US2727121A

US2727121A - Electric heater terminal

Info

Publication number: US2727121A
Application number: US458190A
Authority: US
Inventors: Joseph F Nelson; Robert W Sawyer
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1954-09-24
Filing date: 1954-09-24
Publication date: 1955-12-13
Anticipated expiration: 1972-12-13

Description

Dec. 13, 1955 NELSON ET AL 2,727,121

ELECTRIC HEATER TERMINAL Filed Sept. 24, 1954 lnveMors:

Joseph F. Nelson Roberf W. Sawyer by. Wand:

Their AHorney United States Patent G ELECTRIC nna'raa TERMINAL Joseph F. Nelson, Pittsfield, and Robert W. Sawyer, Lee, Mass, assignors to General Electric (Jempany, a can poration of New York Application September 24, 1954, Serial No. ddthwti 4 Claims. (Cl. 201-67) This invention relates to terminal structures and has particular application to terminal structures for electrical resistance heaters of the encased or sheathed type wherein a resistance heating element is embedded in a compacted mass of electrically insulating, heat conducting material which is, in turn, enclosed by an outer metallic sheath.

It is well known that, in heaters of the above type, it is extremely desirable from the standpoint of obtaining an adequate operating life, that the resistance heating element and its associated heat conducting and electrically insulating material be hermetically sealed within the heater from the deleterious effects of moisture and various harmful atmospheres and gases in which the heater may be required to operate. It will be seen that, with the well known sheathed heater construction wherein an outer sheath of non-porous material is employed, the

problem of hermetically sealing the heater is concerned principally with providing a terminal structure in which a hermetic seal is eifected around the corresponding terminal opening of the heater.

it is accordingly one object of this invention to provide an improved terminal structure for heaters of the encased or sheathed type which terminal structure is of a rugged and inexpensive construction, and which provides a hermetic seal around the corresponding terminal opening of the heater.

Briefly stated, this invention, in accordance with one aspect thereof, employs a sealing member of non-porous ceramic material and provides a construction utilizing butt joints between the ceramic sealing member and the adjacent metallic members to which it is attached in sealing relationship in any suitable manner such as by means of a titanium hydride braze. The requirement for holding close tolerances in the manufacture of the ceramic sealing member is thus eliminated while, at the same time, a mechanically rugged and hermetically sealed construction is provided. In addition, this invention provides, in accordance with one embodiment thereof, a terminal structure which provides a hermetic seal around the terminal opening without exceeding the diameter of the heater sheath. Further, there is provided in one embodiment an arrangement for minimizing mechanical stresses which might otherwise be induced by differences in the temperature coefficients of expansion of the adjoining metal and ceramic parts.

Other objects and advantages of this invention will be apparent from the following description taken in connection with the accompanying drawings, and its scope will be pointed out in the appended claims.

Referring to the drawing, Fig. 1 is a perspective view, shown partly in cross section, of a sheathed heater embodying this invention with one of the terminals being shown in an exploded view to more clearly illustrate the elements of the terminal structure; while Fig. 2 is a perspective view, also shown partly in cross section with an exploded view of one of the terminals, of a sheathed heater embodying this invention and illustrating an arrangement alternative to that shown in Fig. 1.

2,727,121 Patented Dec. 13, 1955 As illustrated in Fig. 1, this invention is particularly applicable to electric resistance heaters of the encased or sheathed type wherein a resistance heating element 1 is embedded in a compacted mass of electrically insulating and heat conducting material 2 which is, in turn enclosed by an outer metallic sheath 3. The heat conducting, electrically insulating material 2 may be a material such as magnesium oxide, which can be loaded into the sheath 3 in powdered form around the heating element 1 and then compacted into a dense mass by means of elongating and reducing the diameter of the sheath 3, usually by swaging or rolling.

in order to ensure that the protective layer of heat conducting and electrically insulating material 2 remains in good condition during the life of the heater, it is very desirable that this material be hermetically sealed within the heater so that it does not become exposed to the deleterious action of moisture and certain harmful atmospheres in which the heater may be required to operate. Thus, with both the resistance heating element and the surrounding heat conducting, electrically insulating material sealed within the heater enclosure, the operating life of the heater can be considerably extended.

As has been pointed out, one of the most difficult areas with respect to providing a hermetically sealed heater of the aforementioned type, lies in the problem of sealing the terminal openings of the heater while at the same time providing an inexpensive and mechanically rugged terminal structure which is physically compact in size.

