US2370400A - Contact materials - Google Patents

Description

Feb. 27, 1945. H. GRAVES, JR

CONTACT MATERIAL Filed Sept. 25, 1941 v M NTOR; BY Was/:01 C ill rx W M a m PatentedjFeb',

uNiTEo? STATES PATENT entice} 1 m .1 C. Pai, asrinor to I. '1. E. Circuit Breaker Company, Philadelphla,-Pa., a corporation of Pennsylvania A plication September 25. 1941. Serial No, 412,215 a ;"4asims. (onto-ice) v My invention relates to an electrical contact themass'are defined by the intersprsedand subcircuitmaking and breaking element and more particularly to a-combination copper and cadmium material tor forming the same.

of another contact element in order to provide a currentpath therehetween.

The contact it comprises a or copper t2, the pores of-whichare however ir'aii, filled wlthfcadmium E3, in a pure or relatively pure state. The mass of copper i2 is substantially laminar in composition with each of the ltions exrelatively porous the two materials are soluble e stantially continuous cadmium particles; "which 1 s are either in a pure or partially alloyed form. l Since the contact I 0 is manufactured y by 'a It is well hnownth'at copberand alloy in such a manner that neither. el be disuished microscopi" can other It this were the case. in a set ofcontacts.- none, of the advantages, hereinafter set forth, obtained by the useoi iree cadmium would be achieved.

It is necessa y tor the correct? operation or the contact that iree cadmium belpresent near the.

mm, in a plane substantia y nareuei to tha 55 earth: the contact, n 9 Prevent of the face M or the contact. The an i "1 hthr words, I

Heretofore, various types of contacts have'been .5 sintering operation from particles of copper{ and proposed and used in an attempt to lower concadmium which, are compressed and"c'aused-to tact resistance, increase the wearing qualities, flow lnto'laminar form, each of thelaminations f H and decrease welding. Thus, contacts made or a of the copper l2 iS'Of minutethickness, and silver graphite combination,or of cadmium plated "w aring or flak ng fl of Surf ce laminations copper 'or silver, or even of a copper and cadlib when the contact is in u e causesno appretlablfl mium allo have been used. wear. v v

But, an object of this inventionis the utiliza- I must be emph sized that the drawlns oi. tion 01 cadmium in a free state or relatively free Figure 2 erely schematic. The laminatiohs state in th contact, K of the finished copper need not be as regular or A further object of this invention is the forw W ll defined as in the drawing.v And the eadmation of a contact material or a substantially mium particles need not-necessarily be in the laminar structure having particles or another form of flakes of minute thickness but may be material dispersed between the laminations. f ar o flas le e fo w e, nevertheless Another objector this invention is the formae g the laminations of the finished tion of a contact of substantially laminar st'rucso The specific operations by which the contact ture with particles of an additional material t l m ufee ured wi l behereinafter described. having a lower melting point than the principal Iheve found t there is a wide euse of material of the-contact dispersed between the t ns under wh h the c pper cadmium laminations, contact of my invention will have y' advan- Still further objects of this invention include td't e s pp r or silver: the formation of a, contact having low contact Whemthe t-c nsists Of a bodyoi pu e resistance, no tendency to weld, high conductiv "copper fil d with pure um.. ity, strength and resistance to physical shock and Where the contact consists of an al v of abrasion, high resistance to erosion, and oflow copper and cadmium fil i fp e admium. t V 3. ere the contact-consists of pure copper These and many other objects of this invenfi led with an alloy of 0996 ndmdmium. tion will in part be apparent and, where not l' t consists copper a apparent, pointed out, in the following descrip 111mm alloy high in pp fi ed w th an alloy .tion and drawing in which: of pe and admium in um. v

Figure 1 is a side view of a contact formed from 35 l f h eg i s e am r tr my novel mp a ture of the type above described has been found Figure 2 is a schematic fragmentary cross-secw be wmnmgwusi mwever' the m i is tional view of a minute portion of a corner of the i we PD I even though this 1 contact of Figure 1. gg is not P t A Referring now to Figures 1 and 2, Ihave here an fadmtage a maftgiial 9 shown inschematic form, a contact member 86' $53 s 1 g;

, constructed in accordance with my invention. tends to educ 5 i L 3% The face ii of the contact member is intended, assisting Sn outing-am e .7 3 during operatiomtc be in contactwith-the face 5 M m the contacts under arcing conditions and to decrease milli-volt drop. Otherwise, if an alloy is used, a quantity of copper must be volatilized before any cadmium is set free.

