US2193987A - Electric heating element - Google Patents

Description

ELECTRIC HEATING ELEMENT Filed latch 9, 1938 Patented Mar. '19, 1940' UNITED I STATES ELECTRIC HEATING ELEMENT Paul Schwarzkopf, Reutte, Tyrol, Austria, assignor to American Electro yMetal Corporation, New York, N. Y., a corporation of Maryland Application March 9, 1938, Serial No. 194,749 VIn Germany March 11, 1937 8 Claims.

This invention relates to electric heating elements for producing high temperatures. Metals having a high melting point, such as molybdenum, tungsten and tantalum, are most suitable for this purpose. Those metals oxidize, however, in the presence of air even at relatively low temperatures between 500 C. and 700 C. and they are therefore to be operated in an atmosphere of reducing or inert gases, or in a vacuum if temperatures of this or considerably higher range are t o be produced by them.

In order to utilize the advantages oered by these high melting metals for producing high temperatures in furnaces or for other heating purposes, it .has been proposed to cover the resistor proper of a metal of the type referred to above with a ceramic sheathing which reliably and completely prevents the access of gases, particularly of atmospheric oxygen to the metallic resistor core. Such a ceramic sheathing has been obtained by sintering suitablev metal oxides having a high melting point which were applied to the core in colloidal form and then fritted or sintered thereon, or a sinteredtubular sheathing was shrunk unto the metallic resistor core. Thereby resistor elements of high melting metals but liable to oxidation at relatively low temperature could be used even in open air.

In order to prevent the access of oxygen to the metallic resistor core within the sheathing, it became necessary to apply closures at the ends of the heating elements in such a manner that diffusion of air to the metallic core was prevented.

To this effect the ends of the resistor were projected outside the ceramic sheathing and sleevelike plugs were slipped over the projecting ends and connected with the ends of the ceramic sheathing. However, .this plug could not effec-y Way.

It is a further object of the invention to provide endclosures a solid portion of which is converted into dough-like state at operating temperature and thereby assists in gas-tightly sealing the ends of the sheathing to those of the resistor.

I t is still another object of the invention to reduce the effect of different temperature expansion of the resistor and its cover.

to increase the gas-tightness of the end closures concerned during operation, by applying suitable cementing substances between the end closure and the sheathing.

These and other objects of the invention will be more clearly understood when the specification proceeds with reference to the drawing in which by way of example diiferent features of the invention are shown in cross section; for conveniences sake', only one end of a heating element is shown in the drawing wherein identical reference numbers indicate parts of similar function. 1

Referring to Fig. 1, a resistor element I in the form of a wire consisting of tungsten, molybdenum, tantalum or another suitable high melting metal of the type referred to above is covered with a ceramic sheathing 2 which extends beyond the end of the resistor I. Into the projecting end of sheathing 2 the end of a metal pin 3 is inserted which for instance consists of copper; The other endv of pin 3 projects beyond the end of sheathing 2, and a sleeve-like plug 4 is slipped thereover. Pin 3 is connected with the resistor ,I in any convenient way, such as by fusion. In order to increase the engaging surfaces of resistor I and pin 3 they may be shaped as engaging tongue and groove as shown in Fig. 1.

The feature shown in Fig.-2 differs from that according to Fig. 1 insofar as the engaging surfaces of resistor I and pin 3 are wedge-shaped.

Pin 3 may'also be made of nickel or an ironnickel-cobalt alloy.

It will be appreciated that a gas-tight closure between pin 3 and the projecting end of sheathing 2 is obtained thereby that the temperature coefcient of expansion of the metal forming pin 3, for instance that of copper, is higher than that of the ceramic materialforming the sheathing so that the copper pin upon heating will be pressed closely against the engaging inner surface of the projecting .end -of the sheathing. Thereby access of air tothe resistor element I during operation at high temperatures is effectively prevented.l This closing effect can be still further increased by keeping the temperature so high at the place where the pin is connected with the resistor that the portion of the solid D good heat conductor may be suitably used for.

the pin, for instance copper, silver, nickel and like metals. For this purpose also an alloy may be used which is a good heat conductor, for instance an iron-nlckel-cobalt alloy which at operating temperatures prevailing at the end oi the resistor obtains suiiicient plasticity so as to bring about a plugging action and to eifect gastight closure.

It will also be appreciated that upon inserting the heating element in an electrical circuit with the plugs 4 as connecting electrodes, the pin is heated and therebyexpands and presses against the inside of the projecting end of the sheathing and if this expanding portion of the pin is rendered plastic or even dough-like, it will closely follow all inavoidable unevenness of the inner surface of the sheathing and tightly ll open spaces, if present, so as to obtain the desired gas-tight plugging effect.

