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

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 (RC). 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 RC. During the 200 °C heat treatment, Sn in the coating is transformed into a Cu3Sn layer, on which an oxide grows that consists essentially of a thin outermost layer of CuO on top of Cu2O, similar to the oxide structure formed on bare Cu. The underlying Cu3Sn 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 Cu3Sn layer is not formed during the 200 °C heat treatment mainly Sn oxide grows and RC is comparatively low.