US2252981A - Resistor and method of making same - Google Patents

Description

Aug. 19, 1941'. R. R. RIDGWAY RESISTOR AND METHQD 0F MAKING SAME Filed May 24, 1939" 10 .100 Percentage of T010). Titania m 0 I EC 0 .P; m m m 4e T n O Raymond Rfijdgfi (p e attorney wilnes s Huber! E-Covey Patented Aug. 19,1941

- UNITED STATES PATENT OFFICE I I 2,252,981 I v nasrsroa AND Mn'rnon or MAKING s Raymond B. moms, Niagara Falls, N. 12., ascignor to Norton Company, Worcester, Mass., a

corporation of Massachusetts Applicatlon May 24,1939, Serial No. 275,366

BClaims.

The inventionrelates to resistor bars, compositions of matter for the manufacture of resistor bars and like products and methods of manufacture thereof.

n e object 'of-the invention is to provide a material .for the manufacture of resistor rods for electric furnaces. Another object of the inventionis to provide resistor rods of long life and good electrical properties. Another object of the invention is to provide a composition for resistor rods for high power furnaces of desired resistivity and great mechanical strength. Another object of the invention is to provide a composition for the manufacture of resistor bars which will make bars of long life.

Another object of the invention is to modify aluminum oxide so that in the fused condition it can be readily poured and on solidification will The invention accordlngly'consists in the features of construction, combinations of elements, molecular and a crystalline structure, arrangements of parts, and in the several steps and arrangement and order of each of said steps to one or more of the others thereof, as will be ex' emplified in the product, method and apparatus to be hereinafter described, and the scope of the application of which will be indicated in the following claims.

In the accompanying drawing:

Fig. 1 is a plan view of areslstor bar;

Fig. 2 isia sectional view of a resistor tube;

Fig. 3 is a resistance-temperature curve for a resistor embodying this invention; and t Fig. 4 is a curve showing the relation of specific resistance to the titania content of the resistor.

b conductive enough for use'as a resistor for electric furnaces and will' still have great strength. Another object of the invention is to provide a refractory conductor which will hold its electric conductive properties without deterioration. Another object of the invention is to provide a resistor capable of resisting a substantial amount of thermal shock. Another object of the invention is to provide a dense homogeneous oxide resistor free from pores and blebs, with a surface free from welds and discontinuities'. Another object of theinventlon is to provide a resistor having one or more of the characteristics indicated which will resist fusion up to a temperature of 1600 C. Another object of the invention is to provide a bar which even at high temperatures may be supported. only which seals the interior thereof against further z Another object of the'invention is oxidation. Another object of the invention is to provide a tube which has a refractory non-conductive coating integral therewith on the inside thereof so that it can act as a refractory container while the body of the tube acts as a resistor and radiates heat.

Another object of the invention is to produce a dense casting free from voids and gaseous inclusions. Another object of the invention is to provide a method of manufacture of the products indicated which is practical. Another object of I the invention isto provide a method of production which will take care of the high shrinkages involved in refractory oxide products of the nature dealt with herein. in part obvious or in part pointed out hereinafter.

Other objects will be In accordance with this invention, an electrical heating resistance element may be made of titania interfused and, crystallized with alumina. The titanium oxide of the resistor may be a mixture of TiO: and T120: which may be in the form of the composition and produced according to the method described and claimed in my copending application Serial No. 275,365.filed of even date herewith. As described in said copendingv application, as the starting material for the production of. the desired mixture of T102 and T120: I may meet and reduce purified titanium dioxide, T10: to form a crystalline mass ',of T10: containing from 5 to of T120 and not over 5% of other refractory oxides selected from the ture of my final products. Commercial ceramic grade T10: ordinarily has thismuch S102 and I analyze samples to ascertain that'my starting products contains such amount of silica. Preferably also my starting material has no more tha'n a trace of any iron oxide, and the aforesaid commercial ceramic grade TiOs fills this speci-r fication. My starting material comes'in the form of a light, fluffy, amorphous'powder having a low apparent'density and is comparatively free from moisture, having been calcined. I prefer a well calcined product. For the manufacture of partially reduced titanium oxide referred to above as a mixture of T102 and T: I provide any suitable open top electric arc furnace such,

