US2806786A - Method of making sintered electrical contact material - Google Patents

Description

United States Patent METHOD OF MAKING SINTERED ELECTRICAL CONTACT MATERIAL Floyd C. Kelley, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York No Drawing. Application May 14, 1954, Serial No. 429,966

3 Claims. (Cl. 75-211) This invention relates to a composition of matter suitable for use as an electric current transmitting contact, and more particularly, to an iron containing silver composite material which, when employed as an electrical contact, does not generate objectionable radio interference, and to the method of making this material.

The generation of radio interference when an electric circuit is made or broken by contact elements such as are used in a switch, for example, has long been a source of annoyance. Attempts to solve this problem in the past either have been directed to suppressing or quenching the arc formed between the contacts just prior to completing the circuit or just after interrupting the circuit by either electrical or mechanical means extraneous to the contact points per se, or have been directed to mechanical means for accelerating the relative motion between the contacts to reduce the duration of the are. My invention is concerned with the provision of a composition of matter, from which at least one of a pair of electrical contact points or surfaces may be fabricated, which virtually eliminates radio interference when an electric circuit is completed or broken across the points or surfaces.

A principal object of this invention is the provision of electric contact points for circuit making and breaking apparatus which virtually eliminate radio interference.

A further object of this invention is the provision of a composition of matter having the ability to act as an arc-forming electrode without generating objectionable radio frequency interference signals.

A further object of this invention is the provision of a method of manufacturing a metallic composite material which does not generate objectionable radio frequency interference when used as electric contact points in circuit making and breaking apparatus.

Briefly stated, I have discovered that the radio frequency signal generated by an arc may be virtually eliminated by providing at least one of the pair of electrodes between which the arc is formed with a contact point or surface at the location of the are composed essentially of a metallic composite material comprising a matrix of substantially pure silver through which is distributed about 2.5 percent to about 30 percent, by weight, iron in a finely divided dispersion, providing the iron particles have been subjected to a reduction treatment in situ, as will be more specifically described later.

The basic problem solved by my invention, as briefly stated supra, is the elimination of radio frequency interference signals which are coincident with the operation of circuit making and breaking apparatus, such as a switch. It is commonly assumed that the are formed between the contact elements of such apparatus during the making and breaking operation is the origin of these signals. When such apparatus is operated and an arc is formed between the contact points or surfaces, the area of the points or surfaces from which the arc is struck tends to become heated. Since no attempt is usually made to exclude the atmosphere from this area or zone, the contact points or surfaces tend to become oxidized and eroded, reducing their efficiency. It is not uncommon to provide certain types of switches and other circuit making and breaking apparatus with contact points made of materials having high resistance to oxidation and high electrical conductivity. Many materials having these characteristics have been so employed in the art, silver, for example, being one which has been quite commonly used. While these previously known contact materials have satisfactorily resisted corrosion and erosion over relatively long periods of time, they have not proven satisfactory from a radio frequency interference standpoint. The magnitude and duration of these interference signals originating from a switching operation are thought to depend upon several electrical and mechanical factors which vary from one type of apparatus to another. For example, a conventional snap-action switch which mechanically provides means to accomplish the making and breaking of the circuit in a very short period of time and thus reduce the time during which an arc may exist, and which usually additionally provides a very firm contact pressure when closed, will generate an interference signal of fairly short duration. 0n the other hand, a switch which operates slower, for example, one in which the movable contact comprises a thermally responsive bimetal element, will tend to are over longer periods of time and, therefore, generate signals of longer duration. There are unquestionably many other factors which infiuence these signals.

I have discovered that an electrical contact material comprising a matrix of substantially pure silver containing a fine dispersion of metallic iron which has been subjected to a reducing treatment in situ has the property of virtually eliminating arc-generated radio frequency interference signals. I have further discovered that the method by which this material is made determines whether or not the resulting composite material has this desirable property. One such method which I have found to be efficacious involves the following procedure. Powdered commercially fine silver is mixed with a solution of ferric nitrate, Fe(NO3)3-9H2O in alcohol to form aslurry. Sufficient ferric nitrate is present so that the iron content of the dry components of the slurry, calculated as pure iron, is in the range of about 2.5 percent to about 30 percent, by weight. The slurry is stirred constantly until dry to prevent caking. The dry mixture is then reduced in dry hydrogen at a temperature of about 500 to 550 C. for sufficient time to accomplish the reduction of the ferric nitrate to iron. The mixture of silver powder and finely divided iron particles is then pressed into a compact and sintered in a reducing atmosphere of pure dry hydrogen at a temperature of about 850 to 900 C. During this step, the iron particles are subjected to a reducing treatment in situ prior to and during the sintering of the silver matrix. The sintered material may then be cold-worked into rods or any other desired form form which contact points may be conveniently fabricated. It may be found necessary to anneal the material between cold-working operations. I have found it desirable to perform such anneals at about 850) C. in an atmosphere of pure dry hydrogen.

