US3522574A

US3522574A - High temperature electric resistance device

Info

Publication number: US3522574A
Application number: US697080A
Authority: US
Inventors: Roger R Giler
Original assignee: Kanthal Corp
Current assignee: Kanthal Corp
Priority date: 1968-01-11
Filing date: 1968-01-11
Publication date: 1970-08-04
Anticipated expiration: 1987-08-04

Description

1970 R. R. GILER 3,522,574

HIGH TEMPERATCRE ELECTRIC RESISTANCE DEVICE Filed Jan. 11, 1968 United States Patent O 3,522,574 HIGH TEMPERATURE ELECTRIC RESISTANCE DEVICE Roger R. Giler, Wilton, Conn., assignor to The Kanthal Corporation, Bethel, Conn., a corporation of Connecticut Filed Jan. 11, 1968, Ser. No. 697,080 Int. Cl. H01c 1/02, 1/08 U.S. Cl. 338-316 4 Claims ABSTRACT OF THE DISCLOSURE The invention relates to the field of high temperature electric resistance device such as jet engine glow plugs, fuel flame ignjters, flame sensing devices and the like requiring the use of an electric resistance element operating at elevated temperatures, usually under conditions of intermittent heating and cooling.

Other than platinum, which is expensive and not always reliable, devices of the above general type have been proposed using materials such as silicon carbide or molybdenum disilicide. Silicon carbide has the disadvantages of being slow acting, its electrical resistivity being highest when cold, of being subject to burning out if run continuously with more or less flame impingement, and of having a relatively short service life in any event. Molybdenum disilicide, and this includes in general the refractory metal silicides, has had the disadvantage that heretofore there was no commerically practical way to equip such an element with the necessary electrically conductive terminals other than by welding on terminals of larger diameter than the element and made of corresponding silicide. The larger diameter makes the terminals electrical resistance lower so that the terminals do not heat so much as the element and, therefore, may be connected with metal conductors at their outer ends, but the result is that the terminals cost is unduly high.

Electric resistors of the refractory metal silicide type have for some years been available commercially from Aktiebolaget Kanthal, Hallstahammar, Sweden. The silicide is mainly molybdenum disilicide, this being about 90% by weight of the material, the balance being ceramics. Anyone unfamiliar with this material may refer to the following patents and others assigned to this company:

July 18, 1961, N. G. Schrewelius, 2,992,959

July 18, 1961, N. G. Schrewelius et al., 2,993,111

Mar. 27, 1962, Nils Gustav Schrewelius et al., 3,027,330 Mar. 27, 1962, K. H. J. Medin, 3,027,332

An attempt to use silicon carbide as a burner flame igniter isshown by the Heyroth Pat. 2,095,253, Oct. 12, 1937.

To briefly summarize the present invention, it should first be noted that unlike silicon carbide, molybdenum disilicide, or the other silicides of its types, are characterized by an electrical resistivity that increases sharply with temperature in a substantially linear manner. With this in mind, the invention comprises making the terminals of a metal which has substantially greater heat conduc- 3,522,574 Patented Aug. 4, 1970 tivity than the silicide element and connecting these terminals to the ends of the elements by fused metal, such as solder or brazing metal alloy, with each terminal preferably, but not necessarily, having a substantially greater cross-sectional size than the portion of the element connected thereto and in all events being proportioned to form a heat sink so that this portion operates cool enough to maintain its electrical resistivity substantially less than the balance of the element when the latter is operating at the elevated temperatures for which it is adapted. When operating as an electrical resistance heating element, this means that the element portions at the terminals act like the enlarged diameter silicide terminals previously described.

Because the element may be connected by the fused metal only to the outer surfaces of the metal terminals, as contrasted to being inserted in holes formed in such terminals, differences in thermal expansion and contraction of the silicide element and metal terminals have no harmful effects.

When the foregoing principles are applied to a fuel igniter, or jet engine glow plug, or a flame sensing device, the terminals and the portions of the element adjacent to these terminals preferably should be kept free from direct impingement by flame. The portions of the terminals remote from the element may be cooled. When in operation with the silicide element heated either by an electric current or by a flame, the terminals draw heat from the adjacent portions of the element. Therefore, these adjacent portions operate at lower temperatures than this balance. This means that even if the fused metal connecting the element and the terminals has a low melting temperature, relative to the normal operating temperature of such an element, the fused metal is protected and remains effective in connecting the parts together.

