| Achoimre: | Nb–47 wt% Ti is the workhorse alloy of the superconducting magnet industry. Fabrication of “perfect” superconducting joints with electrical resistance values < is necessary to operate the magnets in persistent-mode without a power supply, enabling the generation of the high stability magnetic fields required for MRI and NMR applications. Many jointing techniques involve exposure of the highly-reactive metallic filaments to air, and it is well known that exposures of more than a few minutes prevent successful superconducting joint formation. Here we present the first fundamental study of the oxidation kinetics and chemistry of the oxide layers on a superconducting NbTi alloy. We use angle-resolved x-ray photoelectron spectroscopy (AR-XPS) to analyse both the earliest stages of oxidation by gas dosing with pure oxygen in UHV, and also oxidation under industrially relevant conditions by exposure of clean surfaces to laboratory air for timescales up to an hour. The reaction layer is found to be a mixed Nb/Ti oxide, with little change in the Nb:Ti ratio with depth. The oxide formation follows logarithmic growth kinetics, increasing to a limiting thickness of 3.70.3 nm after extended exposure to air.
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