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TUP034 |
Atomic-Scale Characterization of the Subsurface Region of Niobium for SRF Cavities Using Ultraviolet Laser-assisted Atom-probe Tomography | |
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Funding: This research was funded by USDOE (DE-AC02-07CH11359) and LEAP measurements were supported by NSF-MRI (DMR 0420532) and ONR-DURIP (N00014-0400798, N00014-0610539, N00014-0910781) programs. Niobium is the metal of choice for SRF cavities for a linear particle accelerator because it has the highest critical temperature of any element in the periodic table and can be deformed plastically into complex geometries. Differences in the sub-surface chemistry from bulk niobium are believed to determine the high-field Q-drop. In this study, the subsurface chemistry of niobium was characterized utilizing ultraviolet laser-assisted local-electrode atom-probe (LEAP) tomography employing picosecond laser pulsing. The superior spatial resolution and analytical sensitivity of a LEAP tomograph permits us to determine the subsurface composition on an atom-by-atom and atomic {hkl} plane-by-plane basis. The 3-D reconstructions from the LEAP tomographic analyses demonstrate different behaviors for Nb-oxides and Nb-hydrides in pure niobium as well as interactions with structural imperfections, dislocations and grain boundaries in SRF-grade Nb coupon material. Additionally, the chemistry and crystallographic structure of subsurface interstitial atoms were analyzed based on energy shifts of electron energy-loss spectroscopy in conjunction with a scanning transmission electron microscopy. |
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TUP042 |
In-Situ Study of Nb Oxide and Hydride for SRF Cavity Applications Using Aberration-Corrected STEM and Electron Energy Loss Spectroscopy | |
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Funding: supported by the University Research Associate (URA) Visiting Scholars Program at the FNAL. UIC JEOL JEM-ARM200CF is supported by an MRI-R2 grant from the U.S. National Science Foundation We present an atomic-resolution study of the effects that 48hr bake at 120 °C in vacuum has on the high-field properties of Nb-based SRF cavities. This bake results a significant increase in high-field Q, reversely, 800 °C bake for 2hr reduces the Hc3/Hc2-ratio. Several mechanisms have been proposed, including an increased NbOx surface layer thickness and the precipitation of NbHy. Using combination of atomic-resolution Z-contrast imaging and electron energy-loss spectroscopy with in-situ heating and cooling experiments, we examine the atomic and electronic structures of Nb and related oxides/hydrides near the cavity surface. We quantify the oxygen diffusion on surface during bake by measuring the local Nb valence using EELS. We demonstrate that hydrogen atoms incorporated into the Nb crystal, forming β-NbH precipitates, can be directly visualized by annular bright field imaging in our aberration-corrected JEOL ARM-200CF. The effects of baking on the local hydrogen and other impurity will be examined by imaging, EEL spectra and strain analysis. Our results will be combined with atom-probe tomography to develop a 3-D impurity and phase profile of Nb near the SRF cavity surface. "R. Tao, R. F. Klie et al, Journal of Applied Physics, 110, Issue 12, (2011)" "Y.J. Kim, R. Tao, R.F. Klie and D. Seidman, ACS nano 7 (1), pp 732–739, (2013)" |
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| TUP079 | ECR Nb Films Grown on Amorphous and Crystalline Cu Substrates: Influence of Ion Energy | 631 |
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Funding: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177. In the pursuit of niobium (Nb) films with similar performance with the commonly used bulk Nb surfaces for Superconducting RF (SRF) applications, significant progress has been made with the development of energetic condensation deposition techniques. Using energetic condensation of ions extracted from plasma generated by Electron Cyclotron Resonance, it has been demonstrated that Nb films with good structural properties and RRR comparable to bulk values can be produced on metallic substrates. The controlled incoming ion energy enables a number of processes such as desorption of adsorbed species, enhanced mobility of surface atoms and sub-implantation of impinging ions, thus producing improved film structures at lower process temperatures. Particular attention is given to the nucleation conditions to create a favorable template for growing the final surface exposed to SRF fields. The influence of the deposition energy for both hetero-epitaxial and fiber growth modes on copper substrates is investigated with the characterization of the film surface, structure, superconducting properties and RF performance. |
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