Note: Descriptions are shown in the official language in which they were submitted.
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CERAMIC IGNITERS
The present application claims the benefit of U.S. provisional application
number 60/623,478 filed October 28, 2004, which is incorporated by reference
herein
in its entirety.
BACKGROUND
1. Field of the Invention
In one aspect, the invention provides new methods for manufacture ceramic
resistive igniter elements that include extrusion of one or more layers of the
formed
element. Igniter elements also are provided obtainable from fabrication
methods of
the invention are provided.
2. Background..
Ceramic materials have enjoyed great success as igniters in e.g. gas-fired
furnaces, stoves and clothes dryers. Ceramic igniter production includes
constructing
an electrical circuit through a ceramic component a portion of which is highly
resistive and rises in temperature when electrified by a wire lead. See, for
instance,
U.S. Patents 6,582,629; 6,278,087; 6,028,292; 5,801,361; 5,786,565; 5,405,237;
and
5,191,508.
Typical igniters have been generally rectangular-shaped elements with a
highly resistive "hot zone" at the igniter tip with one or more conductive
"cold zones"
providing to the hot zone from the opposing igniter end. One currently
available
igniter, the Mini-IgniterTM, 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 ("AIN"), molybdenum disilicide ("MoSi2"), and
silicon
carbide ("SiC").
Igniter fabrication methods have included batch-type processing where a die is
loaded with ceramic compositions of at least two different resistivities. T-he
formed-- - -
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green element is then densified (sintered) at elevated temperature and
pressure. See
the above-mentioned patents. See also U.S. Patent 6,184,497.
While such fabrication methods can be effective to produce ceramic igniters,
batch-type processing presents inherent limitations with respect to output and
cost
efficiencies.
Current ceramic igniters also have suffered from breakage during use,
particularly in environments where impacts may be sustained such as igniters
used for
gas cooktops and the like.
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 new igniters that have good mechanical
integrity.
SUMMARY OF THE INVENTION
We now provide new methods for producing ceramic igniter elements which
includes extruding ceramic material to thereby form the ceramic element. Such
extrusion fabrication can provide enhanced output and cost efficiencies
relative to
prior approaches such as die cast methods as well as provide igniters of
notable
mechanical strength.
More particularly, preferred methods of the invention include extruding one or
more layers to form a ceramic element. If multiple layers are extruded,
preferably
those layers have differing resistivities to provide regions of distinct
conductivity in
the formed element. For example, an element may be fonned by co-extrusion of
one
or more multiple, sequential layers of 1) an optional insulator (heat sink);
2)
conductive zone; 3) resistive hot zone; and 4) second conductive zone. The
second
conductive zone may be applied for only a portion of the igniter to provide an
exposed resistive hot zone for fuel ignition.
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Preferred methods of the invention also include formation of multiple igniter
elements in a single process which includes a step of extruding cerainic
material.
In one aspect of such preferred methods, a plurality of operational igniter
elements can be produced from one or more billet or tile elements where such
billet or
tile elements are produced by extruding ceramic material. For example, one or
more
ceramic tile elements can be produced by extrusion or co-extrusion of ceramic
material. Thereafter, the tile elements can be thermally treated to remove any
binders
or other carriers used in the extrusion process and optionally densified (such
as at
elevated pressures and temperatures), and then the densified tile element(s)
cut to
provide igniter-shaped elements of desired dimensions. Such steps also may be
conducted in alternate sequence, e.g. prior to densification, the tile
elements(s) may be
cut to form igniter-shaped elements of desired dimensions and the thus
produced
green state igniter elements may then be optionally densified at elevated
pressures and
temperatures.
Fabrication methods of the invention may include additional processes for
addition of ceramic material to produce the formed ceramic element. For
instance,
one or more ceramic layers may be applied to a formed element such as by dip
coating, spray coating and the like of a ceramic composition slurry.
Preferred ceramic elements obtainable by methods of the invention comprise a
first conductive zone, a resistive hot zone, and a second conductive zone, all
in
electrical sequence. In certain preferred embodiments, the first conductive
zone will
be positioned within an inner area of the igniter element and encased or
enveloped at
least in part by the second, outer positioned conductive zone, as further
discussed
below. 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). For at least some preferred applications, at least a
substantial
portion of the first conductive zone does not contact a ceramic insulator
(heat sink).
Such absence of a ceramic insulator region can provide enhanced time-to-
ignition
- temperature performance of the igniter. _
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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.
