U.S. patent number 3,875,476 [Application Number 05/432,385] was granted by the patent office on 1975-04-01 for igniter element.
This patent grant is currently assigned to Honeywell Inc.. Invention is credited to William B. Crandall, Linden E. Shipley.
United States Patent |
3,875,476 |
Crandall , et al. |
April 1, 1975 |
IGNITER ELEMENT
Abstract
A heat-resistant ceramic electric igniter element has a
plurality of compositions in a continuous unitary body structure
including a central or igniting zone of a composition having a
relatively high electrical resistance flanked by end zones of a
composition having a lower electrical resistance to which the leads
are attached. Gradual transitions from one composition to the other
are provided which compensate for any differences in the
coefficient of expansion between the two compositions and minimize
migration between the two compositions.
Inventors: |
Crandall; William B. (Wheaton,
IL), Shipley; Linden E. (Evanston, IL) |
Assignee: |
Honeywell Inc. (Minneapolis,
MN)
|
Family
ID: |
23715940 |
Appl.
No.: |
05/432,385 |
Filed: |
January 10, 1974 |
Current U.S.
Class: |
361/264; 219/270;
219/553; 252/516; 338/330; 431/258 |
Current CPC
Class: |
H01C
7/02 (20130101); F23Q 7/22 (20130101) |
Current International
Class: |
F23Q
7/22 (20060101); H01C 7/02 (20060101); F23Q
7/00 (20060101); F23q 007/10 () |
Field of
Search: |
;338/275,330
;219/552,553 ;252/516 ;264/65 ;13/25 ;431/258 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Mayewsky; Volodymyr Y.
Attorney, Agent or Firm: Mersereau; Charles G.
Claims
The embodiments of the invention in which an exclusive property or
right is claimed are defined as follows:
1. A continuous unitary body formed in a single segment, said body
containing a plurality of heat and oxidation resistent,
electrically conductive ceramic material compositions wherein said
compositions comprise:
An igniting zone having a composition exhibiting a relatively high
electrical resistance;
zones flanking said igniting zone, said flanking zones having a
composition exhibiting a lower electrical resistance than said
igniting zone; wherein the compositional transition between said
igniting zone and said flanking zone is a gradual transition; and
wherein said compositions of both said igniting zone and said
flanking zones comprise principly of silicon carbide and pyrex-type
glass; and
electrical leads attached to said flanking zones.
2. The continuous unitary body of claim 1 wherein said ignition
zone has a negative temperature coefficient of electrical
resistance and wherein said flanking zones have a positive
temperature coefficient of electrical resistance.
3. The continuous unitary body of claim 1 wherein said ignition
zone comprises:
from about 25 to about 88 percent silicon carbide,
from about 1 to about 8 percent ferro-silicon,
from about 1 to about 30 percent of oxides from a group consisting
of oxides of titianium and zirconium,
from about 5 percent to about 30 percent of a pyrex-type glass
and
from about 5 percent to about 30 percent silica;
and wherein said flanking zones comprise:
from about 40 percent to about 75 percent silicon carbide,
from about 1 percent to about 10 percent ferro-silicon,
from about 1 percent to about 20 percent titianium dioxide and
from about 5 percent to about 30 percent of pyrex-type glass.
Description
BACKGROUND OF THE INVENTION
The present invention relates generally to electrical
resistance-type ceramic igniter elements for igniting gaseous fuels
and the like, and, more particularly, to an improved continuous
service element utilizing a plurality of ceramic compositions in a
unitary structure.
DESCRIPTION OF THE PRIOR ART
In the prior art it has long been known to utilize various
heat-resistant oxidation-proof ceramic materials for
resistance-type electric igniter elements. It is also known that an
igniter may be made utilizing a plurality of ceramic compositions
having different electrical resistivities. An example of that
concept is shown in a patent to Schrewelius U.S. Pat. No. 3,321,727
issued May 23, 1967. That disclosure illustrates and describes an
igniter element made up of separate segments of different materials
which are subsequently joined together to provide such a composite
structure.
While such structures have met with partial success, they suffer
from several serious drawbacks. First, at the junction point
between the diverse compositions there is an abrupt compositional
change which does not allow for inherent differences in the
coefficients of expansion between the two materials. This inherent
difference in coefficients of expansion resistance coupled with the
fact that the higher-resistance center portion reaches a far
greater temperature than the end portions when a current is applied
to the structure, may result in a mechanical failure of the igniter
element at one or more of the junction points after a number of
heating and cooling cycles. Second, the use of distinct segments in
the manufacture of such an element, necessitates separate moldings
of the sections and subsequent assembly of the element which adds
to the cost and complexity of its manufacture.
