US2207558A - Electrical heating element - Google Patents

Description

y 9, 1940. F. sNGER 2207558 ELECTRICAL HEATING ELEMENT Filed Nov. 9, 1937 INVENTOR. ;Eliot cs 'qe' BY ATTORNEY.

Patented July 9, 1940 UNITED STATES PATENT OFFICE Application November 9, 1937; Serial .No. 173552 14 Claims.

This invention relates to electrical heating elements suitable for operation in open air and other atmosphere containing substances capable of attacking the resistor element. In particular the invention relates to electrical heating elements to be operated at high temperatures, such as 1400 to 1600 C. and more.

According to this invention the heating element consists of an electrical substantially metallic resistor which is capable of sustaining high temperatures such as 1400 to 1600 C. and more, without softening or melting. suitable substances are in particular tungsten, tantalum, molybdenum and alloys thereof. For lower temperatures, such as l400 C., a resistor element of chromium or an alloy thereof can be used.

It is well known in the art that such resistor elements when operated at high temperatures are subjected to attacks by the oxygen of the surrounding atmosphere or other gases in which the resistor is being operated. Furthermore, if the resistor is submerged into a substance which is to be heated it is liable to be attacked by some of the chemicals contained in those substances to be heated. Thereby the diameter of the resistor is continuously reduced and its resistance increased whereby its heating increases and its life is cut short.

It was suggested therefore to cover such resistor elements with suitable tubes or coatings which are resistant to the surrounding air or other substances and prevent the latter from reacting upon the resistor element at operation temperatures. such covers or coatings were applied either closely upon the resistor or spaced thereirom. In the latter case an inert protective gas, or a vacuum, was applied between the cover and the resistor element.

It has been suggested to use for such cover or coating oxides having a high melting point, such as zirconia, alumina, magnesia, silica, sillimanite and mixtures thereof compounded by heat treatment. It was particularly suggested to use, partly or entirely, vitreous or vitrified substances for such covers or coatings.

such coatings or covers, although suited for temperatures of about 1400" to 1500 C. do not remain gas-tight for an extcnded period of operation, nor do they withstand repeated electrical connecting and disconnecting.

It is an object of the invention to improve electrical heating elements of the type referred to so as to prolong their life when operated at high temperatures.

It is another object of the invention to make operation for a long period of time and notwithv standing repeated switching-in and out within temperature ranges between about 1400 and 1600 C. and higher, without impairing the gastight and other qualities of the cover.

This and other objects of the invention will be more clearly understood when this speciflcation proceeds with reference to the drawing in which Figs. 1 and 2 show a cross-section through different types of resistors to which this invention pertains, without, however, being limited to them since they are cited as exempliflcation only.

According to this invention the cover consists of highly refractory substances, in particular oxides of metals selected from elemlents of the second, third, fourth and sixth group of the periodical system, and these refractory materials' are used in amorphous but substantially other than nitreous form. Thereby a gas-tight cover is obtained which efficiently protects the resistor against the attacks of the surrounding atmosphere and substances, this quality being maintained during operation and heating to high temperatures, such as 1400" to 1600 C. and above.

Referring to the drawing, such a cover may either be made as a tube 3, Fig. 1, of any desired cross-section and the resistor 4 arranged within that tube, spaced from its inside walls. That space may be filled with inert gas either under atmospheric or super-atmospheric pressure or under lower than atmospheric pressure. There may also be applied a vacuum of any desired degree. The coating 5, Fig. 2, may also be applied directly upon the resistor 6, in particular in close touch therewith. The coatings 5 may also be prepared separately and in particular be fritted or finely sintered and then applied to the resistor E. They may also first be fritted or presintered by heating the latter to adequately high temperatures. The highly heated resistor may also be submerged into a more or less plastic mass containing the selected constituents, with a resulting coating adhering to it, being fritted and sintered by that highly heated resistor which, after having been submerged into the mass and after having derived from it an adequate layer, is suitably removed from the mass and heated in open air or another, particularly neutral, atmosphere or vacuum, long enough to permit finely sintering the mass upon the resistor while at high two oxides of the type referred to above areused, sintered into a dense and gas-fight body- I tenerature.

