EP2019946A2

EP2019946A2 - Ceramic heating elements

Info

Publication number: EP2019946A2
Application number: EP07794595A
Authority: EP
Inventors: Taehwan Yu; Suresh Annavarapu
Original assignee: Saint Gobain Ceramics and Plastics Inc
Current assignee: Saint Gobain Ceramics and Plastics Inc
Priority date: 2006-05-04
Filing date: 2007-05-04
Publication date: 2009-02-04
Also published as: EP2019946A4; WO2007130658A3; WO2007130658A2; CN101484755A; CA2651001A1; JP2009535789A; MX2008014158A; KR20090009294A; US20070295708A1; BRPI0711313A2

Abstract

New methods are provided or manufacture ceramic resistive igniter elements that include sintering of the elements in the absence of substantially elevated pressures. Ceramic igniters also are provided that are obtainable from fabrication methods of the invention.

Description

CERAMIC HEATING ELEMENTS

The present applications claims the benefit of U.S. provisional application number 60/798,266 filed May 4, 2006, incorporated by referenced herein in its entirety. BACKGROUND

1. Field of the Invention

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

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 ("MoSi₂"), 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 heating element systems. It would be particularly desirable to have new methods for producing ceramic heating elements. It also would be desirable to have more efficient production methods.

SUMMARY OF THE INVENTION

In one aspect, new ceramic articles are provided which are formed from one or more ceramic powders that have a mean particle size of about 2.5 microns or less.

We have found that ceramic articles made from such small size ceramic materials can be densified under significantly more mild conditions, including under reduced pressures relative to prior procedures.

In another aspect, ceramic articles are provided that are fabricated by treatment of the green state ceramic article by multiple, increasing pressures. Preferably, the ceramic article is treated at a first pressure and then treated at a second pressure which is higher than the first pressure. Preferably, the multi-pressure densification is conducted with use of gas-pressure sintering.

We have found that the multiple-stage pressure treatments can provide a highly dense article (e.g. at least 96, 97, 98 or 99 dense percent) ceramic article under quite mild conditions. For instance, the first pressure treatment suitably may be at about 1000 psi or 500 psi or less and the second pressure treatment may be at 4000 psi or less. Significantly lower pressures also have yielded highly dense articles, such as a first pressure of about 200 psi or less or 150 psi or less and a second pressure treatment of about 3000 psi or less, 2000 psi or less or 1500 psi or less.

In particularly preferred aspects of the invention, ceramic compositions are utilized that comprise one or more metal oxides such as alumina. Preferably, the one or more one or more metal oxides have a small mean particle size as disclosed herein. Particularly preferred are ceramic compositions that comprise alumina with small mean particle size as disclosed herein, such as 2.5 microns or less, 2 microns or less, 1.5 microns or less or 1 micron or less.

In a further aspect of the invention, ceramic compositions are densified in the absence of a so-called sintering aid. Sintering aid additives have included rare earth ■ oxides, such as yttria (yttrium oxide), a gadolinium material (e.g. a gadolinium oxide or Gd₂O₃), a europium material (e.g. a europium oxide or EU2O3), a ytterbium material (e.g. a ytterbium oxide or Yb₂O₃), or a lanthanum material (e.g. lanthanum or La₂O₃)-

Particularly preferred fabrication methods of the invention include forming a ceramic igniter element that comprises one or more small particle size ceramic materials as discussed above and then hardening through a two-stage pressure treatment as discussed above. Suitably, hardening is conducted under elevated temperatures such as in excess of 1400⁰C, more typically in excess of 1600°C such as at least 1700⁰C or 1800⁰C. Preferably, the sintering is conducted under an inert atmosphere, e.g. in an atmosphere of an inert gas such as argon or nitrogen.

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.

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 10 volume percent of silicon carbide or other carbide material based on total volume of the ceramic composition, more typically less than about 9, 8, 7, 6, 5,4, 3, 2, 1 or 0.5 volume percent based on total volume of the ceramic composition.

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 heating 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 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 heating element.

In preferred aspects of the invention, at least three portions of a ceramic heating 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 ceramic heating element are 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.

Particularly preferred heating elements of the invention will 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 igniter to a resistive hot zone). More particularly, preferred ceramic heating elements 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 heating element is particularly preferred. Such rod configurations offer higher Section Moduli and hence can enhance the mechanical integrity of the heating element.

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.

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 igniter element as described herein. Specific heating systems include gas cooking units, heating units for commercial and residential buildings, including water heaters. As referred to herein, the term "ceramic material" includes materials both prior to and after sintering processes. For instance, alumina, Mo₂Si2, SiC₅ AlN and other materials referred to herein are considered ceramic materials including in the pre-sintered state of those materials.

Other aspects of the invention are disclosed infra.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. IA and IB show top and bottom views respectively of a heating element of the invention;

FIG. 2A 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

In a first aspect, new ceramic articles are provided which are formed from one or more ceramic powders that have a mean particle size of about 2.5 microns or less, more preferably a mean particle size of about 2 microns or less, or 1.5, 1.25 or 1 micron or less. Such ceramic materials typically have a mean particle size of at least about 0.2, 0.3, 0.4 or 0.5 microns.