Igniters of the invention may have a variety of configurations. In a preferred
configuration, a conductive shaft element is positioned within a conductive
tube
element and both the shaft and tube elements mate with a hot zone cap or end
region.
Ceramic igniters of the invention can be employed at a wide variety of
nominal voltages, including nominal voltages of 6, 8, 10, 12, 24,120, 220, 230
and
240 volts.
The igniters of the invention are useful for ignition in a variety of devices
and
heating systems. More particularly, heating systems are provided that comprise
a
sintered ceramic igniter element as described herein. Specific heating systems
include
gas cooking units, heating units for commercial and residential buildings,
including
water heaters.
Other aspects of the invention are disclosed infra.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1(includes FIGS. 1 A through 1 C) depicts a preferred production method
of the invention;
FIG. 2 shows a cut-away view along line 1-1 of FIG. 1 C;
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FIG. 3 (includes FIGS. 3A through 3D) depicts a further preferred production
method of the invention;
FIGS. 4 shows a further preferred igniter of the invention; and
FIGS. 5A and 5B shows a further preferred igniter of the invention; FIG. 5B is
a view taken along line 5B-5B of FIG. 5A;
FIGS. 6A and 6B shows a further preferred igniter of the invention; FIG. 6B is
a view taken along line 6B-6B of FIG. 6A; and
FIGS. 7 and 8 show further preferred igniter and fabrication methods; and
FIGS. 9A, 9B and 9C an additional preferred igniter and fabrication method.
DETAILED DESCRIPTION OF THE INVENTION
As discussed above, new methods are now provided for producing ceramic
igniter elements that include extrusion of one or more layers of the element.
As typically referred to herein, the term extrusion, extruding or other
similar
term indicates the general process where a material is forced through or
otherwise
advanced through a shape-inducing member such as a die element, where the die
may
be suitably formed of e.g. a polymer, metal, combinations thereof, etc. In
extrusion
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 through a shape-inducing element. Suitably, the
extruded material may be cured or otherwise hardened after exiting the shape-
inducing element.
Referring now to the drawings, FIGS. lA through 1C show a preferred
__ fabrication method of the invention. As shown in FIG. lA, an igniter
element 10 is
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produced by co-extrusion of multiple layers that have differing resistivities.
In a
preferred system, the inner layer is a conductive layer 12, an intermediate
layer is a
more resistive hot zone layer 14, and the exposed outer layer is a second
conductive
layer 16.
Extrusion of the igniter elements may be suitably conducted by forming a fluid
formulation of a ceramic composition and advancing the ceramic formulation
through
a die element that provides the igniter of desired configuration.
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
MoSiZ,
SiC, 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 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 extruded igniter. For instance, to form a rod-shaped element, a
ceramic
powder paste may be extruded through a cylindrical die element. To form a
stilt-like
or rectangular-shaped igniter element, a rectangular die may be employed.
After extrusion, the shaped igniter 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.
The examples which follow describe preferred extrusion processes to form an
igniter element.
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As shown in FIG. 1 B, a portion of conductive layer 16 can be removed to
expose the resistive hot zone 14. The exposed hot zone length (shown as length
"a" in
FIG. 1B) can be varied to provide optimal performance for a targeted voltage.
The igniter element 10 then may be further processed as desired. For
example, as shown in FIG. 1 C, igniter 10 may be core-drilled to provide inner
void
region 18. The formed igniter 10 also may be further densified such as under
conditions that include temperature and pressure.
A suitable igniter electrical path can be seen in FIG. 2 where electrical
power
enters the igniter system 10 through the interposed conductive core element 12
that
mates with resistive hot zone 14. Proximal end 12a of conductive element 12
and 10a
of conductive element 10 may be 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 12a as
disclosed in U.S. Published Patent Application 2003/0080103. Metallic fixtures
also
maybe suitably employed to encase the igniter proximal end.
As shown in FIG. 2, the igniter's 10 depicted electrical path extends from
conductive core element 12 through resistive hot zone 14 then through outer,
encasing conductive region 16. The igniter also can be configured whereby the
electrical path runs in the opposite direction and extends conductive region
16 through
resistive hot zone 14 and then through the conductive core element 12.
