US 3320039 A
Description (OCR text may contain errors)
y 1967 R. e. HOLLISTER 3,
ELECTRICAL RESISTANCE ELEMENTS AND THE LIKE Filed Nov. 12, 1963 FIG.3
3,320,039 Patented May 16, 1967 3,320,039 ELECTRICAL RESISTANCE ELEMENTS AND THE LIKE Ralph George Hollister, Thorpe Bay, Essex, England, assignor to Johnson, Matthey & Company Limited, London, England, a British company Filed Nov. 12, 1963, Ser. No. 323,032 Claims priority, application Great Britain, Nov. 16, 1962, 43,421/62 4 Claims. (Cl. 29-493) This invention relates to improvements in and relating to electrical resistance elements such as Wires, rods and the like, such as are suitable for use in the manufacture of heater elements for electric furnaces used for melting, heat-treatment and similar operations employed in the metallurgical, glass and ceramic industries and for laboratory purposes, or for use in the manufacture of thermocouple elements.
Electric heater elements used in electric, metallurgical or laboratory furnaces usually consist of a length of relatively thin resistance wire, which is wound on a. refractory tube or like insulating support. The resistance wire may consist of a platinum group metal or a platinum group metal alloy, such, for example, as an alloy of platinum with rhodium. Thermocouples, likewise, usually comprise one limb of a platinum group metal or alloy and the other limb of another platinum group metal or alloy.
It is known that one of the major causes of failure or breakdown of furnace heater elements is deterioration of the mechanical properties of the material of the element, due to the initial fine-grained structure of the pure metal or alloy, of which the element is formed, undergoing exaggerated grain-growth when subjected to high temperatures during use of the heater element. This change from a fine-grained structure into a coarse-grained structure, resulting from the absorption of smaller grains by larger grains, causes slip to occur at the interface of two contiguous large grains resulting in a reduction of the effective cross-section of the wire at the position of slip. The electrical resistance of the wire is, thus, increased and the temperature of the wire, consequently, rises, at this position of reduced cross-section. This rise in temperature further increases the tendency for slip to take place until, ultimately, cleavage or separation of the wire will occur at this point, or the wire will melt, resulting in failure or breakdown of the furnace winding. Such a failure, known generally as a clean-break," may occur at any time during the average operating life of a heater element, even after only a few hours service.
It is well recognised that this graingrowth is largely due to the purity of the metal, that is to say, the substantial absence of inclusions, and, consequently. the obvious way of tackling this problem and increasing the operating life of electrical resistance elements would seem to be to endeavour to prevent the basic cause thereof, namely, the incidence of grain-growth, by the deliberate incorporation of inclusions. With this aim in mind, it has, consequently, already been proposed to incorporate in the metal grain stabilisers in the form of oxide or carbide additions.
Whilst furnace heater elements, formed of grainstabilised metal or alloy, undoubtedly have a longer operating life than elements formed of metals not incorporating such grain-stabilising additions, the applicants experience has been that such a procedure has not proved a complete answer to the problem, but that, whilst failure of the ele ment may be delayed, breakdown will eventually occur.
The applicant has, accordingly, investigated this problem and, during the course of his studies, it occurred to him that, even if grain-growth of the crystals could not be entirely prevented, the ultimate effect of such graingrowth, namely cross-sectional cleavage of the wire, and consequent failure of the element, could, perhaps, be obviated by incorporating in the resistance element a material not subject to grain-growth at the temperature of service of the element. Further experiments along these lines proved the correctness of the applicants surmise and showed that electrical resistance elements having a longer operating life than those hitherto used or proposed could be produced in a relatively simple manner.
The object of this invention, therefore, is to provide an improved electrical resistance element suitable for use as a heater element for furnaces or as a thermocouple element and which is less liable to failure as a result of crystal grain-growth than are existing types of resistance elements.
With the above and other objects in view, the invention broadly provides an electrical resistance element for use as a heater element for furnaces or as a thermocouple element which comprises a refractory ceramic-cored wire of noble metal or noble metal alloy.
In carrying out the invention in practice, the electrical resistance element may comprise a composite material formed of an outer layer of noble metal or noble metal alloy and a core of a sintered refractory ceramic material which will not alloy with the noble metal or alloy layer at the temperature at which the element is designed to operate.
Further, in accordance with the invention, an electrical resistance element for use as a heater element for furnaces or as a thermocouple element comprises a composite material formed of an outer tubular member of noble metal or noble metal alloy and a filling composed of particulated refractory ceramic material which will not alloy with the noble metal or alloy layer at the operating temperature of the element, but which is capable of sintering at the said temperature into the form of a solid rod-like core within said tubular member.
The composite material or refractory-cored tube is advantageously drawn down into wire before sintering.
The invention also embraces a method of making an electrical resistance element which method comprises the steps of filling a tube formed of noble metal or noble metal alloy with a mass of particulatcd refractory ceramic material which will not alloy with the noble metal or alloy tube at the operating temperature of the element, and heating said composite so-filled tube to a temperature sulficient to cause the particles of the refractory ceramic filling to sinter together to form a solid rod-like core within the metal tube.
In the case where the resistance element is intended for use as a heater element for a furnace and to be wound on a tubular insulating former or support, as is usual in metallurgical furnaces, this must be done with the core or filling in the unsintered condition, and sintering can then, advantageously, be carried out when the heater element is first brought to operating temperature during use of the furnace.
The noble metal or noble metal alloy used in carrying out the invention, is preferably a platinum group metal or an alloy of a platinum group metal with another platinum group metal or other platinum group metals or with another metal or metals.
Platinum or an alloy of platinum with lO%-40% rhodium will be found to be particularly suitable for the purposes of the invention.