Since the terminals form heat sinks, when the element joined to them by the fused metal is heated by an electric current, the portions of the element adjacent to the terminals cannot heat when the current first flows and while the balance of the element is heating so that its resistivity is increasing. Therefore these portions retain their low resistivity and remain relatively cool while the balance of the element operates hot.

Specific examples of the invention are illustrated by the accompanying drawings in which:

FIG. 1 schematically shows in vertical cross-section a fuel flame igniter application incorporating the principles of the present invention;

FIG. 2 shows the device itself on an enlarged scale and in perspective views;

FIG. 3 is an enlargement taken from FIG. 2;

FIG. 4 shows in section a modification of FIG. 3;

FIG. 5 is a vertically sectioned view showing another modification; and

FIG. 6 is a cross-section taken on the line 6-6 in FIG. 5.

As an example of the device of the present invention, FIG. 1 shows a burner formed by an air pipe 1 with an internal fuel pipe 2 which together eject a stream of ignitable gas 3. The device embodying the principles of the present invention is shown generally at 4 in FIG. 1 and in FIG. 2 is shown in detail as comprising a block of dielectric material 5 through which terminals 6 extend and which connect with the silicide element 7 by means of fused metal 8. FIG. 3 shows this on an enlarged scale.

The element 7 is made of commercially available molybdenum disilicide compound of about molybdenum disilicide and 10% ceramics, and may be formed by powder metallurgy techniques as explained by the patents previously referred to and others assigned to the same assignor. As contrasted to the normal larger diametered elements commercially sold, this element 7 is wire-like and the invention ordinarily contemplates element diameters not greater than 3 mm. and more usually in the neighborhood of 1 mm. The use of such small diameters is appropriate in the case of jet engine glow plugs, igniters, flame sensors and many other devices.

The terminals 6 may be made of iron or copper or their alloys or any metal that can bond with fused metal and has higher thermal conductivity than the silicide element. The terminals may have a diameter substantially larger than the diameter of the element 7 and may have substantial lengths to provide masses of metal for giving the heat sink effect previously described. The terminal ends may be cooled advantageously. The fused metal 8 may be ordinary solder, so-called silver solder, or brazing or welding alloy. All form a firm connection with the disilicide element. The connection between the element 7 and terminals 6 may be easily effected without special skills or expensive techniques. Standard fused metal connecting methods are practicable. In FIGS. 2 and 3 the element ends are simply abutted against the fiat ends of the terminal and brazed to them by deposited brazing metal.

When used as an igniter, and in FIG. 1 the device 4 is shown powered by low voltage current from a stepdown transformer 9, the element 7 throughout its major portion almost immediately becomes incandescent assuming that the transformer is supplying adequate current at a voltage that would maintain the major portion of the element 7 at the desired high temperature. Keep in mind that the electrical resistivity of the element 7 increases sharply with temperature and that'the voltage should be such that the high operating temperature resistivity of the element maintains the desired temperature.

For an igniter the temperature should be such that the elements main portion is incandescent. The characteristics of commercially available molybdenum disilicide permits its operation anywhere within the range of from 800 C. up to or possibly even higher than 1750 C.

With room temperature low resistivity the result is almost immedaite incandescence of the element 7 as indicated above, excepting that as its portions adjacent to the terminals 6 and the fused metal 8 begin to rise in tempertaure, the large heat sink effect or action of the terminals 6 sucks away the heat and keeps these parts and the adjacent portions of the element 7 relatively cool. These adjacent portions of the element thus maintain to a considerable degree their room temperature low resistivity. This results in concentrating the heat therebeyond within the balance of the resistor element which then quickly becomes incandescent. When observed in operation, only the portion well beyond the terminals 6 is incandescent. In this fashion the fused metal 8 is protected from the high temperature.

When used as a sensing device only the portions spaced from the terminals should be exposed to the flame, the terminals then also functioning to keep the fused metal 8 at an adequately low temperature to prevent the fused metal from softening. By passing a small current through the element 7 and measuring the current flow, flame sensing is possible, keeping in mind that the silicides increase in resistance with temperature in a substantially linear fashion. Correspondigly, temperature measuring is possible.

