11 Flux pinning study of YBa 2 Cu 3 O 7-  coated conductors – ideas for performance enhancement Aixia Xu Department of Mechanical Engineering Applied Superconductivity Center National High Magnetic Field Laboratory Florida State University

22 Outline Background and Motivation Experimental methods Preliminary results Future work

33 YBCO is the only superconductor for application above 77K. Highest H irr Larbalestier et al., 2001 Nature 414, 368 YBCO is the best material for high field magnet applications YBCO superconductor brings up the application of >30T all-superconducting magnet. Highest J c Courtesy of P. Lee at MagLab

44 YBCO coated conductor RABiTS Rolling assisted biaxially textured substrate Provided by AMSC Ion beam assisted deposition Provided by SuperPower <0.1mm

5 High field magnet application of HTS Greg Boebinger presented at 2009 user summer school Sumitomo/MIT Hitachi/NIMS NHMFL/OST Yamada BSCCO YBCO NHMFL 33.8T SuperPower 26.8T

66 High I c requirement of high field magnet  I c is strongly anisotropic in background magnetic fields YBCO layer  Although magnetic field is generally in ab-plane, it is tilted at the ends of the magnet. This limits its performance. High I c is important in the whole  range!

77 How to increase I c Thickness (t) Width (w) Length (l) IcIc Approach I: Enhance Jc  Flux pinning study Approach II: Increase thickness of YBCO layer  Thickness dependence of Jc

8  4.9 //ab//c  11 J c is enhanced significantly by second phase addition. Different doping results in different Jc angular dependence Approach I- J c enhancement at high temperature and low field Harrington et al., 2009 Supercond. Sci. Technol. 22, Feldmann et al., HTS peer review 2009, August 4-6, Alexandria VA 77K/75.5K 1T

99 Motivation I  J c (  ) at low temperature and high fields  What is J c (  ) of YBCO coated conductors  High field magnet application It is important for high field magnet application to cool YBCO coated conductor down to low temperature YBCO CC work in high background magnetic field J c (  ) is an key parameter for magnet design  Flux pinning study J c (  ) is a powerful tool for flux pinning study Theoretical and experimental work has shown that pinning mechanism is significantly different from that at high temperature At very high field, the vortex density may be higher than the density of strong pinning centers.  what are effective pins at low temperature and high magnetic fields?  How to modulate Jc(  ) to meet the requirement of high field magnet? Increase of thermal fluctuation Gurevich, Supercond. Sci. Technol S128 Gutierrez J et al 2007 Nature Mat. 6, 367

10 However, some recent data show almost thickness independent Jc 10 Usually, Jc decreases with thickness in YBCO coated conductor. Some data show t -1/2 like dependence J c (t). Approach II-thick film growth

11 Motivation II  J c (t) study  Can we suppress thickness dependence of J c ? Indeed, there is theoretical model predicting t-independent Jc in the 3D strong pinning regime. Gurevich et al., HTS peer review 2004, July 27-29, Washington DC  How can we obtain effective pins to keep high J c through thickness in thick films?

12 Outline Background and Motivations Experimental methods Preliminary results Discussion Future work

13 Thin film growth and coated conductors J c (t) studyFlux pinning study J c (H, T,  ) measurement Ion milling I c <0.1A I c >0.1A Microstructure analysis Sample growth J c (H,  ) measurement at high temperature J c (H,  ) measurement at low temperature and very high fields

14 Outline Background and Motivations Experimental methods Preliminary results –MOD RABiTS from AMSC – mainly J c (t) –Many SuperPower IBAD-MOCVD – mainly J c (H,  ) at high and low T and high and low H –PLD, ASC-grown thin films to address intrinsic pinning issue Discussion Future work

15 sample t (  m) T c (K)J c (MA/cm 2 ) 77K, sf H irr (T)F p max (GN/m 3 )Substrate FSU RABiTS Preliminary result- J c (t) study of MOD-RABiTS J c is independent on thickness except close to buffer T c and H irr is independent on thickness except close to buffer

16 TEM image of MOD/RABiTS YBCO sample Surface roughness associated with the MOD film process. (rms = 84nm) Interfacial roughness due to localized reactions with the CeO 2 (BaCeO 3 ). (rms = 17 nm) Surface roughness + Voids + Interfacial roughness uncertainty of thin layer thickness TEM by Terry Holesinger voids containing in the sample through thickness.

