US3506437A

US3506437A - Method for making silver/cadmium oxide contact materials

Info

Publication number: US3506437A
Application number: US681097A
Authority: US
Inventors: Childress B Gwyn Jr
Original assignee: Textron Inc
Current assignee: Textron Inc
Priority date: 1967-11-07
Filing date: 1967-11-07
Publication date: 1970-04-14
Anticipated expiration: 1987-04-14

Description

United States Patent O 3,506,437 METHOD FOR MAKING SILVER/CADMIUM OXIDE CONTACT MATERIALS Childress B. Gwyn, Jr., Wethersfield, Conn., assignor, by mesne assignments, to Textron Inc., Providence, R.I., a corporation of Delaware No Drawing. Filed Nov. 7, 1967, Ser. No. 681,097 Int. Cl. C22c 5/00, 1 H01b 1/02 U.S. Cl. 75-206 5 Claims ABSTRACT OF DISCLOSURE A method for the manufacture of silver/ cadmium oxide electrical contact materials, in which a silver/cadmium alloy material is reduced to discrete flat particles having thicknesses not more than about 5 microns. The discrete particles are then oxidized to form a substantially homogeneous silver/ cadmium oxide material.

BACKGROUND OF THE INVENTION (1) Field of the invention carbonate to the oxide. An added disadvantage of the latter two methods is the fact that particle shrinkage during the course of the oxidation-reduction reactions is largely unpredictable.

Yet a fourth method, that of oxidizing silver/ cadmium alloys of the desired composition, in the form of the desired finished contact elements or as shot which may, after compacting, be extruded into the shapes of such elements (see, for example, Haarbye U.S. Patent No. 3,317,991, granted May 9, 1967), is costly in its energy requirements due to the prolonged periods of heating required and the, nevertheless, incomplete oxidation of the cadmium metal component thus effected.

SUMMARY OF THE INVENTION The present invention is directed to an economical method for making substantially homogeneous silver/ cadmium oxide electrical contact materials, which method involves the physical reduction of a silver/ cadmium alloy feed material of the desired composition to discrete, thin particles whose major dimensions, e.g., length, are at least 3 times their thicknesses, and which have substantially uniform thicknesses of no more than about 5 microns. The formation of such fiat, relatively thin alloy particles which may, for example, be provided in either platelet or thread-like form, facilitates subsequent uni- While the electrical properties of such mixtures, which A number of processes have heretofore been proposed to avoid material inhomogeneity of silver/ cadmium oxide electrical contact materials, and to improve the uneconomical methods of achieving such homogeneity.

One such process involves the sintering of a powder mixture of silver and cadmium oxide, as described by Goetzel in his Treatisebn Powder Metallurgy, vol. III, p. 647 (1952). Such technique, while using relatively inexpensive materials, does not result in a composite material of the required homogeneity due to the differences inthe shapes, the sizes, and the specific gravities, of the particles constituting the initial powder mixture.

Another method prescribes the use of an initial mixture of silver and cadmium powders,the cadmium being subsequently oxidized in a suitable arnosphere. In such process difficulties arise from the fact that oxygen diffusion into the cadmium particles, needed for effective oxidation to a certain minimal depth, is slow, so that in actual practice an unoxidized core remains in the cadmium powder. particles. Pure cadmium-metal does not possess those characteristics requisite fora nonform oxidation of the cadmium component of the alloy 'whereby to produce a substantially homogeneous silver/ cadmium oxide contact material. Materials thus produced by the successive physical forming and oxidation steps may thereafter be compacted into the final desired shape for the electrical contact element in question and sintered, if desired.

The method of this invention thus provides an economical technique for producing silver/cadmium oxide electrical contact materials of substantially homogeneous composition, which materials may thus be employed to produce non-welding electrical contact elements. It will be understood that such homogeneity is achieved by the initial formation of discrete, relatively thin silver/cadmium alloy particles, the decreased thicknessof such particles reducing the depth of necessary oxygen diffusion therethrough during oxidation and thereby facilitating the subsequent formation of substantially homogeneous silver/cadmium oxide materials. It will be understood that various procedures for forming the discrete silver/ cadmium particles having thicknesses of no more than 5 microns, generally from about 1 to 3 microns, may be utilized within the scope of the present invention. While several preferred techniques are described hereinafter for forming alloy particles of the desired configuration, it is intended that the method hereof encompasses any such techniques for converting the initial silver/cadmium alloy material to the desired configuration set forth hereinabove, facilitating the formation of substantially homogeneous oxidized electrical contact materials.

