U.S. patent application number 11/800168 was filed with the patent office on 2007-12-27 for ceramic heating elements.
This patent application is currently assigned to Saint-Gobain Ceramics & Plastics, Inc.. Invention is credited to Suresh Annavarapu, Taehwan Yu.
Application Number | 20070295708 11/800168 |
Document ID | / |
Family ID | 38668378 |
Filed Date | 2007-12-27 |
United States Patent
Application |
20070295708 |
Kind Code |
A1 |
Yu; Taehwan ; et
al. |
December 27, 2007 |
Ceramic heating elements
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: |
Yu; Taehwan; (Sudbury,
MA) ; Annavarapu; Suresh; (Somerville, MA) |
Correspondence
Address: |
EDWARDS ANGELL PALMER & DODGE LLP
P.O. BOX 55874
BOSTON
MA
02205
US
|
Assignee: |
Saint-Gobain Ceramics &
Plastics, Inc.
Worcester
MA
|
Family ID: |
38668378 |
Appl. No.: |
11/800168 |
Filed: |
May 4, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60798266 |
May 4, 2006 |
|
|
|
Current U.S.
Class: |
219/260 |
Current CPC
Class: |
H05B 2203/027 20130101;
F23Q 7/22 20130101; H05B 3/141 20130101 |
Class at
Publication: |
219/260 |
International
Class: |
F23Q 7/00 20060101
F23Q007/00 |
Claims
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 densify 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.
Description
[0001] The present applications claims the benefit of U.S.
provisional application No. 60/798,266 filed May 4, 2006,
incorporated by referenced herein in its entirety.
BACKGROUND
[0002] 1. Field of the Invention
[0003] 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.
[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 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
[0010] 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.
[0011] 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.
[0012] 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.
[0013] 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.
[0014] 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.
[0015] 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 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] 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 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.
[0017] 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.
[0018] 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.
[0019] 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.
[0020] 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.
[0021] 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).
[0022] 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.
[0023] 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.
[0024] 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.
[0025] 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.
[0026] 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.
[0027] 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.
[0028] 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.
[0029] 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.
[0030] 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.
[0031] 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.
[0032] 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.
[0033] 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.
[0034] As referred to herein, the term "ceramic material" includes
materials both prior to and after sintering processes. For
instance, alumina, Mo.sub.2Si.sub.2, SiC, AlN and other materials
referred to herein are considered ceramic materials including in
the pre-sintered state of those materials.
[0035] Other aspects of the invention are disclosed infra.
BRIEF DESCRIPTION OF THE DRAWINGS
[0036] FIGS. 1A and 1B show top and bottom views respectively of a
heating element of the invention;
[0037] FIG. 2A shows a cut-away view along line 2A-2A of FIG. 1A;
and
[0038] FIG. 2B shows a cut-away view along line 2B-2B of FIG.
1A.
DETAILED DESCRIPTION OF THE INVENTION
[0039] 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.
[0040] 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.
[0041] As discussed herein, ceramic compositions employed to
produce heating elements of the invention may suitably comprise
two, three or more distinct materials such as Al.sub.2O.sub.3, AlN,
Mo.sub.2Si.sub.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.
[0042] As discussed above, in certain embodiments, use of a small
mean particle size metal oxide such as Al.sub.2O.sub.3 may be
particularly preferred.
[0043] 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.
[0044] 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.
[0045] 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.
[0046] 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.
[0047] 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.
[0048] For at least certain applications, the first pressure
treatment and the second pressure treatment each will not exceed
5000 psi.
[0049] 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.
[0050] Again, without wishing to be bound by theory, it is believed
a first lower pressure treatment can provide an initial
densification 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.
[0051] 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.
[0052] As discussed above, in a further aspect of the invention,
ceramic compositions are densified in the absence of a so-called
sintering aid.
[0053] 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.
[0054] 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.
[0055] 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.
[0056] 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.
[0057] 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.
[0058] 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.
[0059] The examples which follow describe preferred injection
molding processes to form an igniter element.
[0060] Referring now to the drawings, FIGS. 1A and 1B shows a
suitable heating element 10 of the invention.
[0061] 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.
[0062] FIG. 1B shows igniter bottom face with exposed heat sink
region 12.
[0063] Cross-sectional views of FIGS. 2A 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.
[0064] 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 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 heating element proximal end.
[0065] As discussed above, and exemplified by heating element 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 heating element length, such as length x shown in
FIG. 1B. Igniter 10 of FIGS. 1A, 1B, 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. 1B) has a rounded cross-sectional shape; in such
designs, the balance of the heating element length may have a
profile with exterior edges.
[0066] 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.
[0067] 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. 1B) 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.
1B) suitably may be from about 0.2 to about 3 cm.
[0068] 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 a
heating element 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.
[0069] In preferred systems, the hot or resistive zone of a heating
element of the invention will heat to a maximum temperature of less
than about 1450.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.
[0070] 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 Si.sub.3N.sub.4.
[0071] 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 alumina (Al.sub.2O.sub.3).
[0072] 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.
[0073] 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.
[0074] 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.
[0075] 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
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.
[0076] 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.
[0077] 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
[0078] Preferred heating element ceramic materials are disclosed in
the examples which follow.
[0079] 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.
[0080] 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 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.
[0081] 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.
[0082] In other preferred aspects, heating elements are suitably
employed as glow plugs, e.g. as an ignition source in a motor
vehicle.
[0083] Heating elements will be useful for additional specific
applications, including as a heating elements for an infrared
heater.
[0084] 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
[0085] The following materials were admixed to provide a conductive
composition for injection molding fabrication of a heating element:
30 vol % MoSi.sub.2, 7 vol % SiC, and 63 vol % Al.sub.2O.sub.3, and
based on the weight of ceramic materials 2.about.3 wt % of
polyvinylalcohol and 0.3 wt % of glycerol.
[0086] The following materials were admixed to provide an insulator
composition for injection molding fabrication of a heating element:
10 vol % MoSi.sub.2, 90 vol % Al.sub.2O.sub.3, and based on the
weight of ceramic materials 2.about.3 wt % of polyvinylalcohol and
0.3 wt % of glycerol.
[0087] The following materials were admixed to provide a resistive
hot zone composition for injection molding fabrication of a heating
element: 25 vol % MoSi.sub.2, 5 vol % SiC, and 70 vol %
Al.sub.2O.sub.3, and based on the weight of ceramic materials
2.about.3 wt % of polyvinylalcohol and 0.3 wt % of glycerol.
[0088] In each of the three compositions, the Al.sub.2O.sub.3 had a
mean particle size of 1.7 microns. No sintering aids such as yttria
or other such materials were included in the compositions.
[0089] 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 (N.sub.2) at 300-500.degree. C. for 60 hours to remove
the remainder of the residual binder.
[0090] 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.degree. 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.degree. 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 1300.degree. C.
[0091] 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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