MagnetoOptical Study of High Purity Nb for SRF
Magneto-Optical Study of High Purity Nb for SRF Application A. A. Polyanskii, Z. H. Sung, P. J. Lee, A. Gurevich, D. C. Larbalestier ASC, NHMFL, FSU, Tallahassee, USA SSTIN-2010, Jefferson Lab, New. Port News, Virginia, September 22 -24, 2010 This work was supported by the US DOE under grants DEFG 02 -07 ER 41451, FNAL PO 570362, and the Florida State
MOTIVATION • The recent studies showed that superconducting cavity performance is very sensitive to quality of Nb surface. • The penetration of small numbers of flux lines into the cavity surface during RF operation is leading to breakdown of the superconducting state and causing Qdrop or quench. • Q-drop can be triggered by early flux penetration in hotsports leading to thermal breakdown on the cavity surface (1 and 2). (Model by A. Gurevich). • Local study of flux penetration is important
MOTIVATION Magnetization and Magneto-Optical Imaging (MOI) techniques can detect local magnetic flux penetrate in Nb
EXPERIMENTALTECHNIQUE : 1. MAGNETO-OPTICAL 2. VSM 3. SEM 4. ZYGO LIGHT MICROSCOPY by Z. H. Sung (See: www. zygo. com/? /products/nv 6000)
PRINCIPILE OF MAGNETO-OPTICAL TECHNIQUE MAGNETO-OPTICAL INDICATOR ON BASE OF GARNET FILM DOPED BY Bi DETECTS MAGNETIC FIELD Polarized light F=V Bz 2 d GGG Ba, z YIG: Bi M Nb Ba, x d Protective layer Reflective layer
THE FARADAY ROTATION IN A FERRIMAGNETIC Bi. DOPED IRON GARNET INDICATOR FILMS WITH IN-PLANE MAGNETIZATION LIGHT POLARIZATION
MAGNETO-OPTICAL CONTRAST PICKS UP ON PLASTIC REFRIGIRATE CARD: DISTRIBUTION OF STRAY FIELD AROUND MAGNETIC STRIPS
MO IMAGES OF MAGNETIC FIELD IN AUDIO TAPE AND COMPUTER DISCETS 500 m Audio tape 100 m 5. 25 inch disc 3. 5 inch disc
MO IMAGES OF MAGNETIC CODES IN DIFFERENT PLASTIC CARDS (AAA) American Automobile Association Card 500 m Library Vendor Visa Credit Card
MAGNETO-OPTICAL IMAGE OF MAGNETIC SECURITY CODE IN US $100
MAGNETO-OPTICAL IMAGING SETUP FOR SUPERCONDUCTING RESEARCH: electromagnetic system creates magnetic field in two direction: X and Z Polarized Optical Microscope Hext Z Hext X LHe cryostat
MAGNETO-OPTICAL SETUP ON THE BASE OF A POLARIZING MICROSCOPE IN REFLECTIVE MODE
GEOMETRY OF MAGNETO-OPTICAL EXPERIMENT ON Nb SAMPLES FOR CAVITY APPLICATION IN PERPENDICULAR-Z AND PARALLEL-X (IN-PLANE) FIELDS MO indicator with in-plane magnetization detects the normal component of magnetic flux Z MO indicator Nb Nb Nb Hext X Hz=(1/1 -Nz) x Hext Demagnetization factor Nz>0
MAGNETIC FLUX DISTRIBUTION IN RECTANGULAR SAMPLE: EFFECT OF SAMPLE SHAPE WITH DEMAGNETIZATION FACTOR Nz>0. (“roof pattern” or “pillow”) 1 mm Maximum field enhancement at the center of each side. MOI of magnetic flux distribution in uniform square Nb sample
ZERO FIELD COOLED (ZFC) IN PERPENDICULAR FIELD: MO images in uniform rectangular and circle Nb samples (“roof pattern” or “pillow”) M SC MO images Calculated current streamlines 1 mm
FIELD COOLED (FC) IN PERPENDICULAR FIELD: MO images trapped magnetic flux in uniform rectangular and circle Nb samples SC MO images (“roof pattern” Calculated current streamlines
SAMPLES Thomas Jefferson Lab National Accelerator Facility (JLAB): • Nb samples were cut from 1. 8 mm thick large slice from the extremely large grain ingot fabricated by CBMM - Brazil for Jlab: GBs were randomly oriented • In case of cavities some planes of GBs could be parallel to direction of RF magnetic field Fermi National Accelerator Laboratory (FNAL): • Nb pure polycrystalline samples taken through a typical cavity optimization process and cut from regular and weld regions • Samples with artificial grooves and deformed samples • Samples with PIT in weld seam
