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 105 atoms and with typical dimensions of 105 nm3 (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 Nb2O5 at the surface to near-stoichiometric Nb2O 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.