PREFERRED EMBODIMENTS OF THE INVENTION Homogeneous silver/cadmium oxide contact materials are produced in accordance herewith from silver/ cadmium alloy materials having compositions known in the art. While the exact proportions of the silver and cadmium constituents of such alloy materials may be dictated by the electrical and/or mechanical requirements for the desired final contact material, it is preferred to utilize alloys incorporating quantities of the order of by weight silver and 10% by weight cadmium, good results having been obtained by the use of alloys constituted of from about 80 to by weight silver and, correspondingl 20 to 5%. by weight cadmium.

As indicated hereinabove, the alloy materials (which.

may initially be provided in ingot or any other desired form) are converted, in accordance with this invention, to discrete, thin particles which may subsequently be oxidized to form substantially homogeneous silver/cadmium oxide compositions. The discrete particles thus formed have large surface area to volume ratios, in excess of 3 to 1, and have major dimensions (lengths or diameters) of at least .005, and generally from about 3 to 20 times their thicknesses. Such particles, which may possess configurations of extremely thin flakes or platelets, or finely-spun threads, have substantially uniform thick nesses of no more than about 5, and preferably from about 1 to 3, microns. The formation and use of such particles has been found uniquely suited to the preparation of substantially homogeneous contact materials.

Forming of the silver/cadmium alloy stock material may be achieved by various techniques as, for example, by impinging a molten stream of the alloy against a suitable baflle to disintegrate the alloy into discrete particles and flatten and/or comminute such particles into the desired configuration described hereinabove. Alternatively, the solid alloy material, in bar, rod or other desired form, may be mechanically abraded to remove flake-like particles of the desired material suitable for oxidation in accordance herewith. Thirdly, the malleable alloy material may be reduced to the desired discrete, thin paroxygen moleculcshaving only. relatively short distances to diffuse into the center of the individual granules treated as compared with spherical'granules of equal volume.

While it is believed that formation of substantially homogeneous silver/cadmium oxide contact materials by the-present process is achieved by the mechanismproposed hereinabove, it will be understood that the invention is not limitedto such proposed mechanism. Rather,.it is intended that the present process shall not be limited other than as defined in the claims appended hereto;

The following'examplesfurther'illustrate preferred embodiments of the present process for forming silver/cadmiumoxide electrical contact materials: i

EXAMPLE 1 A silver/cadmium alloy'is melted by a source of heat sufficient for'the purpose, and the molten metal alloy is then pressed through a nozzleinto a chamber for expansion into a jet of fine droplets. This jet impinges upon a water barrier, acting as a baifle for instantly cooling and disintegrating the molten stream impinging thereon. As a result, the alloy solidifies into flat platelets-of silver/ cadmium alloy having thicknesses less than 5 microns.

' The particles are then collected, dried, and oxidized in a ticulate form by repeated rolling with no intermediate annealing operations. Alternatively, the physical con version operation may be effected by spinning the molten silver/cadmium alloy material into strands or threads of the desired shape.

The discrete silver/cadmium particles having large surface to volume ratios are thereafter readily oxidized to form the desired silver/cadmium oxide material. Such oxidation, which requires considerably less time than the partial oxidation techniques heretofore employed, may be effected by heating the alloy composition at temperatures of from about 1300 to 1500 F. for periods of from about 2 to 24 hours. The discrete particles are thus oxidized substantially uniformly throughout, no appreciable core of unoxidized cadmium. metal remaining.

The silver/ cadmium oxide contact material thus formed may be readily compacted, extruded into wire or other shapes, and interlocked into desirable forms suitable for electrical contact applications. In various instances the contact materials may be used directly as compacted whereas in other instances it may be desirable to subsequently sinter and/or re-coin or compact the material for particular intended uses. In any event, the contact materials thus produced are characterized by homogeneity, high density, mechanical strength, and superior electrical characteristics. Moreover, as noted hereinabove, such materials may be formed at lower cost and in decreased time as compared with prior techniques employed for converting silver/cadmium alloy stock materials to silver/cadmium oxide electrical contact components.

It is believed that the improved homogeneity effected in accordance with the present invention is produced by virtue of the use of the discrete, thin particles formed in accordance herewith. conventionally made alloy shot, powder or granules are composed of particles which are largely spherical. It is a well known fact that a sphere has the smallest surface area of any body relative to its volume. Since, however, oxidation proceeds by diffusion from the surface inwardly of the body to be oxidized, it is desirable, in the present instance, to rather oxidize alloy materials-having large surface to volume ratios or, otherwise stated, large surface to cross-section ratios. Such is achieved in accordance with the present process by the formation of thin flakes, platelets or strands of the silver/cadmium alloy which have thicknesses of no more than 5 microns and length or diameter tothickness f; I

chamber at 1470 F., containing an oxygen-rich atmosphere.