ereby the resistor obtains its largest Volume, and the mass sintered upon it shrinks to the smallest Volume, and upon cooling the resistor material will either shrink in the same proportlon or to a higher degree than the coating a'pplied to it. In the former case the coating will remain adherent to the resistor; in the letter case the cover-ing will slightly segregate rrom the resistor, creating a. vacuum between it and the resistor. In any case, cracmng of the coating during manufacture and operatlon is successully prevented because the resistor will never he heated to temperatures higher than those at which the coating was finally sintered upon it.

For the coatlng mixtures of oxides of at least without, however, developing glass or vitreous substances to any substantial extent, i. e. ii any vitreous component will develop during manufactu'e they will amount to substantially less than 3% to 10% of the entire mass. Yitreous substances, if present, have the disadvantage that they devitriy after a short period of service, rendering the cover brittle and permeable to gases. This holds true for all vitreou's amorphous substances if present in the cover in substantial anmnts and may be termed an aging phenomenon. Sintered alumina as suggested for covers of electrical heating elements contains usually small amounts only of vitreous amorphous constituents. Nevertheless it is permeable to gases, though to a smaller extent, and the aging phenomenon occurs quickly if such a cover is continually used at high temperatures.

The present' invention established by numerous and extended experiments that sintered mixtures of oxides melting above about 1600 C. selected from oxides of the second, thir-d, fourth and sixth group of the periodical system provide gas-fight, incombustible products if treated in a manner to be described later on. As oxides suitable for the purposes of the invention the following may be mentioned by way of example: SiOz, AlaOz, cmos, CeOz, Tioz, T203, BeO, Mgo, Z'Oz, Thoz, MgO.A12O3, BeO.Al203, mzoszroa, AlzOaSiOz, 3Al2O32Si02, Mgocrzm, Al203.Ti02, MgO.(Al2O3) (cmos) or combinations of these substances.

Two or more of these substances are chosen in suitable proportion and thereupon shaped in powdered state and subjected to compounding under heat treatment. However, this heat treatment is taking place at temperatures where no melting occurs, i. e. sintering. In particular, by sintering a powdered shane mixture according to the present invention it is densified by heat treatment so that the powdered mass substantially shrinks and crystallzation, or recrystallization, just starts. This may also he termed a reaction of the crystals of the powdery substance upon each other while still in solid state, in contradistinction to melting or a partial melting process in which at least part of the substances present is melted and starts-if vitreous in characterto vitrefy. In that latter case the higher melting oxides will remain more or less solid and coalesce to a kind of skeleton the interstices or pores of which are filled with the lower melting` substances which are caused to melt by the heat treatment. These lower melting and actually melted vitreous amorphous products fill the pores of the skeleton, and upon cooling they may entirely or partially devitrify. According to the present inventlon, pure sinterlng is applied which causes an at least beginning crystallization or recrystallization without melting substantial amounts of substances present and, consequently,

without causing any essential vitrifloaon of components of the shaped powdery mixture.

Other conditions deserving particular consideration are the temperatures and duration of heating. According to the invention these two conditions are determined so that at least two different types of crystals will result in the final product which, therefore, is hereinafter and in the appended claims called a heterogeneous product. By this reterogeneity the new product according to this invention is principaly dierentiated from products which consist of asingle type of crystal only such as sintered alumina which is also called sintered corundum.