In preferred ceramic compositions, at least a major portion (e.g. greater than 50, 60, 70, 80 or 90 weight percent) of a specified ceramic material will have a small particle size as disclosed herein. More preferred, the entire portion of the specified ceramic material will have such a small particle size. For example, if a ceramic composition is indicated to include alumina having a mean particle size of 2 microns or less, preferably at least a major portion (such as greater than 50, 60, 70, 80 or 90 weight percent) of the alumina utilized in the ceramic composition will have a mean particle of 2 microns or less, and more preferably the entire portion of alumina present in the ceramic composition will have a mean particle size of 2 microns or less. As discussed herein, ceramic compositions employed to produce heating elements of the invention may suitably comprise two, three or more distinct materials such as AI2O₃, AlN, Mθ2S-2, SiC, and the like. Suitably, one or more of such distinct materials may be employed in small mean particle size as disclosed herein. However, in certain embodiments, not all materials of a ceramic compositions need to be employed in such mean small particle sizes. In this aspect of the invention, at least one material of a multiple-material composition is of such small mean particle size, but more than one or all materials of a multiple-material composition may have such small mean particle sizes if desired.

As discussed above, in certain embodiments, use of a small mean particle size metal oxide such as AI₂O3 may be particularly preferred.

Without being bound by any theory, it is believed that use of such smaller mean size particle materials can facilitate reduced pressure sintering of the formed green state heating element.

In another aspect, as discussed above, new methods are now provided for producing ceramic igniter elements that include hardening (densifiying) of a formed green ceramic element under reduced elevated pressures.

In this aspect, ceramic articles are provided that are fabricated by treatment of the green state ceramic article by multiple, increasing pressures. Preferably, the ceramic article is treated at a first pressure and then treated at a second pressure which is higher than the first pressure.

For at least certain applications, the first and second pressure treatments differ by at least 500 psi, more preferably by at least 1000 psi, 2000 psi or 2500 psi.

For at least certain applications, the first pressure treatment suitably may be at about 3,000 psi or less, 2000 psi or less, 1000 psi or less, 500 psi or less, or 200 psi or less, and the second pressure treatment may be at 6000 psi or less, 5000 psi or less, 4000 psi or less, 3000 psi or less, 2000 psi or less, 1500 psi or less or 1000 psi or less. For at least certain applications, the first pressure treatment and the second pressure treatment each will not exceed 5000 psi.

Other pressures also may be employed for the first and second pressure treatments provided the first pressure treatment is at a lower level than the second pressure treatment.

Again, without wishing to be bound by theory, it is believed a first lower pressure treatment can provide an initial densiflcation that avoids entrapped gases within the article. Once porosity is significantly closed by the first pressure treatment, higher densifications can be achieved in the elevated second pressure treatment.

Preferably, the multi-pressure densification is conducted with use of gas- pressure sintering. Commercial gas phase sintering ovens may be employed.

Preferably, sintering is conducted under an inert atmosphere, such as a nitrogen or argon atmosphere.

As discussed above, in a further aspect of the invention, ceramic compositions are densified in the absence of a so-called sintering aid.

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 heating 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 heating 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 M0S-₂, AI₂O₃, 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 5O⁰C or 6O⁰C 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 heating element 10 of the invention.

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 heating element 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 heating element 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 1 Oa 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 heating element proximal end.

As discussed above, and exemplified by heating element 10 of FIGS. IA, IB,

2 A and-2B, at least a substantial portion of the igniter length has a rounded cross- sectional shape along at least a portion of the heating element length, such as length x shown in FIG. IB. Igniter 10 of FIGS. IA, IB, 2A and 2B depicts 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 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 heating element length (as exemplified by heating element length x in FIG. IB) has a rounded cross-sectional shape; in such designs, the balance of the heating element length may have a profile with exterior edges.

Heating element 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 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 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 heating element 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 a heating element 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 a heating element of the invention will heat to a maximum temperature of less than about 1450⁰C at nominal voltage; and a maximum temperature of less than about 1550⁰C at high-end line voltages that are about 110 percent of nominal voltage; and a maximum temperature of less than about 1350⁰C 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 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 SIaN₄.

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 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 alumina (AI₂O₃).

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 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 10¹⁰ 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 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.

As discussed, heating element 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 AI₂O₃ or other insulating material; optional semiconductor material; and MoSi₂ or other conductive material. However, cold zone regions will have a significantly higher percentage of the conductive materials (e.g., 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 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, heating elements of the invention may suitably comprise a non-conductive (insulator or heat sink) region, although particularly preferred heating elements of the invention do not have a ceramic insulator 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 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/ό of one or more aluminum nitride, alumina and boron nitride

Preferred heating element ceramic materials are disclosed in the examples which follow.

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, a heating element of the invention may be used as an ignition source for stop top gas burners as well as gas furnaces.

