Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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CERAMIC HEATING ELEMENTS
The present application claims the benefit of U.S. provisional application
number
61/009,381 filed December 29, 2007, which is incorporated by reference herein
in its
entirety.
BACKGROUND
1. Field of the Invention
In one aspect, ceramic heating elements are provided that have a recessed
portion
for receiving an electrical lead. Such ceramic heating elements can more
secure
engagement of the heating element with an electrical lead. In a further
aspect, ceramic
heating elements are provided that have a conductive zone of substantially
equal or
increasing cross-section along a length of the element. The present heating
elements are
useful in a variety of application, including e.g. for fuel ignition for gas
cooking
appliances as well as vehicular glow plugs that have strict space constraints.
2. Background.
Ceramic materials have enjoyed great success as heating elements (includes
igniters) in e.g. gas-fired furnaces, stoves and clothes dryers. Ceramic
heating element
production includes constructing an electrical circuit through a ceramic
component a
portion of which is highly resistive and rises in temperature when electrified
by a wire
lead. See, for instance, U.S. Patent Publication 2006/0131295 and U.S. Patents
6,028,292; 5,801,361; 5,405,237; and 5,191,508.
Typical igniters have been generally rectangular-shaped elements with a highly
resistive "hot zone" at the heating element tip with one or more conductive
"cold zones"
providing to the hot zone from the opposing heating element end. One currently
available
igniter, the Mini-Heating element, available from Norton Igniter Products of
Milford,
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N.H., is designed for 12 volt through 120 volt applications and has a
composition
comprising aluminum nitride ("AIN"), molybdenum disilicide ("MoSi2"), and
silicon
carbide ("SiC").
Since these heating elements are resistively heated, each of its ends must be
electrically connected to a conductive lead, typically a copper wire lead.
Ceramic heating
elements have been connected to electrical contact by direct welding or
brazing to wire or
by brazing to an intermediate metal lead frame which is then welded or brazed
to wire.
See U.S. Patents 7,241,975 and 6,933,471.
For heating elements that have cylindrical or other non-rectangular cross-
section
configurations, such attachment of electrical contacts can result in an
increase in the
diameter of the insulating section (where the electrical leads interface with
the heating
element). Such increased dimensions can be problematic for a number of
applications,
such as appliances or automotive environments where tight specifications may
exists for
the outer dimensions of the heating element block of the heating element.
Additionally,
separation of the electrical lead from the heating element can result in
device failure.
It thus would be desirable to have new heating element systems. It would be
particularly desirable to have new heating elements that have cylindrical or
other non-
rectangular cross-sectional configurations and that have comparatively narrow
cross-
sectional dimensions across regions that interface with electrical contacts.
It would be
further desirable to have new heating elements that have secure engagement of
an
electrical lead to the heating element.
SUMMARY
In one aspect, ceramic heating elements are provided that have a recessed
portion
for receiving an electrical lead. Such ceramic heating elements can provide a
reduced
cross-sectional dimension across element regions that interface with
electrical lead(s) as
well as a more secure engagement of the lead(s) to the heating device.
Consequently,
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heating elements can be highly useful in a variety of applications, including
e.g. for fuel
ignition for gas cooking appliances as well as vehicular glow plugs.
In a preferred aspect, a heating element may comprise at least one recess
(e.g.
hole) that can receive an electrical lead, where the recess is positioned at a
bottom face of
the heating element, although the recess also suitably may be situated in
other regions of
a heating element, such as a side portion of an element.
In certain aspects, the recess may be tapered, e.g. inwardly tapered
(decreasing
cross-sectional area), which can further secure an engagement of an electrical
lead with
the heating element.
Preferably, a conductive zone (i.e. region of relatively low resistivity) of
the
heating element forms at least a portion of the wall surface of the recess. As
a
consequence, power from an electrical lead nested within the recess can flow
through the
heating element via such conductive zone.