As can be seen in FIGS. 1 C and 2, the first, inner conductive zone 12 is
segregated through void region 18 from the other igniter areas until mating
with hot
zone 14 at the conductive zone distal portion 12c. Further, as discussed
above, in
preferred systems such as those depicted in FIGS. 1 C and 2, the proximal
portion 12a
of the first conductive zone does not contact a ceramic heat sink (insulator)
area that
has been employed in certain prior systems. For at least many applications,
suitably
the igniter may not contain any insulator or heat sink region and will contain
only two
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regions of the differing resistivity, i.e. the igniter will contain only
conductive (cold)
zone(s) and a higher resistivity (hot) zone.
Such absence of a ceramic insulator from at least a substantial portion of the
first conductive zone length can provide significant advantages, including
enhanced
time-to-temperature performance of the igniter. As referred to herein, "a
substantial
portion of the first conductive zone length" indicates that at least about 40
percent of
the length of the conductive zone as measured from the point of affixation of
an
electrical lead to the mating hot zone (as shown by distance b is FIG. 2) does
not
contact a ceramic insulator material. More preferably, at least about 50, 60,
70, 80, 90
or 95 percent or the entire length of the conductive zone as measured from the
point
of affixation of an electrical lead to the mating hot zone (as shown by
distance b is
FIG. 2) does not contact a ceramic insulator material. In particularly
preferred
systems, at least a substantial portion of the first conductive zone length is
exposed
such as to void area 18 as generally depicted in the igniters exemplified in
FIGS. 1C
and 2.
As referred to herein, the term "time-to-temperature" or similar term refers
to
the time for an igniter hot zone to rise from room temperature (ca. 25 C) to a
fuel (e.g.
gas) ignition temperature of about 1000 C. A time-to-temperature value for a
particular igniter is suitably determined using a two-color infrared
pyrometer.
Particularly preferred igniters of the invention may exhibit time-to-
temperature values
of about 3 seconds or less, or even about 2 seconds or less.
As discussed above, and exemplified in FIG. 1, for at least certain preferred
systems, 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
b shown in
FIG. 2. FIG. 1 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
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igniter length b in FIG. 2) has a rounded cross-sectional shape; in such
designs, the
balance of the igniter length may have a profile with exterior edges.
FIG. 3A through 3D depict a further preferred igniter fabrication method of
the invention where igniter element 20 is fonned by extrusion of a conductive
core
region 22 that is encased within an insulator (heat sink) inner layer 24.
As shown in FIG. 3B, a second outer conductive layer 26 can be applied to the
extruded igniter element followed by application of a resistive hot zone
region 28, as
shown in FIG. 3C. As discussed above, ceramic layers 26 and 28 may be applied
by
any of a number of methods. A preferred application method is dip coating of
igniter
element in a ceramic composition slurry with appropriate masking of non-coated
igniter regions.
For such dip coating applications, a slurry or other fluid-like composition of
the ceramic composition may be suitably employed. The slurry may comprise
water
and/or polar organic solvent carriers such as alcohols and the like and one or
more
additives to facilitate the formation of a uniform layer of the applied
ceramic
composition. For instance, the slurry composition may comprise one or more
organic
emulsifiers, plasticizers, and dispersants. Those binder materials may be
suitably
removed thermally during subsequent densification of the igniter element.
Dip coating may be conducted by immersion of the igniter element in the
ceramic composition slurry. Preferred dip coating processes and ceramic
composition
slurries for dip coating are exemplified in the examples which follow.
As shown in FIG. 3D, ceramic insulator region 24 may be at least partially
removed such as by drilling to provide void regions 30. Thus, as depicted in
FIG. 3D,
interposed first conductive zone 22 extends from a proximal end 22a (which may
have an affixed electrical lead as discussed above) and extends to resistive
zone 28
that mates with second conductive zone 26, positioned above partially removed
insulator layer 24 and interposed void region 30.
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FIG. 4 shows a further preferred igniter 40 of the invention that can be
provided by co-extrusion of an insulator ceramic core with an encasing
resistive zone
42 (such as resistive zone 28 shown in FIG. 3C). Conductive zone 44 then_ may
be
applied such as by dip coating the igniter element in a slurry of the conduc-
tive
ceramic composition. Flats 46 may be formed on igniter faces as generally
shown in
FIG. 4 either by machining after densification or in the green state. As
discussed
above with respect to igniter 20 in FIG. 3D, ceramic insulator region may be
at least
partially removed such as by drilling to provide void region 48, or more
preferably at
least partial removal of the insulator region in the final igniter element may
be
provided by coextruding a hollow tube element.
