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.

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.

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.