In a further aspect, ceramic heating elements are provided that have a
conductive
zone of substantially equal or increasing cross-section from a proximal end of
the heating
element along the element length. In particular, the cross-sectional dimension
of the
conductive zone that forms at least a portion of the wall surface of the
recess for
receiving an electrical lead will have a cross-sectional dimension at a
portion that
contacts the recess that is substantially equal to or greater than the cross-
section
dimension of that same conductive zone further along that conductive zone's
length.
It has been found that such conductive zone configurations can avoid undesired
warpage upon sintering of the heating element.
Preferred heating elements of the invention have an outer or substantially U-
shaped or L-shaped electrical path, i.e. where the electrical path extends
from (i) an outer
conductive zone to (ii) an hot or ignition zone and then through (iii) a
second outer
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conductive zone. Such an outer or U-shaped or L-shaped electrical path is
different than
and distinguished from a co-axial path that contains an interior first
conductive zone that
is encased by an outer conductive zone.
Particularly preferred heating elements of the invention may have cylindrical
or
other non-rectangular cross-section configurations. In a preferred aspect,
preferred
heating elements of the invention have a rounded cross-sectional shape along
at least a
portion of the heating element length (e.g., the length extending from where
an electrical
lead is affixed to the heating element to a resistive hot zone). More
particularly,
preferred heating elements may have a substantially oval, circular or other
rounded cross-
sectional shape for at least a portion of the heating element length, e.g. at
least about 10
percent, 40 percent, 60 percent, 80 percent or 90 percent of the heating
element length, or
the entire heating element length. A substantially circular cross-sectional
shape that
provides a rod-shaped heating element is particularly preferred. The invention
also
provides heating elements that have non-rounded or non-circular cross-
sectional shapes
for at least a portion of the heating element length.
Preferred heating elements comprise multiple regions of differing electrical
resistivity, i.e. preferred ceramic heating elements may comprise a first
conductive zone,
a resistive hot zone, and a second conductive zone, all in electrical
sequence. Heating
elements of the invention may have a variety of electrical configurations. As
discussed,
in preferred systems, the heating element may have a substantially U-shaped
electrical
path, e.g. where opposing conductive zones are separated by an interposed hot
or ignition
zone.
Ceramic heating elements of the invention can be employed at a wide variety of
nominal voltages, including nominal voltages of 6, 8, 10, 12, 24, 120, 220,
230 and 240
volts.
As mentioned, the heating elements of the invention are useful for ignition in
a
variety of devices and heating systems. More particularly, heating systems are
provided
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that comprise a sintered ceramic heating element as described herein. Specific
heating
systems include appliances such as gas cooking units, heating units for
commercial and
residential buildings. Vehicular (e.g. automotive, watercraft) glow plugs also
provided
that comprise a sintered ceramic heating element as described herein.
Other aspects of the invention are disclosed infra.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1 through 4 show schematically preferred heating element systems;
FIG. 5 shows in a cut-away view a further preferred heating element; and
FIG. 6 (which includes FIGS. 6A-6C) shows plan views of a further preferred
heating element. FIG. 6B is a view sliced along line B-B of FIG. 6A, and Fig.
6C is a
view sliced along line C-C of FIG. 6A.
DETAILED DESCRIPTION
As discussed above, in one aspect, ceramic heating element systems are
provided
that include new configurations for mating of electrical lead components. In a
further
aspect, ceramic heating element systems are provided that include conductive
region(s)
that can provide notable benefits, including reduced warpage upon sintering.
Preferred
ceramic heating elements of the invention having a substantially outer or U-
shaped or L-
shaped electrical path.
Referring now to the drawings, FIGS. 1 through 4 show in a schematic cut-away
view a preferred heating element 10 where conductive zones 12A and 12B mate
with
interposed hot (ignition) zone 14 to thereby form an electrical pathway. As
can be seen,
outer conductive zones 12A and 12B together with interposed hot (ignition)
zone form a
substantially U-shaped or L-shaped electrical pathway that traverses an outer
or perimeter
portion of the heating element 10.
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Conductive zone 12A defines in part recess 16 that engages with electrical
lead 18
during use of element 10. In preferred systems, heating element 10 may be
encased with
a metal fixture 20 and affixed therethrough, e.g. via a metal braze 22. The
interior region
24 encased by conductive zones 12A, 12B and ignition zone 14 may be void or
may have
an insulative (heat sink) composition.
