US1787749A - Electrical resistance element and process of manufacturing the same - Google Patents

Description

Y jquent expense and further diiculty Patented Jam i931 A 4Uliu'nio STATES APATEISJT ori-ica ALBERT H. HEYRJOTH, OF WAUWATOSAUWISOONSIN, ASSIGNOR TO GOBAR CORPORA- TION, OF NIAGARA FALLS, NEW YORK, vA CORPORATION OF YORK mineralen nEsIs'rANcr: ELEMENT aNnrnocFss or MANUFACTURING 'rari-sana Application led October 4, 1927. Serial No. 223,947.

` This invention relates to the production of' electrical resistance materials or elements,

more particularly such as are used or various.

' current limiting and regulating purposes and I 5 for the production of heat of various degrees..

1osatisfactory degree of commercial uniformity, as to mechanical and electrical characteristics,

Vwithout involving the rejection of a relatively large number of elements, with a consequent increase in cost of production of such resistance elements. Likewise, where a protective shell or seal is provided for the purpose of protecting the element and maintaining the resistance of the element at a reasonabl constant figure, covering or seal of this character has involved considerable additional handling and consem providin' g for a commercially satisfacto union or electrical connection between theody of the resistance element and highly conductive terminals vof such a character as to insure Y proper and readyconnection ofthe resistance element in a line circuit, with its resistance of the desired value.' Y V l The resent invention contemplates -the production of a resistance element of an 1m- Y proved character in the Vform Vof a heat-treated com 'tion product made from a mixture'of a re ativel v conductive-material and a relativel nsula material, the product being uced in commercial quantities and at re duced cost by aprocesswhich'insures a high degree of' uniformity as to the physical and velectrical characteristics of I the product Vthroughout the Vordinary handling and c ommercial life thereof; andas a moreeparticular feature, the invention contemplates the production of a resistance element from material as an ingredient a suhaance ing among its desirable ,properties the capability of producing a highly desirable surface seal of a protective character on the finished 'ceelement -j I have discovered that through the use of such provision of an e ectivev certain elly-like substances, benw tonite or an. ingredient thereof, as a substance of insulating properties in theoriginal mixture from which the resistance element is formed, particularly desirable results are attained, among which is the fact that a highly satisfactory seal or covering for the resistance element may readily be produced on .the resistance element merely through a heating operation' which may be a part of that used for hardening or adjusting the resistance of the product.

- lIt is an object of the present invention to produce an improved resistance element of the composition type including as ingredients materials of different specific resistance and which may be commercially produced in a relatively simple and inexpensivemanner.

Y A further' object of lthe present invention is the production of a resistance element of this general character from a comminuted mixture includin asan ingrediente inaterialrof relative y high specific resistance which, in addition 'to its other properties, is capable of producing a glaze-like coating on the element which effectively seals the surface thereof, thus insuring constancy of elec- .trical and phgsical'characteristics thereof.

er object of the presentinvention to produce an electrical resistance ele-- Itis a furt ment of the character described above, wherein the relatively conductive medium of the resistance element includes a carbon-containing substance, and; more particularly, to pro- "duce a resistance element of this character which includes silicon carbide as an essential ingredientthereof. A

These' and other objects and advantages are attained by the present invention, various novel features of which will be apparent from the descri tion and drawings herein, disclos ing embo be more particularly pointed out vin the claims. .f I'

In'tlie drawings n Fig. lis a bro tion, of an electrical resistance element einbodyin'g or features of the present invention.

Fi 2 is a sectionalV view in the plane of 4the II-II of Figi 1..

en elevation, partly in sec.

produced in accordance with ments of the invention, and will Figs. 3 and 4 are vlews similar to Figs. 1 and 2, respectively, of a modified embodiment of the invention, Fig. 4being in the plane of the line IV-IV of Fig. 3.

Figs. 5 and 6 are views similar to Figs. l and 2, respectively, of a further modified embodiment of the invention, Fig. 6 being in 4the plane of the line VI-VI of Fig. 5.