JLAB samples with big grains were cut from extremely largegrain ingot Samples rectangular and circle shapes were cut from large grain 1. 8 mm thick material fabricated by CBMM-Brazil § Disc shape sample- to avoid flux penetration due to sample shape § Bi-crystal and Tri-crystal samples § Plane of GBs were perpendicular and random oriented to face of sample: some plane of GBs in cavity wall parallel to RF field
Nb slice was cut from ingot with big grains. Sample shapes: rectangular and circle (disc) with big variety of GBs Tri-crystal RF field in -plane Bi-crystal GB (#2) Angle to Surface GB #2 GB #1 Tricrystal Bi-crystal GB (#1) Normal to Surface Thickness of sheet is 1. 88 mm
GB #1 and GB #2 form triple-point. Plane of of GB #1 twisted in the vicinity of the triple point: Orientation of RF field in cavity may be parallel the plane of any GBs. Samples were taken from different part of GB#1 and GB#2 and in triple-point Top face #9 Overlap top and bottom surfaces of Nb sheet with traces of GBs, Sample #9 Bottom face
Perpendicular and random angle orientation GBs in bi- and tri-crystals samples rectangular and round shapes bi-crystal GB #2 has angle about 30 -35 degree to wide face of sample bi-crystal GB#1 perpendicular to wide face of sample
MO imaging sample on different surface: when GB - 330 angle and after 900 rotation - GB perpendicular to surface GB trace on top face of sample No GB trace on side face This face has been imaged by MO, when sample was turned by 900 H MO indicator GB trace on bottom face of sample 2. 1 7 mm 1. 89 mm 2. 78 mm
Bi-crystal #6 with the 330 angle GB #2 to surface, no remarkable flux penetration along GB Optical, GB#2, angle to surface 330 H=80 m. T T=5. 4 K 1 mm H=100 m. T
MO Image and current distribution in sample with perpendicular plane , ZFC, T=5. 5 K. 170 GB #1 admits magnetic flux and shows obstacle to current flow: distortion of current stream lines 17. 80 GB GB #1 1 mm T=5. 5 K H=80 m. T Current streamline : GB is obstacle to current flow Jb=0. 5 Jc
Misorientation angle between two grains ≈17. 8 o Orientation Imaging Microscopy (OIM): from D. Abraimov 17. 80 GB Misorientation profile
Critical current vs T : bulk Jc and Jb across 17. 80 GB (from MO measurement) Nb 17. 80 GB T=5. 5 K Jb ~ 0. 5 Jc YBCO 50 GB T=7 K Polyanskii, at el, Jb ~ 0. 5 Jc A. A. Phys. Rev. B, 53, 8687, (1996).
MAGNETIC FLUX BEHAVIOR AROUND 3 o, 5 o, and 10 o GRAIN BOUNDARIES in YBCO bi-crystals T = 7 K, H ~ 40 m. T, H||c 3 o GB 10 o GB 5 o GB Jb/Jc =0. 95 A. A. Polyanskii, at el, Phys. Rev. B, 53, 8687, (1996). Jb=0. 95 Jc Jb=0. 065 Jc
After 900 rotation GB is perpendicular to surface. GB is a weak link in ZFC and FC. T=6 K 1 mm GB#2 #7 H=24 m. T #8 H=26 m. T #9 H=28 m. T GB#2 #11 H=32 m. T #13 H=40 m. T #23 H=0 FC T=6 K
GB perpendicular to surface: Top and bottom surfaces of Nb sample after electro-polishing (EP) has different roughness TOP GB is not visible Top surface. After Mechanical polishing + Electropolishing BOTTOM GB hardly visible Bottom surface. After diamond saw + Electropolishing
Nb sample with GB perpendicular to surface. MO image of top and bottom surfaces of Nb sample after (EP): good flux penetration on both surfaces TOP Current streamline on top side: GB is obstacle to current flow BOTTOM ZFC H=60 m. T T=6. 5 K
ROUND JLAB SAMPLE, with random oriented GBs in triple-point Top face Bottom face
Tri-crystal #9 fully processed: 5 steps, top face, no preferential flux penetration along GBs, but flux penetrates faster in some additional local places ? ? GB #1 GB #2 Optical, top face. Shape is not perfectly round. GB #2 #81 H=80 m. T, ZFC 1 mm T=5. 5 K
JLAB #11 with GB #2 with angle 30 -350 , ZFC T=5. 6 K. No flux penetration along GB #2. Flux penetrates in some additional places much faster. What is a reason? ? GB #2 Surface, bottom face 1 mm #33 H=86 m. T