SUMMARY OF THE INVENTION
According to the present invention, the problems associated with
the prior art multi-composition electrical-resistance type igniter
elements are solved by the provision of a unitary structure having
a gradual compositional transition from one composition to another.
This overcomes the problems created by differences in the
coefficient of expansion and molecular migration across the
juncture between adjacent compositions and simplifies the
manufacture of the element by eliminating extra molding and
assembly steps. The heat-resistant ceramic electric igniter element
of the present invention has a plurality of compositions in a
continuous unitary body structure including a central or igniting
zone of a composition having a relatively high electrical
resistance flanked, in gradual transition, by end zones of a
composition having a lower electrical resistance to which the
electrical leads are attached. Thus, when an electrical current is
applied across the element the central or igniting portion will
reach a much higher temperature than the two end portions.
The central zone is normally made of a composition having a
negative temperature coefficient of electrical resistance (an
electrical resistance which decreases with an increase in
temperature) and the end or lead attachment zones of a material
having a positive temperature coefficient of electrical resistance
(an electrical resistance which increases with an increase in
temperature). One successful version of the igniter of the
invention and which is described in the preferred embodiment,
below, has a central or igniting zone having a composition
including from about 25 percent to about 88 percent green SiC
various meshes, from about 1 percent to about 8 percent
ferro-silicon, from about 1 percent to about 10 percent TiO.sub.2,
from about 1 percent to about 20 percent ZrO.sub.2, from about 5
percent to about 30 percent pyrex-type glass (defined below) and
from about 5 percent to about 30 percent fused silica. The end
zones have a positive temperature coefficient of electrical
resistance and are made of a mixture including from about 40
percent to about 75 percent green SiC of various meshes, from about
1 percent to about 10 percent of ferro-silicon, from about 1
percent to about 20 percent TiO.sub.2, and from about 5 percent to
about 30 percent pyrex-type glass.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings:
FIG. 1 illustrates a heating element made in accordance with the
present invention and,
FIG. 2 is a schematic representation of temperature profile
produced with the igniter of FIG. 1.
DESCRIPTION OF A PREFERRED EMBODIMENT
Turning now to FIG. 1, we see a representative illustration of the
igniter of the present invention. The igniter has a unitary body
10, which may be in the shape of a hairpin as illustrated, or in
any other shape desired for the particular application of the
device. The unitary structure 10, is divided by composition into a
central or igniting zone 11 and two end or lead-attachment zones 12
and 13 to which electrical leads 14 and 15 may be attached as at 16
and 17. The changes in composition between zones 12 and 13 and zone
11 have been indicated for illustration purposes along lines 18 and
19 which represent a gradual change from one composition to the
other.
In the selection of the compositions utilized for both the high and
low resistance portions of the igniter of the preferred embodiment,
considerations such as cost, electrical resistance properties, life
or stability, at elevated operating temperatures, and forming
suitabilities had to be considered.
After considerable experimentation involving a large number of
possible components, optimum values for a high resistance mixture
for the ignition zone and a lower resistance mixture for the
lead-attachment zones which would produce a long-life, relatively
inexpensive and easily fabricated igniter have been developed and
can be found in Table I below:
TABLE I ______________________________________ COMPONENT
LEAD-ATTACHMENT IGNITION ZONES ZONES
______________________________________ Component % Component %
Range Optimum Range Optimum ______________________________________
Green SiC 20-35 32.5 13-50 44 (100-140 Mesh) Green Sic 5-10 6.5
2-10 8 (240-325 Mesh) Green Sic 15-30 26 10-28 18 (600 Mesh)
Ferro-Silicon 1-10 5 1-8 3 TiO.sub.2 1-20 10 1-10 1 ZrO.sub.2 --
1-20 2 Pyrex-Type Glass* 5-30 20 5-30 20 Fused Silica -- 5-30 4
______________________________________ *The term Pyrex is a
trademark of Corning Glass Works, Corning, New York and pyrex-type
glass as used herein refers to glass having approximately the
following composition: 81 percent SiO.sub.2 ; 13 percent B.sub.2
O.sub.3 ; 3.6 percent Na.sub.2 O; 0.2 percent K.sub.2 O; 2.2
percent Al.sub.2 O.sub.3 and negligable or indeterminate amounts of
MgO, CaO and ZnO.