Comparative microscopic investigations of rntered alumina and the product according to this invention both in their new state and after having been used for an extended period of time and under high operatng temperatures, explain the different behavior of sintered alumina and of the product according to the invention, although I do not wish to confine myself to any scientic explanation of my invention.`

Uniformly crystallized substances, such as sintered alumina, show before being put to *use and while in the state as obtained by sintering alumina, a. mosaic-like structure. After prolonged continuous heating, however, glidingplanes will form between the-individual crystais which in time will develop more and more and ultimately will reach from one surface to the other of the body. Along these gliding surfaces between thecrystals gas may flow from one side to the other of the body, and im original quality of gas-tightness will gradualy disappear.

With a heterogenecus product according to'the tufe can be observed under the mici-escape. Depending upon the raw materials selected and the fineness of grinding same, urthernore depending upon the range of temperature and duration of heating, a large number of diiferent crystallites are ascertained which did not develop one alongside of the other. as is the case with homogeneous materials dealt with above, but which grow one into the other or one permeates the other. Thereby a mechanically strong structure is obtained which coheres in itself and assures continued high gas-tightness because no gliding planes or sur-faces can develop between crystallites of diii'erent types which According to a particular feature of the inveni tion, materials are added to the initial mixture which accelerate and promote the formation of crystals, which further increase the number of crystal grains and, lastly, assist in the growth ,present invention no uniform crystalline struoot the crystals, thereby causing immediate devitriflcation of components present in the mixture if they happen to vltrify during manufacture. It is to be understood that the last mentioned action realizes quasi in situ during the manufacture of the product. It may be described as an effect which' vitrication removes at the very moment when it starts to occur. By selection and admixture in proper and small amounts of one or more substances of the effect described above, the number of types of crystals created within the product can be increased almost at will. such substances are known as mineralisers. such mineralisers are of particular advantage if they form either fluxes or if they are of themselves highly refractory and capable of entering the crystal lattice of the main constituents used for the product. Ordinarily, small amounts `of such mineralisers suflice for the purpose of the invention. Therefore, in combination with alumina, fluxes may be used which are of a very strong nature because it is entirely immaterial whether the melting point of the final product is reduced by their admixture from 2050 C, to 2000 C. whereas it is of utmost importance that the temperature of manufacturing, i. e. the sintering temperature, be reduced, for example, from 1800 C. to 1600 C. A

Up to the present time one was under the impression tha impurities are to be avoided and that alumina should be used in as pure a state as possible for the manufacture of the sintered alumina (corundum). If, however, the admixtures to alumina are carefully chosen, as it is the case according to the invention, then the above advantages during manufacture can be obtained, and the final product can be rendered highly heterogeneous, containing two o r more diflerent types of erystals which have grbwn into each other and form a type of dense network of extraordinarily high mechancal strength. This network contains no pores or channels reaching from one to the other surface of the body and, consequently, is prefectly gas-tight. Due to this particular feature this gas-tightness is retained during use, and the body is of exceptional mechancal strength. No` aging phenomenon can appear. v

The following are' examples of mixtures and methods of manufacture of coverings or coatings according to the invention. Any shape of the body, and any method of p'oducing and applying the same may be used as outlined in the opening paragraphs of the specification. In particular, tube-like covers may be first finished by sintering and then be applied upon the substantially metallic resistor element. It is to be understood that suitable terminals, or electrodes, are to be connected to the ends of the heating elements in a well known manner which does not form part of this invention.

Example 1 About 93% anhydrous alumina (A12O3) ground to particles of a diameter smaller than one micron is admixed to about bentonite and about 2% fluorspar which are all ground to particles of less than one micron diameter. The mixture is shaped, preferably under pressure.- and then heated to about 1650 C. for about 12 hours. A heterogeneous gas-tight product is obtained which can be used for heating temperatures up to about 1600 C. without aging and without becoming permeable to gases.

The amount of bentonite may be varied between about 2% to 6%, and that of fluorspar between 1% to 5%. The particles may be ground together or individually, and diameters up to about 20 microns are still suitable.

It is to be understood that by reducing said` diameter the period of firing the mixture can be Shortened whereas by increasing the amount of bentonite and fluorspar, which form substantially fiuxes, the firing temperature can be reduced to about 1500 C.