17 Strong flux pinning centers MOD/RABiTS YBCO sample 1.High density of RE 2 O 3 precipitates are strong 3D effective pinning centers pushing Jc into thickness independent regime. 2.Voids, threading dislocations are effective pinning centers for Jc enhancement. 3.high density stacking faults are major correlative pinning centers that responsible to Jc enhancement along ab- plane Void SF (black line) (Y,Dy) 2 O 3 Threading dislocation TEM by T. Kametani

18 Summary of J c (t) MOD-RABiTS study  J c is independent of thickness except close to the buffer layer. T c and H irr show the same thickness dependence as Jc  The high density of RE 2 O 3 precipitates is expected as the source of strong 3D pinning which makes the high thickness-independent J c for the top YBCO layer  The degradation of J c near the interface is not fully understood. In former times, MOD-RABiTS had the inverse of this behavior, good near the bottom and degrading near the top. The conversion process for the MOD-TFA route is complex and kept private from us. The key point is that it IS possible to have a high thickness- independent Jc  The data is consistent with strong 3D pinning models Alex Gurevich et al., HTS peer review 2004, July 27-29, Washington DC Gurevich, Supercond. Sci. Technol S128

19 Sample details Thickness (  m) Tc(K) Jc (MA/cm 2 ) 77K, sf Hirr (T) 77K, //c comment Standard-2.1  m (Y,Gd)BCO RE 2 O 3 and SFs BZO-1.5  m YBCO/BZO RE 2 O 3 and SFs BZO nanorods Double layer BZO-0.9  m M MS (Y,Gd)BCO/ BZO Standard-1.2  m M3-674 FS GdBCO PLD-LTGYBCO/STO SF only as visible pins PLD-HTGYBCO/STO Only ppts as visible pins J c study at low T and high H SuperPower versus my PLD samples Goal: contrast SP samples with many stacking faults with SF-free samples to better understand ab-plane peak Representative samples

20 J c (  ) at 77 K 1 T of SP samples Different samples shows different J c (  ) at high temperature and low field. Strong pins dominate the high temperature pinning because strong thermal fluctuations are present. Slight rewording BZO-containing samples show high J c around c-axis.

21 J c (H) of SP at 4.2K J c is almost independent on the field when field is in ab- plane J c is suppressed for H > 20T because of LHe levitation in a strong field gradient. J c is thickness dependent. Thin film shows higher Jc. The in-plane Ic at 20 T for 4 mm wide CCs is 1.2kA and 1.4kA for double layer and standard-2.1  m sample, respectively, values quite high enough for magnet applications. Sample BZO-0.9  m shows highest I c below 4T even though its thickness is only 1  m. BZO sample decrease faster with the increasing of field.

22 Jc (  ) of SP at 4.2K At 1T, both samples show broad maximum when magnetic field is around ab-plane. Raising the field from <5 T to 30 T at 4 K causes a marked transition from a broad maximum to a marked cusp-like behavior. No measureable c- axis peak is observed BZO-1.5  m show broader Jc(  ) around ab-plane Xu et al., Supercond. Sci. Technol. January 2010

23  is the effective electron mass anisotropy parameter J c (  ) at 1 T follows very well the G-L model. Random pins are dominant. At higher fields, Jc(  ) is failed to fit GL scaling, which strongly suggests that correlative pinning is dominant at high fields around ab-plane. Question I What controls the Jc(  ) at low temperature-GL scaling

24 Where produces the correlated pinning effects in SP samples? BZO-1.5  m 1.RE 2 O 3 precipitate arrays 2.Stacking faults 3.Intrinsic pinning Standard-2.1  m RE 2 O 3 precipitate Stacking faults BZO nanorods RE 2 O 3 precipitate Possible sources TEM by T. Kametani

25 1. Stacking faults or precipitate arrays? Compare PLD samples…………. Stacking faults PLD-LTG No stacking faults or threading dislocations Y2O3Y2O3 PLD-HTG SFs are the only visible pinning centers Ppts are the only visible pinning centers TEM by T. Kametani TEM by F. Kametani

26 Stacking faults are effective pinning centers both at low temperature and high temperature. Mainly stacking-faults Black 1T Red 4T Green 9T Mainly precipitates Precipitates are effective pins along c- axis especially at high temperature Due to high Lorentz force

27 For YBCO, the separation s of the CuO layers is around 0.4nm, while the coherence lengths are  c = 0.3 nm,  ab = 1.6nm at T = 0, Tc=92K and Thus coherence length becomes shorter than separation of the CuO layers  c = 0.4nm = s when T  40K R. M. Schalk et. Al., Cryogenics 1993 vol.33 No Intrinsic pinning? c axis  Kes law It states that only the c-axis component of the applied magnetic field affects the critical current density. T-T model

28 T-T and Kes model fits for Turbo (double layer SP) Intrinsic pinning is not dominant at low field Kes model predicts a higher ab-peak beyond real data. At high fields, He gas bubble heating may explain greater deviation T-T model is a good fit at high fields

29 Question II: Can we make Jc (  ) broader? SP sample with BZO nanorodsSP sample without BZO They show similar J c (  ) to previous three samples. BZO-containing sample show broader maximum around ab-plane. No c-axis peak is observed even at low fields.

