AtomicScale Characterization of SRF Niobium Using UV LaserAssisted
Atomic-Scale Characterization of SRF Niobium Using UV Laser-Assisted Local-Electrode Atom-Probe Tomography Researcher: Dr. Yoon. Jun Kim PI: Prof. David N. Seidman Department of Materials Science and Engineering Northwestern University, Evanston, IL Feb. 16, 2011
Objectives u Current research is focused on obtaining an improved understanding of the origin of weld pits in the SRF Nb cavities. u Project employs nano-scale characterization methods such as: - Dual-Beam Focused-Ion Beam (FIB) Microscopy - Laser-assisted Local Electrode Atom Probe (LEAP) Tomography - Transmission Electron Microscopy (TEM)
Sample Preparation – Material u Pre-treatment at FNAL (1) A cold worked Nb coupon (CW-3) supplied under following conditions: - Buffered chemical polish (BCP): 25 micron in N 2 - Cold work: Rolled from 3. 3 mm down to 1. 8 mm thickness - Electropolish (EP): 31 o. C, 298 min, 189 micron 422 Gloss unit (GU) - Pit density: >20 pits/40 cm 2 2 cm Sample taken for LEAP Normal Cold Work Heavy Cold Work (2) Pure Nb coupons under cold worked and BCPed. - Approximate dimensions of 1 cm x 2 mm
Sample Preparation – FIB/SEM u FEI Helios Nanolab Used for LEAP & TEM sample preparation
Sample Preparation – FIB/SEM (1) Lift-out Pt Deposition 20 mm Trench 20 mm Transfer (2) Mounting 20 mm 3 mm 15 mm Pt Welding 5 mm Initial (3) Tip-Shaping Lift-out 3 mm Final 2. 5 mm Cut-off Final-High Mag 400 nm
LEAP u Cameca (formerly Imago) LEAP 4000 XSI u LEAP Condition (1) Laser pulse rate: 250 – 500 k. Hz (2) Laser pulse energy: 0. 050 – 0. 075 n. J per pulse (3) Temp. : 60 K – 80 K (4) Evaporation rate: 0. 50% (5) Vacuum: ≈10 -11 Torr
LEAP u Mass Spectrum 1 H + 93 Nb 3+ 69 Ga 2+ 16 O+ 93 Nb 2+ 94 Nb. H 2+ 109 Nb. O 2+ 69 Ga+ 105 Count 104 103 102 101 0 20 40 Mass-to-Charge-State Ratio (Da) 60 80
LEAP – Pure Nb u 3 -D Reconstruction - Approximately 2. 5 M ions detected (mostly Nb, H, and Nb. H) - Small amounts of O and Nb. O ions detected
LEAP – Pure Nb u Top Views 40 nm Nb H Nb. H Ga O Nb. O
LEAP – Cold Worked Nb (Normal CW) u 3 -D Reconstruction - Approximately 6 M ions detected (mainly Nb, H, and Nb. H) - Small concentrations of O and Nb. O ions are also observed
LEAP – Cold Worked Nb (Normal CW) u Top Views 40 nm Nb H Nb. H Ga O Nb. O
LEAP – Cold Worked Nb (Heavy CW) u 3 -D Reconstruction - Approximately 1. 5 M ions detected (mainly Nb, H, and Nb. H) - Small concentrations of O and Nb. O ions are also observed
LEAP – Cold Worked Nb (Heavy CW) u Top Views 20 nm Nb H Nb. H Ga O Nb. O
Depth Profiles Pure Nb u Pure Nb and normal cold worked Nb contain an extremely high H concentration (up to 90 at. %). u Heavily cold worked Nb exhibited relatively uniform concentration profiles. u Oxide contribution was almost negligible. Normal Cold Work Heavy Cold Work
LEAP – Summary u Three different samples (pure, normal CW, and heavy CW) were prepared using FIB milling. u Results showed predominant detected atoms and molecules are Nb, H, and Nb. H. Oxygen and oxides were observed in a limited amount. u H and Nb. H in all three samples displayed a unique geometry (for example, 3 -fold symmetry). Pure Nb Normal CW Preferred sites of hydrogen exist? Heavy CW
TEM – Pure Nb u Nb. H precipitates near dendritically shaped dislocations Nb. H ppts Slip band Nb. H ppts Dislocations SAD pattern Grossbeck, Birnbaum , Acta Metall. 25 (1977) pp. 135 Nb 110 zone Simulated SAD pattern Nb H sublattice
Nb-H System u Temperature versus H/Nb u Tetrahedral sites in BCC lattice concentration ratio diagram Makenas, Birnbaum, Acta Metall. 30 (1982) pp. 469 Interstitial sites of H in Nb lattice for b-Nb. H = Face-Centered Orthorhombic Soteros, Hall, Phys Rev B, 42 (1990) pp. 6590 SAD pattern confirms ordering of H superlattice.
Summary and Future Research u High concentration of hydrogen was observed in both pure and cold worked Nb coupon. u Cold-work generates dislocations where interstitial hydrogen atoms can preferably reside. u For pure Nb, LEAP results coupled with TEM observations indicate that ordered Nb. H phase precipitates in the vicinity of dislocations. u More LEAP/TEM works will be performed where pits are observed. u Also, required characterizations will be performed for mechanically polished SRF cavities.
- Slides: 19