In the embodiment of the invention shown in Figs. 1 `and 2 of the drawing, 7 indicates the body of a resistance element of rod-shape. This resistance element is produced from a mixture of suitable ingredients by molding the same under pressure, as by means of the the element is liable to be subjected underr conditions of ordinary operation.

As indicated in the drawings, the resistance element includes an outer skin or shell 2 which is of an insulating character and in the form of a glaze of suitable thickness and which effectively seals the element from the' effects of moisture or other foreign materials which might. in any way affect the resistance of the element. skin or shell 2 is the main body or core portion 7 which is formed of the mixture of the clay-like insulating material and the carbonaceous or conductive material entering into the originalmixture as the relatively conductive ingredient thereof, but in a fired.or

heat-hardened condition, with its consequent effects.

5 indicates acoating of highly conductive material applied `to the exposed end of the resistance element, this conductive coating being in particularly intimate electrical and mechanicalconnectlon with the body portion 7 of the element, and it preferably ex-v tends over the outer sides or surface, that is, outside of the insulating glaze-like shell 2, of the element to' an a preciable extent. 6 and 6` indicate types o? a metallic sleeve or cap of suitable conformation pressed upon the end of the resistance element with an. ample area of the inner surface thereof in intimate electrical engagement with the conductive coating 5 at the end or along. the outer cylindrical surface, or both, of the resistahce element. Direct .contact engagement between the-ca 6 0r6? and the coating 5 may t be relied ony or the desired electrical and mechanical connection of the cap to the end Within the annular outer4 of the resistance element, or these parts may be united by fusion of metal, either direct or through an intermediate brazing or soldering strip or coating.

I have discovered that certain clay-like materials which are of relatively great electrical resistance, in fact more nearly of an insulating character, and are reasonably resistant to heat, and more particularly, a claylike material known as bentonite, are of particularly desirable characteristics as an ingredient of a mixture from which to form resistance elements of the general type with which the present invention is concerned. This material, which is readily obtainable in powdered or colloidaliorm, when present in a substantial amount, particularly in the latter form, as an ingredient of the insulating material of the mixture, increases toa con- .siderable degree the'plasticity and workability of the mixture; and it permits most thorough intermingling of the relatively conductive and insulating ingredients of the mixture. Further, becausev of its composition, bentonite is capable of readily producing a lazelike skin or covering of a vitreous an permanent nature and high di'-electric.strength upon a formed body containiiig the material in a substantial amount, "by heating to a suitable temperature in air, arid this glaze thus lfoimed effectively -seals the surface of the For the ingredient of a relatively conductive nature in the mixture, particularly desirable results may be attained when using silicon carbide alone to 'produce resistance elements of relatively high specific or unit resistance, or in combination with carbon or graphite, with or without an additional conductive material, to produ'ce resistance eley ments of lower specific or unit resistance.

In accordance with a desirableprocess of practicingthe present invention, such as may e used in producing resistance elements of relatively small physical dimensions and vof fairly high ohmic resistance, and adapted to vproduce or withstand appreciable heating` effects, a comminuted mixture maybe produced containing 70 percent of silicon 'car bide, 5 percent of carbon, and 25 percent of bentonite, preferably in the colloidal state. A suflcient amount of Water may be added to form a doughy mixture of the most Vdesirable consistency `to permit squirting the commi-- nuted mixture under high pressure through a die of suitable conformation to` produce thereby a rod-shaped element ofl desired prosssectlonal area and form, usually circu- The properties ofbentonite which (listinguish it from other claysvappear in general to be due toits physical constitution rather than to its chemical composition. Its chemical composition is characterized by a high sllica content (54% to 63.2%), a.4 high-alkali that the particles exist in the form of Hakes, which have dimensions visible under the microscope parallel to the flakes, but have-a third smaller dimension perpendicular to the flakes.

.hydrophylic quality is When suspended in water bentonites often swell to six times their original volume. This one of the principal physical characteristics of bentonite. The fine state of division and the large water content of bentonite result in good lubricating properties which make it easy yto extrude the resistor rods such as,the applicant makes.

