Magneto-Optical (MO) and SEM/FIB characterization of IBAD-MOCVD tapes for cable and magnet applications

Transcription

1 Magneto-Optical (MO) and SEM/FIB characterization of IBAD-MOCVD tapes for cable and magnet applications A.A. Polyanskii, D.V. Abraimov, Y. Xin and D.C. Larbalestier ASC, NHMFL, FSU, Tallahassee, FL USA Y. Zhang, SuperPower Inc., Schenectady, NY USA A.K. Ghosh, NNL, Upton, NY USA MEM-2016/03/23

2 Motivation Development of coated conductor tapes is focused on increasing their length and improvement of electromechanical performance We need to know, which manufactured defects can be produced during technological process and their impact on electromagnetic characteristics of coated conductors We know that quench and damage can happened even in tapes that passed quality control procedures and were used to manufacture different cables and magnets 4 mm Burn area MOI of a burn section of CC tape after a few quenches

3 SAMLES and TECHNIQUE SAMPLES: Coated Conductor REBCO tapes manufactured by SuperPower Characterization and Testing techniques: 1.! Magneto-optical imaging 2. SEM 3. Tensile deformation 4.! Transport measurements

4 Architecture of CC REBCO tape REBCO (1-1.5 µm Hastelloy Substrate

5 Geometry of Magneto-Optical Imaging MO garnet indicator H Ar layer Hastelloy/Substrate REBCO layer Cu layer Advantage of Magneto-optical Imaging: 1. Non-distractive 2. Observation of REBCO structure through copper (Cu) and silver (Ag) layers

6 MOI: typical uniform coated conductor REBCO tape from SuperPower - no macroscopic defects a) b) ZFC, T=77K 4 mm FC, T=77K FC: Tape in Critical state (Bean Model) Current -uniform flow

7 Slitting of 12 mm wide tape creates significant damage along edges of 4 mm tapes and reduced cross-section for transport current (typical for all tapes) Damage in buffer along virgin edges and cracks after slitting are the origin of a-axis grains in REBCO I I Slitting direction I 4 mm 4 mm 4 mm

8 Cracks after slitting are common defects for all REBCO tapes Typical for all old tapes: µm Cracks in 4 mm wide REBCO tape after slitting from 12 mm virgin tape

9 Damage and a-axis grains on virgin tape edge reduce REBCO cross-section OPTICAL on buffer layer: manufactured defects (voids) are an origin of a-axis grains Slitting edge with cracks REBCO a-axis grains 2 mm MOI 60 µm MO image of 2 mm wide tape: flux penetrates along a-axis grains virgin edge SEM on REBCO: a-axis grains along virgin edge Voids in buffer virgin edge: no cracks

10 Slitting creates cracks on tape edge and reduces REBCO cross-section Slitting edge with cracks: significant improvement 10 µm REBCO 50 µm Buffer SEM on REBCO: cracks on tape edges Optical on Buffer layer: cracks on tape edges

11 a-axis grain lines in REBCO in rolling direction (). Grooves in buffer layer. Voids in substrate MOI Groove in ReBCO/Buffer interface Top view of FIB trench 1 mm 2 µm SEM: REBCO Optical: Grooves in buffer 2 µm SEM: a-axis grains 1 µm Voids in substrate

12 Long longitudinal defects in 4 mm wide tape. Origin of ab-grains: scratches in buffer layer SEM: a-axis grains 4 mm 3 µm MO image of 4 mm wide tape: flux penetrates along a-axis grains SEM of REBCO: a-axis grain line scratches in buffer 50 µm Longitudinal defect are crucial for ROEBEL cable

13 Short longitudinal defects in tape in direction. Scratches and cleavage in buffer are origin a-axis grains f) REBCO FIB REBCO 100 µm Short longitudinal defects (scratches) a-axis grains 10 µm Optical voids cleavage 2!m

