US2319364A - Electrical heating conductor and method of producing same - Google Patents

Description

Patented M g' 7 ,4.

ELECTRICAL HEATING coNnUo'roR 'AND METHOD or raonocmqsarm Paul Ziegs, Piorzheim, Germany; vested in the Alien Property Custodian No Drawing. Application April 1 7, 1939, Serial No.

. 'H-Claims.

This invention relates to articles adapted to withstand high temperatures, more particularly electrical heating conductors, and process for the manufacture thereof.

Said electrical heating conductors may be in the form of wires or other profiled members, which can be used for example in resti'stance furnaces for high temperatures. For withstanding temperatures in the range above1200 C.'the materials platinum or plati- Y num metals such as iridium, rhodium, palladium or alloys of platinum metals are the most suitable resistance material. It is known that the durability of resistance wires or similar articles of the aforesaid metals is fairly limited, for even evaporation of the metal occurs, the effect of which is so great at temperatures around 1400 to 1500" C. that aftera relatively short heating time the heating resistances are destroyed. The short life oi." resistances of such metals results in undesirable stoppages in operationand, in

addition, the operation of a furnace becomes expensive through the loss of the evaporated metal of such heating resistances. 1 Various measures have already been adopted in order to prevent the evaporation of heating wires of platinum or platinum metals. Thus attempts were first made to obviate these short comings by the method of alloying, the platinum I being alloyed with other less highly volatile platinum metals. No substantial degreeof success attended these eilorts. 'I'hemethodwas then adopted of applying a protective skin over the wires of platinum or platinum metals, an

268,427. In Germany May 6, 1938 The problem of creating an entirely satisfactory heating wire for high temperatures from platiat temperatures train-4000 -C. -an. appreciable enamel-like protective skin being-selected for the purpose. Although it was possibleby this means substantially to reduce the evaporation, yet the enamel-like-protective' skin had the disadvantage of not adhering firmly'but of chipping off when'subjected to the slightest stress. Since these measures were still not satisfactory, the

method was finally adopted of /forming an alloy .containing platinum metal with small percentages ofearth or alkaline earth metals. It is true that it was possible greatly to reduce the rate of evaporation of such an alloy at high temperatures, but this alloy involves other disadvan tages which detract from its' use for heating wires, i. e. even when a few tenths per centof the earth or alkaline earth metal are added, the v alloy becomes so brittle that it can only with difliculty, be worked up into wires. The wires withstand only S i ht mechanical stresses, particularlybending stresses, so that they easilybreak,

n'um or platinum metals has thus still not yet been satisfactorily solved.

, v The present invention aims at providing a different method of preventing the harmful evaporation eifect in heating conductors consisting entirely, or having only an outer. layer of platinum or platinum metals or platinum metal al- In accordance therewith the outer layer of platinum or the like of the'heating conductor is surrounded by a protective layer of earth or alkaline earth metals which is bonded therewith by ignition and alloys with said outer layer,- i. e. the outer layer of the heating conductor is, so

to speak impregnated by the surface coating and protected against evaporation.

The resulting protective layer then contains from about 20 to 40% of .earthor alkaline earth metal. Calcium or aluminium are advantageously used for the protective layer.

By means of the present invention, a heating conductor is obtained the heat load of which can be about to C. higher, owing to the impregnation and the resulting minimisingof the evaporation, than in the case of the heating conductors of platinum hitherto employed. Since a high percentage ofthe earth or alkaline earth metal is already contained in the alloy of the protective layer, this is a guarantee that even if the wire is damaged, evaporation of the metal of the platinum group is prevented, since the oxide' skin, if cracked, has always the tendency, owing tothe presence of the earth or alkaline earth metal, to close up again. On the. other hand the protective layer can be extremely thin, owing to the high percentage of earth oralkaline earth .metal contained therein, so that the workability and electrical resistance of the heating wire are retained unaltered. Tests have shown that the heating conductor of the present invention has a a life which is a multiple of that of existing types. The advantage or the present invention also extends to base metal wires coated with platinum, i-.- e. to all wires the outer layer of which consists of a material evaporating at high temperatures, such as platinum or the like.

It is already known per se to render heating wires of chrome-nickel or similar alloys more resistant by surrounding them with a protective coating. The heating wires of-chrome-nickel or the like, however, are not used for high temperatures 01' about mom or the like. V The article of the present invention can be produced in various ways. For example, the wire or the like is coated with the earth or alkaline platinum metal lying inside has thus received an armour-like sheathing which protects it against evaporation. The article may also be coated with the eart or alkaline earth metal by plating. The protective outer layer of the earth or alkaline earth 7 metal may also be deposited on the article by electrodeposition. In addition, it is possible to spray the earth or alkaline earth metal on to the article by a known metal spraying method, such as is used, for example, for coating base metals and other substances. Through the subsequent ignition, a portion of the earth, or alkaline earth metal layer applied is undetachably united with the underlying platinum metal.

When using the earth or alkaline earth metal in powder or paste form, it is convenient to employ a. suitable binder. A binder containing manganese resinate has been found advantageous.

The ignition of the article can be carried out either separately or simultaneously in a suitable atmosphere. When the article is used as a heating resistance particularly a heating wire, the ignition-may also only be carried out when the heating wire isalready inits ceramic support. In addition, it is also possible to apply the coating of protective metal to the article only when the latter is embedded in the support of refractory substances.

When the protective coating of the invention line earth metals.

has been applied to the article, for example in the form of bars or the like, and the articles are then worked into wires, bands or the like, the protective coating is still preserved in the same manner and with the same good effect as ii" the finally formed articles themselves were coated with this layer. The articles can thus be coated at any time, i. e. also even subsequently, for example when the heating wire has been built into its support.

I claim:

1. An electrical heating conductor comprising a metal of the platinum group covered by an outer protective layer of an alloy of said metal of the platinum group with from 20% to of a metal of the group consisting of earth and alka- 2. An electrical heating. conductor as defined in claim 1 in which the alloying metal is calcium.

3. An electrical heating conductor as defined in claim 1 in which the alloying metal is aluminum.

4. Method of producing electrical heating conductors which comprises coating a metal of the platinum group with a metal of the group consisting of the earth and alkaline earth metals and heating at a temperature of at least 1000 C. to alloy the coating metal with the platinum metal, the coating metal being applied in quantity sufiicient to yield a surface layer consisting of an alloy of the platinum metal with from 20% to 40% of the coating metal.

5. Method as defined in claim 4 in which the coating metal is applied in the form of a powder.

6. Method as defined in claim 4 in which the coating metal is applied in the form of a paste consisting of the powdered metal and a binder.

7. Method as defined, in claim 4 in which th coating metal is applied by electrodeposition.

PAUL ZIEGS.