for example, as.a Higgins type furnace as disclosed in United States Patent Reissue No. 13,027 to Aldus C. Higgins, but preferably I provide a tilting furnace in order to be able to pour the molten charge. This furnace preferably has graphite electrodes and any'suitable type of regulating mechanism. Any known size of furnace is suitable for'carrying out the present inor bricks. I then crush these blocks or bricks tov a grain or powder. As the result of fusing the chemically precipitated T102 in the open type electric arc furnace there is produced a mixture of'titanium oxides probably consisting mainly of alumina will enable one easiest ture treatment '-,to remove the will. also do for the purpose. The foregoing are be considered as examples and any relatively pure to carry out the present invention. e e s It is preferred to crush the alumina to a powder as well as the reduced titania. I feed the reduced titania-and the alumina preferably in powdered or grain form either separately or mixed to the Higgins. furnace at the rate of about one pound of oxides per eight-tenths kilo-- watt hour input. I usually operate the furnace at about 65 to 75 volts with the electrodes ad- Justed to produce slightly reducing'conditions but not enough to reduce the oxides to metallic alloys. when the charge'is completed I hold the power on until thebath becomes a complete TiO: and the remainder empirically identified as T1203. The fusion described reduces only part of the T102 to the suboxide represented by T120: and when carried out in the manner indicated does not reduce any substantial part of the starting material to the metallic form. With the rate of power input indicated the product will correspond to an analysis of about 85% TiOz and 15% Ti-203. By using an open top electric arc furnace with graphite electrodes there is' produced an atmosphere of carbon monoxide on top of the melt formed by the burning of the electrodes,

and yet this is not pure nor is the atmospheric air so completely excluded that the reduction will go to an extreme extent. This fusion of .titanium oxides in the percentage as indicated when cooled is a fairly coarse crystalline material having a dense black appearance and a conchoidal fracture and it is a conductor of elec- 'tricity, its conductivity being along the order of that of poorly bonded carbon, carbonaceous materials, and the like.

This fusion of titanium oxides in the preferred form identified as having an analysis of about 85%. T10: and 15% T1203 is, so far as the present invention is more specifically concerned, the starting material thereof. While substantially pure 'I'iOz has a resistivity of about 11,730 ohms to the cm the partlyv reduced titanium oxide specified has a resistivity of about 340,000 micro ohms to the cm}. In other words the pure titanium dioxide 'TiO: has roughly thirty-five thousand times the resistance of the 85% TiOz 15% T1203 mixture.

I now charge the same Higgins furnace above referred to with the reduced titanium oxide powder which is the starting material more spehomogeneous thin liquid under the arcs. I have I found in order to produce a denser casting with desired refractory fine grained structure that it is advisable to shut ofi the power and allow the melt to stand in the furnace crucible [for a few moments before pouring. This allows the temperature to fall to a point where it is just above the crystallizing point of the liquid and when the melt is in this condition it is ready for pouring and will'quickly crystallize to form thedesired castings, which may be shaped as shown in Figs. 1 and 2'or otherwise as desired.

In a typical run 100 pounds of the oxides were fused using power at the rate of kilowatts.

The mixture was 11% titanium oxide and 89% aluminum oxide. The pouring temperature selectedwas 1700 C. and the liquid was poured into a mold heated to 1000 C. I The resis orbars are dense and have a fine granular blue-black structure which is mechanically "strong andhas a linear coeilicient of expansion of approximately 80x 10-".

v A microscopic study of the product shows that it consists of two phases, an intergrown phase of crystal alumina surrounded with a continuous neti work of Ti0a.Th0a.Alz0a glass and slag which imparts the conductive properties to the mass. This reduced titania-rich phase seems to difiuse into the alumina crystals imparting a blue coloration to them. y

For electric furnaces whose temperatures range up to 1100" C. oxidation resistant-metal conducciflcally for this present invention and. some chemically pure prefused alumina preferably in the proportions of about 12 parts of the reduced titania to 88 parts of the alumina. Suitable alumina (aluminum oxide, A1203) for use in carrying out this invention may be produced as described in my own prior Patent Reissue'No. 20,457 reissued November 2, 1937.

