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.
The effects of metal oxide additions or dopants on the electrical performance of AgSnO/sub 2/ contact materials
The new contact material AgSnO/sub 2/, which is replacing AgCdO in many applications, requires the addition of doping agents to improve electrical contact performance. These additions effect thermodynamic stability, wettability of the SnO/sub 2/ surface by liquid silver as well as the microstructure of the area melted by the electric arc. This work considers the frequently used dopants WO/sub 3/, MoO/sub 3/, Bi/sub 2/O/sub 3/, CuO and RuO/sub 2/ as well as others: ZnO, Ta/sub 2/O/sub 5/, HgO, RuO/sub 2/, Sb/sub 2/O/sub 3/, TeO/sub 2/ and TaC. DTA and TGA analyses are performed on mixtures of these metal oxides with SnO/sub 2/, Ag and AgSnO/sub 2/. Reaction products, if present, are characterized by X-ray diffraction. The wetting angles of molten silver on surfaces of these mixtures are established. The results are used to interpret the microstructures found in the molten regions of AgSnO/sub 2/ materials doped with these substances. These structures are interpreted and related to performance in arcing contact applications.
Practical, legal, and ethical issues in expanded access to investigational drugs.
The authors review the FDA policies and procedures that permit some patients with serious conditions to receive investigational drugs before formal product approval and examine the legal and ethical issues associated with expanded access.