Oxidation and contact resistance of Sn–Ag coated superconducting strands for the Large Hadron Collider (LHC)

Abstract The oxides formed on the Sn–Ag coated Large Hadron Collider (LHC) superconducting cables during a 200 °C heat treatment in air are described and the oxide composition is compared with the interstrand contact resistance ( R C ). The analysis of more than 250 interstrand contact areas shows that the higher the average Cu content with respect to the Sn content in the oxide, the higher is R C . During the 200 °C heat treatment, Sn in the coating is transformed into a Cu 3 Sn layer, on which an oxide grows that consists essentially of a thin outermost layer of CuO on top of Cu 2 O, similar to the oxide structure formed on bare Cu. The underlying Cu 3 Sn layer acts as an O diffusion barrier that prevents O diffusion into the Cu bulk during the subsequent cable heat treatment under high pressure. On contact zones where the Cu 3 Sn layer is not formed during the 200 °C heat treatment mainly Sn oxide grows and R C is comparatively low.