By platinum group metal is meant platinum, palladium, rhodium, iridium, osmium or ruthenium.
The ceramic material used to form the core or filling is preferably alumina or zireonia but other ceramic materials such as magnesia, which do not alloy with the noble metal, may be used.
In order that the invention may be fully understood, one embodiment thereof will now be described by way of example, as applied to the manufacture of a furnace heater element, by reference to the accompanying diagrammatic drawings, in which:
FIGURE 1 is a sectional elevation of a resistance element suitable for use as a furnace heater element at an intermediate stage of its manufacture;
FIGURE 2 is an end view of FIGURE 1 loolring in the direction of the arrow A;
FIGURE 3 is a similar view of FIGURE 1 but showing a further stage in the manufacture of the element, and
FIGURE 4 is a view of the resistance element after reduction to wire.
Referring now to the drawings and first to FIGURES l and 2 thereof, 1 indicates a tube of about /8" external d ameter and A3" internal diameter and formed of an alloy of 10% rhodium-platinum. The tube 1 is sealed at one end 2 by fusing over the said end with an oxyhydrogen torch, the other end 20 being left open. The bore of the tube is then filled with powdered alumina 3 previously dried at 110 C. which is packed into the tube 1 through the open end 2a thereof, by means of a small ram and compressed under a load of about 2 lb./sq. in.
The so-filled tube 1 is then heated, in order to remove all traces of moisture, to a temperature of about 800 C. after which the Open end 20 of the tube 1 is sealed by fusing in a similar manner to the previously sealed end 2 as shown in FIGURE 3.
The sealed tube 1 is now square rolled to 0.25" square wire, annealed at 1100 C. and finally drawn to wire of a diameter of 0.020" as illustrated by the numeral 4 in FIGURE 4.
If the so-formed alumina-cored wire 4 is to form a thermocouple wire, the composite wire 4 may now be heated to a temperature below the melting temperature of the powdered alumina, to cause the alumina particles to sinter together to form a solid rod-like core within, and supporting, the outer platinum alloy casing In the case where the cored wire 4 is to constitute a furnace heater element, the said wire 4 may, after annealing at about 1350 C., be wound on a cylindrical former to form a helix, which is then placed in position in a metallurgical furnace, and heated to the operating temperature of the furnace, which is below the melting temperature of the alumina, to sinter the alumina particles to a solid core.
If desired, of course, the sintering operation may be carried out on the wound composite heater element prior to its introduction into the furnace, but this involves an additional operation which is unnecessary as no disadvantages result from effecting the sintering during the initial heating up of the furnace.
It will, of course, be readily understood that, where the improved resistance element is not required to be wound on an insulating former, nor drawn down into wire, the sintering of the alumina filling may be carried out on the cored tube.
The improved results obtainable in accordance with the teachings of the invention, will, it is thought, he readily appreciated from a consideration of the foregoing. For example, in the case of a furnace winding, the heater element, during operation of the furnace, attains a relatively high working temperature with the result that grain-growth of the crystals of the platinum alloy tube or wire will occur and some slip may also take place. However. the presence of the alumina core will effectively prevent this grain-growth from ultimately reaching the state necessary to produce cleavage across the entire crosssection of the wire with the result that failure of the winding will be prevented.
It will be understood that the metal used to form the outer tube may be pure metal or metal which has been grain-stabilised by the incorporation of grain-stabilising compounds, such as carbides or oxides. Although not considered necessary, the use of grain-stabilisers will reduce the tendency of the metal to undergo grail-[growth and to that extent tend further to increase the working life of the element.
Whilst in the above, one particular embodiment of the invention has been described by way of example, it is to be clearly understood that the invention is in no way limited thereto or thereby, but that modifications may be made thereto without departing from the scope of the invention. For example, the tubular casing need not be formed of a 10% rhodium-platinum alloy, but any other platinum group metal or alloy may be used. Moreover, the core may be formed of any other suitable refractory ceramic material than alumina.
What I claim is:
1. An electrical resistance wire comprising a composite material formed of an outer tubular member composed of a metal selected from the group consisting of platinum group metals and alloys of platinum group metals and a filling within said tubular member composed of particular refractory ceramic material selected from the group con sisting of alumina Zirconia and magnesia which will not alloy with the metal of said tubular member at the operating temperature of said wire but which is capable of sintering at the said temperature into the form of a solid rod-like core within and supporting said tubular member.
2. An electrical resistance wire according to claim 1 in which the outer tubular member is formed of an alloy of platinum with 10%40% rhodium.
3. An electrical resistance wire comprising a composite structure formed of a supporting core of a sintered refractory ceramic material selected from the group consisting of alumina, zirconia and magnesia and, surrounding said supporting core, a sheath of. a metal selected from the group consisting of platinum group metals and platinum group metal alloys.
4. An electrical resistance wire comprising a composite structure formed of a supporting core of sintered alumina and, surrounding said supporting core. an outer sheath formed of an alloy of platinum with 10% rhodium.
References Cited by the Examiner UNITED STATES PATENTS 377,316 1/1888 Marshall 29l91.6 X 1,358,311 11/1920 Harris 161-175 X 2,548,423 4/1951 Cumming 338238 X 2,930,105 3/1960 Budd 161175 2,947,114 8/1960 Hill 161-225 X 2,964,839 12/1960 Marafioti 29195 3,091,561 5/1963 Marzocchi 161-196 HYLAND BIZOT, Primary Examiner.
UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,320,039 May 16, 1967 Ralph George Hollister It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.
Column 4, line 31, for "particular" read H particulate Signed and sealed this 14th day of November 1967..
Edward M. Fletcher, Jr. EDWARD J. BRENNER attesting Officer Commissioner of Patents