Note: Descriptions are shown in the official language in which they were submitted.
CA 02651001 2008-10-31
WO 2007/130658 PCT/US2007/010975
CERAMIC HEATING ELEMENTS
The present applications claims the benefit of U.S. provisional application
number 60/798,266 filed May 4, 2006, incorporated by referenced herein in its
entirety.
lo BACKGROUND
1. Field of the Invention
In one aspect, the invention provides new methods for manufacture ceramic
heating elements that include substantially pressureless sintering of the
formed green
igniter element. Igniter elements also are provided, including such elements
obtainable from fabrication methods of the invention.
2. Background.
Ceramic materials have enjoyed great success as igniters in e.g. gas-fired
furnaces, stoves and clothes dryers. Ceramic igniter production includes
constructing
an electrical circuit through a ceramic component a portion of which is highly
resistive and rises in temperature when electrified by a wire lead. See, for
instance,
U.S. Patents 6,582,629; 6,278,087; 6,028,292; 5,801,361; 5,786,565; 5,405,237;
and
5,191,508.
Typical igniters have been generally rectangular-shaped elements with a
highly resistive "hot zone" at the igniter tip with one or more conductive
"cold zones"
providing to the hot zone from the opposing igniter end. One currently
available
igniter, the Mini-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. The
formed
green element is then densified (sintered) at elevated temperature and
pressure. See
the above-mentioned patents. See also U.S. Patent 6,184,497.
CA 02651001 2008-10-31
WO 2007/130658 PCT/US2007/010975
While such fabrication methods can be effective to produce ceramic igniters,
the protocols can present inherent limitations with respect to output and cost
efficiencies.
.
It thus would be desirable to have new heating element systems: It would be
particularly desirable to have new methods for producing ceramic heating
elements.
It also would be desirable to have more efficient production methods.
SUMMARY OF THE INVENTION
In one aspect, new ceramic articles are provided which are formed from one or
more ceramic powders that have a mean particle size of about 2.5 microns or
less.
We have found that ceramic articles made from such small size ceramic
materials can be densified under significantly more mild conditions, including
under
reduced pressures relative to prior procedures.
In another aspect, ceramic articles are provided that are fabricated by
treatment of the green state ceramic article by multiple, increasing
pressures.
Preferably, the ceramic article is treated at a first pressure and then
treated at a second
pressure which is higher than the first pressure. Preferably, the multi-
pressure
densification is conducted with use of gas-pressure sintering.
We have found that the multiple-stage pressure treatments can provide a
highly dense article (e.g. at least 96, 97, 98 or 99 dense percent) ceramic
article under
quite mild conditions. For instance, the first pressure treatment suitably may
be at
about 1000 psi or 500 psi or less and the second pressure treatment may be at
4000 psi
or less. Significantly lower pressures also have yielded highly dense
articles, such as
a first pressure of about 200 psi or less or 150 psi or less and a second
pressure
treatment of about 3000 psi or less, 2000 psi or less or 1500 psi or less.
In particularly preferred aspects of the invention, ceramic compositions are
utilized that comprise one or more metal oxides such as alumina. Preferably,
the one
2
CA 02651001 2008-10-31
WO 2007/130658 PCT/US2007/010975
or more one or more metal oxides have a small mean particle size as disclosed
herein.
Particularly preferred are ceramic compositions that comprise alumina with
small
mean particle size as disclosed herein, such as 2.5 microns or less, 2 microns
or less,
1.5 microns or less or 1 micron or less.
In a further aspect of the invention, ceramic compositions are densified in
the
absence of a so-called sintering aid. Sintering aid additives have included
rare earth
oxides, such as yttria (yttrium oxide), a gadolinium material (e.g. a
gadolinium oxide
or Gd203), a europium material (e.g. a europium oxide or Eu203), a ytterbium
material (e.g. a ytterbium oxide or Yb203), or a lanthanum material (e.g.
lanthanum or
La203).
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 C, more typically in excess of 1600 C
such as
at least 1700 C or 1800 C. Preferably, the sintering is conducted under an
inert
atmosphere, e.g. in an atmosphere of an inert gas such as argon or nitrogen.