Tri-crystal #12, fully processed: 5 steps, top face, no preferential flux penetration due to GBs. Asymmetric flux penetration GBs Optical, top surface #15 ZFC H=72 m. T
Nb tri-crystal JLAB #4: GBs with random orientation, thickness is 1. 88 mm Optical, Surface of tri-crystal 1 mm 3 D MODEL of GBs on the base GB traces on top and bottom. Random orientation (Peter Lee)
Nb tri-crystal JLAB #4: no penetration at to GB with random orientation, no evidence of weak link, but traces of GBs are visible Visible traces of GB 1 mm T=5. 5 K H=120 m. T Remn T=7 K H=0 after H=24 m. T
MO conclusion on Jlab samples with big grains 1. GBs can accept magnetic flux in case when plane of GB is parallel to external magnetic field 2. Some additional flux penetration has been found on surface of many fully processed samples.
SAMPLES FROM FNAL samples-square shape 3. 75 x 1. 5 mm were cut from the same 2. 8 mm thick sheet (RRR~ 450) but in two different places: • Samples from fine-grained Nb sheet (regular), where grain sizes were small ~ 50 m, • Samples from weld region, where grain sizes were big > 1 mm • Both types of samples taken through a typical cavity optimization process: • The processing sequence includes 5 steps: • (1) cold work produced by the sample machining process, followed by degreasing, • (2) ~100 m BCP etch, • (3) HT 5 hours at 750°C in a vacuum < 10 -6 Torr, • (4) ~20 m BCP etch • (5) bake 50 hours at 120°C in a vacuum < 10 -6 Torr
SOME EXAMPLE: OPTICAL IMAGES OF Nb samples cut feom REGULAR AND WELD regions 1 mm REGULAR AREA: small grain size WELDED AREA: large grain size Machine marks (like grooves), large grains and steps at GBs are well visible
VSM Magnetization of Regular and Weld samples taken through a typical cavity optimization process: 5 steps. Magnetization hysteresis is significantly reduced after step 4, the large reduction Hc 2 and the small increase in the first field of flux penetration Regular: small grains Weld: big grains
MOI of Regular samples with small grains taken through a typical cavity optimization process: 5 steps 1. CW, degrease 2. 100 µm etch 3. HT 5 hr/7500 C 4. 20 µm etch 5. Bake 50 hr/1200 C 60 м. Т 30 м. Т 34 м. Т 40 м. Т 80 м. Т 34 м. Т 40 м. Т 48 м. Т Optical ZFC Then H applied FC in 110 m. T, then H=0 applied 1 mm
MOI of Weld samples with big grains taken through a typical cavity optimization process: 5 steps 1. CW, degrease 2. 100 µm etch 3. HT 5 hr/7500 C 4. 20 µm etch 5. Bake 50 hr/1200 C 60 м. Т 48 м. Т 36 м. Т 31 м. Т 80 м. Т 40 м. Т 48 м. Т Optical ZFC Then H applied FC in 110 m. T then H=0 H applied 1 mm
We have observed unusual central flux penetration in weld sample after HT and 20 min BCP (second etching), step 3, 4 Details of flux penetration
Details of a flux nucleation on surface weld sample in perpendicular field: demagnetization factor of sample Nz 0 Optical, machinery marks (grooves) and large grains are well visible Enhancement of magnetic flux at surface defect 32 m. T T=5. 6 K 1 mm Hz Hx
Details of a flux nucleation on surface W 2 -5 in perpendicular field: Expansion of central flux penetration vs H ZFC T=5. 6 K #27 H=52 m. T 1 mm #28 H=54 m. T
Details of a flux nucleation on surface W 2 -5 in perpendicular field: Farther expansion of central flux penetration in weld sample ZFC T=5. 6 K Flux profile taken across sample #30 ZFC H=56 m. T 1 mm
Surface weld sample and profile: “dome” shape. Good correlation with MO central flux penetration. Form of penetrated flux correlates with surface shape T=5. 6 K 48 m. T Surface profile taken along marked line (ZYGO microscope). Max step height 1 mm is about 10 m
Topological defects on surface weld sample • MO STUDY IN IN-PLANE FIELD. • OPTICAL STUDY THE SURFACE OF WELD SAMPLE BY USING ZYGO LIGHT MICROSCOPE (See: www. zygo. com/? /products/nv 6000) (by SUNG).