As a result of the relatively low boiling point of pure cadmium and also the low ionization temperature, a contact made thereof will tend to maintain the arc stream at a relatively low temperature. However, cadmium cannot be used alone owing to the high temperatures present during the arc. The advantages of the use of cadmium may however be obtained if a quantity thereof is set free during the drawing of the arc.

During engagement, the circuit breaker contacts provide low resistance causing a normal temperature rise. Upon the occurrence of a fault, the increased current will generate heat which is a product of the square of the current value and the resistance. Moreover, the resistance between the contacts increases rapidly and the heat generated further increases correspondingly.

The are is drawn at a single point on the contact. The temperature of this point is extremely high, above the vaporizing point of any contact material. Heat radiates from this point through a hemisphere of material, the temperature gradient depending upon the thermal conductivity of the contact material. Consequently, there are surface temperature zones of metal in different conditions: solid, plastic, molten, vaporizing. Under these conditions, contacts composed of pure metals or alloys are very apt to weld or stick together.

When, however, there exists (as in the case of the present contact) directly behind an area of plastic material, a film or layer of material (such as cadmium) that, at the same temperature or even at a considerably lower temperature, is in a vaporizable or molten state, then the force pushing the contacts apart will be able to pull off metal flakes within the plastic area and free the contacts. This is more particularly true when the contact structure is a laminar one as above described.- Local explosions of the highly volatile material may occur at the points of high current density corresponding to points of flnal contact separation and assist the flaking process.

Contacts of this type thus tend to shed flakes of metal upon each other during heavy current interruption. Due to the fine grain structure employed, this produces relatively little roughness on the surface. Chemical analysis indicates that this process has a tendency to reduce very slightly the percentage of the secondary material. The effectiveness of the contacts is not, however, altered.

The explosive action of the cadmium has a As soon as the cadmium vapor comes into contact with air it is oxidized. The oxide is a good insulator and will not provide a low resistance path for dielectric breakdown. This is in contrast to the current path formed along adjacent insulators by particles of silver or carbon when contacts containin such materials are used.

The cadmium oxide formed on the surface of the contact as a result of arcing is extremely friable and is easily removed from the surface by the wiping action of the contacts. This is particularly true when the surfaces have been slightly roughened by current interruption. The high points of the contacts are wiped clean and the oxide may be transferred to the depressions. Since the laminar construction above described enhances non-welding qualities of my contact, it becomes important to utilize particles for forming the same which are as small and thin as possible. Since the particles of copper are held together by a solder (cadmium) which melts at a lower temperature, local heating of the contact permits small areas of copper to flake off, remaining engaged to the other contact block. The smaller the particles and the thinner they are, the less the roughness caused at the contact surface by the flakin off. And, if the particles are in the form of flakes lying in planes parallel to the plane of the contact surface, any roughness causedby the flaking off will be greatly reduced. In one method of manufacture of my contacts, the thinnest possible flakes of copper (produced in a ball mill and annealed) are plated with cadmium by the standard barrel electro-plating process and aligned, asfar as possible, in parallel planes by vibrating the loose mass. The cadmium deposit may be approximately five to ten percent by weight of the total. The thickness of the plating film should, however, be suflltendency to blowout the arc and decrease the amount of arcing once the arc is struck. The very low melting point and boiling point of cadmium makes possible the evolution of cadmium vapor at a fairly low temperature. The heat of vaporization of cadmium is such that it does not require a great deal of energy to vaporize a given .amount of cadmium. The cadmium may be doubly ionized at fairly low voltages so that the charge on the ions is +2e, where e is the charge of the electron.- Because of its fairly high atomic weight (112) it willcmove relatively slowly in the are stream.

As a result of these characteristics, cadmium is able to absorb considerable energy in the arc without causing undue stress on the-contacts or on the surrounding materials.

cient to produce the flaking off effect and are snufllng results previously set forth. Additional free cadmium may be added if it is deemed necessary for particular uses of the contact or electrodes. 1 i I The mass of plated flakes is then compressed to a substantially solid block preferably by a pressure of the order of 60,000 pounds per square inch to eliminate most of the voids in the mass.