Preferably, the hardening treatment provides a ceramic element that is at
least
95 percent dense, more preferably a ceramic element that is at least 96, 97,
98 or 99
percent dense. The hardening process which includes the noted elevated
temperatures
is conducted for a time sufficient to achieve such densities, which may be
several
hours or more.
Particular ceramic compositions and method of forming the green ceramic
element may be utilized to facilitate producing a dense ceramic element in the
absence of substantially elevated pressures.
More specifically, preferred ceramic compositions employed to form a
ceramic element may be at least substantially free or completely free of
silicon
carbide, or other carbide material. As referred to herein, a ceramic
composition is at
3
CA 02651001 2008-10-31
WO 2007/130658 PCT/US2007/010975
least substantially free of silicon carbide or other carbide material if it
contains less
than 10 volume percent of silicon carbide or other carbide material based on
total
volume of the ceramic composition, more typically less than about 9, 8, 7, 6,
5,4, 3, 2,
1 or 0.5 volume percent based on total volume of the ceramic composition.
For sintering a ceramic element that comprises alumina, preferably sintering
of the element is conducted in an atmosphere that is at least substantially
free of
nitrogen (e.g. less than 5 volume % nitrogen based on total atmosphere), or
more
preferably at least essentially free of nitrogen (e.g. less than 2 or 1 volume
% nitrogen
based on total atmosphere), or more preferably completely free of nitrogen.
For
instance, sintering may be conducted in an Argon atmosphere.
For sintering a ceramic element that comprises AIN, preferably sintering of
the
element is conducted in an atmosphere that contains at least some nitrogen,
e.g. at
is least about 5 volume percent of nitrogen (i.e. at least 5 volume % nitrogen
based on
total atmosphere), or higher levels such as at least about 10 volume percent
of
nitrogen (i.e. at least 10 volume. % nitrogen based on total atmosphere).
It also may be preferred to form the ceramic elements through an injection
molding process. As typically referred to herein, the term "injection molded,"
"injection molding" or other similar term indicates the general process where
a
material (here a ceramic or pre-ceramic material) is injected or otherwise
advanced
typically under pressure into a mold in the desired shape of the ceramic
element
followed by cooling and subsequent removal of the solidified element that
retains a
replica of the mold.
In injection molding formation of heating elements of the invention, a ceramic
material (such as a ceramic powder mixture, dispersion or other formulation)
or a pre-
ceramic material or composition may be advanced into a mold element.
hi suitable fabrication methods, an integral igniter element having regions of
differing resistivities (e.g., conductive region(s), insulator or heat sink
region and
4
CA 02651001 2008-10-31
WO 2007/130658 PCT/US2007/010975
higher resistive "hot" zone(s)) may be formed by sequential injection molding
of
ceramic or pre-ceramic materials having differing resistivities.
Thus, for instance, a base element may be formed by injection introduction of
a ceramic material having a first resistivity (e.g. ceramic material that can
function as
an insulator or heat sink region) into a mold element that defines a desired
base shape
such as a rod shape. The base element may be removed from such first mold and
positioned in a second, distinct mold element and ceramic material having
differing
resistivity - e.g. a conductive ceramic material - can be injected into the
second mold
to provide conductive region(s) of the igniter element. In similar fashion,
the base
element may be removed from such second mold and positioned in a yet third,
distinct
mold element and ceramic material having differing resistivity - e.g. a
resistive hot
zone ceramic material - can be injected into the third mold to provide
resistive hot or
ignition region(s) of the heating element.
In preferred aspects of the invention, at least three portions of a ceramic
heating element are injection molded in single fabrication sequence to produce
a
ceramic component, a so-called "multiple shot" injection molding process where
in
the same fabrication sequence where multiple portions of an igniter element
having
different resistivity values (e.g. hot or highly resistive portion, cold or
conductive
portion, and insulator or heat sink portion). In at least certain embodiments,
a single
fabrication sequence includes sequential injection molding applications of a
ceramic
material without removal of the element from the element-forming area and/or
without deposition of ceramic material to an element member by a process other
than
injection molding.