MO contrast in in-plane field on topological defects, when field changes direction. MO contrasts are different on different defects: double and monochrome. Steps at GBs have only monochrome contrast H Machinery marks H have double MO contrast (black and 1 mm white) H H in-plain =60 m. T
Enhancement of normal component Hz on steps at GBs, well visible on profile by Zygo microscope H in 1 4 2 3 H in 1 2 3 5 5 2 In-plane field 60 m. T, T=5. 7 K 1 mm 4 3
MECHANICAL POLISHING AND BCP ON WELD SAMPLE Mechanical polishing with 0. 05 m Alumina Suspension BCP condition -HF(49%): HNO 3(69. 5%): H 3 PO 4(85%) = 1: 1: 2 -Temp; ~13˚C, Time; ~7 min
Weld sample after mechanical polishing with 0. 05 m Alumina Suspension and BCP Before mechanical polishing After mechanical polishing: traces of GBs are still visible, but machinery marks (grooves) disappeared After BCP: grains and GBs are well visible, but machinery marks (grooves) disappeared
Zygo light microscope images and surface profiles of weld sample before and after mechanical polishing with 0. 05 m Alumina Suspension and BCP Before mechanical After mechanical polishing After BCP
Weld sample after mechanical polishing and BCP Mechanical polishing H=60 m. T BCP H=60 m. T
SEM on BCP weld sample : BCP produces steps at GBs. As longer BCP as bigger steps TOP FACE Nb O Al Si 41. 29 % : 56. 73% : 1. 62% : 0. 36%
Our study found influence of topological features which are on the surface of fully processed Nb weld sample: • The second chemical etch (BCP) creates steps at GBs (10 -15 m ) and cause the surface to “dome” shape. • Concentration of magnetic flux much stronger at surface steps than at grooves due to different sizes. The surface steps may be are one of the reasons the hotsport on the cavity surface, where nonuniform thermal breakdown actually happened. • Sample with demagnetization factor Nz>0 in perpendicular field deforms external magnetic field and creates in-plane components. In-plane components concentrate magnetic flux at GB steps and ignites unusual flux penetration into the center of weld Nb. The “dome” surface contributes the propagation flux toward the edge. • Mechanical polishing and BCP removed steps and “dome” surface and restored classical flux behavior.
JLAB single crystal with artificial defect (groove) on the surface 0. 5 mm Depth of notch is not the same on both edges 0. 5 mm Surface, artificial notch
Artificial defect (groove) has small impact on flux distribution ZFC H=40 m. T T=7 K Remn H=80 m. T T=7 K
MO images of trapped flux in Nb discs deformed by compression at T=6 K (samples deformed at Fermi Lab and received from A. Romanenko) no deformation 1 mm 35% deformation 46% deformation
Pit in weld seam (Fermi from L. Cooly) Sample for MO Weld Pit 1 Slice cut from welding area with PIT for MO Pit 1 contour map: “moat” around the peaks 1 mm
MO images in PIT area: obviously more stronger resistivity to flux penetration Pit 1 in weld seam Optical, crosssection area with PIT #20 T=7 K ZFC H=40 m. T #27 T=7 K ZFC H=100 m. T #28 T=7 K H=0 Remn after H=100 m. T 1 mm
SUMMARY • MOI is a sensitive tool to check superconducting property of Nb for cavity application • GBs can admit magnetic flux and depress superconductivity, if magnetic field parallel to plane of GBs • GBs do not admit magnetic flux in case of random orientation of GB. • Some additional flux penetration has been found on surface of many fully processed samples. We do not know the reason. It will takes some more study. • In-plane orientation of magnetic field: detect enhancement of magnetic field on topological defects • Topological defects stimulate the nucleation of magnetic flux in Nb and may depress superconductivity and Q-drop can be triggered by early flux penetration
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