In the particular combination of components selected for both the
lead-attachment zones and the ignition zones of the igniter of the
present invention, a composite is produced which may be regarded as
consisting of two intercalated networks, one of which supplies the
electrical conduction mechanism and the other bonding mechanism.
The bulk of the electrical conduction mechanism is provided by the
silicon carbide and the ferro-silicon components and the basic
bonding mechanism provided by the titanium dioxide, zirconium
dioxide and glass components. It has also been theorized that the
oxides of titanium and zirconium do contribute some electrical
conductivity to the final glassy silicate bond, especially at
elevated temperatures.
To be compatible with most safety circuitry, the mixtures of
components of the ignition zone and lead attachment zones were also
selected such that the ignition zone has a negative coefficient of
electrical resistance with increasing temperature and the
lead-attachment zones have a positive coefficient of electrical
resistance.
While it is contemplated that other possible mixtures of components
which provide the necessary combination of properties required for
the igniter of the present invention including some containing
MoSi.sub.2 or other compounds not found in Table I, the
combinations illustrated for the preferred embodiment have been
found to provide an excellent igniter combining low power
consumption with a desired temperature operating range in both the
ignition zones and lead-attachment zones.
In addition to providing an excellent bonding material, pyrex glass
utilized in the composition of both the igniter and lead-attachment
zones of the igniter of the present invention is thought to provide
an additional advantage which is of great benefit in adding to the
longevity of the igniter. The elevated temperatures wherein the
igniters are normally operated, it appears that the inclusion of
what amounts to a film borosilicate glass aids in inhibiting any
further oxidation of the system which would lead to a degradation
of the igniter composition and ultimate failure of the igniter. In
this manner, the inclusion of the basically borosilicate-pyrex
composition seems to actually extend the life of the igniter.
In FIG. 2 there is pictured an experimentally-obtained picture
profile of a typical igniter fabricated in accordance with the
present invention. The profile illustrated was obtained by
electrically energizing such an igniter allowing sufficient time
for the igniter to reach a state of temperature equilibrium. The
particular profile shown in FIG. 2 was obtained by applying 60 Hz,
AC (90 v. rms and 0.34 a rms) power to an igniter similar to that
of FIG. 1. In normal ignition operation, the igniter is operated at
a peak ignition zone temperature of about 1,200.degree. C but a
substantially similar temperature profile obtains.
It can readily be seen by the temperature profile of FIG. 2 that
the temperature varies from approximately 1,250.degree. F at a
point close to the tip of the igniter. The reason for the highest
temperature not being precisely located at the tip is not fully
understood; however, that variation is probably well within the
limits of experimental error. The effect of the gradual transition
from the lead attachment zone composition to that of the ignition
zone is readily reflected in the general slope noted in the
temperature profile. As explained above, this eliminates any abrupt
change in temperature between adjacent segments of the igniter and
prevents any problems associated with such abrupt changes.
In the fabrication of the igniter of the invention, the proper
mixtures for both the ignition and lead-attachment zone are
premixed in the cold state. Quantities of these mixtures are then
placed in a pressing die normally made of graphite, in the desired
shape of the igniter in a manner which allows overlap of the
components as illustrated in FIG. 1. The die is then raised to
pressing temperature of approximately 1,500.degree. C and the
igniter is pressed in the die at a pressure of approximately 5,000
psi for about 15 minutes in a well known manner. The
temperature-pressure-time combination utilized in the hot pressing
step also enables the igniter to achieve the desired density of
greater than 99 percent theoretically possible density and it is
this high density which is theoretically responsible for much of
the excellent oxidation resistance achieved by the igniter of the
present invention. The formed igniters are then allowed to cool in
the dies and are subsequently removed and the electrical leads
attached by one of several techniques which are well known in the
art.
As explained above the preferred method of fabricating the igniter
of the invention is a hot pressing process. Other methods of
fabrication have been attempted with less success. Thus, some
experimental igniters have been fabricated by cold pressing and
subsequent sintering. Igniters made in that manner although
exhibiting the appropriate resistivity in both the lead-attachment
zones and the ignition zone, had a high porosity which severely
limited the useable life of such igniters as by oxidation. Other
techniques such as chemical vapor deposition, pack cementation and
hot isostatic pressing were also attempted as methods to fabricate
the igniter of the invention. The third technique, hot isostatic
pressing is also an acceptable method, but appears to be much more
expensive than the hot pressing technique described with regard to
the preferred embodiment.
* * * * *