For instance, in one aspect, a first insulator (heat sink) portion can be
injection
molded, around that insulator portion conductive leg portions then can be
injection
molded in a second step, and in a third step a resistive hot or ignition zone
can be
applied by injection molding to the body containing insulator and resistive
zones.
In another embodiment, methods for producing a resistive ceramic heating
element are provided, which include injection molding one or more portions of
a
5
CA 02651001 2008-10-31
WO 2007/130658 PCT/US2007/010975
ceramic element, wherein the ceramic element comprises three or more regions
of
differing resistivity.
Fabrication methods of the invention may include additional processes for
addition of ceramic material to produce the formed ceramic element. For
instance,
one or more ceramic layers may be applied to a formed element such as by dip
coating, spray coating and the like of a ceramic composition slurry.
Preferred ceramic elements obtainable by methods of the invention comprise a
first conductive zone, a resistive hot zone, and a second conductive zone, all
in
electrical sequence. Preferably, during use of the device electrical power can
be
applied to the first or the second conductive zones through use of an
electrical lead.
Particularly preferred heating elements of the invention will have a rounded
cross-sectional shape along at least a portion of the heating element length
(e.g., the
length extending from where an electrical lead is affixed to the igniter to a
resistive
hot zone). More particularly, preferred ceramic heating elements may have a
substantially oval, circular or other rounded cross-sectional shape for at
least a portion
of the igniter length, e.g. at least about 10 percent, 40 percent, 60 percent,
80 percent,
90 percent of the igniter length, or the entire igniter length. A
substantially circular
cross-sectional shape that provides a rod-shaped heating element is
particularly
preferred. Such rod configurations offer higher Section Moduli and hence can
enhance the mechanical integrity of the heating element.
Ceramic heating elements of the invention can be employed at a wide variety
of nominal voltages, including nominal voltages of 6, 8, 10, 12, 24,120, 220,
230 and
240 volts.
The heating elements of the invention are useful for ignition in a variety of
devices and heating systems. More particularly, heating systems are provided
that
comprise a sintered ceramic igniter element as described herein. Specific
heating
systems include gas cooking units, heating units for commercial and
residential
buildings, including water heaters.
6
CA 02651001 2008-10-31
WO 2007/130658 PCT/US2007/010975
As referred to herein, the term "ceramic material" includes materials both
prior to and after sintering processes. For instance, alumina, Mo2Si2, SiC,
AIN and
other materials referred to herein are considered ceramic materials including
in the
pre-sintered state of those materials.
Other aspects of the invention are disclosed infra.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. lA and 1B show top and bottom views respectively of a heating
element of the invention;
FIG. 2A shows a cut-away view along line 2A-2A of FIG. lA; and
FIG 2B shows a cut-away view along line 2B-2B of FIG. 1A.
DETAILED DESCRIPTION OF THE INVENTION
In a first aspect, new ceramic articles are provided which are formed from one
or more ceramic powders that have a.mean particle size of about 2.5 microns or
less,
more preferably a mean particle size of about 2 microns or less, or 1.5, 1.25
or 1
micron or less. Such ceramic materials typically have a mean particle size of
at least
about 0.2, 0.3, 0.4 or 0.5 microns.
In preferred ceramic compositions, at least a major portion (e.g. greater than
50, 60, 70, 80 or 90 weight percent) of a specified ceramic material will have
a small
particle size as disclosed herein. More preferred, the entire portion of the
specified
ceramic material will have such a small particle size. For example, if a
ceramic
composition is indicated to include alumina having a mean particle size of 2
microns
or less, preferably at least a major portion (such as greater than 50, 60, 70,
80 or 90
weight percent) of the alumina utilized in the ceramic composition will have a
mean
particle of 2 microns or less, and more preferably the entire portion of
alumina present
in the ceramic composition will have a mean particle size of 2 microns or
less.
7
CA 02651001 2008-10-31
WO 2007/130658 PCT/US2007/010975
As discussed herein, ceramic compositions employed to produce heating
elements of the invention may suitably comprise two, three or more distinct
materials
such as A1203, AIN, Mo2Sia, SiC, and the like. Suitably, one or more of such
distinct
materials may be employed in small mean particle size as disclosed herein.