The fine state of division is also conducive to coating of the silicon carbide grains. with a thin c oating of bond which shrinks a great deal during curing. This. condition is such that small electricarcs may pass between the silicon carbide grains through the thin coatings or through the cracks formed during the curing. I

After sure, as by being squirted under pressure through a die, the formed elements, in any 'desired length to Vpermit ready and economical handling, may be received on plates or trays and dried inthe presence of air at a temperature ranging from 200 F. to 300 F. These rod-shaped elements, more particularly if they are of relatively great cross-sectional area, may be more or less completely embedded, during this drying operation, in relatively coarse sand or the like which will afford suitable support for the rods ainst distortiomwhile still permitting the passage of air and gases-at a suitably gradual or retarded rate, insuring" a uniform drying throughout.' The period of drying need be only sufficient to remove all moisture from thev elements, land this period will depend upon the cross-sectional ,areaof the element and the freedom with which moisture may be driven therefrom.

After these rods or other shaped bodies or Y elements have been suitably dried, they may be heated or fired in neutral or inert, that is, non-oxidizing, atmosphere or surroundings, at a suliciently high temperature and for a suicient period of time to thoroughly consolidatethe particles of and to heat-harden' the element. A satisfactory temperature for this operationis- 1800 F. valthough .fairly satisfactory results may be secured with temperatures ranilngbetween 1500. F. to 22.00

. or slightlyV ghe r; andthe peroid of heating may'ran e from a veryk slight period to a considerab eperiod, dependirg -upon the beingformed to shape under pres? cross-sectional area and .somewhat upon the desired final resistance of the element. -For elements of three-eighths inch in diameter formed from the above described mixtures,

l heating at a temperature of 1800 F. for ap-I proximately twent -iive minutes gives satisfactory results. ith this heating operation carried on in a neutral atmosphere, there is no resultant oxidation of the/carbon or other oxidizable ingredient of the mixture lfrom which the element is formed, and thus any lack-of uniformity in the resistance of the final products due to differences in the degree to which oxidation has occurred, is avoided.

`Having beenheated to the desired extent .in the manner described, the formed element,

after cooling, may be calibrated to determine its specific or unit resistance; and if the resistance value suitably corresponds to that de: sired for the final resistance product, theresistance element may then be heated in the presence of air for a short interval at a temperature of vapproximately 2150o F. to 2450o F., five minutes heating at this temperature being ordinarily suiiicient to tire or cause the production of a glaze-like shell or covering o n all exposed surfaces ofv the element, this effect being an incident of heating the body formed of the original mixture from which y the element is formed,more particularly the relatively insulating ingredient. This glaze Y constitutes a very effective sealing envelope which protects the enclosed body of the resistance relement from further change in resistance due to oxidation or absorption of any material or substance which might affect the resistance of the element.

A modification of the aboveV described process of heat-hardening or firing the resistance element, and one which in many ref spects is preferable from the standpoint of economical. commercial production, involves the single step of heating 1n oxidizing surroundings the formed and dried element-at a temperature sufficiently high to produce the desired surface glaze or vitreous seal thereon,

a temperature approximately 2200 F. tol

2500i" F. being quite satisfactory for this purpose. Continued heating at this temperature produces during the'irst stages the surface glaze-likecoating and then the desired consolidation and heat-hardening of the particles of the mixture from which the element `is formed, the incidental effect of, continued heating being to reduce the resistance of the formed element at a decreasing rate. For the satisfactory production of heating elements ofv thespecific physical and electrical" characteristics mentioned hereinabove, heating at the temperature specified for a period of approximately 30 to40 minutes is satisfactory, and gradual application of full heat is desirable.

los

The heat-hardened' resistance elementsv i providedv ,With-the vitreous sealover all exfusion through an intermediate conductive coating, indicated at 5l in the? forcing a stream of drawings, prefera by esirably in a molten metal particles, more condition, into intimate mechanical and electrical engagement with the ends of the element, this conductive coating preferably being applied in such a manner as to have a portion thereof extending to a suitable extent ovver the insulating glaze or shell on the outer surface of the elements. This conductive terminal coating thus applied may be of any desired thickness, and it may be relied upon as the only conductive terminal serving to connect the resistance element to a line circuit or, as indicated in the drawings, a separate conductive terminal sleeve or cap 6 or 6 may be secured over the end of the resistance element, with the conductive coating 5 thereon, by pressing the cap directly into intimate electrical and mechanical engagement with the conductive coating; and likewise, the conductive terminal cap may be connected to the conductive coating 5 by direct fusion of these metal arts or by razing or soldering material. n g

