US2208342A - Method for producing high temperature heating resistors - Google Patents

Description

July 16, 1940. R REICHMANN 2,208,342

METHOD FOR PRODUCING HIGH TEMPERATURE HEATING RESISTORS Filed May 16, 1939 P/ated E WITNESSES: I INVENTOR fiflv 7 1M Re/n/w/d Pet/imam jaw/"M ATTORNEY Patented July 16 1940 METHOD FOR rnonuomc men remnan- TUBE HEATING misisrons Reinhold Beichmann, Berlin, Germany, assignor to Westinghouse Electric & Manufacturing Company, East Pittsburgh, Pa a corporation of Pennsylvania Application May 16, 1939,

. In Germany May 6, 1938 Serial No. 273,891

This invention relates to the adaptation of metallic substances having a high melting point for electric heating units to be operated at high temperatures, and generally of the straight bar type. In order to employ the substances in heating units,'it is necessary to protect them against oxidation, and this is accomplished by providing a gas-impervious shell or tube around the substance.

Heretofore heating units for high temperatures (over 1400 C.) of the type involved were produced, for example, by covering the metallic heating resistance with a non-durable layer and then enclosing it with a protective shell by ceramic means and methods. The whole assembly is then burnt or fired while in a vacuum or while enveloped by hydrogen by external heating in a furnace. The thickness of the nondurable layer is so selected that it corresponds 30 to the shrinkage of the ceramic shell during the firing. After the firing, the ceramic shell is gastight, and lies in a gas-tight manner on the heating resistor,protecting it against oxidation during the use of the assembly as a heating unit. The tight engagement of the protective shell with the heating resistor yields a good heat transfer and an eflicient heating body. The production of such heating units, however, is accompanied with many manufacturing difficulties, in particular, requiring expensive equipment for buming the ceramic shell deposited on the heating conductor, and frequently is accompanied by erratic results.

In accordance with my invention, heating units are made of a metallic conductor and a ceramic protective shell or tube, the latter consisting of oxides of silicates (sillimanite, Alzsim, or mullite AlzOaSiQa). The tube is first fired and then slipped over the heating resistor which is in the form of an electric conductor, The heating unit is constructed by selecting a fired shell with an inner cross-section related to the cross-section of the heating conductor that is inserted therein in such manner that the heating conductor may thereafter be heated by electric current passed through it. As a consequence the metal of the conductor recrystallizes but the bar shape is It is a further object of my invention to pro- 5 vide a heating unit of the type described which can be inexpensively manufactured, and which is durable in use.

A further object of my invention is to provide in a heating unit of the type described a good bond between the metallic conductor which is the core of the unit and the outer'ceramic shell whereby there obtains a good heat transfer from the core to the'shell, and without the creation of hot spots.

For the heating conductor, tungsten, or molybdenum, or alloys of these metals are preferably used while the ceramic protective shells may consist of oxides that fuse at a high temperature, such as, for example, aluminum oxide, magnesium oxide, or the like, or may be made out of materials which contain substantial amounts of aluminum silicate ormagnesium silicate. If the heating conductor is now slippedinto the previously-fired shell, and is heated by current passing through the conductor to temperatures which are above 1500 C., the heating conductor recrystallizes and this is accompanied by a volume increase. The original intermediate space between the conductor and the protective tube is completely filled, and, at the same time, under certain circumstances, a small amount of metal may sublimate into the tube. It is of importance that the intermediate space between the metal conductor and the inner wall of the protective shell be not too large. If it is too large, hollow spaces may be formed in the heating conductor, and a structure of non-uniform cross-section may result. Consequently, inuse, such a heating conductor is over-heated at certain points and the whole arrangement becomes useless after a short time.

For this reason, in accordance with my invention, the inner cross-section of the protective shell is selected in relation to the cross-section of the heating conductor. Generally, the former should not be more than 25% greaterthan the latter, and in this manner heating units are obtained in which the metal, somewhat lightened by recrystallization, adheres so tightly to the I ceramic shell that the junction of the tube is maintained .even for temperatures at which the protective shell becomes soft, and when freely suspended, bends. In this event, the metal follows the bending without any fissure resulting in the ceramic protective shell. The junction of the two parts is much better than that obtained when the protective shell is shrunk onto the heating conductor as in the previous constructions. Also variations in length of the heating conductor with heating and cooling are taken up better.

n the heating bars should be operated with water cooling at the ends, sealing caps are mounted in a known manner at the ends to prevent penetration of air. The smallamount of air which is substantially enclosed is" harmless for the formation process.- It is even possible to have the formation take place after the building in of the heating unit into the furnace in which it is to be later used.

. to operate the heating unit without water cooling.. "This results in'a considerable economy since no heat is withdrawn from the heating bars at its ends. If the heating unit'is loaded with current while in a vacuumpr enveloped in hydro-,- gen until the metal, by recrystallization, has filled the hollow space of the ceramic tube, then the heating bar is protected against the penetration of air and the detrimental oxygen contained therein.

