The effect of various silver tin oxide materials on contact performance under motor load (a.c.)
The new contact material AgSnO/sub 2/ which is replacing AgCdO in many applications, requires the addition of additives to improve electrical contact performance. These additives such as copper oxide (Cu/sub 2/O) and bismuth oxide (Bi/sub 2/O/sub 3/), can significantly affect contact performance. An investigation was undertaken to compare the changes in contact performance between silver cadmium oxide (internally oxidized) and three silver tin oxide based materials containing (1) no additive, (2) copper additive and (3) bismuth plus copper additives. Performance changes and physical characteristics of each material such as formability (tensile properties), contact resistance and contact material erosion (arc erosion) were evaluated. The silver tin oxide based materials containing copper additive exhibited significantly higher resistance to arc erosion than silver cadmium oxide based material and other silver tin oxide materials evaluated. The resistance to welding and changes in electrical resistance were dependent on the chemistry of each material. The results suggested that the chemistry of the material was the chief reason silver tin oxide based material with copper additive performed better than silver cadmium oxide based material under identical motor load.
Mass material transfer of Ag-SnO2 contact material in DC level
A composite contact materials Ag-SnO2 have been used in low current switching devices for its good anti-welding properties and resistance to arc erosion. But the mass material transfer occurred at large surge current in DC switching now and then. Mass material transfer have an increased probability of early failures because the effective contact gap reduces or two contacts lock together. In this research, the shape and distribution of crystals at the contact surfaces were observed to analyse the reason of mass transfer. It was found that the shape and distribution of the crystals are like sunflower at the contact surfaces. The equal axes crystals distributed over the surfaces center, and the radial club-shaped crystals distributed around the center. One test showed that the melting phenomena at the interface of crystals had precedence over that in other sections at large surge current for the composite contact materials Ag-SnO2, it result in individual crystal transfer or several crystals transfer together.
Effect of additions of indium, tin, bismuth, and tungsten oxides on the properties of a silver-cadmium oxide composite
ConclusionsThe addition of 5% of bismuth oxide to an Ag-10% CdO powder composite enhances the erosion resistance of contacts made from it. Similar additions of tungsten and indium oxides adversely affect, and one of tin oxide has but little effect on, the erosion resistance of Ag-15% CdO composite contacts. Bismuth oxide and, to an even larger extent, indium oxide improve the resistance to welding of these contacts. The effect of the oxides on the structure and resistance to erosion and welding of the contacts is determined by their ability to react with the components of the matrix and by the properties of the reaction products.