Material studies on HighT Doping Mrton Major and

Material studies on High-T Doping Márton Major and Ruben Grewe Technische Universität Darmstadt, Germany 05. 02. 2020 | TTC 2020 | CERN | Márton Major

Possible superconducting materials for RF cavities Material Critical Temp. Tc [K] Normalstate resistivity ρn [μΩcm] Critical Field Hc(0) [T] Lower Critical field Hc 1(0) [T] Upper Critical field Hc 2(0) [T] Penetration depth (0) [nm] [me. V] Type Nb 9. 23 2 0. 18 0. 28 40 1. 5 II Pb 7. 2 0. 08 N/A 48 Nb. N 16. 2 70 0. 23 0. 02 15 200 -350 Nb. Ti. N 17. 3 35 Nb 3 Sn 18 20 V 3 Si 17 Mo 3 Re 15 Mg. B 2 40 YBCO Pnictides 0. 03 0. 54 0. 05 I 2. 6 150 -200 II, B 1 comp. 30 80 -100 3. 1 II, A 15 24. 5 179 II, A 15 0. 43 0. 03 3. 5 140 0. 43 0. 03 3. 5 -60 140 2. 3/7. 2 II- 2 gaps 93 1. 4 0. 01 100 150 20 d-wave 30 -55 0. 5 -0. 9 30 >100 200 10 -20 0. 1 -10 Whistler 2015, SRF Tutorial - Pushing Nb Performance - A. -M. Valente-Feliciano 05. 02. 2020 | TTC 2020 | CERN | Márton Major | 2

Possible superconducting materials for RF cavities Material Critical Temp. Tc [K] Normalstate resistivity ρn [μΩcm] Critical Field Hc(0) [T] Lower Critical field Hc 1(0) [T] Upper Critical field Hc 2(0) [T] Penetration depth (0) [nm] [me. V] Type Nb 9. 23 2 0. 18 0. 28 40 1. 5 II Pb 7. 2 0. 08 N/A 48 Nb. N 16. 2 70 0. 23 0. 02 15 200 -350 Nb. Ti. N 17. 3 35 Nb 3 Sn 18 20 V 3 Si 17 Mo 3 Re 15 Mg. B 2 40 YBCO Pnictides 0. 03 0. 54 0. 05 I 2. 6 150 -200 II, B 1 comp. 30 80 -100 3. 1 II, A 15 24. 5 179 II, A 15 0. 43 0. 03 3. 5 140 0. 43 0. 03 3. 5 -60 140 2. 3/7. 2 II- 2 gaps 93 1. 4 0. 01 100 150 20 d-wave 30 -55 0. 5 -0. 9 30 >100 200 10 -20 0. 1 -10 Whistler 2015, SRF Tutorial - Pushing Nb Performance - A. -M. Valente-Feliciano 05. 02. 2020 | TTC 2020 | CERN | Márton Major | 3

Brauer, J. Less-Common Met. 2 (1960) 131 Phase a: Nb-N solid solution, bcc b: Nb 2 N, hexagonal g: Nb 4 N 3, tetragonal d: Nb. N, cubic Na. Cl type : Nb. N, hexagonal Tc Temperature (°C) Nb–N phase diagram 9. 2 K 9. 5 K 12. 2 K 16. 5 K (17. 3 K - film) < 1. 2 K (11. 6 K*) Springer Handbook of Condensed Matter and Materials Data (W. Martienssen, H. Warlimont, Eds. , 2005) *Zou et al. , Sci. Rep. 6 (2016) 22330 05. 02. 2020 | TTC 2020 | CERN | Márton Major | 4 Layout of the Nb-N phase diagram ( ) Brauer and Kern, Z. anorg. Allg. Chem. 507 (1983) 127 Musenich et al. , J. Alloys Compd. 209 (1994) 319