I may also use alumina produced as described in Patents No. 1,798,261 and No. 1,971,793 to Basil, T. Horsiield. Furthermore any alumina such as produced by chemical precipitation and rather high temperalowered so that it would bethe same as coppertors have usually constituted the heating elements. Above this'range, however, metal resistors have not in general been very useful be-v cause they deteriorate rapidly with use. Above the range of 1100 C. silicon carbide resistor rods have been chiefly used but such resistor-rods while more suitable than metal products nevertheless deteriorate'with use since they slowly oxidize resulting in the gradual rising of theresistance. Resistor bars made according to the present invention form in use a thin film of oxidized material, that is to say the T120: is probably re-oxidized to T102; This film which maybe of the order of one thirty-second of an inch thick protects the remainder of the resistor bar and over a long period of time no further oxidation takes place and the resistance value remains substantially constant.

It will be appreciated that in any resistor bar there is a desired vrangeof resistivity. For example, one would not want the resistance to be 7 under certain power conditions, not would one want the resistivity to be as high as that of aluminum oxide. have found that by adding aasaosr approximately 12% of the starting product for this'invention, which itself constitutes only about 15% T1201. a resistor bar is achieved which has the desired resistivity for a high power electric furnace so that it can be embodied in bars whose cross-section is large enough to give adequate strength and whose length is about right for all practical considerations and furthermore the large proportion of alumina insures a strong resistor rod which will resist mechanical stresses and strains. At the same time a bar made as herein described is adequately resistant to thermal shock. In the resistor bar the conductive material is the 'IJla-Os which is well dispersed as aforesaid.

Alumina is more.refractory than titania and the large percentage of alumina in the resistor bars makes them. highly refractory and also chemically stable. A typical resistor bar as described will resist softeningor fusion up to 1600 C. and will absorb two kilowatts of energy and while supported only at its ends. The resistor rod or bar is a vitreous fused oxide mixture and it has a clear metallic ring when struck with a hammer. Although as aforesaid it is resistant to heat shock this is a relative matter and being a I r the mold.

. v a 3 to avoid quick chilling of Using a graphite mold which has been preheated to 1000 C. or thereabouts I have formed dense, homogenous, well knit resistor bars. These may be shaped as shown in Fig. 1 to prorefractory container vide a central cylindrical Portion iii of a diamradiate it to the interior of an electric furnace vitreous or ceramic object it should be heated and cooled slowly. Its modulus of rupture at room temperatures has been determined to be 20,000 pounds a square inch. It has a specific gravityof from 4 to 4.1. Its specific resistance is .3 ohm/in. at room temperatures.

The curve of Figure 3 shows the temperature coemcient of the resistance. The curve of Fig. 4 shows the changes in resistance of different compositions of alumina with varying percentages of titania, of which 21% is T1203. For different specific uses the amount of the ingredients A1203; T10: and T1203 will be varied. In order to produce resistor bars which are strong, refractory, and have the desired electrical characteristics, the quantity of total titaniamay range between 5 and 20 per cent, the balance being mainly alumina. Products having more than 50% alumina are highly refractory and have a chemical stability which makes them useful for many industrial applications. The starting material for this present invention and therefore also the final resistorbar preferably contains -a small quantity of silica which causes the mass to crystallize as a fine grained structure and results in a smoother and denser casting of improved conductivity. Zirconia acts also to improve the crystal structure.

With regard to the formation of the oxide film I have found that when the resistor bar is placed on power and raised. to a temperature between l200-and 1400 C. the oxidation of the partially reduced material forms a glaze which seals the interior of the structure against further oxida tion. This glaze may be between one thirtysecond and one sixteenth of an inch thick and in a life test of a particular bar heated for ten thousand hours at 1100 C. the film at the end eter of ilve-eighths of an inch, cylindrical end portions ll of a diameter of flfteen-sixteenths of an inch connected by frusto-conical portions II, the longitudinal dimensions being 13% inches overall length with the end portions 2 inches and the central portion 6% inches, long. Also, as shown in Fig. 2, the bar may be shaped as a hollow tube I 0 having face. I! and an inner cylindrical surface 20, or any other suitable shape may be made. I may form tubes of the type'shown in -Fig.,2, and I have found that the I 20 of the tubes and the resistor bar'- is hence a refractory container. For many heat treatment products it is a decided advantage to have a resistor rod which is also a refractory container since dissipation of heat and Power can readily be reduced. For example I might pack the resistor tube within mflgnesia cement-and thus prevent it from radiating very much heat outwardly. As theheat would then radiate inwardly I could heat treat small articles with a minimum amount of power consumption.

