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Electron Microscopy Imaging of Flux Pinning Defects in Ybco Superconductors

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Electron Microscopy Imaging of Flux Pinning Defects in Ybco Superconductors ELECTRON MICROSCOPY IMAGING OF FLUX PINNING DEFECTS IN YBCO SUPERCONDUCTORS BY Anne-Hélène PUICHAUD A thesis submitted to the Victoria University of Wellington in fulfilment of the requirements for the degree of Doctor of Philosophy Victoria University of Wellington 2016 Abstract High-temperature superconductors are of great interest because they can transport electrical current without loss. For real-world applications, the amount of current, known as the critical current Ic, that can be carried by superconducting wires is the key figure of merit. Large Ic values are necessary to off-set the higher cost of these wires. The factors that improve Ic (microstructure/performance relationship) in the state-of-the-art coated conductor wires based on YBa2Cu3O7 (YBCO) are not fully understood. However, microstructural defects that immobilise (or pin) tubes of magnetic flux (known as vortices) inside the coated conductors are known to play a role in improving Ic. In this thesis, the vortex-defect interaction in YBCO superconductors was investigated with high-end transmission electron microscopy (TEM) techniques using two approaches. First, the effect of dysprosium (Dy) addition and oxygenation temperature on the microstructure and critical current were investigated in detail. Changing only the oxygenation temperature leads to many microstructural changes in pure YBCO coated conductors. It was found that Dy addition reduces the sensitivity of the YBCO to the oxygenation temperature, in particular it lowers the microstructural disorder while maintaining the formation of nanoparticles, which both contribute to the enhancement of Ic. In the second approach, two TEM based techniques (off-axis electron holography and Lorentz microscopy) were used to study the magnetic flux vortices. Vortex imaging was attempted with a TEM operated at 300 kV on both a YBCO crystal as well as a YBCO coated conductor. Many challenges were encountered including sample preparation, inhomogeneity, and geometry, in addition to the need to perform measurements at cryogenic temperatures. Although vortices were not able to be observed in the coated conductors, tentative observation of vortices in a YBCO crystal was made using Lorentz microscopy. Improvements for future electron holography experiments on YBCO at low voltage are suggested. This work represents a pioneering step towards directly imaging vortices in YBCO using more widely available microscopes with the aim of better understanding flux pinning to ultimately boost Ic in superconducting wires. i ii À mes parents iii iv Acknowledgements It is a pleasure for me to acknowledge many people that have contributed throughout the last three years and without whom all this work would not have been the same. First I would like to express my deep gratitude to my supervisors: Dr Ruth Knibbe, for her advice, support and for believing in me from the beginning. I am grateful for her time; despite her busy schedule, she found moments for personal and professional guidance. I want to thank my co-supervisor, Dr Stuart Wimbush, for his input in this project, the useful discussions and suggestions. From the Robinson Research Institute, I am thankful to the many staff members who helped me and I am especially grateful to Dr Evgeny Talantsev for his assistance in coated conductors synthesis and X-ray diffraction. Thank you to Dr James Storey for his help on so many levels and to Sarah Spencer for her support. I want to thank David Flynn, at Victoria University of Wellington (VUW), for his help with electron microscopy, and Martin Ryan and Dr Bridget Ingham, at Callaghan Innovation. This work was funded by the Royal Society of New Zealand with a Marsden Fund. I consider myself lucky for having had the opportunity to work with many amazing scientists around the world. I want to thank Dr Takeshi Kasama from the Center for Electron Nanoscopy at the Technical University of Denmark (DTU Cen), Dr Sadegh Yazdi (now at Rice University, United States) and Dr James Loudon, from the University of Cambridge, for their technical assistance, and valuable discussions during my time at DTU Cen. The holography work presented in Chapter 5 would not have been possible without them. Thank you to Zoltán Balogh for training and help on specimen preparation using FIB-SEM. Thank you to Andrew Borrow and to all the staff members of DTU Cen for making me feel welcomed in your research centre. I am very grateful to Dr Laure Bourgeois and Dr Peter Miller for generously welcoming me at Monash Centre for Electron Microscopy, Australia. Thank you to Dr Jisheng Ma and Dr Amelia Liu for their assistance in specimen preparation. A special thanks to all my family and friends from either side of the world for their unconditional support, to the PhD students I met along the way that became friends. A great thank you to Sophie, Clarisse and Pauline, for never doubting me, to Geoff, Kristina, v Julia, Chris, Tushara, Sarah, Maryam, Camille, Simon, Hélène, Françoise, Laure, Judy, and Dale. Finally, I want to express my deepest thank you to Matt for his endless love, support and reassurance. vi Table of contents ABSTRACT .................................................................................................................................... I ACKNOWLEDGEMENTS ............................................................................................................... V TABLE OF CONTENTS ................................................................................................................ VII LIST OF FIGURES ......................................................................................................................... XI LIST OF ABBREVIATIONS AND ACRONYMS ............................................................................... XIV LIST OF SYMBOLS ...................................................................................................................... XV INTRODUCTION ........................................................................................................................... 1 BACKGROUND ..................................................................................................... 5 Superconductivity ............................................................................................................................................. 5 YBCO coated conductors ................................................................................................................................. 7 1.2.1 Synthesis .............................................................................................................................................................................. 7 1.2.2 Electrical properties ....................................................................................................................................................... 9 Yttrium barium copper oxide ....................................................................................................................... 9 1.3.1 Structural characteristics ............................................................................................................................................. 9 1.3.2 Defects and precipitates in YBCO coated conductors ................................................................................... 11 Conventional imaging using transmission electron microscopy ................................................... 15 1.4.1 TEM and STEM ............................................................................................................................................................... 16 1.4.2 3D electron tomography ............................................................................................................................................ 17 Vortex characteristics in YBCO ................................................................................................................... 18 1.5.1 Penetration depth ......................................................................................................................................................... 18 1.5.2 Coherence length .......................................................................................................................................................... 20 1.5.3 Vortex spacing ................................................................................................................................................................ 22 Vortex imaging using TEM electron holography techniques .......................................................... 23 1.6.1 Principle of electron holography ........................................................................................................................... 23 1.6.2 In-line electron holography (Lorentz microscopy) ....................................................................................... 24 vii 1.6.3 Off-axis electron holography ................................................................................................................................... 25 1.6.4 Vortex imaging using electron holography in the literature ..................................................................... 30 1.6.5 Local applied field in holography experiments ............................................................................................... 33 Background summary and aims of the thesis ....................................................................................... 34 EXPERIMENTAL METHODS
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