US2202054A - Electric contact element - Google Patents

Description

Patented May 28, 1940 UNITED STATES PATENT FHE ELECTRIC CQNTACT ELEMENT Franz R. Hansel and Kenneth L. Emmert,

Delaware No Drawing; Application May 2, 1939, Serial No. 271,282

5 Claims.

The present application is a continuation in part of copending application Serial No. 223,506, filed August 6, 1938, now Patent #2161253, issued June 6, 1939.

This invention relates to electric contacts.

An object of the invention is to improve the characteristics of electric contact elements containing silver.

Another object of the invention is to provide a silver base contact material which is highly resistant to tarnishing, and therefore will not form a layer of high contact resistance.

A still further object is to provide a new contact material which can be used under severe electrical conditions without welding.

It is a further object of the invention to provide a contact capable of operating on high frequency at heavy current values, without objectionable contact metal transfer.

Other objects of the invention will be apparent from the following description, taken in connection with the appended claims.

The present invention comprises the combination of elements, methods of manufacture and the product thereof, brought out and exemplified in the disclosure hereinafter set forth, the scope of the invention being indicated in the appended claims.

While a preferred embodiment of the invention is described herein, it is contemplated that considerable variation may be made in the method of procedure and the combination of elements, without departing from the spirit of the invention.

The present invention comprises an improvement in silver electric contact elements. According to the invention, the contacts are improved by the addition of arsenic.

The contacts may be composed of silver and arsenic accordingly. The ingredients may be present in approximately the following proportions Per cent Arsenic .05 to 4.5 Silver Balance It is likewise possible to improve the characteristics of silver alloys containing additional ingradients, by the addition of arsenic; thus, silver alloys containing beryllium. magnesium, zinc, cadmium, copper, gold, nickel, manganese, thorium, palladium group elements, platinum group elements, silicon, titanium, zirconium, tin, indium, can be improved by the addition of arsenic,

- in substantially the proportions indicated above.

We have found that silver and arsenic, up to 4.5% arsenic, alloy readily, and can be cast into ingots of desirable shape. The arsenic forms with the silver in alpha solution up to approximately 4.5%. We have therefore used this percentage as the high limit of the arsenic content. The alloys with a percentage of approximately 2% arsenic, have a liquldus of over 900 degrees C., and a solidus above 800 degrees C. If the specified maximum content of arsenic is exceeded, then a liquid phase Will be present, at such low temperatures as 600 degrees C. This makes it very difiicult to work alloys of higher arsenic content, either hot or cold. The alloys of the present invention, with the limited percentage range of arsenic, however, can be swaged and rolled into shapes from which contacts may be prepared. An alloy containing 2.5% forms a very satisfactory material.

The arsenic is preferably added in the form of a master alloy, which is prepared by mixing arsenic powder and silver powder in suitable proportions, pressing same to a high density and then adding such pressed body to the silver melt. By using this procedure, a high arsenic recovery is obtained.

In a number of contact applications. silver is mixed with refractory materials, taken from the group of tungsten and molybdenum, or their compounds, and with metals taken from the iron group, such as nickel, cobalt and iron. These materials are prepared either by mixing the powders together, pressing same, and hot and cold working same into suitable forms from which contacts can be prepared. It is also possible to produce such materials by first pressing the fractory or iron group metals, and then impregnating same with a silver-arsenic alloy. Since arsenic is easily available in powder form, it can be readily incorporated into the powder mix, and during elevated temperatures, will alloy and diffuse into the silver resulting in a homogeneous product. Certain care has to be taken in producing these products, in order to avoid sublimation of arsenic.

Contact tests which were made with silver containing arsenic, showed an outstanding advantage in material transfer of such compositions. These alloys were tested on a comparison basis, wherein contacts of similar physical dimensions 59 were tested on a resistive inductive circuit at 470 cycles per minute, and wherein the current flowing in the circuit was increased periodically to obtain definite current values of alloys, of the nature described above. The amount of material M transfer from one contact to the other was used as the main means of comparing. Fine silver or silver alloy contacts show in this test a transfer from the positive to the negative contact.

Of all the combinations that were tested silverarsenic showed a very pronounced transfer in the reverse direction, namely, from the negative to the positive. This fact is of great importance,

because failures in electric contacts are primato operate thealloy of the present invention,

against, for instance, fine silver, with the fine silver being negative, because fine silver has a tendency to transfer to the negative and therefore, if operated at such a polarity, would not tend to transfer. On the other hand, silver-arsenic has a tendency to transfer to the positive and again,

,if operated on the positive, it will have no tendency to transfer from such a polarity.

Tests of this nature were carried out in which silver-arsenic was used on the positive contact and fine silver or silver-cadmium or silver-'magnesium, on the negative polarity, because the latter three materials have a tendency to transfer from the positive to the negative. The results which we obtained, showed that with such combinations, a negligible transfer of .001 to the negative was obtained. By comparing this extremely low transfer value with values that are obtained when the alloys are operated against each other, it was found that the transfer was reduced.

The alloys of the present invention have a very satisfactory hardness. As a matter of fact, the alloys cold work very readily by applying very light cold reductions, and hardness values such as to Rockwell F, can be readily obtained.

' With such hardness values, the contacts will show The fact that a ical wear in the order of a very small fraction of an inch, will mean failure.

The alloys were also tested for tarnish resist- .ance by placing same in a chamber through which hydrogen sulphide was passed. For comparison, fine silver and some of the standard silver alloys were tested. It was found that while fine silver would show a veryheavy sulphide coat, within a period of 24 hours, the alloys of the present invention maintained a bright surface for the same length of time, and showed only a slight tarnish film at the end of 72 hours. When l testing the contact resistance, it was found that 'ried out in specific embodiment thereof, it is not desired to be limited thereby, but it is intended to cover the invention broadly within the spirit and scope of the appended claims.

What is claimed is:

1. An electric contact member, containing as essential ingredients, silver and arsenic.

2. An electric contact member, containing as an essential ingredient, silver, to which has been added arsenic in quantities ranging from .05 to 4.5%.

3. An electric contact member consisting of an alloy of .05 to 4.5% arsenic, balance substantially all silver.

4. An electric contact member consisting of a refractory metal base and a silver alloy, containing .05 to 4.5% arsenic.

5. An electric contact member consisting of an iron group metal base, and a silver alloy, containing .05 to 4.5% arsenic.

FRANZ R. HENSEL. KENNETH L. EMMERT.