- Although the self-forming glaze, which has the refractory propertiesof fused alumina for example as sold under the trade-mark Alundum.

rent flows freely into and out of it. Although I have described the-production of rods and tubes in detail it should be understood that I may also make other shapes for example plates which can used in furnaces as electric heating agents. Also although the articles described are electrical I resistors for power furnaces. nevertheless the composition may be put to other uses.

Although silica has been described as the other.

ingredient besides two kinds of titanium oxide" and alumina, I might substitute zirconia. silica is used the amount thereof. if the product is to be used as a resisto will be limited to a few per cent since otherwise the resistance is too reat.

I have found definite eflects from prefusing, casting and crushingthe titanium oxide as thereby a fine grained structure results. There is a distinct advantage, therefore, flowing from the method involving two fusing processes. However, so far as many features of the invention are concerned, I may, still using two fusings. mix precipitated titania with alumina, fuse and reduce to form an ingot, then crush this ingot and refuse toform the final melt which is poured into the mold. In this modification of the process there are still two fusing operations of the original ingredients. 1

It will thus be seen that there has been provided by this invention a method and an article in which the various objects hereinabove set forth together with many. thoroughly practical advantages are successfully achieved. As many possible embodiments may be made of the above invention and as many changes might be made in the embodiment above set forthyit is to be understood that all matter hereinbefore set an outer cylindrical .sur-

glaze forms on the insides,

' self-bonded crystalline 4- I 7 forth, or shown in the accompfl-livi s drawing.

is to be interpreted as illustrative and not in a limiting sense, I claim: i I

. '1. An' electrical resistor'formed as a shaped.

self-bonded, coherent, dense, continuous, monolithic, prefused, crystalline structure having alumina crystals as the major phase intergrown with a phase of crystals containing essentially.

titanium dioxide and a lower oxide and formin an electrically conductive refractory body. 7

2. An electrical resistor formed of an intergrown crystalline structure comprising. essenoxide, said ingredients being associated-as crystais intergrown from a fused mass thereof.

- 5. An electrical resistor formedas a shaped, self-bonded, prefused crystalline body having alumina crystals as the major phase intergrown with an electrically conductive phase of crystals tially a major phase of crystalline alumina in- I terspersed with and surrounded by a lesser amount of an electrically conductive glassy phase containing T102, Th: and A120: crystallized-from a premised mixture of said oxides as a dense, self-bonded, monolithic structure.

.8. .An electrical resistor formed as a shaped self-bonded crystalline structure, 95% of which 1 is composed of crystalline alumina forming 80 to 95% of the total mass. and titania. forming 5 to 20% of the mass. and wherein 50 to 95% of the titania is T: and the balance is a partially reduced titania empirically represented by the formula ThOa, said ingredients being associated as crystals intergrown from a fused mass thereof. a

4. An electrical resistor formed as a shaped, structure containing from 75 to 95% of-aluminsa. from 5 to 20% of titania and from zero to 5% of ingredients selected from the groupconsisting of silica and zirconia, the titania consisting of 50 to 95% of TiOrand the balance being a lower titanium containing essentially titanium dioxide and a lower oxide, said body having a protective surface glare oi titanium dioxide thereon.

6. An electrical resistor formed of a crystalline mass, 95% of which is'composed of alumina constituting so to 95% oi the total and titania constituting 5 to 20% oi the total, the titanic. consisting of from to of titanium dioxide and the balance being a partially reduced titania empirically represented by the formula T1203. said resistor being a'shaped dense body having a conchoidal fracture formed by crystallization in a mold of a fused mixture of said ingredients '7. The method-of making an electrical resistor 7 portion thereof to a lower oxide, fusing said product with a larger amount of alumina and crystallizing the interfusion as a shaped mass having an intergrown crystalline structure.

RAYMOND R. RIDGWAY.

Images