In some cases it may occur, however, that the portion of the pin .functioning as a stopper in the way described above is heated locally and suddenly to such a degree that its expansion substantially exceeds that of the adjacent portion of the surrounding end of the ceramic sheathing, whereby the latter may burst, causing rapid destruction of the heating element.

In the feature of the invention shown in Fig. 3 a metal pin is used forming a body composed of a number of parts 3', 3", 3 each being of uniform heat conductivity. Thev conductivities of the parts inter se are, however, different and increase towards the end of the 'heating element.

Thus the temperatures prevailing at the end of the heating element can be controlled in such a way that the deeper the body forming the pin extends into the end of the ceramic sheathing, the lower is its heat conductivity. As a result, the portions of the projecting end of the sheathing lying close to the end ofthe heating element will be heated to lower temperatures than portions close to the resistor because outer portions of better heat conductivity of the composite pin will convey away better and faster the heat developed than inner portions of lower heat conductivity, and as a result the temperatures to which the end of the sheathing is heated, is materially reduced and its bursting eiectively prevented.

Fig. 3 illustrates the portions of which the pin-like body is composed as of different metals having different heat conductivity in order to obtain the desired effect, to wit, that the heat conductivity of the composite body increases to.

wards the end of the sheathing due to relatively better conductivities ofthe parts of which it is composed.

According to fthe feature shown in Fig. 4 the body forming the pin is composed of different parts 3', 3"' fand 23 each of which is of uniform heat conductivity -but their individual heat conductivities increase again towards the end of the heating element. To this eiect these parts have different diameters, and the diameters increase towards the end of the heating element.

Fig. 4 exemplifies a stepwise increase of the diameters fhnug'h .a gradual increase can be obtained -by shaping' of these parts of the body as will be miderstood by anyone skilled in the art. f'

Here again 'the Y:better conductivity of the parts of which the pin-like body is composed can be obtained by applying different metals the heat conductivity of which is chosen so that the individual heat conductivities of the parts increase towards the end of the heating element.

Instead of or simultaneously with such a choice of metals for the parts of the pin-like body, by enlarging the diameters towards the end of the heating element'stepwise or gradually and thereby the conducting surfaces between the pin-like body and the sheathing, the desired effect of better conducting away the heat towards the end of the sheathing can be effectively obtained.

Care should be taken, however, that the end portions of the pin-like body lying close to the resistor element and. thereby nearer the center of the heating element are not heated beyond their liquefying point, in order to prevent undue expansion.v To this effect the dimensions of the parts of the pin-like body clo'se to the resistor should be made such that this portion upon heating will not expand to a greater dimension than vthe internal diameter of the ceramic sheathing.

In order to obtain a gradual diminution of the heat developing at the places Where adjacent parts of the pin-like body engage each other, those places should be stepped in diameter.

As can be seen from Figs. l to 3, a sleeve-like plug 4 is slipped over the end of the pin-like body 3; in Fig. 4 the plug I is closed at its outer end.

From the above it will be appreciated that the pin-like body connecting the plug with theresistor element proper is to be dimensioned in such a way that it will be heated at its outer end lying within the plug only to such a low temperature that a tight joint between the plug and the end of the pin-like body lying therein is obtained, Whereas the portions of the pin-like body lying within the projecting end of the ceramic sheath-` ing will be locally heated to such an extent that a part or portion thereof becomes plastic or even dough-like for the purpose and to the effect described above.

Also additional means may be used for tightly joining the plug with the end of the sheathing, particularly cements which at temperatures oc'- curring -at this joint do not soften at all or at least not beyondthe state of dough-like plastcity. Suitable means for this type are in the first place inorganic cements. In particular, metallic cements such as solders containing tin, lead, brass, silver may be applied. Since solders of the metallic type do not adhere sufdciently to ceramic materials it'is advisable to metallize the ceramic material at the places to be soldered before the solder is applied.

In addition to thecements mentioned above silver chloride, lead oxide with an addition of glycerine, asbestos-waterglass, quartz-waterglass, soft and hard glasses have been found partclllarly suitable.

' under the locally occurring temperatures during operation, and that it adheres equally well to ceramic materials and metal and does not react with them to any undesired extent.

It is to be understood that the invention is not limited to any of the particular examples given above but to be derived in its broadest aspects from *he claims.