It also may be preferred to include an exterior insulative layer 25 on heating
element portions that contact metal fixture 20. Such an exterior insulative
layer may be
suitably formed by dip coating or other application of an insulative ceramic
composition.
FIG. 5 shows in a partial cut-away view a particularly preferred heating
element
where conductive zone 12A forms a portion of walls 16A of recess 16 that
receives an
electrical lead. In this preferred configuration, conductive zone 12A and 12B
mate with
interposed hot (ignition) zone 14 to thereby form an electrical pathway. The
heating
element also includes central insulator region 24 with outer insulator 25A
that encases at
least a portion of the first conductive zone 12A as well as insulator 25B that
encases at
least a portion of conductive zone 12B.
As depicted in FIG. 5, in a preferred configuration, recess 16 contacts
conductive
zone 12A whereby walls 16A of the recess are formed by conductive zone 12A. In
other
less preferred embodiments, the entire surface of the walls that define recess
16 are part
of the conductive zone 12A.
In the depicted preferred configuration, by only a portion of the walls that
define
recess 16 being part of the conductive zone 12A, that conductive zone 12A can
have a
substantially equal or increasing cross-section along a length of the element.
Thus, as
shown in FIG. 5, the cross-section dimension of conductive zone 12A at the
element
proximal end (as shown by dimension a) is substantially the same as or less
than the
cross-sectional dimension of that conductive zone for the substantial portion
of that
zone's length (length y as depicted in FIG. 5), e.g. for at least about 50,
60, 70, 80, 90, 95
or even 100 percent of that zone's length y as shown in FIG. 5. Thus, as shown
in FIG.
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5, dimension a will be the about the same as or less than the depicted
dimensions a' or
a". As referred to herein, references to the first conductive zone having
"substantially the
same" or "about the same" (or other similar phrase) cross-section along its
length means
that the cross-section dimension (such as a relative to a' and a" as shown in
FIG. 5) does
not vary by more than 5, 10 or 20 percent. In certain aspects, references to
the cross-
section dimension of the conductive zone does not include the interface of
that
conductive zone and the mating ignition zone.
As discussed above, this configuration of the first conductive zone cross-
sectional
dimension has provided notable benefits, including reduced undesired warpage
upon
sintering of the heating element.
FIG. 5 also shows a preferred configuration of recess 16, where recess 16
inwardly tapers, i.e. recess 16 has a decreased cross-section along its
length. Such a
tapered configuration can provide more secure engagement of an electrical lead
nested
within the recess.
In use, an electrical lead is nested within recess 16 and provides power
through
the depicted electrical pathway (see pathway as shown by arrows in FIG. 5)
that extends
from conductive zone 12A to hot (ignition) zone 14. Interior ceramic insulator
24 can
provide further mechanical strength to the heating element.
FIGS. 6A through 6C show a further heating element 10 in a preferred
configuration where only a portion of recess 16 contacts conductive zone 12A.
That is,
only a portion of the surface that defines recess 16 is a component of
conductive zone
12A. In the system depicted in FIG. 6, the balance of the walls defining
recess 16 is a
component of insulator region 24. In exemplary preferred systems, up to about
20, 30,
40, 50, 60, 70, 80 or 90 percent of the surface area of the walls of recess 16
may be a
component of the component of conductive zone 12A, with the balance of the
surface
area of the walls of recess 16A being a component of the central or interior
insulator
region 24.
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As with the element of FIG. 5, in use, an electrical lead is nested within
recess 16
and provides power through the depicted electrical pathway from conductive
zone 12A,
through hot (ignition) zone 14 and then through conductive zone 12B to provide
a
substantially U-shaped or L-shaped outer electrical pathway.
As shown in FIGS. 5 and 6A, in preferred systems, the length of first
conductive
zone 12A that contacts recess 16 is greater than the length of second
conductive zone
12B that is on the distal side of resistive (ignition) zone 14. For example,
in certain
preferred configurations, the length of second conductive zone 12B (shown as
y' in FIG.