However,
in certain embodiments, not all materials of a ceramic compositions need to be
employed in such mean small particle sizes. In this aspect of the invention,
at least
one material of a multiple-material composition is of such small mean particle
size,
but more than one or all materials of a multiple-material composition may have
such
small mean particle sizes if desired.
As discussed above, in certain embodiments, use of a small mean particle size
metal oxide such as A1203 may be particularly preferred.
Without being bound by any theory, it is believed that use of such smaller
mean size particle materials can facilitate reduced pressure sintering of the
formed
green state heating element.
In another aspect, as discussed above, new methods are now provided for
producing ceramic igniter elements that include hardening (densifiying) of a
formed
green ceramic element under reduced elevated pressures.
In this aspect, ceramic articles are provided that are fabricated by treatment
of
the green state ceramic article by multiple, increasing pressures. Preferably,
the
ceramic article is treated at a first pressure and then treated at a second
pressure which
is higher than the first pressure.
For at least certain applications, the first and second pressure treatments
differ
by at least 500 psi, more preferably by at least 1000 psi, 2000 psi or 2500
psi.
For at least certain applications, the first pressure treatment suitably may
be at
about 3,000 psi or less, 2000 psi or less, 1000 psi or less, 500 psi or less,
or 200 psi or
less, and the second pressure treatment=may be at 6000 psi or less, 5000 psi
or less,
4000 psi or less, 3000 psi or less, 2000 psi or less, 1500 psi or less or 1000
psi or less.
8
CA 02651001 2008-10-31
WO 2007/130658 PCT/US2007/010975
For at least certain applications, the first pressure treatment and the second
pressure treatment each will not exceed 5000 psi.
Other pressures also may be employed for the first and second pressure
treatments provided the first pressure treatment is at a lower level than the
second
pressure treatment.
Again, withoiut 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.
Preferably, the multi-pressure densification is conducted with use of gas-
pressure sintering. Commercial gas phase sintering ovens may be employed.
Preferably, sintering is conducted under an inert atmosphere, such as a
nitrogen or
argon atmosphere.
As discussed above, in a further aspect of the invention, ceramic compositions
are densified in the absence of a so-called sintering aid.
As discussed above, ceramic elements may be preferably formed by injection
molding techniques. Thus, for instance and as discussed above, a base element
may
be formed by injection introduction of a ceramic material having a first
resistivity
(e.g. ceramic material that canfunction 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.
9
CA 02651001 2008-10-31
WO 2007/130658 PCT/US2007/010975
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 defmes 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 AIN in a fluid composition of water optionally together with one
or
more organic solvents such as one or more aqueous-miscible organic solvents
such as
a cellulose ether solvent, an alcohol, and the like. The ceramic slurry also
may
contain other materials e.g. one or more organic plasticizer compounds
optionally
together with one or more polymeric binders.
A wide variety of shape-forming or inducing elements may be employed to
form an igniter element, with the element of a configuration corresponding to
desired
shape of the formed igniter. For instance, to form a rod-shaped element, a
ceramic
powder paste may be injected into a cylindrical die element. To form a stilt-
like or
rectangular-shaped igniter element, a rectangular die may be employed.
After advancing ceramic material(s) into a mold element, the defined ceramic
part suitably may be dried e.g. in excess of 50 C or 60 C for a time
sufficient to
remove any solvent (aqueous and/or organic) carrier.
CA 02651001 2008-10-31
WO 2007/130658 PCT/US2007/010975
The examples which follow describe preferred injection moldiing processes to
form an igniter element.
Refen-ing now to the drawings, FIGS. IA and 1B shows a suitable heating
element 10 of the invention.
As can be seen in FIG. 1 A, 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
funetion as resistive hot zone 20.
FIG. 1B shows igniter bottom face with exposed heat sink region 12.
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.
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.
As discussed above, and exemplified by heating element 10 of FIGS. lA, 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
11
CA 02651001 2008-10-31
WO 2007/130658 PCT/US2007/010975
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. 1 B) has a rounded cross-
sectional
shape; in such designs, the balance of the heating element length may have a
profile
with exterior edges.