As illustrated in this first example, the element 7 extends between the terminals 6 so as to include at least one complete loop 7a. It has been found that with a silicide element of 3 mm. or less such a loop has an inherently long service life even under the conditions of intermittent heating and cooling, representing service for which a silicide element is ordinarily not recommended in the case of the heavier units used for industrial heating purposes.

In FIG. 4 the element 7 is formed with an inside complete loop 7b in the manner of the old cal bon filament, electric incandescent lamp. Another difference is that the elements ends are laid diagonally on the corners of the terminals 6 and are brazed or soldered to these corners. In the first instance the terminal ends are in a'butting relation to the terminal ends.

Finally, when a silicide element of U-shape is used, as shown by FIG. 5, the element 70 may be brazed or soldered to terminals 6a with the block 5a in this case provided with clearance 5b between it and the terminals 6a with respect to the holes through the block and. through which the terminals 6a extend. The terminals 6a may be made of rectangular sape, as shown by FIG. 6, so that they are somewhat like flat leaf springs having lateral flexibility while providing sufficient mass for the heat sink effect.

In all of the foregoing instances, the idea is to provide flexibility permitting movement of the silicide element either due to thermal expansion and contraction effects or possibly due to current surges.

In the case of the heavier industrial molybdenum disilicide units, it is necessary to start a cold element with a low voltage high amperage current, and, as the element heats, to increase the voltage to compensate for the sharp increase in resistivity.

However, in the case of the units of the present invention, when the molybdenum disilicide element has a diameter not greater than 3 mm., and more often in the neighborhood of 1 mm. in the case of an igniter or the like, the cold element may receive the full voltage required to keep it at its desired operating temperature, which may well be in the area of 1450 or 1500 C. This applies regardless of the length of the element.

Keeping in mind that ordinary electrical engineering is involved in selecting the voltage required for the desired operating temperature and considering the resistivity of the element at that temperature, and its small size, the initial current surge, as from the transformer 9 in the case of FIG. 1, can be applied directly. The small diameter molybdenum disilicide element reaches incandescence in as little as one second. Only the portions adjacent to the terminals 6 or 6a remain cool and black and free from incandescence or temperatures harmful to the fused metal 8. This presupposes that these portions of the element and the terminals are kept free from direct flame impingement. In other words, the terminals 6 must be free to dissipate the heat to the atmosphere or possibly to a cooling medium such as in the case of water cooling. Naturally, the fused metal 8 will be chosen for the operating temperatures to be encountered, but heretofore with the elements of larger diameter and in ordinary furnace applications even the highest temperature melting brazing alloy could not be used. It is the heat sink effect of the present invention, due to the large or high heat conductivity metal terminals, which permits the use of fused metals in general.

Commercially available molybdenum disilicide resistor elements in the large diameters normally provided have been reported to be under some conditions subject to lowtemperature oxidation known as pest, within the temperature range of from 300-800 C. This, together with reported tendencies for the silica coating that forms on such elements to occasionally fragment with thermal expansion and contraction, has indicated that such elements have a considerably shorter service life when subjected to intermittent operation.

In the case of elements of 3 mm. diameter and smaller, with the heat sink effect of the terminals as described, there presumably exists a zone between the incandescent portion of the element of the cooled portions at the terminals where the temperatures range within this SOD-800 C. range. Also, the incandescent portion with its silica layer plainly must be subjected to severe thermal expansion and contraction effects during intermittent operation.

Although pest and silica coating fragmentation might have been expected, long time intermittent tests of devices made according to the present invention have shown that they possess an extremely long service life, much longer than platinum which is about the only comparison that can be used.

