US2944239A - Electrically conductive element for use at elevated temperatures - Google Patents

Description

y 1960 N. G. SCHREWEL I I Jfi 2,944,239

ELECTR LY CONDUCTIVE FOR USE ELEVATED TEMP TURES Filed Jan. 26, 1959 2 Sheets-Shut 1 y 1960 N. G. SCHREWELIUS 2,944,239

ELECTRICALLY CONDUCTIVE ELEMENT FOR USE AT ELEVATED TEMPERATURES Filed Jan. 26, 1959 2 Sheets-Sheet 2 IIIIIIIIII United States Patent O ELECTRICALLY 'CONDUCTIVE ELEMENT FOR USE AT ELEVATED TEMPERATURES Nils Gustav Schrewelius, Hallstahammar, Sweden, as-

signor to Aktiebolaget Kanthal, Hallstahammar, Sweden, a corporation of Sweden Filed Jan. '26, 1959, Ser. No. 789,118

Claims priority, application Sweden Jan. 27, 1958 8 Claims. (Cl. 338-322) This invention relates to electrically conductive elements for use at elevated temperatures. Elements according to this invention may find various applications, such as for heating of furnaces, as tube type furnaces, as sources of radiation, and as thermo-couples wherein a hot junction is arranged within the element. However, as such elements are particularly well adapted for producing very high furnace temperatures they will be discussed herein with reference made to furnace elements as a non-limiting example.

It is already known that certain highly refractory oxides become electrically conductive at temperatures above about 900 C. Examples of such material are toria and zirconia either singly or in mixture with oxides of rare metals, such as yttria, ceria and lanthana. Furthermore it has been suggested to use such highly refractory oxides and oxide mixtures in the resistor proper of electrical resistance heating elements as well as to clamp such resistors between contact blocks. Thereby the dimensions and the composition of these contact blocks may be chosen to provide for such a temperature gradient across each contact block from the resistor to the lead-in electrodes that the latter is protected from the high temperatures which prevail in the resistor and which otherwise would cause the lead-in electrodes to fuse.

Although it is known that very high temperatures may be produced with the aid of electrical resistance heating elements of the class described, and although certain provisions have been made to prevent the lead-in electrodes from fusing, such elements have, however, as far as known not found any practical application.

.The principal object of the present invention is to provide an electrically conductive element for use, in practise, at elevated temperatures in air, for example temperatures of up to 2200' C., and more.

According to the invention this object is realised with electrically conductive elements of the class described by employing as lead-in electrodes only those composed essentially of highly temperature resistant silicides. Examples of such silicides are silicides of molybdenum, titanium, tungsten, niobium or zirconium, either singly or in combination, and possibly with minor portions of other metallic or nonmetallic substances. The silicide content should be at least 30 percent by volume and is preferably 75 percent by volume, or more. Lead-in electrodes composed of molybdenum disilicide or containing molybdenum disilicide to an essential proportion have-been found particularly advantageous. A particular advantage of employing molybdenum disilicide is to be found in that the strongly positive temperature coefiicientof this material acts in a balancing sense with respect'to the strongly negative temperature coefiicient of the oxide resistor. This has a favourable eifect on the operation characteristics and the usable life time of the element.

'Iheilead-in electrodes may be insulated in a suitable manner; Preferably, however, oxides of the class which are electrically nonconducting at very high temperatures are used for the purpose. Examples of such oxides are baryllia and alumina, either singly or in combination. The insulating layer may be applied in any convenient manner such as by hot spraying or the like. The insulation may also consist of a tube such as of a ceramic material of the above mentioned class which tube is disposed to surround the lead-in electrode.