Example 2 70% to 90% alumina, 6% to 26% magnesia, 2% to 6% bentonite are ground to particles the diamete of which is below 30 to 3 microns, and the mixture is fired to about 1680 C. for 24 hours. A particularly satisfactory result is obtained by using 70% alumina, 26% magnesia and 4% bentonite.

Example 3 1% to 5% chromium oxide, 1% to 6% bentonite, 1% to 4% magnesium phosphate, balance alumina, are ground to less than 30 micron diameter and heated at 1600 C. for 36 hours. Particularly satisfactory results are obtained with 2% chromium oxide, 92% alumina, 4% bentonite and 2% magnesium phosphate.

Example 4 1% to magnesia, 1% to chromium oxide, 2% to 8% beidellite, balance alumina, ground to less than about 20 microns diameter of the particles are heated to 1620 C. for about hours'. Particularly good results are obtained with 4% magnesia, 10% chromium oxide, 80% alumina, and 6% beidellite (c. I". Journ. of the American Ceramic Society, issue of August 1938, p. 276/7) 0 Example 5 20% to 60% sillimanite, 1% to 10% chromium oxide, 2% to 8% beidellite, 2 to 6% calcium borate, balance alumina, are ground to particles of less than 30 microns in diameter and fired at 1520 C. for about 48 hours. Particularly Satisfactory results are obtained with 30% sillimanite,

56% alumina, 40% chromium oxide, 6% beidellite and 4% calcium borate.

Example 6 Example 7 10% to 30% cerium oxide, 1% to 8% magnesium phosphate, 2% to 8% beidellite, balance alumina, are ground to less than 20 microns diameter of the particles and fired. Particularly good results are obtained with 20% chromium oxide, 70% alumina, 5% magnesium phosphate and 5% beidellite, ground to less than' about 6 microns diameter of the particles and fired for 72 hours at 1600 C.

Example 8 About 5% be'yllium oxide, about chromium oxide, about 45% alumina, about 5% bentonite are ground to less than 20 microns diameter of the particles and fired for about 24 hours ,at about 168o c Example 9 5% to 15% talc, 5%, to 25% cerium oxide, 2% to 8% bentonite, balance chromium oxide, are ground to less than 20 microns in diameter of their particles (talc to about 325 mesh), mixed 'and fired for about 24 hours at about 1620 C.

Particularly good results are obtained with about 65% chromium oxide,`about 10% talc, about 20% cerium oxide and about bentonite.

Example 11 70% to 90% mom-203, 3% to 8% zirconium silicate, 2% to 8% beidellite, balance alumina, are ground to less than about 20 microns in diameter of their particles and fired for about 48 hours at about 1620 C. Particularly good results are obtained with 80% MgOCI'aOs,

alu'nina, 5% zirconium silicate and 5% beidel- Example 1 2 10% to 30% thoria, 10% to 30% chromium oxide, 2% to 8% bentonite, 1% to '6% talc, balance alumina, are ground to less than' about microns diameter of the particles and talc to 325 mesh, respectively, and fired for about 24 hours at about 1650 C. Particularly favorable results are obtained with 20% thoria, 20% chromium oxide, 53% alumina, 5% bentonite and 2% talc. K

It is to be understood that the invention is not limited to any of the examples given above but is to be derived in its broadest scope from the appended claims.