In Figs.'3 and 4, there is' shown a modification of the resistance element shown in ,Figs.. 1 and 2, and wherein the resistance element is in the form of a hollow tube having an annular body portion 7 provided with a glaze-likev seal 2 on its outer surface and 2 on its inner surface. A resistance element of this form may be pressed or squirted from the original mixture insubstantially the same m'anner as described hereinabove in connection'l with the resistance element ofcircular cross-section shown in Figs. 1 and 2; Of course, a core of snitable form-niust be provided in the molding or the squirting operation to insure the de-. sired hollow center rof the linal element. Likewise, the original drying and heating operation in connection with an element of this form niay be the same as described hereinabove, independently of whether the heating operation is one wherein the desired heat-hardening and consolidation of the particles of the mixture are secured initially 'and the vitreous glaze is then fired upon the surface of the elenent, or the firing of the I(vitreous glaze is an initial incident of the heat-hardening operation at the -temperatures mentioned hereinabove. With .the ends of the bore of the formed element open to the Vpassage of air therethrough, the inner surface of the element will be provided with' the vitreousglaze which is produced Athereon `subsequent to heat-hardening in accordance with the lprocess first described hereinabove or is automatically produced onthe resistresistance element of Fig. 1, this conductive preferably extending to some degree over t esurface of insulating or glaze- like shells 2 and 2; and this conductive coating. by itself, may serve as the finishedl terminal 'device for the resistance element, or an additional sleeve or cap 6b may be provided, thls cap extending across the entire end of the heating. element, as in the casevof that shown in Fig. 1, or bein centrally apertured as more particularl in icated in Fig. 3.

In Figs. 5 and 6, t element embodying the external insulating glaze-like shell or covering sleeve 2 about an annular body portion 7 of the same character asthatI of the resistanceelement of Figs. 3 and 4; but the inner surface of the element of Figs. 5 and 6 is not provided with any vitreous seal or laze, corresponding to that indicated at 2 at this inner surface being ofsubstantially the same composition as the 'remainder'of the body portion 7 of the resistanceelement. A resistance element of this character is formed in the manner described hereinabove in connection with the resistance element of Figs. 3 and 4, except that prior to the heathardening or firing operation, the ends of the bore are closed by the insertion therein of a, small portion'of some plastic mixture, which may be of substantially the same constitution coating as the body of the resistance element, or by,

pinching the ends of the formed annular body while the same is in plastic condition. With the bore thus effectively sealed during the heat-hardening operation, there is no oxidation from the inner surface and there is no formation of the glaze or vitreous seal thereon. Atv the termination of the heathardening operation of the general character described fabove, the end seals or closures of the bore in the element may be removed; and tthe conductive coating 5% is then applied as described hereinabove, along with the'metal cap 6 which may be secured in position in the manner heretofore described. i

An element of the character of that shown in Figs. 5 and 6 has particular utility as a variable resistance element, one terminal, corresponding to thatformed by Ya conductive coating 5'* and metallic cap 6b thereon constituting a. line terminall connection, and a ere is shown a resistancey contact device being disposed Within the bore of the element and adjustable so as to contact with any point on this' inner surface,