14 REBCO: Multiple longitudinal defects different length and width. Intensity of MOI is vary on different types of defects 4 mm Optical, REBCO Overlap: REBCO/MOI

15 REBCO: a-axis grain lines. Buffer layer: scratches, splitting and bending in buffer layer on SEM images REBCO 50 µm buffer Splitting in buffer 1 µm FIB 1 µm 10 µm a-axisgrains Bending in buffer

16 REBCO: Transverse defects are a-axis grain lines. They reduce tape cross-section Area of MO flux penetration REBCO: transverse defects 1 mm Overlap REBCO and MOI

17 REBCO: Transverse defects are a-axis grain lines. Buffer: scraches, bending and disruption Area of MO flux penetration REBCO: ab-grain line Buffer-transverse scratches SEM/FIB CROSS-SECTION: damage in buffer FIB 1 µm Bending 2 µm 50 µm Disruption

18 MOI: ellipsoid shape defect in 12 mm wide tape Area of MO flux penetration 12 mm defect 1 cm

19 REBCO: a-axis grains. Buffer: strong damaged area a-axis grains a-axis grains 500 µm REBCO REBCO 10 µm buffer: strong damage 50 µm

20 Buffer layer: longitudinal lines of cracks parallel tape rolling direction MOI Buffer layer 1 mm 0.5 mm

21 REBCO: MO image of flux penetration. Surprise-no longitudinal ab-grains line. Buffer: cracks in REBCO/buffer interface MOI 1 mm Tape axis REBCO Cracks only in REBCO/ Buffer interface 10 µm REBCO: no cracks Top view of FIB window on REBCO 10 µm Top view of FIB window on REBCO/buffer

22 FIB trench crosses cracks in buffer 50 µm REBCO 10 µm REBCO Cracks in buffer

23 Most cracks locate in buffer, than in substrate Buffer SEM: Top view Buffer Area of MO flux penetration Substrate 2 µm SEM/FIB : cross-section Buffer Substrate 1 µm

24 Formation cracks in coated conductor REBCO tape REBCO Buffer REBCO/Buffer Interface Substrate

25 Tolerant test of 12 mm tape. Tensile deformation: MOIno damage between 0.0% and 0.8% deformation Tension load 12 mm MO MO MO Tension load Sample for MOI Tensile deformation: 0.0% - original virgin tape 0.6% -no damage 0.8% -no damage 1.0% - cracks are well visible on MO image 1.5% -cracks are well visible on MO image

26 MO images of REBCO tape with line of flux penetration SEM image of REBCO: no peculiarities in REBCO Original tape: no deformation 12 mm MOI Strong defect SEM of REBCO: no peculiarities

27 Cracks in buffer layer, where magnetic flux penetrates Original tape: no deformation

28 Delamination in REBCO Original tape: no deformation

29 Defects in REBCO Original tape: no deformation mm 1 2

30 Defects in Buffer mm

31 MO images of flux penetration in 1.0% and 1.5% tensile deformed REBCO 12 mm wide tapes 12 mm 1 mm 1.0% tensile deformation 1.5% tensile deformation

32 Buffer layer: Transverse cracks after 1.0% and 1.5% tensile deformed 12 mm wide tapes 100 µm 1.0% tensile deformation 1.5% tensile deformation

33 MOI: damage across 4 mm wide RECO tape after quench REBCO: manufactured scratches and cracks after quench 4 mm Defected line after quench scratches Cracks after quench Cracks after quench 1 mm in REBCO 0.5 mm Optical 5 µm SEM REBCO

34 REBCO: cracks in damaged area after quench significantly reduced tape cross-section and I c 1 µm I c

35 Conclusion Key points: 1.Quality of buffer layers are very important for REBCO tapes 2. Magneto-optical technique is a very sensitive tool and in combination with SEM plays a large role in controlling properties of coated conductor tapes. 3. MOI often shows the existences of different obstacles to critical current that are not visible on REBCO surface in the light microscope and even in SEM