5) is no more than 90, 80, 70, 60, 50, 40, 30, 20 or even 10 percent the
length of the first
conductive zone 12A (length y is FIG. 5). In certain preferred configurations,
second
conductive zone 12B does not contain or contact recess 16, as shown in FIGS.
5, 6A and
6B.
As discussed above, and exemplified in FIGS. 5 and 6, preferably, at least a
substantial portion of the heating element length has a rounded cross-
sectional shape
along at least a portion of the heating element length, such as length a shown
in FIG. 6A.
FIGS. 5 and 6 depict a particularly preferred configuration where heating
element 10 has
a substantially circular cross-sectional shape for about the entire length of
the heating
element to provide a rod-shaped heating element. However, as discussed above,
preferred
systems also include those where only a portion of the heating element has a
rounded
cross-sectional shape, such as where up to about 10, 20, 30, 40, 50, 60, 70 80
or 90
percent of the heating element length, e.g. where in such designs, the balance
of the
heating element length may have a profile with exterior edges.
Also, while a rounded cross-sectional shape is preferred for many
applications,
preferred heating elements of the invention also may have a non-rounded or non-
circular
cross-sectional shape for at least a portion of the heating element length,
e.g. where up to
or at least about 10, 20, 30, 40, 50, 60, 70 80 or 90 percent of the heating
element length
(as exemplified by heating element length a in FIG. 6A) has a cross-sectional
shape that
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is non-rounded or non-circular, or where the entire heating element length (as
a heating
element length is exemplified by length a in FIG. 6A) has a cross-sectional
shape that is
non-rounded or non-circular.
Dimensions of heating elements of the invention may vary widely and may be
selected based on intended use of the heating element. For instance, the
length of a
preferred heating element (length a in FIG. 6A) suitably may be from about 0.5
to about 5
cm, more preferably from about 1 about 3 cm, and the heating element maximum
cross-
sectional width may suitably be from about (width b in FIG. 6A) suitably may
be from
about 0.2 to about 3 cm.
Similarly, the lengths of the conductive and hot zone regions also may
suitably
vary. Preferably, the length of first conductive zone (length c in FIG. 6A and
y in FIG. 5)
of a heating element depicted in FIG. 6A may be from 0.2 cm to 2, 3, 4, or 5
more cm.
More typical lengths of the first conductive zone will be from about 0.5 to
about 5 cm.
The height of a hot zone (length d in FIG. 6A) may be from about 0.1 to about
2, 3, 4 or 5
cm, with a total hot zone electrical path length (shown as the dashed line in
FIG. 6A) of
about 0.5 to 5 or more cm, with a total hot zone path length of up to about
0.5 to 1, 2 or 3
cm generally preferred.
In preferred systems, the hot or resistive zone of a heating element of the
invention will heat to a maximum temperature of less than about 1450 C at
nominal
voltage; and a maximum temperature of less than about 1550 C at high-end line
voltages
that are about 110 percent of nominal voltage; and a maximum temperature of
less than
about 1350 C at low-end line voltages that are about 85 percent of nominal
voltage.
A variety of compositions may be employed to form a heating element of the
invention. Generally preferred hot zone compositions comprise at least three
components
of 1) conductive material; 2) semiconductive material; and 3) insulating
material.
Conductive (cold) and insulative (heat sink) regions may be comprised of the
same
components, but with the components present in differing proportions. Typical
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conductive materials include e.g. molybdenum disilicide, tungsten disilicide,
nitrides
such as titanium nitride, and carbides such as titanium carbide. Typical
semiconductors
include carbides such as silicon carbide (doped and undoped) and boron
carbide. Typical
insulating materials include metal oxides such as alumina or a nitride such as
A1N and/or
Si3N4.
As referred to herein, the term electrically insulating material indicates a
material
having a room temperature resistivity of at least about 1010 ohms-cm. The
electrically
insulating material component of heating elements of the invention may be
comprised
solely or primarily of one or more metal nitrides and/or metal oxides, or
alternatively, the
insulating component may contain materials in addition to the metal oxide(s)
or metal
nitride(s). For instance, the insulating material component may additionally
contain a
nitride such as aluminum nitride (AIN), 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 (A1N).