The compressed mass is then sintered.

The cadmium plate then forms a homogeneous bond between the copper flakes. The sintered material may then be repressed at a low temperature to break up the continuous cadmium bond and to cause it to crumble; by which means, a grain structure is produced in the' contact which may be caused to be substantially parallel to the plane of the contact face. This latter step is, however, not essential.

It may be difficult, however, to cause the copper flakes to arrange themselves in substantially parallel relation.

Accordingly, an alternative method comprises the steps of utilizing grains of copper in a form approximating a sphere, plating them as above, and compressing them-into flat plates. A cake of the plated grains is formed and then subjected to pressure as above, allowing the edges of the cake to fiow out. While the flakes thus produced are not as thinas in the case of ball mill flakes, a better alignment is obtained. The previous plating of the copper with cadmium provides a lubricant to assist the. flow. The mass may then be sintered and treated in the manner above described.

In another method, no plating operation is perasrdcco formed, but the finely divided copper and cadmium are intimately mixed, compressed, sintered and repressed as above set forth. In this process, the cadmium, as it wets the surface of the copper, flows through the copper mass by capillary action. The bond thus produced may not be as good as thatformed'by the plating method, but provides local areas of cadmium. These lat ter will cause an additional action'during current interruption when their temperatures exceed the vaporizing point. Local explosions may thus occur which will assist in the flaking operation. I

In any of these sintering operations, a small quantity of finely divided graphite may be intermixed with the mass to lubricate the particles and to facilitate the flow of metal during compression and sintering and to decrease the tendency to weld during operation. Such graphite particles are shown schematically at id of Fig. 2.

An analysis of the cadmium copper contact thus formed by any of these methods shows a matrix of pure copper interspersed-with a substantially continuous binder of pure cadmium.

The method for forming abontact described above may be extended to various materials other than copper as a basic material. Thus it is possible to employ such refractory materials as tungsten or molybdenum and form a contact of such material combined with a low boiling point metal such as cadmium. Similarly materials such as silver, nickel, iron,

cobalt, gold, platinum or palladium may be employed to replace the copper used as the basic material in my-contact; and, further, the cadmium itself may be replaced by other metals of low boiling pointwhich assist in snufiing an arc.

Thus it may be seen that I may employ various granulatedmetals in combination and by a process including the steps of compression, sintering and recompres sion, form a contact having a laminated construction which isdesirable for circuit interrupters in high current circuits. Furthermore, if one of the finely divided metals employed is a metal of low boiling point, the

arc extinguishing properties of the circuit inter-- rupter are enhanced by the contact itself.

Hence many modifications of my contact and the means for fabricating the same will be obvious to those skilled in the metal arts. Thus I prefer to be bound not by the specific disclosures herein but only by the appended claims.

I claim:

1. An electrical contact having a surface for engaging a second contact, said contact compris ing a quantity of finely divided conducting copper, a quantity of a second finely divided conducting material of a lower melting point, said materials being compressed and sintered together in a compact mass, a pluralitytcf the particles of said first material being in the form of flakes extending in planes approximately parallel to said surface.

2, An electrical contact having a surface for engaging a second contact, said contact comprising a quantity of finely divided conducting material, ,a quantity of a second finely dividednconducting pure metallic badmium of a. lower melting point and a quantity of finely divided carbon, said materials being compressed and sintereitogether in a compact mass, a plurality of the particles of said first material being in the form of flakes extending in planes approximately parallel to said surface. i

3. An electrical contact having a surface for engaging a second contact, said contact comprising a quantity of finely divided metal of high electrical conductivity, a quantity of finely divided metallic cadmium, said materials being compressed and sintered together in a compact mass, the particles (if said metal of high conductivity being predominantly in the form of flakes extending in planes parallel to .the surface.-

An electrical contact comprising a quantity of finely divided copper in the form of elongated particles extending in planes substantially parallel to the surface of the contactand a quantity of a second finely divided metallic conducting.

material of a lower melting point than cbpper, said materials being compressed and sintered together in a compact mass. 1

' HERBERT C. GRAVES, JR.

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