U.S. patent application number 11/347674 was filed with the patent office on 2006-08-24 for ceramic igniters.
This patent application is currently assigned to Saint-Gobain Ceramics & Plastics, Inc.. Invention is credited to Suresh Annavarapu, John D. Pietras, Taehwan Yu.
Application Number | 20060186107 11/347674 |
Document ID | / |
Family ID | 36793578 |
Filed Date | 2006-08-24 |
United States Patent
Application |
20060186107 |
Kind Code |
A1 |
Annavarapu; Suresh ; et
al. |
August 24, 2006 |
Ceramic igniters
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.
Inventors: |
Annavarapu; Suresh;
(Somerville, MA) ; Yu; Taehwan; (Sudbury, MA)
; Pietras; John D.; (Sutton, MA) |
Correspondence
Address: |
EDWARDS & ANGELL, LLP
P.O. BOX 55874
BOSTON
MA
02205
US
|
Assignee: |
Saint-Gobain Ceramics &
Plastics, Inc.
Wocester
MA
|
Family ID: |
36793578 |
Appl. No.: |
11/347674 |
Filed: |
February 3, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60650396 |
Feb 5, 2005 |
|
|
|
Current U.S.
Class: |
219/270 |
Current CPC
Class: |
H05B 3/141 20130101;
H05B 3/42 20130101; F23Q 7/22 20130101 |
Class at
Publication: |
219/270 |
International
Class: |
F23Q 7/22 20060101
F23Q007/22 |
Claims
1. A method for producing a resistive igniter, comprising: forming
a ceramic igniter element comprising a ceramic composition that
comprises one or more sintering aid materials; and sintering the
element at temperature in excess of 1400.degree. C. in the absence
of substantially elevated pressures.
2. The method of claim 1 wherein the ceramic igniter element is
formed by injection molding.
3. The method of claim 1 wherein the ceramic igniter element is
sintered at a temperature in excess of about 1600.degree. C.
4. The method of claim 1 wherein the ceramic igniter element is
sintered in an inert atmosphere.
5. The method of claim 1 wherein the sintering aid materials
comprise one or more rare earth oxides.
6. The method of claim 5 wherein the sintering aid materials
comprise yttria.
7. The method of claim 1 wherein the ceramic igniter element is
formed of a composition that does not comprises silicon
carbide.
8. The method of claim 1 wherein the ceramic element comprises two
or more regions of differing resistivity.
9. The method of claim 1 wherein the ceramic element
comprises-three or more regions of differing resistivity.
10. A ceramic igniter element obtainable by the method of claim
1.
11. The ceramic igniter element of claim 10 wherein the element
comprises two or more regions of differing resistivity.
12. The igniter element of claim 10 wherein the igniter element has
a substantially rounded cross-sectional shape for at least a
portion of the igniter length.
13. The igniter element of claim 10 wherein the igniter element has
a non-circular cross-sectional shape.
14. A method of igniting gaseous fuel, comprising applying an
electric current across an igniter an igniter of claim 10.
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 an igniter of claim 10.
Description
[0001] The present application claims the benefit of U.S.
provisional application No. 60/650,396, filed Feb. 5, 2005, which
is incorporated herein by reference in its entirety.
BACKGROUND
[0002] 1. Field of the Invention
[0003] 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.
[0004] 2. Background
[0005] 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. Pat. Nos. 6,582,629; 6,278,087; 6,028,292; 5,801,361;
5,786,565; 5,405,237; and 5,191,508.
[0006] 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.TM., 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.sub.2"), and silicon carbide
("SiC").
[0007] 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. Pat. No. 6,184,497.
[0008] While such fabrication methods can be effective to produce
ceramic igniters, the protocols can present inherent limitations
with respect to output and cost efficiencies.
[0009] 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
[0010] 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.
[0011] 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.degree. C., more typically in excess of 1600.degree. C. such
as at least 1700.degree. C. or 1800.degree. 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.
[0012] 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.
[0013] 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.
[0014] 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.
[0015] 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.sub.2O.sub.3), a europium material (e.g. a europium
oxide or Eu.sub.2O.sub.3), a ytterbium material (e.g. a ytterbium
oxide or Yb.sub.2O.sub.3), or a lanthanum material (e.g. lanthanum
or La.sub.2O.sub.3).
[0016] 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.
[0017] 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.
[0018] Preferred ceramic compositions employed to form a ceramic
element through the low pressure densification processes of the
present invention may advantageously comprise alumina
(Al.sub.2O.sub.3) and/or aluminum nitride (AlN).
[0019] 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.
[0020] 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).
[0021] 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.
[0022] 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.
[0023] 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.
[0024] 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.
[0025] 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.
[0026] 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.
[0027] 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.
[0028] 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.
[0029] 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).
[0030] 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.
[0031] 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.
[0032] 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.
[0033] Other aspects of the invention are disclosed infra.
BRIEF DESCRIPTION OF THE DRAWINGS
[0034] FIGS. 1A and 1B show top and bottom views respectively of an
igniter of the invention;
[0035] FIG. 2A shows a cut-away view along line 2A-2A of FIG. 1A;
and
[0036] FIG. 2B shows a cut-away view along line 2B-2B of FIG.
1A.