As referred to herein, a semiconductor ceramic (or "semiconductor") is a
ceramic
having a room temperature resistivity of between about 10 and 108 ohm-cm. If
the
semiconductive component is present as more than about 45 v/o of a hot zone
composition (when the conductive ceramic is in the range of about 6-10 v/o),
the
resultant composition becomes too conductive for high voltage applications
(due to lack
of insulator). Conversely, if the semiconductor material is present as less
than about 10
v/o (when the conductive ceramic is in the range of about 6-10 v/o), the
resultant
composition becomes too resistive (due to too much insulator). Again, at
higher levels of
conductor, more resistive mixes of the insulator and semiconductor fractions
are needed
to achieve the desired voltage. Typically, the semiconductor is a carbide from
the group
consisting of silicon carbide (doped and undoped), and boron carbide. Silicon
carbide is
generally preferred.
As referred to herein, a conductive material is one which has a room
temperature
resistivity of less than about 10"2 ohm-cm. If the conductive component is
present in an
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amount of more than 35 v/o of the hot zone composition, the resultant ceramic
of the hot
zone composition, the resultant ceramic can become too conductive. Typically,
the
conductor is selected from the group consisting of molybdenum disilicide,
tungsten
disilicide, and nitrides such as titanium nitride, and carbides such as
titanium carbide.
Molybdenum disilicide is generally preferred.
In general, preferred hot (resistive) zone compositions include (a) between
about
50 and about 80 v/o of an electrically insulating material having a
resistivity of at least
about 1010 ohm-cm; (b) between about 5 and about 45 v/o of a semiconductive
material
having a resistivity of between about 10 and about 108 ohm-cm; and (c) between
about 5
and about 35 v/o of a metallic conductor having a resistivity of less than
about 10"2 ohm-
cm. Preferably, the hot zone comprises 50-70 v/o electrically insulating
ceramic, 10-45
v/o of the semiconductive ceramic, and 6-16 v/o of the conductive material. A
specifically preferred hot zone composition for use in heating elements of the
invention
contains 10 v/o MoSi2, 20 v/o SiC and balance AIN or A1203.
As discussed, heating elements of the invention contain a relatively low
resistivity
cold zone region in electrical connection with the hot (resistive) zone and
which allows
for attachment of wire leads to the heating element. Preferred cold zone
regions include
those that are comprised of e.g. AIN and/or A1203 or other insulating
material; SiC or
other semiconductor material; and MoSi2 or other conductive material. However,
cold
zone regions will have a significantly higher percentage of the conductive and
semiconductive materials (e.g., SiC and MoSi2) than the hot zone. A preferred
cold zone
composition comprises about 15 to 65 v/o aluminum oxide, aluminum nitride or
other
insulator material; and about 20 to 70 v/o MoSi2 and SiC or other conductive
and
semiconductive material in a volume ratio of from about 1:1 to about 1:3. For
many
applications, more preferably, the cold zone comprises about 15 to 50 v/o A1N
and/or
A1203, 15 to 30 v/o SiC and 30 to 70 v/o MoSi2. For ease of manufacture,
preferably the
cold zone composition is formed of the same materials as the hot zone
composition, with
the relative amounts of semiconductive and conductive materials being greater.
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A specifically preferred cold zone composition for use in heating elements of
the
invention contains 20 to 35 v/o MoSi2i 45 to 60 v/o SiC and balance either AIN
and/or
A 1203.
For any of the ceramic compositions (e.g. insulator, conductive material,
semiconductor material, resistive material), the ceramic compositions may
comprise one
or more different ceramic materials (e.g. SiC, metal oxides such as A1203,
nitrides such as
A1N, Mo2Si2 and other Mo-containing materials, SiA1ON, Ba-containing material,
and
the like). Alternatively, distinct ceramic compositions (i.e. distinct
compositions that
serve as insulator, conductor and resistive (ignition) zones in a single
heating element)
may comprise the same blend of ceramic materials (e.g. a binary, ternary or
higher order
blend of distinct ceramic materials), but where the relative amounts of those
blend
members differ, e.g. where one or more blend members differ by at least 5, 10,
20, 25 or
30 volume percent between the respective distinct ceramic compositions.