Significantly, methods of the invention can facilitate fabrication of igniters
of
a variety of configurations as may be desired for a particular application. To
provide
a particular configuration, an appropriate shape-inducing die is employed
through
which a ceramic composition (such as a ceramic paste) may be extruded.
For instance, a die with a substantially square profile may be employed to
produce the igniter element 50 depicted in FIGS. 5A and 5B which comprises a
rectangular-like or a silt-like core conductive zone 52 with angular cross-
sectional
shape (more particularly, substantially square cross-sectional shape as
clearly depicted
in FIG. 5B) and similarly angular outer conductive zone 54 and hot zone (hot
zone not
shown in cut-away view of FIG. 5A).
A die with an irregular rounded shaped profile may be employed to form an
element 60 as shown in FIGS. 6A and 6B with core conductive zone 62 ax;}d
outer
conductive zone 64 each having irregular rounded cross-sectional shapes.
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 b in FIG. 2) suitably may be from about 0.5 to about 5 cm, more
pzeferably
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from about 1 about 3 cm, and the igniter cross-sectional width may suitably be
frorn
about (length c in FIG. 2) suitably may be from about 0.2 to about 3 cm.
Similarly, the lengths of the conductive and hot zone regions also may
suitably
vary. Preferably, the length of a first conductive zone (length d in FIG. 2)
of an
igniter of the configuration depicted in FIG. 2 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 e in FIG. 2) may be from about
0.1 to
about 2 cm, with a total hot zone electrical path length (length f in FIG. 2)
of about
0.2 to 5 or more cm, with a total resistive zone path length (shown as the
dashed line
in FIG. 2) of about 0.5 to 3.5 cm generally preferred.
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 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.
As discussed above, preferred methods of the invention also include
formation of multiple igniter elements in a single general process which
includes a
step of extruding ceramic material.
For example, as generally illustrated in FIG. 7, a ceramic tile element 70 can
be provided by extruding ceramic material though a corresponding die element,
with
the tile element preferably formed through co-extrusion of multiple layers or
regions
of the tile with each layer or region having differing electrical resistivity.
Thus, as
exemplified in FIG. 7, multiple regions of the tile element are co-extruded to
provide
conductive region 72 and comparatively more resistive "hot" or ignition region
74.
The arrow shown in FIG. 7 depicts the direction of co-extrusion of the tile
element. The ceramic tile element 70 may have a variety of dimensions and
suitably
maybe sliced to provide 5, 10, 15, 20, 30, 40, 50, 80, 100 or more discrete
igniter _
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elements. For certain embodiments, the tile element suitably may have a
thickness
(dimension "a" in FIG. 7) that is the same thickness as the igniter elements
formed
after slicing of the tile. In other embodiments, such as discussed below with
respect to
FIGS. 9A through 9Cwhere multiple tiles are aggregated, the tile element
thickness
may be only a portion of the thickness of the subsequently formed igniter
elements.
Following extrusion, binder(s) and other organics present in the extruded
ceramic material may be removed such as by thermal treatment. Thereafter,
igniter
elements may be formed in desired dimensions such as by slicing igniter
element 70
in a direction perpendicular to the direction of extrusion (that extrusion
direction
shown by the depicted arrow in FIG. 7). As discussed above, the igniter
elements
may be densified if desired at elevated pressures and temperatures. Also, if
desired,
the igniter element may be further processed as desired, e.g. where an
internal area is
removed to form a so-called "slotted" igniter design around which slot an
electrical
path is provided.
FIG. 8 illustrates another preferred method where tile element 90 is provided
through extrusion of a ceramic material though a corresponding die element,
with the
tile element preferably formed through co-extrusion of multiple layers or
regions of
the tile with each layer or region having differing electrical resistivity.
Thus, as
exemplified in FIG. 8, multiple regions of the tile element are co-extruded to
provide
conductive areas 86 and comparatively more resistive "hot" or ignition areas
88.
The arrow shown in FIG. 8 depicts the direction of co-extrusion of the tile
element. The phantom line 82 depicts where the formed tile element 80 can be
cut to
two separate components. Thereafter, each tile component can be further sliced
(such
as perpendicular to depicted arrow) to provide individual igniter elements. In
the
embodiment depicted in FIG. 8, the formed igniter element includes an internal
heat
sink or insulator region 84 (rather than a slotted configuration discussed
above with
respect to FIG. 7). Either before or after slicing of the tile element the
ceramic
material may be densified such as at elevated pressures and temperatures.