Of course, the electrically conductive element according to the invention may be constructed and designed in any suitable manner according to the particular field of use. I p In one case, however, a resistance heating element according to the invention is made up in several parts, viz. the following parts: two straight rod or tube. shaped leadin electrodes of uniform cross-section, composed of molybdenum disilicide, Mosi and as by flame spraying provided with an electrical insulation of one or more oxides which are nonconducting at very high temperatures, a resistor portion, composed of an oxide of the above mentioned behaviour or a mixture of such oxides, as for instance a mixture of thoria and ceria, and two contact blocks disposed between the resistor and said lead-in electrodes respectively, and between which the resistor is clamped.

The contact blocks have been provided in order to improve the electrical contact between the lead in electrodes and the resistor proper, and preferably said contact blocks are composed of zirconia to an essential proportion.v

Preferably each lead-in electrode is let into a bore in its corresponding contact block. As is well known, however, there gradually develops a thin silicon oxide coating on the silicide lead-in electrode, and as this coating develops little by little there arises the danger of interruption of the current flow, due to the very high specific resistance of the coating layer. To avoid this drawback the silicide lead-in electrode according to the invention.

such that they each comprise two different portions, one

portion which is composed essentially of zirconia or another oxide which is electrically conductive at elevated temperatures whereas the other portion is composed essentially of zirconia and an additive of the above mentioned kind, for instance molybdenum disilicide. Thereby, the lead-in electrode of molybdenum disilicide or a similar silicide in secured to the silicide containing portion of-the contact block as by sintering. The contact blocks which have been formed by a powder-metallurgical process preferably should be subjected to a pro-oxidation,-

whereby the silicide particles are coated with a thin silicon oxide layer which reacts with the admixed zirconia into zirconium silicate. In order to prevent the development of insulating boundary layers the transition from the silicide rich to the oxide rich portion of the contact block is advantageously made diffuse or continuous along a certain' length. It is possible, however, to give the electrically conductive element according to the invention the shape of a tube with enlarged end portions in each of which there are fitted several spaced silicide lead-in electrodes of reduced cross section.

Now it has been found difficult in certain cases to obtain satisfactory electrical connections with the termi tioned kind, Such electrodeshave very hard surf-aces so that'they exhibit substantially no deformation and terminals, such as clamping terminals, used thereon for Patented July 5, 1960 g connecting them to asource of electrical energy in certajn cases have not been able to provide the desired intimate electrical contact with the entire surface area of the corresponding surfaces of the electrodes. Contact is made only'by spot contact on localized areas greatlyconeentrating thecurrent applied on these areas resultingin anunfavorable reaction between the materials of the.

connecting members and the lead-in electrode material and, arcing as well as embrittlement of the lead-in elec: trodes material eventually, resultingin the destruction of the connection.

While attempts have been made to solve the contact surface problem by accurately finishing the terminal surface of the lead-in electrodes still the service life ofthe lead-in electrode has remained greatly reduced. The matching of clamping terminals and the lead-in electrodesha'snot helped very much, Several materials, as

or xamp e, m ium te n nd e ik h e been used for the clamping terminals, however, the aforementioned unfavorablereactionstill takes place between these different materials and' the lead-in electrode maeri ls- Bonding the terminal clamping members to the leadin electrodes by. the use of hard solders or welding them to, the electrodes has not provided a solution to the problem since the deposition of the weld metals and the shrinking that takes place upon the cooling thereof results in the formation of internal stresses in the base material of the electrodes of such a magnitude that eventually fractures develop destroying the connection. It is a secondary object of the invention to provide a lead-in electrode of the above discussed classwherein the just mentioned limitations areovercome.

A feature of the lead-in electrode in accordance with the invention is that an electrically conductive metallic coating isappliedbyflame spraying to the terminal portion thereof. When used for forming termlnals for clamping terminals such coating is preferably built-up to a sufficient thickness for machining and then it is machined, for example by turning, to finish the surfaces thereof. The metallic coating is preferably a layer of aluminium or some similar material that is deformable;

and comparatively soft at room temperature and will thus provide contact over the entire surface of the contactarea of the coating. In this manner a: terminal zoneis formed on the lead-in electrode that will provide intimate contact with clamping terminals and the like over. theentire contact. area. thereof. The thickness of the. finished metallic coatingmust be at least. .2 mm, preferably greater than .5 mm. On the other hand, said thickness should preferably not exceed 3.0 mm.