A heat sink or insulator may suitably mate with a conductive zone or a hot
zone, or
both. Preferably, a sintered insulator region has a resistivity of at least
about 1014 ohm-cm
at room temperature and a resistivity of at least 104 ohm-cm at operational
temperatures
and has a strength of at least 150 MPa. Preferably, an insulator region has a
resistivity at
operational (ignition) temperatures that is at least 2 orders of magnitude
greater than the
resistivity of the hot zone region. Suitable insulator compositions comprise
at least about
90 v/o of one or more aluminum nitride, alumina and boron nitride. A
specifically
preferred insulator composition of an heating element of the invention
consists of 60 v/o
A1N; 10 v/o A1203; and balance SiC. Another preferred heat sink (insulator)
composition for use with an heating element of the invention contains 80 v/o
A1N and 20
v/o SiC.
For certain systems, it may be desirable to include a power booster or
enhancement zone of intermediate resistance in the electrical circuit pathway
between the
most conductive portions of that pathway and the highly resistive (hot)
regions of that
pathway. Such booster zones of intermediate resistance are described in U.S.
Patent
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application Publication 2002/0150851 to Willkens. Generally, booster zones
will have a
positive temperature coefficient of resistance (PTCR) and an intermediate
resistance that
will permit i) effective current flow to a hot zone, and ii) some resistance
heating of the
booster region during use of the igniter, although preferably the booster zone
will not
heat to as high temperatures as the hot zone during use of the heating
element.
If employed in a heating element, preferred booster zone compositions may
comprise the same materials as the conductive and hot zone region
compositions, e.g.
preferred booster zone compositions may comprise e.g. AIN and/or A1203, or
other
insulating material; SiC or other semiconductor material; and MoSi2 or other
conductive
material. A booster zone composition typically will have a relative percentage
of the
conductive and semiconductive materials (e.g., SiC and MoSi2) that is
intermediate
between the percentage of those materials in the hot and cold zone
compositions. A
preferred booster zone composition comprises about 60 to 70 v/o aluminum
nitride,
aluminum oxide, or other insulator material; and about 10 to 20 v/o MoSi2 or
other
conductive material, and balance a semiconductive material such as SiC. A
specifically
preferred booster zone composition for use in igniters of the invention
contains 14 v/o
MoSi2, 20 v/o SiC and balance v/o A1203. A specifically preferred booster zone
composition for use in igniters of the invention contains 17 v/o MoSi2, 20 v/o
SiC and
balance A1203. A further specifically preferred booster zone composition for
use in
igniters of the invention contains 14 v/o MoSi2, 20 v/o SiC and balance v/o
AIN. A still
farther specifically preferred booster zone composition for use in igniters of
the invention
contains 17 v/o MoSi2, 20 v/o SiC and balance AIN.
The processing of the ceramic component (i.e. green body and sintering
conditions) and the preparation of the heating element from the densified
ceramic can be
done by conventional methods and as discussed above. See U.S. Patent 5,786,565
to
Wilkens and U.S. Patent 5,191,508 to Axelson et al.
A preferred fabrication method includes use of injection molding techniques.
Thus, for instance, a base element may be formed by injection introduction of
a ceramic
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material having a first resistivity (e.g. ceramic material that can function
as an insulator
or heat sink region) into a mold element that defines a desired base shape
such as a rod
shape. The base element may be removed from such first mold and positioned in
a
second, distinct mold element and ceramic material having differing
resistivity - e.g. a
conductive ceramic material - can be injected into the second mold to provide
conductive
region(s) of the igniter element. In similar fashion, the base element may be
removed
from such second mold and positioned in a yet third, distinct mold element and
ceramic
material having differing resistivity - e.g. a resistive hot zone ceramic
material - can be
injected into the third mold to provide resistive hot or ignition region(s) of
the igniter
element.