Nb–N phase diagram Lengauer et al. , Acta mater. 48 (2000) 2633 05. 02. 2020 | TTC 2020 | CERN | Márton Major | 5

High temperature UHV furnace Ø Built in 1983 in Wuppertal by G. Müller Ø Now located at S-DALINAC Ø Two vacuum systems Ø Insulating vacuum (green) Ø Hot-pot vacuum (red) Ø < 10 -4 mbar at 1750 ˚C Ø Hot pot and heat shield made of niobium Ø Heaters made of tungsten Ø 40 k. W power supply Ø Water cooled 05. 02. 2020 | TTC 2020 | CERN | Márton Major | 6

Niobium samples (source: RI) 5 x 5 x 2. 7 mm 3 (Also 10 x 2. 7 mm 3) 05. 02. 2020 | TTC 2020 | CERN | Márton Major | 7

Niobium nitride, first process in “Wuppertal” oven 05. 02. 2020 | TTC 2020 | CERN | Márton Major | 8

Nb. N new procedure (constant N-flow) 05. 02. 2020 | TTC 2020 | CERN | Márton Major | 9

SIMS of N-doping (IKP) new procedure Nitrogen Oxygen 05. 02. 2020 | TTC 2020 | CERN | Márton Major | 10

XRD of N-doped Nb samples (IKP, new procedure) Int. (a. u. ) b-Nb 2 N a-Nb Int. (a. u. ) d-Nb. N 2 (deg) 05. 02. 2020 | TTC 2020 | CERN | Márton Major | 11

Nb 220 Nb 310 Nb 211 Nb 200 Nb 110 X-ray diffraction: phases and texture 05. 02. 2020 | TTC 2020 | CERN | Márton Major | 12

Nb 220 Nb 211 Nb 200 Nb 110 Int (a. u. ) 2 (deg) 05. 02. 2020 | TTC 2020 | CERN | Márton Major | 13 Annealed in UHV at approx. 1200 o. C (nominal) for approx. 1 h

Pole figures

T 1 (Not polished Nb): Pole figures of 200 Bragg-peak Virgin After annealing (1200 °C 1 h) Log colour scale 05. 02. 2020 | TTC 2020 | CERN | Márton Major | 15

2 N 180005 -2: 1 h 100 mbar N 2, 1550 °C Nb 200 reflection (2 = 55. 7°) Linear colour scale 05. 02. 2020 | TTC 2020 | CERN | Márton Major | 16 Log colour scale

2 N 180005 -2: 1 h 100 mbar N 2, 1550 °C Nb. N-111 reflection (2 = 35. 67°) Linear colour scale 05. 02. 2020 | TTC 2020 | CERN | Márton Major | 17 Log colour scale

X-ray diffraction texture check 1550 °C, 100 mbar, 10 min a-Nb 200 Int. (a. u. ) d-Nb. N 200 b-Nb 2 N 100 2 (deg) 05. 02. 2020 | TTC 2020 | CERN | Márton Major | 18

X-ray diffraction texture comparison 1550 °C, 100 mbar, 10 min a-Nb 200 peak (2 = 55. 7°) b-Nb 2 N 100 peak (2 = 33. 85°) 05. 02. 2020 | TTC 2020 | CERN | Márton Major | 19 d-Nb. N 200 peak (2 = 41. 15°)

XRD of N-doped Nb samples (IKP, new procedure) 1450°C 50 mbar 2 min 1550°C 100 mbar 10 min 1550°C 100 mbar 30 min 1550°C 100 mbar 60 min 05. 02. 2020 | TTC 2020 | CERN | Márton Major | 20