The application of the coating or layer of alutniniurn byifiame spraying precludes the generationofmternal; stresses in the base electrode material and ellmmates theaforementioned unfavorable reaction causing-embrittlement since the application of the coating material does not result in the high temperatures: necessary for-the deposition of weld metals thereby eliminating the possibility of rupture of the electrode material and the possible destruction of the terminal during service operation. Moreover, such' coating makes it possible to connect one or. more silicide lead-in electrodes to distribution barsor the like, for instance by flame spraying.

To start up .a. resistance heating element of this kind it -is necessary to heat the resistor proper-to about 900 C. fPreferably this heating is performedby means of molybdenum disilicide elements. Such elements may eitherfbe stationary or adapted to .be removed: from .the

furnace as soon asthe ceramic resistor proper has attained the necessary startingtemperature at, which it becomes conductive.

, Hitherto, it has been considered necessary to control resistors of'highly refractory oxides separately in order to. .avoid that the current-due. to the-highly negative.

temperaturecoefiicient ofsuch materials- -channels down one, resistor only which mightlead to overheating. This serious limitation may be ovcrcome'according to the invention as the lead-in electrode material exhibits a very highly positive temperature coelficient. Thus, it may now be found possible to control several elements of the class discussed herein simultaneously. This necessitates, however, that the resistance of the lead-in electrodes corresponds, at least generally, to that of the oxide resistor proper.

Other, objects, features and advantages of the: invention will be understood from the following description and claims in conjunction with the accompanying drawings which illustrate by way. of example some embodiments of the resistance heating element and the termi nal connection, and in which Fig. 1 is a fragmentary side view of a resistance heating element according to the invention;

Fig. 1a is a fragmentary side view of a slightly modified resistance heating element according to the invention and is similar to that in Fig. l;

Fig. 2 isa fragmentary longitudinal axial section of second embodiment of a resistance heating element according to the invention installedin a furnace;

Fig. 2a is a cross section along line 2a--2a in Fig. 2;

Fig. 3a is a fragmentary side view of a lead-in electrode prior to forming thereon a terminalaccording to the invention; and

Fig. 3b is a fragmentary side view showingtheend portion of the lead-in electrode in Fig. 3a after the diameter thereof is reduced in preparation for forming; a

terminal thereon; and

Fig. 3c is a fragmentary side viewof a lead-in-elec trode with a terminal in accordance with the invention and-is illustrative of the thickness to which the terminal is built up; and

Fig. 3d is a fragmentary side viewof a lead-inelectrode showing the terminal thereon partly in section and is illustrative of-the finished surface of the contactarea:

of the terminal according to the-invention; and

Fig. 4a is a fragmentary sideview of a lead-in electrode similar to Fig. 3a; and

'Fig. 4b is a fragmentary side. view showing the end: portion of the lead-in electrodein Fig. 4a prepared for formation of a terminal thereon; and

Fig. 4c is a fragmentary side View of the lead-in electrode shown in Fig. 4b and is illustrative of a second, em-

bodiment of a terminal for lead-in electrodesaccordingto the invention; and

Fig. 4d is a fragmentary side view partly. in section of the lead-in electrode in Fig. 4c andis illustrative ofv an.

assembled terminal connection according. to the invention; and

Fig. 5 is a fragmentary side view of a third embodiment of'a terminal according to the present invention.

In Fig. 1 the reference 1% denotes the resistor proper of the element. This resistor 10 is composed, for instance, of thoria in mixture with a small amount of ceria. The reference 11 denotes av contact block of for instance zirconia or zirconia and molybdenum disilicide.