In one preferred aspect of the invention, heating elements of the invention may be configured and/or utilized as resistive igniters elements 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 80O⁰C, 900⁰C or 1000⁰C 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 1200⁰C, 1300⁰C or 1400⁰C 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.

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

In other preferred aspects, heating elements are suitably employed as glow plugs, e.g. as an ignition source in a motor vehicle. Heating elements will be useful for additional specific applications, including as a heating elements for an infrared heater.

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

The following materials were admixed to provide a conductive composition for injection molding fabrication of a heating element: 30 vol% MoSi₂, 7 vol% SiC, and 63 vol% AI₂O₃, and based on the weight of ceramic materials 2~3 wt% of polyvinylalcohol and 0.3 wt % of glycerol.

The following materials were admixed to provide an insulator composition for injection molding fabrication of a heating element: 10 vol% MoSi₂, 90 vol% Al₂O₃, and based on the weight of ceramic materials 2~3 wt% of polyvinylalcohol and 0.3 wt % of glycerol.

The following materials were admixed to provide a resistive hot zone composition for injection molding fabrication of a heating element: 25 vol% MoSi₂, 5 vol% SiC₅ and 70 vol% AI2O3, and based on the weight of ceramic materials 2~3 wt% of polyvinylalcohol and 0.3 wt% of glycerol. In each of the three compositions, the AI₂O₃ had a mean particle size of 1.7 microns. No sintering aids such as yttria or other such materials were included in the compositions.

The above three compositions of differing resisitivity were loaded into separate barrels of a co-injection molder. To form the rod-shaped igniter element with internal insulator region of the general configuration shown in FIG. 1 of the drawings, a first shot filled a half-cylinder shaped cavity with insulating paste forming the insulating paste extruded from the cavity. The part was removed from the first cavity, placed in a second cavity and a second shot filled the volume bounded by the first shot and the cavity wall core with the conductive paste. The part was then removed from the second cavity, placed in a third cavity and a third shot filled the top portion of the part with the resistive (hot zone) paste. The thus molded rod-shaped part was then partially debindered at room temperature in an organic solvent dissolving out 10 wt% of the added 10-16 wt%. The part was then thermally debindered in flowing inert gas (N₂) at 300-500⁰C for 60 hours to remove the remainder of the residual binder.

The debindered rod-shaped part was densified through a two-stage process using gas-phase sintering. Thus, the rod-shaped part was placed in a gas sintering oven which was filled with argon gas at a pressure of 150 psi. The oven was maintained at 1725°C for about 1.5 hours. The oven was then allowed to cool to room temperature and then pressure increased to 3000 psi and held at 1725°C for about 2 hours. The oven was then allowed to cool to room temperature. The treated rod- shaped part had a density of greater than 98 percent. The dense element was connected to a power supply of 24 volts and the hot zone attained a temperature of about 1300⁰C.

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

What is claimed is:

1. A resistive ceramic heating element comprising: prior to sintering, one or more ceramic materials having a mean particle size of 2.5 microns or less.

2. The heating element of claim 1 wherein the heating element comprises prior to sintering one or more metal oxides having a mean particle size of 2.5 microns or less.

3. The ceramic heating element of claim 1 wherein the heating element comprises prior to sintering alumina having a mean particle size of 2.5 microns or less.

4. The heating element of claim 1 wherein the one or more ceramic materials have a mean particle size of 2 microns or less.

5. The heating element of claim 1 wherein the one or more ceramic materials have a mean particle size of 1.5 microns or less.

6. A method for producing a resistive heating element, comprising: treating a ceramic composition at a first pressure; and thereafter treating the ceramic composition at a second pressure that is greater than the first pressure to thereby density the ceramic composition.

7. The method of claim 6 wherein prior to treatment at the first pressure the ceramic composition comprises one or more ceramic materials having a mean particle size of 2.5 microns or less.

8. The method of claim 6 wherein prior to treatment at the first pressure the ceramic composition the ceramic composition comprises one or more metal oxides having a mean particle size of 2.5 microns or less.

9. The method of claim 6 wherein prior to treatment at the first pressure the ceramic composition comprises alumina having a mean particle size of 2.5 microns or less.

10. The method of claim 6 wherein the first and second pressures differ by at least 1000 psi.

11. The method of claim 6 wherein the second pressure is about 5000 psi or less.

12. The method of claim 6 wherein the first pressure is about 1000 psi or less.

13. The method of claim 6 wherein the first pressure is about 250 psi or less.

14. The method of claim 6 wherein the first and second pressures are applied as a gas phase sintering process.

15. The method of claim 6 wherein the ceramic igniter element is formed of a composition that has less than 10 volume percent silicon carbide.

16. The method of claim 6 wherein the ceramic element comprises two or more regions of differing resistivity.

17. The method of claim 6 wherein the ceramic element comprises three or more regions of differing resistivity.

18. A ceramic igniter element obtainable by the method of claim 6.

19. A method of igniting gaseous fuel, comprising applying an electric current across an igniter of claim 18.

20. A heating apparatus comprising an igniter of claim 18.