30 BZO-containing sample has broader J c (  ) at 10T and 4.2K Blue BZO-0.9  m Green: standard-2.1  m Magenta: double layer-2.1  m

31 Summary for J c (  ) study  J c (  ) at low temperature and high background magnetic fields  All samples show similar Jc(  ) even if they are very different at high temperature (77K) and low field (1T).  At low fields (  1T), there is a broad maximum around ab-plane.  The broad maximum evolves to a cusp which becomes sharper with increasing magnetic field.  No measureable c-axis maximum is observed regardless of sample and fields.  GL fitting of the angular dependence of J c  At low fields, random pinning centers are dominant in the all angle range.  At high fields, correlative pinning along ab-plane takes over.  J c (H) at low temperature (4.2 K)  Jc is almost magnetic field-independent along ab-plane.  Jc decreases significantly with increasing field along c-axis.  TEM images show that SP CCs have SFs, RE 2 O 3 precipitate arrays and intrinsic pinning as potential ab-plane correlated pinning centers  Stacking faults are effective pins from 4.2K to 77K and fields below 9T.  Intrinsic pinning is negligible above 30K but becomes stronger at lower temperature.  Jc can be greatly enhanced by precipitates except around ab-plane.  BZO-containing samples have reduced anisotropy and broader peak around ab-plane

32 What we have done  Can we eliminate thickness dependence of J c in sample with strong 3D pinning?  Thickness independent J c is obtained in Dy-doped MOD/RABiTS coated conductor except the significantly degradation of Jc near buffer layer.  Dy 2 O 3 nanoparticls is attributed to the strong 3D pins for the t-independent J c  Uncertainty of current-carrying cross-section due to the roughness of thin YBCO layer is the possible reason for lower Jc near buffer layer.  J c (  ) Study at low temperature and very high magnetic fields  What is J c (  ) at low temperature at background magnetic fields?  At low field, J c (  ) is GL-like regardless of sample.  GL-like J c (  ) evolves to cusp-like with the increasing of magnetic fields.  What are effective pins at low temperature?  3D random pins, atomic disorder are dominant at low field  Correlative pins, RE 2 O 3 precipitate arrays, stacking faults and intrinsic pinning are dominant at higher fields around ab-plane Stacking faults are effective pinning centers around ab-plane at temperature regime from 77K to 10K below 9T Precipitate arrays enhance J c except around ab-plane at temperature regime from 77K to 10K below 9T Pinning effect from intrinsic pinning become evidence at low temperature  Can we modulated J c (  ) at low temperature and high magnetic fields?  It is possible to obtain high J c along ab-plane by enhance the density of stacking faults  BZO-containing sample make J c (  ) broader

33 What is our next work  Systematic study of J c (t)  Understand what cause Jc degradation near the buffer layer  Set of samples, grown by different process with various additions on multiple substrates will be studied.  Systematic study of J c (  )  Perform J c (  ) measurement at low temperature and very high fields above 9T.  Samples with stacking faults only  Samples with precipitates arrays only  Samples with BZO nanorods  Intrinsic pinning Intrinsic pinning  Extend the measurement regime of temperature and external magnetic fields.  Span the sample set with different pins  YBCO thin film growth  Facility  1kA capability probe for high field and high Ic measurement;   100A rotator for small sample angular dependence Jc measurement at low temperature and very high fields;

34 What we want to know  What is the general J c (t)? Does it dependent on the process, second-phase addition or substrates?  Is there other effective strong pins to eliminate thickness dependence of Jc.  RE 2 O 3 nanoparticls  2D correlative pins, for example, BZO nanorods, are potential effective pins based on Feldmann’s work.  How can we keep high J c through thickness by a homogenous microstructure with a high density effective strong pins?  What are effective pins at very high magnetic fields?  Are stacking faults effective at very high magnetic fields?  What role does intrinsic pinning play at very high magnetic fields?  Does BZO nanorods affect J c (  ) at very high fields?  How the correlative pins modulated J c (  ) and why?  Where is the pinning effect from? the second phases themselves, the strain corresponding to nanoparticles or other defects ?  Is there other pins exist to modulate J c (  )?  What is the relation between J c at different temperature and different fields regime?  J c (  ) at high temperature and low fields, like 77K, 1T and Jc(  ) at low temperature and very high magnetic fields  J c (H) and J c (  )

35 Thanks for your attention

36 Fields Samples 1T3T4T5T10T15T20T25T30T Standard BZO Double layer % Zr doping49 M3-674 FS M MS FWHM values of samples measured at 4.2K SamplesStandardBZODouble layer 10% ZrM3-674 FSM MS BZO nanorod NOYesNOYesNOYes BZO-containing sample has higher FWHM at low field regime low temperature