IOAP-A Novel Process for the Manufacture of Long-Life Silver-Cadmium Oxide AC Contact Material

This paper describes a new more versatile internal oxidation technique for the manufacture of silver-cadmium oxide contact tips. The process allows precise control in producing a fine homogeneous microstrueture, it avoids the occurrence of a central denuded core in contacts and uses oxidation times which are several orders of magnitude less than in the conventional method. This novel technique essentially combines internal oxidation and powder metallurgical methods of compaction and densification. By correct control of the processing of the powder into high-density contact parts, a wide range of microstructures may be obtained from a single powder. The first stage of the process is the formation of a silvercadmium alloy powder of fine particle size by the low-temperature reduction and alloying of a silver and cadmium oxide powders mixture. This powder is produced in spherical aggregates in preparation for the second internal oxidation step. Internal oxidation of the fine alloy powder is then carried out at relatively low temperatures in times typically 103 times shorter than are required in conventional internal oxidation. This results in powder which contains extremely fine cadmium oxide particles in a simple orientation relationship with the matrix within and on the surface of each powder particle. This internally oxidized alloy powder (IOAP) is then compacted to provide green compacts for sintering and coining to provide high-density contact tips. During the sintering stage of the IOAP process, the ultrafine cadmium oxide particles within each powder particle grow by an Ostwald ripening process. The extent of this coarsening is governed by the time and temperature of the sintering operation and the oxygen potential of the furnace atmosphere: these parameters can be controlled to provide a specific final material selectable from a very wide range of possible microstructures. The coining operation increases the density to a value approaching the theoretical. The IOAP process also allows the addition of selected elements or compounds to the basic material at several stages in the process and retains the ability to control the microstructure independently from the chemical composition. This feature of the IOAP process is in sharp contrast to more common internal oxidation techniques.