Heating element of a variety of configurations may be fabricated as desired
for
a particular application. Thus, for instance, to provide a particular
configuration, an
appropriate shape-inducing mold element is employed through which a ceramic
composition (such as a ceramic paste) may be injected.
Dimensions of heating elements of the invention may vary widely and may be
selected based on intended use of the heating element. For instance, the
length of a
preferred heating element (length x in FIG. 1B) suitably may be from about 0.5
to
about 5 cm, more preferably from about I 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.
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. lA 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.
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
12
CA 02651001 2008-10-31
WO 2007/130658 PCT/US2007/010975
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 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 AIN 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
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 alumina (A1Z03).
As referred to herein, a semiconductor ceramic (or "semiconductor") is a
ceramic having a room temperature resistivity of between about 10 and 108 ohm-
cm.
If the semiconductive component is present as more than about 45 v/o of a hot
zone
composition (when the conductive ceramic is in the range of about 6-10 v/o),
the
resultant composition becomes too conductive for high voltage applications
(due to
lack of insulator). Conversely, if the semiconductor material is present as
less than
about 10 v/o (when the conductive ceramic is in the range of about 6-10 v/o),
the
resultant composition becomes too resistive (due to too much insulator).
Again, at
higher levels of conductor, more resistive mixes of the insulator and
semiconductor
fractions may be needed to achieve the desired voltage.
13
CA 02651001 2008-10-31
WO 2007/130658 PCT/US2007/010975
As referred to herein, a conductive material is one which has a room
temperature resistivity of less than about 10-2 ohm-cm. If the conductive
component
is present in an amount of more than 35 v/o of the hot zone composition, the
resultant
ceramic of the hot zone composition, the resultant ceramic can become too
conductive. Typically, the conductor is selected from the group consisting of
molybdenum disilicide, tungsten disilicide, and nitrides such as titanium
nitride, and
carbides such as titanium carbide. Molybdenum disilicide is generally
preferred.
In general, preferred hot (resistive) zone compositions include (a) between
about 50 and about 80 v/o of an electrically insulating material having a
resistivity of
at least about 1010 ohm-cm; (b) between about 0 (where no semiconductor
material
employed) and about 45 v/o of a semiconductive material having a resistivity
of
between about 10 and about 108 ohm-cm; and (c) between about 5 and about 35
v/o of
a metallic conductor having a resistivity of less than about 10"2 ohm-cm.
As discussed, heating element of the invention contain a relatively low
resistivity cold zone region in electrical connection with the hot (resistive)
zone and
which allows for attaclunent of wire leads to the igniter. Preferred cold zone
regions
include those that are comprised of e.g. AIN and/or A1203 or other insulating
material;
optional semiconductor material; and MoSi2 or other conductive material.
However,
cold zone regions will have a significantly higher percentage of the
conductive
materials (e.g., MoSi2) than the hot zone. A preferred cold zone composition
comprises about 15 to 65 v/o aluminum oxide, aluminum nitride or other
insulator
material; and about 20 to 70 v/o MoSi2 or other conductive and semiconductive
material in a volume ratio of from about 1:1 to about 1:3. For ease of
manufacture,
preferably the cold zone composition is formed of the same materials as the
hot zone
composition, with the relative amounts of semiconductive and conductive
materials
being greater.
At least certain applications, heating elements of the invention may suitably
comprise a non-conductive (insulator or heat sink) region, although
particularly
preferred heating elements of the invention do not have a ceramic insulator
that
14
CA 02651001 2008-10-31
WO 2007/130658 PCT/US2007/010975
contacts at least a substantial portion of the length of a first conductive
zone, as
discussed above.
If employed, such a heat sink zone may mate with a conductive zone or a hot
zone, or both.. Preferably, a sintered insulator region has a resistivity of
at least about
1014 ohm-cm at room temperature and a resistivity of at least 104 ohm-cm at
operational temperatures and has a strength of at least 150 MPa. Preferably,
an
insulator region has a resistivity at operational (ignition) temperatures that
is at least 2
orders of magnitude greater than the resistivity of the hot zone region.