Alternatively, rather than such use of a plurality of distinct mold elements,
ceramic materials of differing resistivities may be sequentially advanced or
injected into
the same mold element. For instance, a predetermined volume of a first ceramic
material
(e.g. ceramic material that can function as an insulator or heat sink region)
may be
introduced into a mold element that defines a desired base shape and
thereafter a second
ceramic material of differing resistivity may be applied to the formed base.
Ceramic material may be advanced (injected) into a mold element as a fluid
formulation that comprises one or more ceramic materials such as one or more
ceramic
powders.
For instance, a slurry or paste-like composition of ceramic powders may be
prepared, such as a paste provided by admixing one or more ceramic powders
with an
aqueous solution or an aqueous solution that contains one or more miscible
organic
solvents such as alcohols and the like. A preferred ceramic slurry composition
for
extrusion may be prepared by admixing one or more ceramic powders such as
MoSi2,
A1203, and/or A1N 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
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one or more polymeric binders.
A wide variety of shape-forming or inducing elements may be employed to form
an igniter element, with the element of a configuration corresponding to
desired shape of
the formed igniter. For instance, to form a rod-shaped element, a ceramic
powder paste
may be injected into a cylindrical die element. To form a stilt-like or
rectangular-shaped
igniter element, a rectangular die may be employed.
After advancing ceramic material(s) into a mold element, the defined ceramic
part
suitably may be dried e.g. in excess of 50 C or 60 C for a time sufficient to
remove any
solvent (aqueous and/or organic) carrier.
Thereafter, the heating element may be further densified (e.g. to greater than
95,
96, 97, 98 or 99 percent) by thermal treatment such as in excess of 1500 C,
1600 C,
1700 C or 1800 C. A single or multiple thermal treatments may be conducted as
desired
to achieve final densities.
Heating elements of the invention may be used in many applications, including
gas
phase fuel ignition applications such as furnaces and cooking appliances,
baseboard
heaters, boilers, and stove tops. In particular, an heating element of the
invention may be
used as an ignition source for stove top gas burners as well as gas furnaces.
As discussed above, heating elements of the invention will be particularly
useful
where rapid ignition is beneficial or required, such as in ignition of a
heating fuel (gas)
for an instantaneous water heater and the like. Heating elements also may be
employed
as glow plug in a variety of vehicles (automotive, watercraft).
The following non-limiting examples are illustrative of the invention. All
documents mentioned herein are incorporated herein by reference in their
entirety.
EXAMPLE 1: Heating element fabrication.
CA 02711015 2010-06-28
WO 2009/085311 PCT/US2008/014094
Powders of a resistive composition (20 vol% MoSi2, 5 vol% SiC, 74vo1% A1203
and 1 vol% Gd203), a conductive composition (28 vol% MoSi2, 7 vol% SiC ,
64vo1%
A1203 and 1 vol% Gd203) and an insulating composition (10 vol% MoSi2, 89 vol%
A1203 and 1 vol% Gd203) are mixed with 10-16 wt% organic binder (about 6-8 wt%
vegetable shortening, 2-4 wt% polystyrene and 2-4 wt% polyethylene) to form
three
pastes with about 62-64 vol% solids loading. The three pastes are loaded into
the barrels
of a co-injection molder. A first shot filled a cavity that has an hour-glass
shaped cross-
section with the insulating paste forming the supporting base. The part is
removed from
the first cavity and placed in a second cavity. A second shot fills the bottom
half of the
volume bounded by the first shot and the cavity wall with the conductive
paste. The part
is removed from the second cavity and placed in a third cavity. A third shot
filled the
volume bounded by the first shot, second shot and the cavity wall with
resistive paste
forming a hair-pin shaped resistor separated by the insulator and connected to
conductive
legs and having the configuration shown in FIG. 5. The part is then thermally
debindered
in Ar or N2 at 500 C for 24h to remove the remaining binder and densified to
95-97% of
theoretical at 1750 C in Argon at 1 atm pressure.
The invention has been described in detail with reference to particular
embodiments thereof However, it will be appreciated that those skilled in the
art, upon
consideration of this disclosure, may make modification and improvements
within the
spirit and scope of the invention.
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