I claim:

1. An electric heating element comprising a resistor of metal having a high melting point as exemplified by molybdenum, tungsten and tantalum, a ceramic gas-tight sheathing surrounding said resistor and substantially projecting beyond its ends, pin-like. bodies of metal of substantially better electrical conductivity than said high melting metal selected from the group consisting of silver, copper, nickel, iron, cobalt, said bodies snugly fitted into said projecting portions of said sheathing and contacting the ends of said resistor, and a sleeve-like plug fitted upon a portion of each of said bodies outside said sheathing in contact with the latter. l

2. An electric heating element comprising a resistor of metal having a high melting point as exemplified by molybdenum, tungsten and tantalum, a ceramic gas-tight sheathing surrounding said resistor and substantially projecting beyond its ends, pin-like bodies of metal of substantially better electrical conductivity than said high melting metal snugly fitted into said projecting portions of said sheathing and contacting the ends of said resistor, the portions of said bodies fitted into said projecting portions dimensioned and arranged to contact said ends at places so as to be heated in operation to doughlike plasticity, and a sleeve-like plug fitted upon a portion of each of said bodies outside said sheathing in contact with the latter.

3. An electric heating element comprising a resistor of metal having a high melting point as exemplified by molybdenum, tungsten and tantalum, a ceramic gas-tight sheathing surrounding said resistor and substantially projecting beyond its ends, pin-like bodies snugly fitted into said projecting portions of said sheathing and contacting the ends of said resistor, each of said bodies composed of a number, two as minimum, of parts arranged in series Within said projecting portions, said parts each of substantially uniform electrical conductivity substantially .better than that of said resistor, the conductivities of said parts being different inter se and increasing towards the 'ends of said sheathing so that the conductivity of said bodies also increases towards said ends, and a sleeve-like plug fitted upon a portion of each of said bodies outside said sheathing in ycontact with the latter.`

4. An electric heating element comprising a resistor of metal having a high melting point as exemplied by molybdenum, tungsten and tantalum, a ceramic gas-tight sheathing surrounding said resistor and substantially projecting beyond its ends, pin-like bodies snugly fitted into said projecting portions of said sheathing and contacting the ends of said resistor, each of said bodies composed of a number, two' as minimum, of parts arranged in series within said projecting portions, said parts each of substantially uniform electrical conductivity substantially better than that of said resistor, the individual conductivities and the diameters of said parts and thereby the conductivity of said bodies increasing gradually towards the ends of said sheathing,A

and a sleeve-like plug fitted upon a portion of each of said bodies outside said sheathing in contact with the latter.

5. An electric heating element comprising a resistor of metal having a high melting point as exemplified by molybdenum, tungsten and tantalum, a ceramic gas-tight sheathing surrounding said resistor and substantially projecting beyond its ends, pin-like bodies of metal of substantially better electrical conductivity than said high melting metal snugly fitting into said projecting portions of said sheathing, means for gas-tightly connecting each of said bodies with an end each stantially better electrical conductivity than said' high melting metal snugly fitted into said projecting portions of said sheathing and contacting the ends of said resistor, a sleeve-like plug fitted upon a portion of each of said bodies outside said sheathing, and means for gas-tightly connecting said plug with said sheathing, said means capable of being heated to dough-like' plasticity at operation temperatures.

7. An electric heating element comprising al resistor of metal having a high melting point as exemplified by molybdenum, tungsten and tantalum, a ceramic gas-tight sheathing surrounding said resistor and substantially projecting beyond its ends, pin-like bodies of metal of substantially better electrical conductivity than said high melting metal fitted into said projecting portions of said sheathing andA contacting the ends of said resistor, a sleeve-like plug f'ltted'upon a portion of each of said bodies outside said sheathing in contact with the latter, and cementing means for gas-tightly connecting said bodies with said sheathing selected from a group of cementing substances consisting of silver chloride, lead oxide admixed with glycerine, asbestos waterglass, quartz waterglass, soft glass, hard glass, and organic cement including articial resin.

8. An electric Yheating element comprising a resistor of metal having a high melting point as exemplified by molybdenum, tungsten and tantalum, a ceramic gas-tight sheathing surrounding said resistor and substantially projecting beyond its ends, pin-like bodies of metal of substantially better electrical conductivity than said high melting metalsnugly fitted into said projecting portions of said sheathing and contacting the ends of said resistor, a sleeve-like plug fitted upon a portion ofeach of said bodies outside said sheathing, and cementing means for gas-tightly connecting said plug with said sheathing selected from a group of cementing substances consisting of silver-chloride, lead oxide admixed with glycerine, asbestos waterglass, quartz Wa'terglass, soft glass, hard glass,v and organicrcement including artificial resin.

PAUL SCHWARZKOPF.

Cil

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