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FIGS. 9A through 9C illustrate yet another preferred fabrication process where
multiple tile or billet elements are produced and then aggregated to form a
plurality of
igniter elements. More particularly, as generally illustrated in FIG. 9A,
ceramic tile
or billet element 90 is formed by co-extrusion of ceramic material through a
corresponding die and includes a conductive region 92 and comparatively more
resistive "hot" or ignition region 94. As shown in FIG. 9B, a separate ceramic
tile or
billet element 96 is formed by co-extrusion of ceramic material through a
corresponding die and includes a insulator or heat sink region 98. The arrows
shown
in FIGS. 9A and 9B depict the direction of co-extrusion of the tile element.
Binder(s)
and other organics of the extruded ceramic materials can be removed by thermal
treatment of the formed tile elements.
Then multiple tile elements may be aggregated, such as assembly of a three-
tile stack of an element 96 between two elements 90 as shown in FIG. 9c. That
assembly 100 then be cut to provide individual igniter elements. As discussed
above,
the igniter elements may be densified if desired at elevated pressures and
temperatures.
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,
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 igniters of the invention may be
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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 (A1N), 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 10$ ohm-
cm.
If the semiconductive component is present as more than about 45 v/o of a hot
zone
composition (when the conductive ceramic is in the range of about 6-10 v/o),
the
resultant composition becomes too conductive for high voltage applications
(due to
lack of insulator). Conversely, if the semiconductor material is present as
less than
about 10 v/o (when the conductive ceramic is in the range of about 6-10 v/o),
the
resultant composition becomes too resistive (due to too much insulator).
Again, at
higher levels of conductor, more resistive mixes of the insulator and
semiconductor
fractions 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 amount of more than 35 v/o of the hot zone composition, the
resultant
ceramic of the hot zone composition, the resultant ceramic can become too
conductive. Typically, the conductor is selected from the group consisting of
molybdenum disilicide, tungsten disilicide, and nitrides such as titanium
nitride, and
carbides such as titanium carbide. Molybdenum disilicide is generally
preferred.
In general, preferred hot (resistive) zone compositions include (a) between
about 50 and about 80 v/o of an electrically insulating material having a
resistivity of
at least about 1010 ohm-cm; (b) between about 0 (where no semiconductor
material
employed) and about 45 v/o of a semiconductive material having a resistivity
of _
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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 igniters of the invention contains
10 v/o
MoSiZ, 20 v/o SiC and balance A1N or A1203.
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. A1N and/or A1203 or other insulating material; SiC or
other
semiconductor material; and MoSiZ 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
AIN
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.
A specifically preferred cold zone composition for use in igniters of the
invention contains 20 to 35 v/o MoSi2, 45 to 60 v/o SiC and balance either A1N
and/or
A1203.
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.
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If employed, such a heat sink zone may mate with a conductive zone or a hot
zone, or both. Preferably, a sintered insulator region has a resistivity of at
least about
1014 ohm-cm at room temperature and a resistivity of at least 104 ohm-cm at
operational temperatures and has a strength of at least 150 MPa. Preferably,
an
insulator region has a resistivity at operational (ignition) temperatures that
is at least 2
orders of magnitude greater than the resistivity of the hot zone region.
Suitable
insulator compositions comprise at least about 90 v/o of one or more aluminum
nitride, alumina and boron nitride. A specifically preferred insulator
composition of
an igniter of the invention consists of 60 v/o AIN; 10 v/o A1203; and balance
SiC.
Another preferred heat composition for use with an igniter of the invention
contains
80 v/o AIN and 20 v/o SiC.
The igniters of the present invention may be used in many applications,
including gas phase fuel ignition applications such as furnaces and cooking
appliances, baseboard heaters, boilers, and stove tops. In particular, an
igniter of the
invention may be used as an ignition source for stop top gas burners as well
as gas
furnaces.
Igniters of the invention also are particularly suitable for use for ignition
where liquid fuels (e.g. kerosene, gasoline) are evaporated and ignited, e.g.
in vehicle
(e.g. car) heaters that provide advance heating of the vehicle.
Preferred igniters of the invention are distinct from heating elements known
as
glow plugs. Among other things, frequently employed glow plugs often heat to
relatively lower temperatures e.g. a maximum temperature of about 800 C, 900 C
or
1000 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 C, 1300 C or 1400 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
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volts, such as 60 volts or more or 120 volts or more including 220, 230 and
240 volts,
whereas glow plugs are typically employed only at voltages of from 12 to 24
volts.
