Silver-Metal Oxide Contact Materials Fabricated by Spray Coprecipitation

This paper describes the development of a highly versatile technique for fabricating improved silver-metal oxide contact materials. The spray coprecipitation method, invented at The Allen Clark Research Centre and patented [1] for Plessey Inc., offers both economic and technical advantages over existing processes for making silvercadmium oxide such as conventional internal oxidation of fabricated alloy, conventional powder metallurgy, and internal oxidation of alloy powder/shot. The main technical advantages are the following: a) controlled microstructural perfection in the uniform fineness and long-range homogeneity of the oxide dispersion; b) unlimited composition with respect to both quantity and type of oxide (not limited to oxides of metals with solid solubility in silver); c) ease of doping with beneficial ternary additons without influencing the microstructure, which is simply controlled by the sintering time and temperature. The method relies on converting, to some extent, the intimate dispersion provided on a molecular scale in mixed aqueous solutions to the solid state by instantaneous coprecipitation from small sprayed droplets of mixed nitrates solutions. This paper concentrates in the main on the application of the process to the fabrication of traditional silver-cadmium oxide, but examples are also given of the production of silver-zinc oxide, silver-tin oxide, and silver-lanthanum oxide materials which demonstrate the versatility and simplicity of the method. IEC AC-4 erosion test results are presented for spray coprecipitated Ag-15 weight percent CdO.