Study of N Doped Samples before and after Electrochemistry Wydglif Dorlus Supervisor: Anna Grassellino GEM Final Report 08/06/18
Outline • SRF Cavities • Treatments • Methods • LSCM Results • AFM Results • Conclusion 2 8/4/18 Wydglif Dorlus | Surface Roughness Study
What are SRF Cavities? o Superconducting Radiofrequency • Key technology of the particle accelerators • Used to accelerate particle beams by storing electromagnetic energy • Energy given to particle beams as they pass through the cavity • Low energy loss 3 8/4/18 Wydglif Dorlus | Surface Roughness Study
SRF Cavities Properties & Requirements • Typically made of Nb, 𝑂𝑐 # 𝑇𝑜 • Can operate at Tc<T • Elliptical shape § Clean and uncontaminated § Smooth Surface 4 8/4/18 Wydglif Dorlus | Surface Roughness Study
Treatments • Buffer Chemical Polishing (BCP) – Mixture of Hydrofluoric acid (HF), Nitric acid ( 𝐼𝑂𝑃 # ), Phosphoric acid ( 𝐼 # 𝑄𝑃 ) ) • Electropolishing – Current applied to cathode through electrolyte contain mixture of HF and Sulfuric Acid ( 𝐼 + 𝑇𝑃 ) ) • Cavity High T Vacuum Baking (H degassing) • High Pressure Rinsing (HPR) • Nitrogen Doping – Bake 800ºC/3hr in High Temperature Vacuum Bake – Nitrogen Diffusion/2 min – N redistribution/6 min 5 8/4/18 Wydglif Dorlus | Surface Roughness Study
N-Doping - Goal • Discovered by Anna Grassellino • Poor performance due to Nitrides • After additional treatment (EP) to remove nitrides → 𝑅 - ≈ factor up to 4 • Hypothesis to test: Accelerating gradient 30% less due to surface roughness? ? Doped 120C bake 5 8/4/18 Wydglif Dorlus | Surface Roughness Study
Study steps Coupon From Nb sheet Bulk EP 150 µm HPR Ethanol Rinse N Doping 2/6 Ethanol Rinse Ethanol Rinse HPR HPR HPR Ethanol Rinse BCP 5 µm EP 5 µm 7 8/4/18 Wydglif Dorlus | Surface Roughness Study
Method of investigations • High resolution optical and electron microscopes used to capture images and roughness measurement of grain boundaries • Laser Scanning Confocal Microscope – Laser beam pass through light source aperture, focused by objective lens (50x) into small area on surface – Image build up pixel by pixel 8 8/4/18 Wydglif Dorlus | Surface Roughness Study
• Atomic Force Microscope – Laser intensity and cantilever height difference – Contact and non-contact mode – Gwyddion software for analysis 9 8/4/18 Wydglif Dorlus | Surface Roughness Study
Roughness Parameters • Rp = 𝒏𝒃𝒚𝒔 𝒍 Highest peak • Rv = 𝒏𝒋𝒐𝒔 𝒍 Lowest valley • Rz = Rp + Rp Roughness height • Ra = 𝟐 𝒐 𝒐 ∑ 𝒔 𝒍 Roughness average 𝒍8𝟐 𝟐 𝒐 𝒔 𝟑𝒍 𝒐 ∑ • Rq = Roughness root mean square 𝒍8𝟐 • Rku = 𝟐 𝒐 𝒔 𝟓𝒍 𝒐𝑺 𝟓 ∑ Kurtosis (spikiness of peaks and valleys) 𝒍8𝟐 10 8/4/18 Wydglif Dorlus | Surface Roughness Study
Coupon From Nb sheet Bulk EP 150 µm HPR Ethanol Rinse N Doping 2/6 Ethanol Rinse Ethanol Rinse HPR HPR HPR Ethanol Rinse BCP 5 µm EP 5 µm 11 8/5/18 Wydglif Dorlus | Surface Roughness Study
Nb before electrochemistry Nb before Treatments 180 160 140 120 100 µm 80 60 40 20 0 Rp Rv Rz Ra Rq Rku Laser intensity image from LSCM Roughness parameters Rough surface and spiky pits and bumps, due to machining of the surface 12 8/4/18 Wydglif Dorlus | Surface Roughness Study