Conclusions and plans Conclusions Bake-out in vacuum and N-doping decreased hydrogen level. Ndoping increased N content. New procedure (continuous N 2 flow in Wuppertal oven), resulted in Nb-N phases, but not phasepure d-Nb. N yet. Recrystallization of Nb at high temperature treatment, texture change for other Nb-N phases. Future Plans Continue N-doping of Nb test samples in Wuppertal oven, optimize conditions (temperature, nitrogen pressure) with help of structural investigations. Treat 3 GHz single cell cavities under optimized conditions. Measure quality factor of the single cell cavities before and after N-doping in the vertical bath cryostat (TU Darmstadt IKP). SRF’ 19 MOP 022 & MOP 028 05. 02. 2020 | TTC 2020 | CERN | Márton Major | 21

Acknowledgement Lambert Alff Matthias Mahr Norbert Pietralla Jens Conrad Michaela Arnold u o y k n a Th Stefan Flege ! n o i t n e t t a r u o y r o f Funded by BMBF Projects 05 H 15 RDRBA and 05 H 18 RDRB 2 05. 02. 2020 | TTC 2020 | CERN | Márton Major | 22 Florian Hug

Backup slides 05. 02. 2020 | TTC 2020 | CERN | Márton Major | 23

Quality factor measurement 05. 02. 2020 | TTC 2020 | CERN | Márton Major | 24

Quality factor measurement Variable input coupler 05. 02. 2020 | TTC 2020 | CERN | Márton Major | 25

Quality factor measurement on single cell cavity 05. 02. 2020 | TTC 2020 | CERN | Márton Major | 26

Quality factor measurement (baseline) Q 0 T = (2. 1 0. 1) K T = (2. 0 0. 1) K Eacc (V/m) 05. 02. 2020 | TTC 2020 | CERN | Márton Major | 27

XRD (comparison) Inetnsity (a. u. ) b-Nb 2 N g-Nb 4 N 3 30° 40° 50° 60° 2 70° 80° 90°

Recrystallization of Nb Fig. 1. Plot of Vickers Hardness Number HV, against percentage reduction in thickness for cold-rolled niobium. TM = 2477 °C Fig. 2 a. Plot of hardness against annealing temperature for niobium deformed by 94% and annealed for five hours. Recrystallization starts ~ 590 °C, maximum ~ 630 °C (0. 32 TM) Thompson and Flewitt, J. Less-Common Metals, 21 (1970) 456 05. 02. 2020 | TTC 2020 | CERN | Márton Major | 29

Sample holder Sample After some cycles Virgin 05. 02. 2020 | TTC 2020 | CERN | Márton Major | 30 Pulverised after 10 th N-cycle

Nitridation: 1500°C, 1 bar N 2, 1 h M. Mahr, bachelor thesis 05. 02. 2020 | TTC 2020 | CERN | Márton Major | 31

Optical metallography on cutout of sampleholder M. Mahr, Bachelor thesis (2017) Lengauer et al. , Acta mater. 48 (2000) 2633 05. 02. 2020 | TTC 2020 | CERN | Márton Major | 32

Nb. N coating of SRF cavities Sample T (˚C) P (mbar) t (h) Tc (K) 1 1427 40 3. 5 13. 1 2 1427 130 3 14. 6 3 1427 200 1. 5 14. 3 4 1427 200 3 14. 9 5 1727 130 3 15. 6 Table. Reaction conditions for the niobium nitride samples. The cavity was nitrided together with the sample No. 2. Test samples 0. 1 mm thick, cavity wall 1 mm thick. Furnace: T up to 2000 ˚C, pressure: UHV (10 -8 mbar) – atmosphere. Cleaning: 2 h UHV @ 1427 ˚C, then nitridation (Table). The resulting nitrides have a light gold colour and are very hard and brittle. 05. 02. 2020 | TTC 2020 | CERN | Márton Major | 33 No d-Nb. N was found only g, and phases with lower N content (a, b). High residual resistance. Possible cause: columnar growth of nitrides with amorphous material between the grains. Seen previously for sputtered Nb. N films on stainless steel [Wagner et al. , J. Appl. Phys. 45 (1974) 465] Fabbricatore et al. , J. Appl. Phys. , 66 (1989) 5944