In one experiment flakes incorporating 90.5 weightpercent silver and 9.5 weight percent cadmium, having a thickness 'of between 2 and 5 microns, were oxidized to substantially 100 of the flakes cadmium content at a temperature of 1550" F. within a period of 10 hours. By varyingthe thickness of the particles by means of the parameters of the spray apparatus, e.g., the pressure, temperature, nozzle diameter, jet velocity, composition of the coolant, the time and temperature required for complete oxidation may be varied at will.

The above processing time compares favorably with that required for oxidizing a silver/ cadmium bar of similar composition and of dimensions /1 inch thick x 2 inches wide x 10 inches long, which may require from 75 to 500 hours at 1500 F; to produce an internal oxidation of between 50% and ofthe cadmium.

EXAMPLE 2 The silver/cadmium alloy of Example 1 isconverted to a granulate form, and the granules are propelled at great velocity" against a hard surface, such as a stainless steel plate. By repetition of such procedure, the granular particles are reduced in size as Well as flattened, the-number of repetitions'of the, process being dictated by the actual particle size found suitable for the particular conditions prevailingin the oxidation chamber used. Oxidation is effected in the same manner as described in Example 1. Representative dimensions'of final particles are mesh, or in the range of l to 3 microns thickness. I

EXAMPLE 3 A solid body of silver/cadmium alloy of the above composition is subjected to'abrasionby a file, a rasp, or similar tool capableof reducing the alloy to chips or filings or powder withoutitself contributing .to the granulate thus formed. Inone" proposed embodiment the tool and the alloy body may usefully be wheel shaped and rotated in the same direction. while their circumferential faces are in contact under pressure, in order to facilitate the'continuous production of thegranulate. This granulate may either be feddirectly to the oxidizing chamber, or it may be subjected to further treatment as described in Example 2, if desired, to further comminute the discrete particles produced.- 4 v M EXAMPLE 4 are produced in'a-direct operation by subjecting'a bar made. of the alloy to repeated rolling under pressure without annealing the alloy between the passes of the roller. The process gives the alloy a brittle surface due to strain hardening, and the surface is then easily broken up into a powder consisting of wafer-shaped particles. Oxidation follows, as described above.

Further modifications and refinements of the process of this invention will be obvious to those skilled in the art. The above description and examples should not, therefore, be construed in a limiting sense.

I claim:

1. A method for making silver/cadmium oxide electrical contact materials, which comprises converting a silver/cadmium alloy stock material incorporating from 80 to 95 weight percent silver and from to 20 weight percent cadmium, to discrete thin particles whose major dimensions are at least three times their thickness, and which have a substantially uniform thickness of no more than 5 microns, and heating said particles in an oxygen rich atmosphere to oxidize the cadmium content thereof and produce a substantially homogeneous silver/ cadmium oxide contact-forming composition.

2. The method of claim 1, in which the discrete particles of the silver/cadmium alloy are formed by melting said alloy stock material, and impinging a stream of said molten material under pressure against a substantially planar surface to flatten and comminute the particles to the desired dimensions.

3. The method of claim 1, in which a solid body constituted of the silver/cadmium alloy is reduced to said discrete silver/cadmium particles by abrading said bed: to remove flake-like particles having thicknesses of fron 1 to 3 microns therefrom.

4. The method of claim 1, in which a solid body constituted of the silver/cadmium alloy and having a substantially planar and smooth surface is subjected to repeated rolling under pressure, without intermediate annealing, to form the desired flake-like alloy particles.

5. The method of claim 1, wherein said particles are heated at temperatures of from 1300 to 1550 F. for periods of from 2 to 36 hours to eifect the substantially complete oxidation of the silver/cadmium alloy to a homogeneous silver/cadmium oxide material.

References Cited UNITED STATES PATENTS 2,796,346 6/1957 Stumbock -173 2,861,155 11/1958 Farnham 75173 X 3,114,631 12/1963 Sistare 29-182.5 X 3,317,991 5/1967 Haarbye 75--206 X BENJAMIN R. PADGE'IT, Primary Examiner A. J. STEINER, Assistant Examiner US. Cl. X.R.