What I claim 'is:

1. A refractory ceramic gas-tight cover for a substantially metallic electrical resistor element,

comprising a layer consisting of uniformly distributed and flnely divided oxide compound and refractory substance other than said oxide compound, said oxide compound formihg the major portion of said layer and being selected from oxide compounds, including oxide, melting above about 1600 C. of elements of the second, third, fourth and sixth group of the periodical system, w id refractory substance forming the minor portion of said layer and comprising substantial amounts of an agent selected from iiuxing and nineralizing agents, said oxide compound and substance sintered into a heterogeneous essentially crystalline structure substantially free of 3. A refractory ceramic gas-tight cover for a substantially metallic electrical resistor element, comprising a layer consisting of uniformly distributed and finely divided substantially amorphous oxide compound in a 'minimum amount of about 80% and a balance o! refractory substanca other than said oxide compound, said oxide compound selected from oxide compounds melting above about 1699 c. of elements of the second, third, fourth and sixth group of the periodical system, said refractory substance i'orming the balance being capable oi decreasing the sintering temperature of said oxide compound and 'exemplifled by talc, said substance and oxide compound sintered into a heterogeneous essentially crystalline structure substantially free of a vitreous phase.

A refractory ceramic gas-fight cover for a substantially metallic electrical resistor element, comprising a layer consisting of uniformly distributed and finely divided oxide compound and refractory substance other than said 'oxide compound, said omde compound forming the major portion, as exempliied by a minimum of about 80%, of said layer and selected from oxide compounds melting above about 1600 C. of elements of the second, thirri, ourth and sixth group of the pericdical system and being present as amorphous particles of less than 5 to 30 microns diameter, said refractory substance :forming the minor portion of said layer and substantiallynon-vitreous, said substance comprising substantial amounts of an agent selected from fiuxing and mineralizing agents,`said oxide compound and substance sintered into a heterogeneous crystalline structure substantially free of a vitreous phase.

5. A refractory ceramic gas-fight cover for a substantially metallic electrical resistor element,

consisting of uniformly distributed and flnely divided oxide compound and a. refractory substance, said om'de compound present in a minimum amount of about 80% and crystalline substantially non-'vitreous particles of less than 5 to 30 microns diameter and being selected :from oxide compounds meltng above about 1600 C. of elements of the second, thi'd, fourth and sixth group of the pericdical system, said refractory substanca forming the balance and consisting of essentially non-vitreous material substantially selected from a group consisting of bentonite, beidellite, talc and fiuorspar, said cover sintered into a heterogeneous essentially crystalline structure substantially free of a vitreous phase.

6. In a refractory cover as described in claim 'substantially metallic electrical resistor element,

comprising a layer consisting of uniformy distributed and finely divided essentially crystalline non-vitreous oxide compound and highly refractory substanca, said oxide compound forming a major portion exemplified by ai minimum of about 80% and selected from oxide compounds,

including oxide, melting above about 1600 C. of

elements of the second, third, fourth and sixth group of the periodical system, said highly refractory substance fol'ming the balance and consisting of appreciable amounts of agents selected from fluxing and mineralizing agents and less than 3% to 10% vitreous material, said oxide compound' and said highly refractory substance 27% to about 97% .alumina, about 1% to about 11% materialet fluxing and mineralizing qualities as exemplified. by bentonite, beidellite,"talc and fluorspanabalance' substantially ox'ide compound, including oxide, .other than alumina selected from oxide compounds inelting 'above about 1600 C. of'elen'ents of the' second, third, fourth and sixth :group of the ;perodical system.

9. A refractory eramic gas-tight cover for a substantially metallic electrical resistor element, consisting substantially of a sintered heterogeneous essentially crystalline mass substantally free of a vitreous phase and consisting of about to 60% sillimanite, about 1% to 10% chromium oxide, about 2% to 8% beidellite, about 2% to 6% borate, balance alumina.

10. A sintered refractory ceramic gas-fight cover for a substantally metallic electrical resistor element.. comprislng a layer of uniformly divided and distributed' oxide' compound and highly refractory substance, said oxide compound forming the major portion as exemplifled by a minimum of 80% of said layer and consisting of at least two oxide compounds selected from oxide compounds, including oxide, meting above about 1600 C. of elements of the second, third, fourth and sixth group of the perodical system. said oxide compounds present in heterogeneous essentially crystalline structure, said refractory suhstance forming the balance of said layer and containing substantlal amounts of an agent selected from fluxing and mineralizing agents capable of entering the crystal lattice of said major portion, said oxide compounds and agent heat treated to substantially permeate each other.