Coupon From Nb sheet Bulk EP 150 µm HPR Ethanol Rinse N Doping 2/6 Ethanol Rinse Ethanol Rinse HPR HPR HPR Ethanol Rinse BCP 5 µm EP 5 µm 13 8/5/18 Wydglif Dorlus | Surface Roughness Study
Pure Nb → Bulk EP 150 um removal Bulk EP 12 10 8 µm 6 4 2 0 Rp Rv Rz Ra Rq Rku Laser intensity image from LSCM Roughness parameters Roughness height decreased by a factor of up to 30, smooth surface due to electropolishing 14 8/4/18 Wydglif Dorlus | Surface Roughness Study
Coupon From Nb sheet Bulk EP 150 µm HPR Ethanol Rinse N Doping 2/6 Ethanol Rinse Ethanol Rinse HPR HPR HPR Ethanol Rinse BCP 5 µm EP 5 µm 15 8/5/18 Wydglif Dorlus | Surface Roughness Study
Bulk EP → Nitrided surface N-Doped 6 5 4 µm 3 2 1 0 Rp Rv Rz Ra Rq Rku Laser intensity image from LSCM Roughness parameters Roughness decreased by a factor of 2 Nitrides added on surface 16 8/4/18 Wydglif Dorlus | Surface Roughness Study
Coupon From Nb sheet Bulk EP 150 µm HPR Ethanol Rinse N Doping 2/6 Ethanol Rinse Ethanol Rinse HPR HPR HPR Ethanol Rinse BCP 5 µm EP 5 µm 17 8/5/18 Wydglif Dorlus | Surface Roughness Study
Nitrided surface + 5um EP à N doped surface EP Post N-Doping 4.5 4 3.5 3 2.5 µm 2 1.5 1 0.5 Laser intensity image from LSCM 0 Rp Rv Rz Ra Rq Rku Roughness parameters Roughness is roughly the same Nitrides removed off surface 18 8/4/18 Wydglif Dorlus | Surface Roughness Study
Coupon From Nb sheet Bulk EP 150 µm HPR Ethanol Rinse N Doping 2/6 Ethanol Rinse Ethanol Rinse HPR HPR HPR Ethanol Rinse BCP 5 µm EP 5 µm 19 8/5/18 Wydglif Dorlus | Surface Roughness Study
Nitrided surface + 5um BCP BCP Post N-Doped 16 14 12 10 µm 8 6 4 2 Laser intensity image from LSCM 0 Rp Rv Rz Ra Rq Rku Roughness parameters Roughness drastically increased; correlates with performance; doping + BCP gives very poor quench field 20 8/4/18 Wydglif Dorlus | Surface Roughness Study
Single Grains analysis Treatments do not affect grains significantly, but grain boundaries 21 8/4/18 Wydglif Dorlus | Surface Roughness Study
AFM contact mode image of nitrided surface Height Length Nitrides are 54 nm tall 22 8/4/18 Wydglif Dorlus | Surface Roughness Study
EDS using SEM Atoms get excited by electrons which release x- rays that get picked up by an EDS detector 23 8/4/18 Wydglif Dorlus | Surface Roughness Study
N doped surface (nitrides + 5 um EP) Surface got smoother, and nitrides disappeared EP AFM Image 24 8/4/18 Wydglif Dorlus | Surface Roughness Study
Nitrides plus BCP Rz = 680nm ≠ 13µm Not sure why…(maybe small secondary nitrides phases?) BCP AFM image 25 8/4/18 Wydglif Dorlus | Surface Roughness Study
Conclusion • Found opposite: Surface does NOT get rougher in N doping (EP) but seems to be actually smoother compared to Bulk EP • Surface gets rougher for BCP treatment à correlates with performance • Surface roughness (EP doped) is not a factor why Eacc is 30% less than 120C bake • BCP is not a good additional treatment option for doped cavities 26 8/4/18 Wydglif Dorlus | Surface Roughness Study
Acknowledgements I would like to thank the GEM Committee, my supervisor Anna Grassellino for advising me, and my mentors. 27 8/5/18 Wydglif Dorlus | Surface Roughness Study
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