To gain some idea of size, units are made as shown by FIG. 5 wherein the molybdenum disilicide element 7c had a diameter of .016", the loop was approximately from to A from the bottom to top of the U, and from 7 to /2" between the shanks of the loop. The material was 90% molybdenum disilicide and ceramic additions and was made by the previously mentioned company to the size required by the present invention. Units of this type were tested by being turned full on so that the main portion of the element was heated to the 1400 C. range, where the flame igniter must ordinarily operate, and then turned completely off with the element flashing to incandescence and then immediately cooling due to its small diameter. The unit was turned on and off repeatedly and showed a completely satisfactory service life. The form shown by FIGS. 1 to 4 gave indications of an even better service life. Incidentally, during the tests mentioned the terminals were mounted by the mounting blocks rigidly and not as shown in FIG. 5, this latter arrangement being believed to provide an even more extended service life, particularly when the loop is of the simple U-shape.

The exact relationship between the size of the terminals and the size of silicide elements to which they are afiixed by the fused metal, may be determined easily by testing. The thermal conductivity of the terminal metal, the size ranges of the terminals and the manner in which they are mounted and the extent to which they can dissipate the heat, melting or softening temperature of the fused metal used and other factors determine this size relationship. In general, using a resistor element of the molybdenum disilicide type commercially available, with this element having a diameter of 3 mm. or less, ordinary fused metal connecting methods may be used to affix the terminals to the element. With the element heated to its desired operating temperature, either through its electric resistance or by direct flame impingement, melting of the fused metal immediately shows either that the terminals do not have adequate mass or that a higher temperature melting connecting metal should be used. For example, a shift may be made from low temperature melting solder to one of the silver solders or possibly to brazing metal, or the. terminals may 'be increased in mass or artificially cooled.

In general, fused metal is used. This may even constitute fused metal resulting from welding the terminals to the ends of the element by using only the terminal metal and/ or the silicide of the element itself. Although the silicide elements illustrated are of short lengths, their length is not limited. Longer lengths of silicides may be provided with terminals in the manner described when the intended use requires such longer lengths.

The main thing is that once the teaching of the present invention is known, it becomes possible for the first time to provide a relatively inexpensive device of the character described using ordinary metals for the terminals and ordinary fused metal connecting techniques. The need for providing the element with terminals made of correspondign silicides with the attendant expense, has been eliminated.

It is to be understood that the usual soldering or brazing fluxes should preferably be used. They are adequate, apparently functioning to remove any silica coating on the molybdenum disilicide so that the solder or brazing metal forms a firm bond. During spot welding, fragmentation of the silica apparently occurs automatically, but it may be removed mechanically or chemically.

The heat sink effect described hereinabove may have advantages even when not required for the protection of a fused metal bond. For example, it permits the connection of terminals made of material which cannot operate satisfactorily at the operating temperature of the element itself.

A molybdenum disilicide element of small size when heated highly sometimes tends to change its shape so it becomes stressed when its ends are held by terminals too rigidly. When using solder or brazing metal as a bond, the element may be heated to temperatures above 1600 C. for a short-time period and then allowed to cool. Then the bonding metal of one terminal at least is heated above its melting point, allowing the molybdenum disilicide element to assume a shape free from stresses. The bonding metal is then allowed to cool, holding the element now free from stress.

What is claimed is:

1. A device including a refractory metal silicide elongated electric resistance element and an electrically conductive terminal connected to one end of this element; wherein the improvement comprises the element having a diameter not greater than 3 mm. and the terminal being made of a metal having a substantially greater heat conductivity than the silicide and connected to the element by fused metal with the terminal having a cross-sectional size related to the portion of said element adjacent thereto forming a heat sink keeping this portion cool ennough to maintain its electrical resistivity substantially less than the balance of this element when the latter is operating at the elevated temperatures for which it is adapted.

2. The device of claim 1 in which the fused metal is solder or brazing or welding metal.

3. The device of claim 1 in which the other end of said element also has a terminal of the character described and the element extends between the terminals in the form of at least one complete loop.

4. The device of claim 1 in which the other end of the element also has a terminal of the character described and at least one of the terminals is mounted so its portion connecting with the element can move to accommodate movement of the element.

References Cited UNITED STATES PATENTS 1,715,824 6/1929 Duersten 338-51 2,319,323 5/1943 Heyroth 338329 2,708,253 5/1955 Cohn 317-79 2,913,695 11/1959 Borghult 1325 1,019,390 3/ 1912 Weintraub. 2,745,928 5/ 1956 Glaser 338-332 ELLIOT A. GOLDBERG, Primary Examiner U.S. Cl. X.R. 33 85 1, 329