The following non-limiting examples are illustrative of the invention. All
documents mentioned herein are incorporated herein by reference in their
entirety.
EXAMPLE 1: Igniter fabrication.
Powders of a resistive composition (15 vol% MoSi2, 20 vol% SiC, remainder
A1203) and an insulating composition (20 vol% SiC and 80 vol % A1203) were
mixed with about 16 wt% water and 5wt% Methyl Cellulose (Dow A4M) to form two
pastes. The two pastes were loaded into the barrel of a piston extruder with
the
insulating paste forming a cylindrical core and the conducting paste forming a
cylindrical sheath. The two mixes were co-extruded to form a coaxially clad
rod of
about 0.300" diameter. The rod was then cured at 65 C to remove the moisture
and
cut to 1-3" lengths. The samples were dip-coated to coat one-half of the
length with a
slurry of conducting composition (30 vol% MoSi2, 20 vol% SiC, remainder
A1203).
The slurry contains dispersants and a low viscosity base fluid containing
isopropyl
alcohol, PEG 400 (emulsifier; reaction product of stearic acid), SANTICIZER
160
(plasticizer; butyl benzyl), BUTWAR B76 (Monsanto; polyvinyl butyral), 11 1M
dispersant (DARVAN). The coated sample was pre-sintered in Argon atmosphere at
1200 C to bum out the binders, coated with boron nitride and densified at 1750
C for
1 hour under a glass-hot isostatic press. The densified parts were cleaned by
grit-
blasting and an electrical circuit was fonned by cutting grooves on opposite
sides of
the curved surfaces. The grooves were 1/8" to'/4" short of the part length at
the
uncoated end. The two faces of the coated end now separated by the groove form
the
two legs of the igniter and when connected to a power supply at a voltage of
60 volts
attained a temperature of about 1200 C.
Example 2: Igniter fabrication.
Powders of a conducting composition (30 vol% MoSi2, 20 vol% SiC,
remainder A1203) and an insulating composition (20 vol% SiC and 80 vol %
A1203)
were mixed with about 16 wt% water and 5wt% Methyl Cellulose (Dow A4M) to
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form two pastes. The two pastes were loaded into the barrel of a piston
extruder with
the conducting paste forming a cylindrical core and the insulating paste
forming a
cylindrical sheath. The two mixes were co-extruded to form a coaxially clad
rod of
about 0.300" diameter. The rod was then cured at 65 C to remove the moisture
and
cut to 1-3" lengths. The samples were dip-coated to coat one-half of the
length with a
slurry of conducting composition (30 vol% MoSi2, 20 vol% SiC, remainder A1203)
and the remaining half with a slurry of resistive composition (15 vol% MoSi2,
20
vol% SiC, remainder A1203). The slurry contains dispersants and a low
viscosity
base fluid such as containing isopropyl alcohol, PEG 400, SANTICIZER 160,
BUTVAR B76, 111 M dispersant. The coated sample was presintered in Argon
atmosphere at 1200 C to burn out the binders, coated with boron nitride and
densified
at 1750 C for 1 hour under a glass-hot isostatic press. The densified parts
were
cleaned up grit-blasting and an electrical circuit was formed by cutting 1/8"
from tip
of the rod at the end coated with the conducting layer. The core and the outer
surface
at the cut end separated by the insulating layer form the two legs of the
igniter and
when connected to a power supply at a voltage of 60 volts attained a
temperature of
about1200 C.
Example 3: Further igniter fabrication
Admixed ceramic powders to form a resistive composition (15 vol% MoSi2,
20 vol% SiC, remainder A1203) and an insulating composition (20 vol% SiC and
80
vol % A1203) were separately mixed with about 16 weight % water and about 5
weight percent methyl cellulose (available from Clariant) to form two ceramic
pastes.
The two pastes were loaded into the barrels of two-piston extruders assembled
to feed
into a two-layer coextrusion die. The insulator paste formed the core and the
resistive
paste formed a skin layer of about 0.015 inches thickness. The coextruded rod
had an
outer diameter of about 0.25 inches. The rod was then cured at about 65 C to
remove
moisture and cut to 1.25 inches lengths. The samples were debinded and
densified.
The densified parts were cleaned by grit blasting and an electrical circuit
formed by
cutting grooves on opposite sides to 1/8 inches from the end.
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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.
