Atom-probe tomography analyses of niobium superconducting RF cavity materials

We present the first atom-probe tomographic (APT) measurements of niobium superconducting RF (SCRF) cavity materials. APT involves the atom-by-atom dissection of sharply pointed niobium tips, along with their niobium oxide coatings, via the application of a high-pulsed electric field and the measurement of each ion’s mass-to-charge state ratio ( m/ n) with time-of-flight (TOF) mass spectrometry. The resulting atomic reconstructions, typically containing at least 10 5 atoms and with typical dimensions of 10 5 nm 3 (or less), show the detailed, nanoscale chemistry of the niobium oxide coatings, and of the underlying high-purity niobium metal. Our initial results show a nanochemically smooth transition through the oxide layer from near-stoichiometric Nb 2O 5 at the surface to near-stoichiometric Nb 2O as the underlying metal is approached (after ∼10 nm of surface oxide). The underlying metal, in the near-oxide region, contains a significant amount of interstitially dissolved oxygen (∼5–10 at.%), as well as a considerable amount of dissolved hydrogen. The experimental results are interpreted in light of current models of oxide and sub-oxide formation in the Nb–O system.