 10 of the invention that has been produced through injection molding of regions of differing resistivities.

As can be seen in FIG. IA, igniter 10 includes a central heat sink or insulator region 12 which is encased within region(s) of differing resistivity, namely conductive zones 14 in the proximal portion 16 which become more resistive where in igniter proximal portion 18 the region has a comparatively decreased volume and thus can function as resistive hot zone 20.

FIG. IB shows igniter bottom face with exposed heat sink region 12.

Cross-sectional views of FIGS. 2 A and 2B further depict igniter 10 which includes conductive zones 14A and 14B in igniter proximal region 16 and corresponding resistive hot zone 20 in igniter distal zone 18.

In use, power can be supplied to igniter 10 (e.g. via one or more electrical leads, not shown) into conductive zone 14A which provides an electrical path through resistive ignition zone 20 and then through conductive zone 14B. Proximal ends 14a of conductive regions 14 may be suitably affixed such as through brazing to an electrical lead (not shown) that supplies power to the igniter during use. The igniter proximal end 10a suitably may be mounted within a variety of fixtures, such as where a ceramoplastic sealant material encases conductive element proximal end 14a as

disclosed in U.S. Published Patent Application 2003/0080103. Metallic fixtures also maybe suitably employed to encase the igniter proximal end.

As discussed above, and exemplified by igniter 10 of FIGS. IA, IB, 2A and 2B, at least a substantial portion of the igniter length has a rounded cross-sectional shape along at least a portion of the igniter length, such as length x shown in FIG. IB. Igniter 10 of FIGS. IA, IB, 2 A and 2B depicts a particularly preferred configuration where igniter 10 has a substantially circular cross-sectional shape for about the entire length of the igniter to provide a rod-shaped igniter element. However, preferred systems also include those where only a portion of the igniter has a rounded cross- sectional shape, such as where up to about 10, 20, 30, 40, 50, 60, 70 80 or 90 of the igniter length (as exemplified by igniter length x in FIG. IB) has a rounded cross- sectional shape; in such designs, the balance of the igniter length may have a profile with exterior edges.

Igniters of a variety of configurations may be fabricated as desired for a particular application. Thus, for instance, to provide a particular configuration, an appropriate shape-inducing mold element is employed through which a ceramic composition (such as a ceramic paste) may be injected.

Dimensions of igniters of the invention may vary widely and may be selected based on intended use of the igniter. For instance, the length of a preferred igniter (length x in FIG. IB) suitably may be from about 0.5 to about 5 cm, more preferably from about 1 about 3 cm, and the igniter cross-sectional width may suitably be from about (length y in FIG. IB) 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 a first conductive zone (length of proximal region 16 in FIG. IA) of an igniter of the configuration depicted in FIG. IA 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 total hot zone electrical path length (length fin FIG. IA) suitably may be about 0.2 to 5 or more cm.

In preferred systems, the hot or resistive zone of an igniter of the invention will heat to a maximum temperature of less than about 1450 ⁰ C at nominal voltage; and a maximum temperature of less than about 155O ⁰ C at high-end line voltages that are about 110 percent of nominal voltage; and a maximum temperature of less than about 1350 ⁰ C at low-end line voltages that are about 85 percent of nominal voltage.

A variety of compositions may be employed to form an igniter of the invention. Generally preferred hot zone compositions comprise two or more 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, and nitrides such as titanium nitride. Typical insulating materials include metal oxides such as alumina or a nitride such as AlN and/or Si ₃ N ₄ .

As referred to herein, the term electrically insulating material indicates a material having a room temperature resistivity of at least about 10 ¹⁰ ohms-cm. The electrically insulating material component of igniters 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 10 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 may be needed to achieve the desired voltage.

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 10 ¹⁰ ohm-cm; (b) between about 0 (where no semiconductor material employed) and about 45 v/o of a semiconductive material having a resistivity of between about 10 and about 10 ⁸ 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.

As discussed, igniters 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 igniter. Preferred cold zone regions include those that are comprised of e.g. AlN and/or Al ₂ O ₃ or other insulating material; optional semiconductor material; and MoSi ₂ or other conductive material. However, cold zone regions will have a significantly higher percentage of the conductive materials (e.g., MoSi ₂ ) 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 MoSi ₂ or other conductive and semiconductive material in a volume ratio of from about 1:1 to about 1:3. 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.