11. A refractory ceramic gas-tight cover for a substantially metallic electrical resistor element, substantially consisting of about 1% to 5% chromium oxide, about 1% to about 6% bentonite, about 1% to about 4% magnesium phosphate, balance aluminum oxide, sintered into a heterogeneous essentially crystalllne structure substantlally free of a vitreous phase.

12. A refractory ceramic gas-fight cover for a vitreous-phasei 13. A method of manufacturing gas-tight ceramic covers and oatings suitable for electrical heating elements `adated for operation in open air andunder other conditions detrimental to the resistor material at operation temperature, comprising the* steps of comminuting a major'portion of oxide compound selected from oxide compounds melting above about 1600 C. of elements of the second, third, fourth and sixth group of the perodical system and a minor portion of an agent selected from fluxing and mineralizing agents exemplified by bentonite, beidellite', talc and fluorspar to less than about 5 to 30 mcrons diameter of their particles, intimately mixing, shaping and firing the same between about 1500 C. and 1750 C. for about 10 to 120 hours until a sintered body of substantially heterogeneous crystallne structure and containing essentially less than about 3 to 10% vitreous substanca is obtained.

14. A method of manufacturing gas-tight ceramic covers and coatlngs suitable for electrical heating elements adapted for operation in open air and under other conditions detrimental to theresistor material at operation temperature, comprising the steps of forming an intimate powdery mixtu're of amorphous particles of less than 5 to 30 microns diameter of at least two oxide compounds, including oxide, selected from oxide compounds, including oxide, melting above about 1600 C. of elements of the second, third, fourth and sixth group of the perodical system, as a major portion exemplified by a minimum of 80%, and of highly refractory material comminuted to at least 320 mesh as a minor portion, shaping and flring the mixture between about l500 C. and 1750 C. until crystallization and permeation of said' oxide compounds without melting them occurs and a sintered body of substantially heterogeneous crystalline structure and in which at least said oxide compounds substantially permeate each other is obtained.

FELIX SINGER.

CERTIFICATE OF CORRECTION. Patent No. 2,2o7,558. Jul 9, 9l o.

- FELIX SINGER.

It is-hereby Certified that error appear in fiheebove numbered patent requiring correction as follows: In the grant, line' 1, and in the heading to the prnted specification, line 5, residence of inventor, for New York N. Y." read --Southv Croydon, Surrey, England--; page LL, second column, line 9, claim 3, for 1699 C." read --1600 C.--; line 62, claim 7, for "unifonny" read --un1form1y--; page 5, first column, line 32, claim 10, for "meting" read --melt1ng--; same page, second column, line 25, claim 15, for

"crystaline" read --crysta1l1ne--; and that the said Letters Patent should be read with this correction therein that the same may conformto the record of the case' in the Patent Office.

Signed and ealed ths 2L.th day of September, A. D 1940.

Henry Van Arsdae, (Seal) Acting Commissioner of Patents.

I CERTIFICATE OF CORRECTION. Patent No. 2,2o7,558. July 9, 9lo.

- FELIX SINGER.

It is-hereby certified that error appear q thebove numbered ;Satent requiring correctonas follows: In the grant linel, and in the heading to the prnted specification, line 5, re sdence of inventor, for "New York N. Y." read --South.Croydon, Surrey, England--gpage h, second column, line 9, claim 5, for "1699 C." read --1600 C.--; line 62, claim 7, for "finiformy" read --uniform1y--; page 5, first column, 1'1ne 32, claim 10, for "miting" read --melt1ng--; same page, second column, line 25, claim 15,' for "crystalne" read --crystal11ne--; and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case' in the Patent Office.

Signed and eaed ths zuth day of September, A. D, 19l0.

. 'Henry Van Arsdae, (seal) Acting Commissioner of Patents

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