Contact points made from material processed in the above manner where installed in a switch and it was observed that while arcing at the contact points during the operation of the switch was not reduced, the radio frequency interference was virtually eliminated. Other contacts having approximately the same chemical constituency but made by another process did not exhibit a correspondingly low level of interference when tested in the same manner. The microstructures of the material made by my process and the unsatisfactory but otherwise identical material were compared at a magnification of 500 diameters, but no significant physical differences were observed. A probable theory has been proposed which might account for the significantly different electrical behavior of the materials. It is suspected that a thin film of iron oxide may surround the iron particles in the unsatisfactory material, and, in some unexplained manner, cause the objectionable noise signals.

I do not consider the specific procedure recited above to be the only means by which this result may be obtained. For example, it would appear that any iron salt capable of being reduced by hydrogen to metallic iron could be substituted for the ferric nitrate cited above. Further, while the above method depends upon precipitation of the iron salt out of an alcoholic solution to obtain good distribution, it is obvious that it would be possible to obtain a similar distribution by dry mixing silver powder and a finely divided iron salt. I do, however, regard as important the step of subjecting the iron particles to a reducing treatment in situ during the sintering operation. While I have no explanation for the subsequent behavior of the material, I believe that the factors controlling the radio frequency interference of this material are related to the optimum distribution of the iron, the fine particle size of the iron and to the probable absence of an oxide film surrounding the iron particles.

From the foregoing, it is apparent that I have discovered a composite metallic material suitable for use as an electric contact in circuit making and breaking apparatus which virtually eliminates radio frequency interference occurring when the apparatus is operated, and a method of making such a material.

While I have disclosed that the material of my invention may be advantageously employed as the contacts of circuit making and breaking apparatus, such as a switch, for example, I also contemplate its usefulness to extend to such applications as a current carrying brush for motors and generators, wiper-arm type contactors for rheostats and slide-wire type potentiometers and the like. In short, my invention is applicable to any electrical apparatus having current carrying components which are relatively movable with respect to each other in the normal operation of the apparatus and between which, incidental to the operation of the apparatus, arcs are formed which tend to generate objectionable radio frequency interference signals.

The specific embodiments set forth in the specification are illustrative of my invention, it being understood that certain changes and modifications might occur to a person skilled in the art without departing from my invention in its broader aspect, and I aim, therefore, in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A method of making electrical contact material comprising the steps of preparing a dry mixture composed of finely divided, substantially pure silver having a finely divided compound of iron capable of being reduced by pure dry hydrogen thoroughly distributed therethrough, the amount of the compound of iron constituting about 2.5 to about 30 percent, by weight, of the whole, calculated as pure iron, subjecting the dry mixture to a reducing treatment in an atmosphere of pure dry hydrogen at from 500 to 550 C., forming the treated mixture into a compacted mass and sintering the compacted mass into a composite in an atmosphere of pure dry hydrogen at a temperature between 850 to 900 C., the particles of the compound of iron thereby being subjected to a reducing treatment in situ.

2. The process recited in claim 1 in which the preparation of the dry mixture therein recited is accomplished by first preparing a slurry of powdered silver and a solution of a soluble iron salt and simultaneously drying and agitating the slurry.

3. The process recited in claim 1 in which the iron content of the compound of iron constitutes from about 5 to 20 percent, by weight, of the whole.

References Cited in the file of this patent UNITED STATES PATENTS 1,093,614 Gilson Apr. 21, 1914 1,919,730 Koenig July 25, 1933 2,119,489 Beer May 31, 1938 2,227,445 Driggs Jan. 7, 1941 2,300,286 Gwyn(A) Oct. 27, 1942 2,311,436 Gwyn(B) Feb. 16, 1943 2,365,249 Comstock Dec. 19, 1944 2,394,501 Weiller Feb. 5, 1946