The joint 12 between the resistor 10. and the contact.

insulated by acoating 15 of for instance beryllia, preferably applied by spraying. The end portion of the lead-inelectrode is provided with a gas tight glazing 16 of zir-.

conium silicate;

- In the embodiment as shown in Fig. 1a the contact block 10 is made by a powder metallurgical process anrl composed of molybdenum disilicide, portion 17, molybdenum disilicide in mixture with zirconia, portion 18,

and zirconia, portion 19, in a continuously varying composition. In the. portion 18 there also existsszirconium silicate, which has been developed at the preoxidation of This. consists. of a straight rod 14 of molybdenum disilicidewhich is.

the contact block. In this embodiment the lead-in electrode is connected to the contact block by means of a welded joint 20.

The resistor proper of the heating element in Figs. 2 and 2a is in the form of a tube 21 of thoria or zirconia, either singly or in combination with admixtures to small proportions. The end portions 22 and 23 of the tube shaped resistor 21 are enlarged so that a considerable cooling elfect is obtained. In said end portions there are introduced the inner ends of several spaced lead-in electrodes 24 which are composed mainly of molybdenum disilicide and which are of a rather small cross sectional area. The lead-in electrodes 24 are disposed in a portion of the resistor body which in operation attains a temperature not exceeding l700 C. but not below about 900 C. The other end of said lead-in electrodes protrude from the resistor tube 21 and are connected to a source of electric current by means of an annular plate 25 of, for instance, aluminium. Preferably, such protruding ends are provided with a flame sprayed coating of aluminium, and these ends may be connected to the annulus 25 by a further flame spraying operation. Such flame spraying is preferred for making this connection but it is, of course, also possible to introduce the aluminium flame sprayed terminal ends into bores in the annulus 25. The inner ends of the lead-in electrodes may be provided with a thin coating of zirconium silicate. Between the annular plate 25 and the end surface of the resistor tube 21 there is disposed a ceramic brick 26 of the kind which is electrically insulating at high temperatures, e.g. composed of A1 or BeO. The advantage of using several molybdenum disilicide lead-in electrodes of reduced cross sectional area instead of single electrode is mainly to be found in that such electrodes make it possible to considerably reduce the thermal stresses between the metallic and the oxidic materials. Moreover, the danger of insufiicient electrical contact between the metallic and the oxidic materials is considerably reduced in as much as the contact surface has been enlarged. Finally, the advantage of an even current supply is secured whereby the tendency for all of the current to channel down one side only of the resistor proper is considerably reduced.

The construction as shown in Figs. 2 and 2a makes it possible to start up the element in a very simple manner, i.e. to heat it to a temperature at which it becomes conductive. As illustrated such starting up is performed by introducing into the resistor tube 21 a resistance heating element 27, preferably essentially consisting of molybdenum disilicide and preferably having the shape of a hair-pin, and heating said element 27 to a high temperature. Thereby, the resistor tube 21 rapidly attains the necessary temperature of 900 C. or more. Of course, the starting element 27 is removed immediately as the resistor tube 21 starts up, whereafter an insulating plug 28 is inserted in place of the starting element to prevent losses.

With reference now to Figs. 3a-3d, 4a-4d, and Fig. 5, which illustrate preferred methods of forming terminals on lead-in electrodes of resistance heating elementsof the class discussed above, particularly with reference to Figs. 1 and In, on the drawing, a lead-in electrode 30 made of molybdenum disilicide, or a similar material having a hard surface, and having any given diameter, for example 14 mm., is machined on a portion on which a terminal is to be formed, as for example, an end portion 31 to finish the surfaces thereof in preparation for forming a terminal on the end of the lead-in electrode.