Referring now to the terminal structure illustrated in Fig. l and embodying this invention, it will be seen that the heater shown is provided with a pair of terminal membets 4 which extend into and make electrical contact with the opposite ends of the heating element 1, and which extend out from the ends of the tubular sheath 3. For purposes of description, the elements illustrated in the exploded view at the right hand portion of Fig. 1 are represented by the same reference numbers as like elements shown in the assembled terminal structure at the left hand end of the heater of Fig. 1.

Referring now to the assembled terminal structure at the left hand end of the heater of Fig. 1, the open end of the sheath 3 is sealed by means of an assembly comprising a non-porous, electrically insulating disk 5 of ceramic or other heat refractory material and a metallic cover disk 6. The ceramic disk 5 is attached in hermetically sealing relationship to the edge portion of the sheath 3 by means of any suitable method for attaching ceramic to metal, such as by means of a titanium hydride braze or by the titanium shim method, which is the method illustrated in the exploded view at the right hand portion of the heater of Fig. 1.

The sealing disk 5 may be formed of any suitable nonporous, electrically insulating and heat refractory material such as a ceramic material. By way of example, it has been found that the disk 5 will perform satisfactorily it formed of a ceramic material commonly known as alumina ceramic, which is composed predominately of aluminum oxide with small percentages of clay and talc acting as binder and fiux respectively.

The disk 5 is provided with a central aperture 7 through which the terminal member 4 extends. The aperture 7 may be considerably larger than the diameter of the terminal member 4 since, as will be seen from the following description, it is not necessary that a seal be effected between the disk 5 and the terminal member 4 along the diameter of the terminal member 4.

The seal between the terminal member 4 and the ceramic disk 5 is effected by means of the cover disk 6 which is brazed or otherwise suitably attached around an aperture 6 therein to the terminal member 4, and which is also attached in sealing relationship to the abutting face of the ceramic disk 5 by means of any suitable method of attaching metal to ceramic such as discussed above.

It will be seen that with the relationship of parts just described allsealed joints or connections between the ceramic disk 5 and the adjoining metal parts are made by means of butt joints and that, consequently, any requirement for holding close tolerances on the ceramic disk 5 is eliminated. It is therefore possible to mold or cast the ceramic disk 5 directly into its final shape so that it can be utilized in the assembly of the terminal structure without any further machine or grinding operations.

In addition, it will be observed that by attaching the cover plate 6 to the terminal member 4 by means of brazing or by some other process involving heating of the terminal structure to a fairly high temperature, the ceramic disk is then put under axial compression upon cooling of the terminal assembly by reason of the corresponding contraction of the terminal member 4 along the axial directioh. The aforementioned axial compressive force results from the fact that the coefiicient of thermal expansion of the material of the terminal member 4 is greater than the thermal coeflicie'nt of expansion of the ceramic member 5. Therefore, when the assembly is heated in the brazing operation, the terminal member 4 expands outwardly in the axial direction by a greater amount than does the ceramic disk 5. Then, upon cooling of the assembly, the brazed joint between the cover disk 6 and the terminal member 4 solidifies to form a solid connection and further cooling results in compression of the ceramic disk 5.

The ceramic disk 5 remains under a compressive force even when the terminal reaches normal operating temperature since, for most applications, the operating temperature of the terminal structure would not exceed 400 F. or so whereas the brazing operation is normally carried out at temperatures in the vicinity of 1100 F. to l500 F. Consequently, even at normal operating temperature, the terminal assembly is still considerably cooler than the brazing temperature so that the ceramic disk 5 remains in compression.

The fact that the ceramic disk 5 is continuously under a compressive force results in a more rugged terminal structure since, as is well known, ceramic materials are relatively strong in compression but quite weak in tension so that it is desirable for that reason to place the ceramic material under compression and thereby provide a prestressed condition. The net result is that the terminal structure just described is less subject to mechanical failure since, before any tensile stresses can be induced, the initial compressive stresses must first be overcome.

The exploded View at the right hand of Fig. l is inchided to show one method of manufacturing the heater terminal structure illustrated at the left hand end of the heater of Fig. l. The method illustrated includes the utilization of the titanium shim method for attaching metal to ceramic, which method comprises clamping the metal and ceramic parts together with a thin titanium shim therebetween and then heating the clamped assembly to a high temperature either in a vacuum or controlled atmosphere to effect a bond between the parts.