DETAILED DESCRIPTION OF THE INVENTION
[0037] 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.
[0038] 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.
[0039] 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.
[0040] 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.
[0041] 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.
[0042] 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.
[0043] 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.sub.2, Al.sub.2O.sub.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.
[0044] 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.
[0045] After advancing ceramic material(s) into a mold element, the
defined ceramic part suitably may be dried e.g. in excess of
50.degree. C. or 60.degree. C. for a time sufficient to remove any
solvent (aqueous and/or organic) carrier.
[0046] The examples which follow describe preferred injection
molding processes to form an igniter element.
[0047] Referring now to the drawings, FIGS. 1A and 1B shows a
suitable igniter element 10 of the invention that has been produced
through injection molding of regions of differing
resistivities.
[0048] As can be seen in FIG. 1A, 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.
[0049] FIG. 1B shows igniter bottom face with exposed heat sink
region 12.
[0050] Cross-sectional views of FIGS. 2A 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.
[0051] 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.
[0052] As discussed above, and exemplified by igniter 10 of FIGS.
1A, 1B, 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. 1B. Igniter
10 of FIGS. 1A, 1B, 2A 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. 1B) has a rounded cross-sectional
shape; in such designs, the balance of the igniter length may have
a profile with exterior edges.
[0053] 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.
[0054] 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. 1B) 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. 1B) suitably may be from about 0.2 to
about 3 cm.
[0055] 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. 1A) of an
igniter of the configuration depicted in FIG. 1A 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 f in FIG. 1A) suitably may be
about 0.2 to 5 or more cm.
[0056] 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.degree. C. at nominal voltage; and a maximum
temperature of less than about 1550.degree. C. at high-end line
voltages that are about 110 percent of nominal voltage; and a
maximum temperature of less than about 1350.degree. C. at low-end
line voltages that are about 85 percent of nominal voltage.
[0057] 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.sub.3N.sub.4.
[0058] As referred to herein, the term electrically insulating
material indicates a material having a room temperature resistivity
of at least about 10.sup.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).
[0059] As referred to herein, a semiconductor ceramic (or
"semiconductor") is a ceramic having a room temperature resistivity
of between about 10 and 10.sup.8 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.
[0060] As referred to herein, a conductive material is one which
has a room temperature resistivity of less than about 10.sup.-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.
[0061] 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.sup.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.sup.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.sup.-2 ohm-cm.
[0062] 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.sub.2O.sub.3 or other insulating
material; optional semiconductor material; and MoSi.sub.2 or other
conductive material. However, cold zone regions will have a
significantly higher percentage of the conductive materials (e.g.,
MoSi.sub.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.sub.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.
[0063] 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.
[0064] 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.sup.14
ohm-cm at room temperature and a resistivity of at least 10.sup.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
[0065] Preferred igniter ceramic materials are disclosed in the
examples which follow.
[0066] 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.
[0067] 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.
[0068] 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.degree. C.,
900.degree. C. or 1000.degree. 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.degree. C., 1300.degree. C. or
1400.degree. 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.
[0069] 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
[0070] Powders of a resistive compositions (22 vol % MoSi.sub.2,
and 78 vol % Al.sub.2O.sub.3) were mixed with 1.about.2 wt % of
Y.sub.2O.sub.3, 2.about.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.sub.2O.sub.3, followed by
pressureless (i.e., not elevated above 1 atmosphere) sintering at
1850.degree. C. in Ar atmosphere for up to 8 hrs. After sintering,
electrical resistivity was measured to be .about.0.1 ohms-cm at
room temperature and it increased to .about.0.4 ohms-cm at
1400.degree. C.
EXAMPLE 2
Additional Igniter Fabrication
[0071] Powders of a resistive compositions (20 vol % MoSi2, and 78
vol % Al.sub.2O.sub.3) were mixed with 1.about.2 wt %
Y.sub.2O.sub.3 and 10-16 wt % binder (6-8 wt % vegetable
shortening, 24 wt % polystyrene and 2-4 wt % polyethylene). Rods
were formed by injection molding at 175-200.degree. C. Rods were
solvent debindered in n-propyl bromide, loaded in graphite crucible
with powder bed of SiC and Al.sub.2O.sub.3, and thermal debindered
in N.sub.2 at 300-500.degree. C. for 60 h., followed by
pressureless (i.e., not elevated above 1 atmosphere) sintering at
1800.degree. C. in Ar atmosphere for up to 4 hrs. After sintering,
electrical resistivity was measured to be .about.0.1 ohms-cm at
room temperature and it increased to .about.0.4 ohms-cm at
1400.degree. C.
EXAMPLE 3
Additional Igniter Fabrication
[0072] Powders of a resistive composition (20 vol % MoSi2, 5 vol %
SiC and 75 vol % Al.sub.2O.sub.3) were mixed with 2 wt %
Gd.sub.2O.sub.3, 2-3 wt % polyvinyl alcohol and 0.3 wt % glycerol.
Tiles were formed by dry-pressing at 3000 psi 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 1750.degree. C. in Ar atmosphere for up to 4
hrs. After sintering, electrical resistivity was measured to be
.about.0.1 ohm-cm at room temperature increasing to about 0.375
ohm-cm at 1400.degree. C.
[0073] 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.
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