The surfaces of the'end portion may be prepared by roughening them by sand blasting or other means prior to forming a terminal thereon. A metallic, electrically conductive coating or covering layer 32 is then applied circumferentially around the end portion 31 and over the end of the lead-in electrode so that coat 32 extends the full length of portion 31. The coating 32 is applied by flame spraying on portion 31 a suitable conductive material which is relatively deformable at room temperature, as for example aluminium, silver, copper or some other material. The thickness of the coating 3 when applied must be sufliciently thick to permit finishing the surface thereof, as for example by machining it.

Lead-in electrodes formed in this manner are connected to a source of current, not shown, by connecting a lug or conductor thereto with a clamping terminal and a bolt assembly. The relatively soft or deformable coat provides intimate electrical contact between the corresponding surfacm of the lead-in electrode terminal and the clamping terminal when bolt is tightened.

Referring to the drawings (Figs. 4a-4d) a second embodiment of a terminal, according to the invention, is formed on a lead-in electrode 30 by matching an end portion 33 thereof and tapering the tip or end of the leadin electrode in the manner shown in Fig. 4b. A connector element in the form of a plug 34 having an internally threaded central bore 37, as shown, (Fig. 4c) is placed and held in end-to-end relationship with the end portion 33' for fixed assembly with the resistor 30. A coating of conductive metal 25 is then sprayed over the end portion 33 circumferentially around the plug 34 and in an axial direction thereof. The thickness of the coating 35 is such that the plug is well embedded therein.

The plug 34 may be made of aluminium or of some suitable material and is preferably provided with serrations or corrugations 36 on the tapered portion thereof in order to insure a greater bonding area between the plug 34 and the metallic coat 35.

The end of the plug meeting the tapered tip of the lead-in electrode is recessed to accept the tip of the lead-in electrode to allow ease of alignment of "the members to be assembled and maintain them coaxial during assembly.

Subsequent to assembly coat 35 is preferably machined to finish dimensions. The finished lead-in electrode is connected to a current source, not shown, by clamping a lug terminal or conductor 38 across the end of the assembled plug with a threaded bolt 39, as shown.

Referring to Fig. 5 wherein is shown a third embodient of the invention a plug connector 34' provided with corrugations 36 in the manner heretofore discussed with respect to the embodiment shown in Fig. 4c is assembled with the lead-in electrode 30 by coating the plug connector and the lead-in electrode end portion to form a terminal in the manner heretofore discussed. In this instance, the connector element 34 is provided with an exterior thread 40 on which is threaded a conductive adapter 41 to which an electrically conductive lug or conductor 38 is connected by a threaded bolt 39' (in the same manner as shown in Fig. 4d). The adapter 41 provides additional contact surface between the plug connector 34 and element 38' thereby allowing maximum currents to be applied through the terminal.

In order to improve the bond between the metallic coat and the connector elements 34 and 34' the connectors are preferably sprayed with a thin metal coating of a suitable material, as for example, molybdenum, iron, stainless steel or zinc and the like before assembly in end-to-end relationship with the lead-in electrode 39.

In each of the embodiments of the invention the lead-in electrode 30 can be pre-glazed prior to forming the terminal portion therein in order to insure the serviceability of the lead-in electrode under all kinds of atmospheric conditions. Prior to machining, the material is heated, at

a rapid rate to a high enough temperature, under oxidizing conditions to form the characteristic vitreous glaze of these materials. The glazing effectively seals the pores precluding the penetration of oxygen under service conditions thereby forming an effective anti-oxidation covering on the lead-in electrode. In order to improve the anti-oxidation cover glazing constitutents or additives may be employed in the composition of the lead-in electrode material, as for example boric oxide (B 0 been illustrated and. described, it will be understood that the invention is in no way limited to these embodiments and that many changes may be made within the spirit and scope of the invention as defined by the following claims:

What I claim and desire to secure by Letters'Patentis:

1. An electrically conducting element for use at elevated temperatures, wherein the resistor proper is composed of a highly refractory oxide of the kind which becomes electrically conductive at temperatures above about 900 C., and wherein the lead-in electrodes are composed at least to an essential proportion of a highly temperature resistant silicide, and are provided With an insulation of at least one oxide of the kind which is electrically non-concluctingat very high temperatures.