The ends of the heating bar, however, do not become as hot as the intermediate portions, and, consequently, the recrystallization with the increasing volume .at the ends is much smaller andmay not exist at all, so 'that the unit may not have a tight jointure at its ends. To preclude the posibility of oxidation of the ends in operation without enclosing them specially, the ends can be coated or plated with noble metals (platinum, rhodium or the like) or chromium, and this zfiay be done electrolytically. This coating should extend along the ends of the conductor at least for the length of the conductor which is not expected to obtain the fullvolume increase in the shell. Obviously, to complete the conductor, current connections may be fastened to the ends which may also be, in addition, provided with cooling ribs.

In the drawing, Figure 1 is an outside elevation view of a heating unit made in accordance with my invention; and

Fig. 2 is an enlarged cross-sectionalview on the line HrII of Fig. 1. "The outer shell or tube is first made of one or more of the non-metallic refractoriespre- -viously mentioned in any common manner involving firing at hightemperatures to yield a uniform gas-impervious fused ceramic mass in the form of a hollow tube having a central bore. Within this bore is placed a pre-formed solid metallic tube 2 of the class of refractory metals or alloys. also mentioned previously. This core isslongenough to protrude beyond both ends of the shell and has threaded end portions 3 for the reception of the usual terminal bolts and.

lugs. The end portions 3 are preferably plated for a short length toward the longitudinal center of the coreto extend a. short distance inside of the shell. e

'The cross-sectional area of the bore of the shell should preferably be somewhat greater than that of the core before insertion, but as a general rule should not exceed the cross-sectional area In many'instances; however, it is desirable a fine grain-texture. With this metallic rod inserted into the pre-formed shell, heavy current is then passed through the rod with the generation of a great amountof heat. This causes the metal to undergo a change in texture, large crys- 'in the metal, but, apparently, because the ceramic shell resists longitudinal movement of the rod, the effect is almost entirely one of an increased cross-section of the rod, which fills entirely the space within the tube. Actually the filling-in of the space in the ceramic tube is somewhat more complicated than simple expansion of the metallic rod. When the rod attains a high temperature, the metal at the surface ai parently vaporizes and condenses on the walls of the ceramic tube, even penetrating the walls somewhat as indicated at 4. With continued heating, more metal evaporates and condenses, producing in place of a clean-cut coaxial space between the metal and the tube an annular region somewhat spongy in appearance. As the high temperature is maintained, it appears that the recrystallization process with increase in volume occurs in both the metal deposited on the inner wall of the tube and the metal core, both parts gradually expanding until the metal becomes a solid mass completely filling the interior of the tubh.

While my invention has been particularly described, it is obvious that many equivalents may be employed therein. Thus, the cross-section of the bore) of the tube or shell, and that of the conductor, must obviously be chosen in accordance with the desired sizes, the substances use and their characteristics.

I claim as my invention:

1. As an article of manufacture, a heating unit comprising a substantially gas-tight ceramic tubular member having a bore, and a refractory metal core which recrystallizes when heated accompanied by increased volume, swelled in the bore of said tubular member by heating to fill the bore of said member for a substantial portion of its length. V 2. As an article of manufacture, a heating unit comprising a substantially gas-tight tubular member of a refractory comprising heat-resistant metallic oxides and silicates, said member having a bore, and a core of a refractory metal which recrystallizes by heat-treatment with increased volume, said core being originally of less size than said bore, but swelled in the bore of said member by recrystallization by heattreatment to fill the bore of said memberfor a substantial portion of its'length.

3. In making; an electric resistor unit by employing a hollow substantially gas-tight tubular member of a non-metallic refractory, and a core of a metallic refractory of a cross section similar to that of the bore of the said tubular members, but of somewhat smaller area; the method which comprises inserting said core in said tubular ,member, and heating said core by electric power to cause it. to swell bygrecrystallization into intimatecontact with said tubular member.-

4. In the making an electric, resistor unit by which comprises inserting said core in said tubular member, passing electric current through the core to heat it whereby it recrystallizes with increase of volume to tightly engage the shell.

5. As an article of manufacture, a bar heating unit comprising a substantially gas-tight, substantially straight, tubular outer shell of prefired refractory comprising heat-resistant metallic oxides and silicates, said shell having a bore, and a core of a refractory metal having an original cross-sectional size less than said bore, said core being swelled in said bore by recrystallization accompanied by increased volume at high temperatures to fill the bore of said shell for a substantial portion of its length, and partially sublimated into the portion of the shell in proximity to the core. I

6. As an article of manufacture, a bar heating unit comprising a substantirlly gas-tight, prefired, hollow tubular member of a refractory comprising heat-resistant metallic oxides and silicates, said member having a bore, and a rod of a refractory metal from the group comprising tungsten, molybdenum and their alloys, said rod being swelled in said bore by recrystallization at high temperatures to fill the bore of said member for a substantial portion of its length.

7. As an article of manufacture, a bar heating unit comprising a pre-formed, substantially gastight, hollow tubular member of non-metallic refractory. said member having a bore, and a rod of a refractory metal from the group comprising tungsten, molybdenum and their alloys, said rod being swelled in said bore by recrystallization by heat-treatment to fill the bore of said member for a substantial portion of its length.

annmom REICHMANN.

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