In manufacturing the terminal assembly of the heater shown in Fig. l in accordance with the titanium shim method just discussed, one procedure which has been found to be satisfactory comprises clamping the

elements

5 and 6 over the terminal 4 to the terminal end of the sheath 3 with a titanium shim 8 between the

elements

5 and 6 and a second titanium shim 9 between the ceramic member 5 and the sheath 3. The clamped assembly is then heated in a vacuum or controlled atmosphere to effect a bond between the ceramic and metal parts as described above. In the same operation, the cover disk 6 is brazed or otherwise suitably attached to the terminal member 4, thereby completing the terminal structure and effecting the seal in a single heating operation.

In a second embodiment of this invention, which is illustrated in Fig. 2, provision is made for compensating for possible differences in thermal coefiicients of expansion between the metal of the sheath and the ceramic disk. The advantages inherent in the construction of Fig. l are, of course, present in the construction illustrated in Fig. 2 in that there is no requirement for close tolerances in the manufacture of the ceramic sealing disk and further in that a rugged, inexpensive and hermetically sealed terminal structure is provided. It will be noted also that the ceramic sealing member 5 of the construction of Fig. 2 is placed under a pre-stressed compressive force as described in connection with the structure of Fig. 1 thereby yielding the advantages discussed above. The construction of Fig. 2 therefore is illustrated to show an alternative arrangement which may be advantageously employed in cases where, by reason of certain limitations on selection of materials, problems of differences in thermal coefficients of expansion of the ceramic sealing disk and the metallic sheath exist. 7

Referring now to the assembled terminal structure at the left hand end of the heater of Fig. 2, in which the elements thereof are identified by the same reference numerals as like elements in the construction of Fig. l, the construction illustrated is similar to that of Fig. 1 except that a cylindrical ring 10 of metallic material is provided interconnecting the ceramic sealing disk 5 and the sheath 3. The cylindrical ring 10 is of tubular form and may have a cross sectional shape substantially the same as that of the sheath 3 so that when the ring 10 is attached to the sheath 3 it forms in shape an extension thereof. The cylinder 10 is of a metallic material which has a thermal coefiicient of expansion closely corresponding to the coefiicient of expansion of the ceramic disk 5 or, in the alternative, it may be of a pure ductile metal, such as pure copper, iron or nickel, which, although not having a coeflicient of expansion closely corresponding to that of the ceramic disk 5, provides a yielding connection between the disk 5 and the sheath 3, thereby avoiding the occurrence of any high mechanical stresses by reason of differences in thermal expansion characteristics.

With this construction it will be seen that if the metal which forms the cylindrical ring 10 is selected so as to have a thermal coefficient of expansion closely corresponding to that of the ceramic disk 5, the difference in physical expansion between the disk 5 and the sheath 3 will appear at the joint between the metallic ring 10 and the sheath 3, which joint can be much stronger than the connection between the metallic ring 10 and the disk 5 so that the mechanical stresses induced by differences in thermal coeflicients of expansion do not reach an objectionable level.

In the event that the disk 5 is composed of alumina ceramic, which, as stated above, is composed predominately of aluminum oxide, the metallic ring 10 may be manufactured of a metallic material identified as a 14% nickel-iron alloy which has expansion characteristics similar to those of alumina ceramic up to about 800 F. Good results may also be obtained if, as mentioned above, the ring 10 is manufactured of a pure ductile metal such as copper, iron, or nickel in a pure or substantially pure state.

It may be found convenient to manufacture the thermal assembly illustrated in Fig. 2 in a similar manner to that described in connection with the structure of Fig. l, or it may be found more desirable to employ a subassembly method. In such a method, the subassembly indicated by the bracket a, which comprises the metallic ring 10, the ceramic sealing member 5, and the cover plate 6, is first formed by utilizing the titanium hydride process, the titanium shim method many other suitable method or process for attaching metal and ceramic materials. In the event that it is desired to employ the titanium shim method, the

elements

10, 5 and 6 are first clamped together with the intermediate titanium shims 8 and 9 as illustrated. This subassembly is then heated to a high temperature in a vacuum or protective atmosphere in order to eifect a hermetically sealed bond between the abutting ceramic and metal surfaces.

When the subassembly a is formed, it is then inserted over the terminal member 4 with the edge of the metallic member 19 being brought into abutting relationship with the edge of the sheath 3 and the terminal member 4 extending through the apertures 7 and 6 in the

members

5 and 6 respectively. The member 10 is then brazed or otherwise attached to the sheath 3 to form a hermetically sealed connection therewith and, in the same operation, the cover disk 6 is then affixed to the terminal member 4 by means of brazing or some other suitable process.