At least certain applications, igniters of the invention may suitably comprise a non-conductive (insulator or heat sink) region, although particularly preferred igniters

of the invention do not have a ceramic insulator insular that contacts at least a substantial portion of the length of a first conductive zone, as discussed above.

If employed, such a heat sink zone may mate with a conductive zone or a hot zone, or both.. Preferably, a sintered insulator region has a resistivity of at least about 10 ¹⁴ ohm-cm at room temperature and a resistivity of at least 10 ⁴ 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

Preferred igniter ceramic materials are disclosed in the examples which follow.

The igniters of the present 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 igniter of the invention may be used as an ignition source for stop top gas burners as well as gas furnaces.

Igniters of the invention also are particularly suitable for use for ignition where liquid fuels (e.g. kerosene, gasoline) are evaporated and ignited, e.g. in vehicle (e.g. car) heaters that provide advance heating of the vehicle.

Preferred igniters of the invention are distinct from heating elements known as glow plugs. Among other things, frequently employed glow plugs often heat to relatively lower temperatures e.g. a maximum temperature of about 800 ⁰ C, 900 ⁰ C or 1000 ⁰ C and thereby heat a volume of air rather than provide direct ignition of fuel, whereas preferred igniters of the invention can provide maximum higher temperatures such as at least about 1200 ⁰ C, 1300 ⁰ C or 1400 ⁰ C to provide direct ignition of fuel. Preferred igniters of the invention also need not include gas-tight sealing around the element or at least a portion thereof to provide a gas combustion chamber, as typically

employed with a glow plug system. Still further, many preferred igniters of the invention are useful at relatively high line voltages, e.g. a line voltage in excess of 24 volts, such as 60 volts or more or 120 volts or more including 220, 230 and 240 volts, whereas glow plugs are typically employed only at voltages of from 12 to 24 volts.

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

Example 1 : Igniter fabrication Powders of a resistive compositions (22 vol% MoSi ₂ , and 78 vol % Al ₂ O ₃ ) were mixed with 1~2 wt% OfY ₂ O ₃ , 2~3 wt% of polyvinylalcohol and 0.3 wt % of glycerol. Tiles were formed by dry-pressing at 3,000 psi and cold isostatic pressing at 30,000 psi. Tiles were loaded in graphite crucible with powder bed of SiC and Al ₂ O ₃ , followed by pressureless (i.e., not elevated above 1 atmosphere) sintering at 185O ⁰ C in Ar atmosphere for up to 8 hrs. After sintering, electrical resistivity was measured to be ~0.1 ohms-cm at room temperature and it increased to ~0.4 ohms-cm at 1400 ⁰ C.

Example 2: Additional igniter fabrication Powders of a resistive compositions (20 vol% MoSi2, and 78 vol % Al ₂ O ₃ ) were mixed with 1-2 wt % Y ₂ O ₃ and 10-16 wt% binder (6-8 wt% vegetable shortening, 2-4 wt% polystyrene and 2-4wt% polyethylene). Rods were formed by injection molding at 175-200 ⁰ C. Rods were solvent debindered in n-propyl bromide, loaded in graphite crucible with powder bed of SiC and Al ₂ O ₃ , and thermal debindered in N ₂ at 300-500 ⁰ C for 6Oh., followed by pressureless (i.e., not elevated above 1 atmosphere) sintering at 1800 ⁰ C in Ar atmosphere for up to 4 hrs. After sintering, electrical resistivity was measured to be -0.1 ohms-cm at room temperature and it increased to ~0.4 ohms-cm at 1400 ⁰ C.

Example 3: Additional igniter fabrication

Powders of a resistive composition (20 vol% MoSi2, 5 vol% SiC and 75 vol% Al ₂ O ₃ ) were mixed with 2 wt% Gd ₂ O ₃ , 2-3 wt% polyvinyl alcohol and 0.3 wt% glycerol. Tiles were formed by dry-pressing at 3000psi and cold isostatic pressing at

30000 psi. Tiles were loaded into a graphite crucible in a sintering bed and pressureless (i.e. at about 1 atmosphere pressure) sintered at 175O ⁰ C in Ar atmosphere for up to 4hrs. After sintering, electrical resistivity was measured to be -0.1 ohm-cm at room temperature increasing to about 0.375 ohm-cm at 1400 ⁰ C.

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.