The conductive vingredients of the original mixture from which the resistance element 1s formed may be so selected that certain of such Athe contact being insulated from allV parts ingredients compensate for the negative reof the resistance element except that which sistance coeicient imparted to the final prodbeais upon the inner surface at the desi1'ed,\ uct .by the carbon or silicon carbide, to the point, constituting or other lineJ terminal. It will be apparent that mately zero resistance coeflicient. by adjustment of the inner contact, any de` 'lhe conductive coating such as 5 or 5, sired resistance may be included between the alone or witli-the,cap 6, 6EL or 6*?,- may be exterminals. tended from one or both ends toward the Thilo the oxidation of any conductive parmiddle of the element, as to length, to any ticles at the surface of the formed body may desired extent, and thusconsiderable capacibe an incident of the formation of a .most tance maybe introduced into the Circuit by effective insulating glaze on the surface of `reason 0f the condenser eleot of, the 'outer the resistance element, nevertheless, Where Conductive Shell and the inner relatively conthe ring or heat-hardening is done ata temductive body portion 7 or 7, with the interpeiature Substantially belOW 22000 E, there mediate di-electric separator in the form of is danger of this sealing skin or shell not the insulating' Sleovog, being produced 1111011 the formed bOdy In subjecting the formed elements tolieatquickly enough to prevent OXldlZlIlg t0 100 ing at temperatures corresponding to those greet en extent the Carbon o1' Other OXdZa-ble requiredto produce the desired consolidation'- conductive medium near the surface of the 0f particles and heat-hardoning of the eleformed body.- And if the flln iij 011e at ment, it is desirable that the latter be brought' too h gh a 'temperature the 111811 atlllg gla-Ze up to such temperatures gradually, so as. to or skin produced 011 the f0lm ed body 1S avoid any strains'or rupture that might occur liable to become StlCkyaIld 000 hlghly fluent, if the elements were suddenly subjected to which niay cause the production of a rather the higher temperatures, rough aha uneven Shffeoe which iS hot Pel' Itis noticeable that where the proportion of tiehllly desirable from it Commercial Stendcarbon is-relatively large, or Vthe proportion poin `With the Present invention may be Varied et within the range specified; and vice versa, Wllly throughmh 9- leonsldel'eble Tange- Fol with smaller proportions of carbon and largthe higher resistance values, the amount of or proportions of bentonite, Satisfactory roeerhoh meyhe Very ,Smell and Plfeeteehy sults may-bel secured at the lower temperazero, the silicon carbide being. relied upon tures within the Specified range.` as the conductive ingredient of the compositloh, and the bentonite Vel'ylhg from ahum' tion, it is possible that certain of the desirable mum of about 2o Percent-to ahyvehle desued., results produced in the final product are due although there 1S ho o'dlhaly oooeslohhf ox' to thel probable action of the bentonite, vor oeed1hg 40 Percoht- For loWeI'- SPeolho o1' some constituent thereof, as a catalyst, possi- Umt l'eSlSteheeS ofthe hhal'pl'ofhlet, the Per bly because of the highl aluininous nature oehtego of bentonite may he reduced below of bentonite, to promote, reakin'g down oa 25 Percent and the carbon o1' Carbon plus portion of the silicon carbide throughout the Some other COIldllCtlVe meterla-1 lha-Y 1h' resistance element, resulting in the produc- Cl'eeeed to Peleeht o1 more, Wlth slhooh t-ion of material of greater conductivity;^and,' Calblde makmg 1P the helehoe of the mlX possibly, there is some further re-arrangeture; However. I hafe found that geherttuy ment of the particles of silicon carbide in the satisfactory eommercialresults are more diifusion of bentonite, or constituent thereof, Cult o ettelhmehta Partloulol'ly lh tho matter resulting in decreased resistance of the -eleof a suitable degree of plasticity or workment ,r u Y Aehlhty of the mhfthe ahd the lhost ready for.' Resistance elements produced in accordllatlOn of e Setlefe-otol'y Seehhg Surface o1' ance with the invention described hereinabove shell on` the element, if the amount of bentO- are chamoterized particularly by llo groot fw lute iS reduce@ meterle-hy he1oW25 Pel'eeht; cility with whicli'a commercialdegree of uniand finost densirable mechanical characterisformity 'of the elements may bo initially ge.' ties are not readily imparted to the iinal recured, and stability of the characteristics' 'of sista-nce element if the proportion of carbon the element in handlingand commercial use in the mixture is in excess of about'20 permaybe insured. While the invention has cent. Further, by varying the size vof the been described hereinabove more particularly grains ,of the silicon carbide used in the mixin connection with resistance elements of io'd Y ture,- further variation in the resistance. of or tubular shape, it will be aparent thatthe the final product may be secured. invention may be readilyutihzed in connecof bentonite is relatively small, most desirable 'lhe specific or unit resistance of electrical results are 'secured by firing 0r heat hard-' resistance elements produced 1n accordance oning the, elements at the highertemperatures Duringthe heat-hardening or firing operabeing connected to the end that the nal product may have approxi- 'v tersPatent the body portion of said 'a conduct-ive coating to 1. A heat-hardened electricalI resistance element formed of a mixture containing asilicon' carbide and carbon and bentonite, the latter ,being present in an amount equal to from 20 to 45 per cent, the body portion of said element being of a substantiallyl homogeneous nature throughout, and said element having an outer insulating and sealing shell which is a reaction product of said bentonite.