Suitable
insulator compositions comprise at least about 90 v/o of one or more aluminum
nitride, alumina and boron nitride
Preferred heating element ceramic materials are disclosed in the examples
which follow.
Heating elements of the invention may be used in many applications,
including gas phase fuel ignition applications such as fiunaces and cooking
appliances, baseboard heaters, boilers, and stove tops. In particular, a
heating element
of the invention may be used as an ignition source for stop top gas burners as
well as
gas furnaces.
In one preferred aspect of the invention, heating elements of the invention
may
be configured and/or utilized as resistive igniters elements distinct from
heating
elements known as glow plugs. Among other things, frequently employed glow
plugs
often heat to relatively lower temperatures e.g. a maximum temperature of
about
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 volts, such as 60 volts or more or 120 volts or more
including
CA 02651001 2008-10-31
WO 2007/130658 PCT/US2007/010975
220, 230 and 240 volts, whereas glow plugs are typically employed only at
voltages
of from 12 to 24 volts.
Heating elements of the invention also are particularly suitable for use for
ignition where liquid (wet) fuels (e.g. kerosene, gasoline) are evaporated and
ignited,
e.g. in vehicle (e.g. car) heaters that provide advance heating of the
vehicle.
In other preferred aspects, heating elements are suitably employed as glow
plugs, e.g. as an ignition source in a motor vehicle.
Heating elements will be useful for additional specific applications,
including
as a heating elements for an infrared heater.
The following non-limiting examples are illustrative of the inveintion. All
documents mentioned herein are incorporated herein by reference in their
entirety.
Example 1: Igniter fabrication
The following materials were admixed to provide a conductive composition
for injection molding fabrication of a heating element: 30 vo1% MoSi2i 7 vol%
SiC,
and 63 vol% A1203, and based on the weight of ceramic materials 2-3 wt% of
polyvinylalcohol and 0.3 wt % of glycerol.
The following materials were admixed to provide an insulator composition for
injection molding fabrication of a heating element: 10 vol% MoSi2, 90 vol%
A1203,
and based on the weight of ceramic materials 2-3 wt% of polyvinylalcohol and
0.3 wt % of glycerol.
The following materials were admixed to provide a resistive hot zone
composition for injection molding fabrication of a heating element: 25 vo1%
MaSiZ, 5
vol% SiC, and 70 vol% A1203, and based on the weight of ceramic materials 2-3
wt%
of polyvinylalcohol and 0.3 wt% of glycerol.
16
CA 02651001 2008-10-31
WO 2007/130658 PCT/US2007/010975
In each of the three compositions, the A1203 had a mean particle size of 1.7
microns. No sintering aids such as yttria or other such materials were
included in the
compositions.
The above three compositions of differing resisitivity were loaded into
separate barrels of a co-injection molder. To form the rod-shaped igniter
element
with internal insulator region of the general configuration shown in FIG. 1 of
the
drawings, a first shot filled a half-cylinder shaped cavity with insulating
paste forming
the insulating paste extruded from the cavity. The part was removed from the
first
cavity, placed in a second cavity and a second shot filled the volume bounded
by the
first shot and the cavity wall core with the conductive paste. The part was
then
removed from the second cavity, placed in a third cavity and a third shot
filled the top
portion of the part with the resistive (hot zone) paste. The thus molded rod-
shaped
part was then partially debindered at room temperature in an organic solvent
dissolving out 10 wt% of the added 10-16 wt%. The part was then thernYally
debindered in flowing inert gas (N2) at 300-500 C for 60 hours to remove the
remainder of the residual binder.
The debindered rod-shaped part was densified through a two-stage process
using gas-phase sintering. Thus, the rod-shaped part was placed in a gas
sintering
oven which was filled with argon gas at a pressure of 150 psi. The oven was
maintained at 1725 C for about 1.5 hours. The oven was then allowed to cool to
room
temperature and then pressure increased to 3000 psi and held at 1725 C for
about 2
hours. The oven was then allowed to cool to room temperature. The treated rod-
shaped part had a density of greater than 98 percent. The dense element was
connected to a power supply of 24 volts and the hot zone attained a
temperature of
about 1300 C.
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.
17