It will be realized of course that in the construction of Fig. 1 the sheath 3 may be formed entirely out of a material having thermal expansion characteristics similar to those of the ceramic disk 5 or, on the other hand, the sheath 3 may be formed of a ductile material in order to prevent the occurrence of high mechanical stresses which might otherwise be induced by differences in thermal expansion characteristics between the abutting metal and ceramic materials. it will also be realized that in certain applications, such as in those where the operating temperature of the heater is relatively low, the problem of diiferences in thermal expansion between the metal and ceramic materials may not arise, in which case the construction of Fig. 1 may be advantageously employed without any special restrictions on the selection of materials.

It will be understood that the terminal structure of the present invention may be manufactured in other ways than in the particular manner illustrated and described herein and that the present method of manufacture is set forth as representative of one method which has been found to be satisfactory. In addition, the ceramic and metal parts may be attached together by means other than the titanium shim method which is set forth herein for purposes of illustration only.

It will be apparent from the foregoing that various modifications, changes, substitutions and combinations may be employed in accordance with the teachings set forth herein without departing from the scope of this invention in its broader aspects as defined in the appended claims.

What We claim as new and desire to secure by Letters Patent of the United States is:

1. A terminal structure for a tubular sheathed heater comprising a metallic terminal member extending from one end of said sheath, a tubular member formed of a metallic material and having a cross sectional shape sulvstantially the same as that of said sheath, said tubular metallic member being attached in sealing relationship to the edge of said sheath so as to form in shape an extension thereof, a non-porous ceramic sealing member shaped so as to abut against the edge of said tubular member opposite the edge affixed to said sheath, said ceramic sealing member being attached in abutting relationship to the said opposite edge of said tubular member and having an opening therein through which said terminal member extends, and a metallic cover member tached in abutting relationship to said ceramic member on the side thereof opposite from the side attached to said tubular member, said cover member extending around said terminal member and being attached in sealing relationship thereto.

2. A terminal structure for a sheathed heater comprising a terminal member for said heater extending from one end of said sheath, a tubular member formed of a substantially pure ductile metal and having a cross sectional shape substantially the same as that of said sheath, said tubular member being attached in sealing relationship to the edge of said sheath so as to form in shape an extension thereof, a non-porous ceramic sealing member shaped so as to abut against the edge of said tubular member opposite the edge afiixed to said sheath, said ceramic sealing member being attached in abutting relationship to the said opposite edge of said tubular member and having an opening therein through which said terminal member extends, said tubular member thereby providing a yieldable sealed connection between said ceramic member and said sheath and acting to reduce mechanical stresses occurring by reason of differences in thermal coefiicients of expansion between said sheath and said ceramic member, and a metallic cover member attached in abutting relationship to said ceramic member on the side thereof opposite from the side attached to said tubular member, said cover member extending around said terminal member and being attached in sealing relationship thereto.

3. A terminal structure for a tubular sheathed heater comprising a metallic terminal member for said heater extending from one end of said sheath, a tubular member formed of a metallic material and having a cross sectional shape substantially the same as that of said sheath, said tubular member being attached to and extending from the terminal edge of said sheath, a non-porous ceramic sealing member shaped so as to abut against the edge of said tubular member opposite the edge aflixed to said sheath, said ceramic sealing member being attached in abutting relationship to the said opposite edge of said tubular member and having an opening therein through which said terminal member extends, said tubular member and said ceramic sealing member having closely corresponding thermal expansion characteristics over a substantial portion of the range of operating temperatures for minimizing mechanical stresses induced by reason of differences in said thermal expansion characteristics, and a metallic cover member attached in abutting relationship to said ceramic member on the side thereof opposite from the side attached to said tubular member, said cover member extending around said terminal member and being attached in sealing relationship thereto.

4. A terminal structure for a tubular sheathed heater comprising a metallic terminal member extending from one end of said sheath, a tubular member formed of a metallic material attached in sealing relationship to the edge of said sheath so as to form in shape an extension thereof, a non-porous ceramic sealing member shaped so as to abut against the edge of said tubular member opposite the edge afiixed to said sheath, said ceramic sealing member being attached in abutting relationship to said opposite edge of said tubular member and having an opening therein through which said terminal member extends, and a metallic cover member attached in abutting relationship to said ceramic member on the side thereof opposite from the side attached to said tubular member, said cover member extending around said terminal member and being attached in sealing relationship thereto.

References Cited in the file of this patent UNITED STATES PATENTS 2,003,175 Daly May 28, 1935