2. The method of manufacturing electrical resistance elements which comprises formlng a shaped body from amixture including a car on-containing conductive material and a relatively insulatinv self-fluxing c1ay-like material in finely divided form, and heating said element at a temperature between 2100 F. and 2500F. under oxidizing .surroundings for a suflicient time to insure the formation ofan insulating sealing shell in the form of a reaction product of said clay-like Ina terial upon the surface of sai'd element and the heat--hardening and consolidation of inteior particles of said element.

3. The method of manufacturing ro'dshaped electrical resistance elements, which comprises forming a shaped body from ya mixture including a carbon-containin conductive material and a relatively insu ating self-fluxing clay-like material in finely divided form, heating said element at a temperature between 2100o F. and 2500 F. under oxidizing surroundings for a sufficient time to insure the formation of'an insulating shell upon the surface of said element and to insure the heat-hardening and consolidation of interior particles of said element, applying the ends of said heating element in the form of a pressure spray of minute molten particles ofemetal into intimate mechanical and electrical union with element and over the slildtlalsurfaces of a portion of said insulating s e x 4. The method of manufacturing a heathardened electrical resistance. element, which comprises forming a l ture including silicon ca r de between and 45 percent carbon between `zero fandv 20-per-L cent, and bentonite between 25and 45 percent, heating said formed body at a temperature between 2100 F..and 2500 F. for a period of time sufficient to insure the producdisk, l

,tions which shaped. body of a mix-y tion von the surface of said element of an insulating sealing shell which is areaction product of one or more of the ingredients of the body, and continuingA said heating operation for aperiod sufficient to consolidate and heat-harden interior portions of said element. Y

5. vThe method of manufacturing electrical resistance elements, which comprises forminga shaped body froni a mixture including silicon carbide as a main constituent and a relatively insulatin self-fluxin claylike material in finely divided form, an heating said'element at a temperature between 2100 F. and l2500o F. under oxidizing sur- -Y roundings and'for a suflicient time to insure the formation of an insulating sealing shell in the form of a reaction product of said clay-like material upon the surface of said element and the heat-hardening and con-. solidation of interior particlesv of said element.

' 6. The method as described'in claim 5 in which the silicon carbide is admixed with a more conducting material and the clay-like material is bentonite.'

7. A method of producing resistors in large quantities which comprlses making a comminuted mixture of finely divided silicon carbide, a more conductive material, bentonite and water; extruding the mixture onto trays under conditions which will keep the extruded portions straight; drying the extruded bodies in air at approximately the normal boiling-point of water under condipreclude distortion; firing t-he dried rods at a temperature exceeding 1500 F. to heat-harden thel said rods and to adjust their specific resistance; and further ring the rods at tem eratures exceeding 2100 F. for a relative y much shorter pe riod to glaze the exterior surface.

8. The method described in claim 7 in which the glazed rods are cut into suitable lengths, impregnated as to their terminall portions with fused metal which is driven into the pores of said terminalportions, and' finally capped with metal sleeves which are in intimate contact with said terminal metal impregnations.

In witness whereof I aix my signature hereto. f

ALBERT H. HEYROTH,

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