US3644692A - Osmium plated contacts - Google Patents

Abstract

Contacts for use in nonoxidizing atmospheres are electroplated with a thin coating of osmium to provide longer life and more reliable performance.

Description

United States Patent Greenspan 1 Feb. 22, 1972 [54] OSMIUM PLATED CONTACTS [56] References Cited [72] Inventor: Lawrence Greenspan, New York, NY. UNITED STATES PATENTS [7 3] Assignee: Engelhard Mineral & Chemical rp ra- 3,146,328 8/1964 Mason et al ..200/166 C tion Primary Examiner-H. 0. Jones [221 Attorney-Miriam w. Left and Samuel Kahn [21] Appl. No.: 79,662

ABSTRACT Contacts for use in nonoxidizing atmospheres are elec- [52] US. Cl. ..200/l66 C, 29/630 C troplated with a min Coating of osmium to provide longer life [51] Int. Cl .H01h 1/66, HOlh 1/02 and more reliable performance [58] Field of Search ..200/166 C; 29/630C 4 Claims, 2 Drawing Figures 'FIGZZ //v l/EN TOR LA WRENCE GRE E NSPA N wal /8 v A 7' TORNE V O SMIUM PLATED CONTACTS BACKGROUND or THE INVENTION This invention relates to improvements in the construction of metallic contact devices which are used in nonoxidizing atmospheres. It is particularly useful for encapsulated contacts such as dry reed switches and relays.

Reed switches of many designs are known, as illustrated in U.S. Pat. Nos. 3,495,061 and No. 3,400,348. Generally they consist of two more resilient flexible thin magnetically susceptible reeds sealed into a glass envelope. The envelope is provided with an inert atmosphere, e.g., nitrogen or mixtures of nitrogen with argon or helium. The reeds extend from opposite ends of the container and the free ends, which are flattened at the tips, serve as contacts. The contacts overlap with a narrow gap between them and they open and close in response to a magnetic field.

With respect to the contact portions there are certain well known problems which shorten the life and reliability of the switches. Among the main problems are abrasion due to mechanical wear, voltage breakdown at high-current densities, and welding or sticking at high temperatures caused by arcing. Also, often thereeds are made of a magnetic material which has roughly thesame thermal coefficient of expansion as the glass envelope in order to avoid the problems caused by differences in expansion at the seal. Nickel-iron alloys, for example, which are suitable with respect to the expansion characteristics do not have proper contact properties. Generally the problems are interrelated and they result in failure or unreliability of the switches. In order to obviate the problems at the contacts, the contact portions of the reeds are usually coated with a thin layer or layers of a nonmagnetic metal or metals. The coating must be thin, uniform, and highly conductive. Precious metals are among the material used to improve the life and wear of the contact portions of the reed switches and relays. In particular rhodium, gold, silver, and platinum have been proposed for this purpose. And, in fact, these precious metals did improve the contacts substantially. However, it has now been found that a thin osmium plating as a coating on the contacts further improved the contacts, and this improvement is very substantial with respect to both the life and the reliability of the contacts.

THE DRAWINGS FIG. 1 is a vertical cross-sectional diagram of a reed switch according to the invention, and

FIG. 2 is a blownup perspective view of one of the contacts of the switch.

THE INVENTION In accordance with this invention contacts for use in nonoxidizing atmosphere are provided with a thin coating of osmi- In a preferred embodiment of this invention the coating of osmium is applied to the contact portion of reeds used in dry reed switches.

Referring to the FIGURES there is shown a cross-sectional representation of a reed switch. The switch consists essentially of a sealed glass envelope 12 within which is contained an inert atmosphere 14. One atmosphere utilized in reed switches and relays is a mixture of helium and nitrogen. A pair of reeds 16 extend into the glass enclosure, one from each end thereof. The reeds 16 are sealed at one end in the glass 12 with their inner ends overlapping slightly. The reeds 16 are made of a magnetic material such as nickel-iron. The overlapping portions 18 of the reeds 16 produce a rather high-contact resistance.

The present invention provides a thin, uniform layer of osmium 20 on the opposing overlapping portions 18 of the reeds 16.