2. An electrically conductive element for use at elevated temperatures comprising, a resistor of a highly refractory oxide of the kind which becomes electrically conductive at temperatures above about 900 C., means for holding the resistor and providing electrical current thereto comprising, a lead-in electrode having a composition cdmprising silicides of the group consisting of molybdenum, titanium, tungsten, niobium and zirconium, and the silicide content of each lead-in electrode being at least 30 percent by volume of the composition of the lead-in electrode.

3. An electrically conductive element for use at elevated temperatures comprising, a resistor of a highly refractory oxide of the kind which becomes electrically conductive at temperatures above about 900 C. comprising oxides of 'the group thoria and zirconia, means for holding the resistor and providing electrical current thereto comprising, a lead-in electrode having a composition comprising silicides of the group consisting of molyb denum, titanium, tungsten, niobium and zirconium, and the silicide content of each lead-in electrode being at least 30 percent by volume of the composition of the lead-in electrode.

4. An electrically conductive element for use at elevated temperatures comprising, a resistor of a highly refractory oxide of the kind which becomes electrically conductive at temperatures about about 900 C. com prising oxides of the group thoria and zirconia, means for holding the resistor and providing electrical current thereto comprising, a lead-in electrode having'a composition comprising silicides of'the group consisting of molybdenum, titanium, tungsten, niobium and zirconium,

the silicide content of each lead-in being at least 30 percent by volume of the composition of the lead-in electrode, and said lead-in electrode having a coating of zirconium silicate along'a portion thereof in contact with said holding means.

5. An electrically conductive element for use at elevated temperatures comprising, a resistor of a highly refractory oxide of the kind which becomes electrically conductive at temperatures above about 900 C., electrically. conductive contact blocks holding'the resistor extending axially between them, lead-in electrodes electrically connected to respective blocks, each lead-in electrode comprising silicides of the group consisting of molybdenum, titanium, tungsten, niobium and zirconium and the silicide content of each lead-in electrode being at least 30 percent by volume of the lead-in electrode.

6. An electrically conductive element for use at elevated temperatures comprising, a resistor of a highly refractory oxide of the kind which becomes electricallyconductive at temperatures above about 900 C. comprising oxides ofthe group thoria and zirconia, electrically conductive contact blocks holding the resistor extending axially between them, lead-in electrodes electrically connected to respective blocks, each lead-in electrode comprising silicides of the group consisting of molybdenum, titanium, tungsten, niobium and Zirconium and the silicide content of each lead-in electrode being at least 30 percent by volume of the lead-in electrode.

7. An electrically conductive element for use at ele-I vated temperatures comprising, a resistor of a highly refractory oxide of the kind which becomes electrically conductive at temperatures above about 900 C., electrically conductive contact blocks holding the resistor extending axially between them, and comprising separate.

layers of the group molybdenum disilicide, a mixture of molybdenum disilicide and zirconia, and zirconia, leaddn electrodes electrically connected to respective blocks;

fractory oxide of the kind which becomes electrically conductive at temperatures above about 900 C. 'com prising oxides of the group consisting of thoria and zirconia, electrically conductive contact blocks holding the resistor extending axially between them and comprising in separate layers molybdenum disilicide, a mixture of molybdenum disilicide and zirconia, lead-in electrodes electrically connected to respective blocks, each lead-in electrode comprising silicides of the group consisting of molybdenum, titanium, tungsten, niobium and zirconium and the silicide content of each lead-in electrode being at least 30 percent by volume of the lead-in electrode.

References Cited in the file of this patent UNITED STATES PATENTS 1,538,972 Conradty May 26, 1925 1,550,206 Church Aug. 18, 1925 2,412,373 Wejnarth Dec. 10, 1946 FORElGN PATENTS 759,476 France Dec. 16; 1933 t l i i i

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