The osmium may be applied by any technique. Preferably, however, the osmium is deposited by electroplating. More preferably it is deposited from an electroplating bath of the type disclosed in copending Patent application Ser. No. 843,174 filed July I8, 1969, wherein the plating bath is comprised of an aqueous alkaline solution having dissolved therein an octavalent osmium compound and an alkali metal salt of sulfamic acid. The bath is prepared by dissolving the octavalent osmium compound, e.g., osmium tetroxide, in an aqueous alkaline solution containing an alkali metal sulfamate, e.g., potassium sulfamate and heating the solution to effect a color change. The yellow solution which forms upon heating is an effective plating solution. Various additives may be included in the bath, e.g., an alkali metal hydroxide, buffering agents, etc. The bath is advantageously maintained at a pH in the range of 13 to 14 and operated at a temperature in the range of 50 to C. and at a current density of about 2 to 40 amperes per square foot. Baths of this type deposit smooth uniform coatings of osmium of a thickness as high as microinches and even higher.

In magnetically responsive reed switches the coatings are preferably thin, of the order of about 25 to 100 microinches, so that they do not interfere with the magnetic field or response time of the switch. Also, since the osmium is expensive it is desirable to keep the coating as thin as possible. It is an advantage of the osmium coating that-it is so durable even as a very thin coating. It was found, for example, that contacts having an osmium coating of 50 microinches, formed from u plating bath of the above-mentioned type, gave exceptionally good performance and had long life.

The osmium coating is formed on base metals chosen for their conductivity, magnetic susceptibility and other properties dependent on the use of the material. In the case of reed switches, for example, the reeds may be constructed of or contain a layer of magnetically susceptible metal such as iron, nickel, cobalt and combinations thereof. As indicated above, preferred materials have thermal expansion characteristics which match the encapsulating glass. The osmium may be plated directly on the magnetically responsive reed material or an intermediate flash coating, for example, of gold may be used.

The following comparative tests illustrate the substantial improvement in performance of reed switches made in accordance with this invention over those using contact materials of the prior art.

Reed switches were prepared using nickel-iron reeds which were electroplated with a 50 microinch coating of osmium over a flash coating of gold at the contact tips. The osmium coating was prepared using an electroplating bath composed per liter of l g. Os as a complex with sulfamic acid, 20 g. KOH, and 39 g. K HPO,. The pH was adjusted to 13.7 with KOH. The osmium complex with sulfamic acid was prepared from OsO sulfamic acid, and KOH in accordance with the method of the aforementioned copending patent application. The plating was effected at a temperature of 7080 C. and a current density of 15-25 amperes per square foot. The switches made with reeds having the osmium plated contact tips were subjected to a cyclical make and break test run at l0 watts, and the results compared with prior art reed switches using gold plated and rhodium plated contact tips. Generally. the switches made with gold plated contacts failed at about 10 X 10 cycles. The switches made with rhodium plated contacts generally failed at about 25X 10 cycles. Using the same test procedure on 40 reed switches made with contacts which were osmium plated in accordance with this invention, the test was discontinued when the switches had reached the following cycles:

No. of switches No. of cycles in test Reached I3 147 X ll) None of the contacts failed. The test was discontinued because the expense of continuing it was high. Based on exprising a thin uniform layer of osmium on the surface of each of said contact portions.

2. A reed-type switch of claim 1 wherein the surface of the contact portion of each reed is coated with a thin uniform layer of electroplated osmium.

3. A reed-type switch of claim 2 wherein said electroplated osmium has a thickness ofabout 25 to microinches.

4. A reed-type switch of claim 2 wherein said electroplated osmium is deposited on a flash coating of gold.

Claims (3)

1. 2. A reed-type switch of claim 1 wherein the surface of the contact portion of each reed is coated with a thin uniform layer of electroplated osmium.
2. 3. A reed-type switch of claim 2 wherein said electroplated osmium has a thickness of about 25 to 100 microinches.
3. 4. A reed-type switch of claim 2 wherein said electroplated osmium is deposited on a flash coating of gold.

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