Flux Pinning Study of REBa₂Cu₃O₇₋[subscript δ] Coated Conductors for High Field Magnet Applications
Xu, Aixia, 1978- (author)
Larbalestier, David (professor directing dissertation)
Jaroszynski, Jan (professor co-directing dissertation)
Brooks, James (university representative)
Hellstrom, Eric (committee member)
Van Sciver, Steven (committee member)
Department of Mechanical Engineering (degree granting department)
Florida State University (degree granting institution)
REBa2Cu3O7-&delta (REBCO, RE = rare earth) coated conductor (CC) holds great promise for high field magnet applications owing to its strong irreversibility field (Hirr), low electromagnetic anisotropy (&gamma2), and high critical current density (Jc). The work of this thesis is tightly related to the development of the funded 32 T, all-superconducting magnet project at the ASC/NHMFL. My concern is thus for understanding the optimizing of the working parameters of REBCO CC at low temperatures T, and very high magnetic fields H, focusing on how to enhance Ic and to reduce its angular dependence. Increasing the active cross-section is a direct and economical strategy to enhance the current-carrying capability for REBCO coated conductors. Unfortunately, the high Jc in thin REBCO layers is seldom sustained in thick layers because of difficulties of thick film growth control. In the presence of strong 3D (pin separation far less than film thickness) pins, a high and thickness-independent (Jc) should result. One of major tasks of this thesis is to explore what are the effective strong 3D pins that develop a high and thickness-independent Jc. High and weak thickness-dependent Jc at 77 K is obtained on most recent coated conductors, and BZO nanorods and RE2O3 nanoparticles are identified as strong 3D pins contributing to this respectable Jc performance. At 77 K, we found that the strong pinning of BZO nanorods remains at least up to 9 T, whereas the strong pinning of RE2O3 nanoparticles gradually evolves to weak collective pinning as the irreversibility field is approached. The second principal part of this thesis concentrates on understanding and minimizing the angular dependence of Jc. Our study is based on the following procedure. First, we investigated the angular dependence of Jc (Jc(&theta)) in the working condition of the future 32 T all-superconducting magnet, i.e. 4.2 K and high magnetic field up to 31 T. Our work shows that the low temperature Jc(&theta) is Ginzburg-Landau-like at low fields and cusp-like towards the ab-plane at high fields. More interestingly, the typically observed Jc c-axis peak of BaZrO3 nanorods (BZO)-containing REBCO at high temperatures disappears at T &le 40 K. We observed that Jc(H||c) follows well a power law with exponent &alpha &approx 0.5 for coated conductors without BZO nanorods and &alpha &approx 0.7 for coated conductors containing BZO nanorods. More importantly, BZO-containing coated conductors show higher and broader Jc(&theta) at least up to 31 T, which is strongly beneficial to high field magnet applications. Finally, we performed Jc(&theta) study over a broad temperature domain, 4.2 K to Tc and magnet fields up to 31 T. We found that weak uncorrelated pinning dominates the low temperature Jc. It raises and broadens Jc in the full angular range. We conclude that BZO nanorods induce dense random defects, like oxygen vacancies, atomic disorder etc., which can exert a large pinning effect at low temperatures where thermal fluctuations are small. Near the ab-plane there is clear evidence for strong correlated defects, which we deduce is due to intrinsic pinning by the Cu-O charge reservoir layers The last section discusses the pinning design relevant to coated conductors. Two types of thin films made by pulsed laser deposition (PLD) with well designed defects were studied: one shows Y2O3 precipitates and a high strain which also generates dense point pinning; the second one presents dense stacking faults. The main conclusion is that Jc(&theta) can be modulated by tailoring the strain introduced by the mismatch between second phases and REBCO layer. Intrinsic pinning governs Hirr(H||ab), and stacking faults govern Jc(H||ab) and enhance Hirr(H||ab) at T &ge 40 K. The thesis contains 8 chapters. The first chapter introduces the background and motivation of this work. Chapter 2 presents thickness dependence of Jc studies. Chapter 3 through chapter 6 present Jc(H, T, &theta) characterization and pinning studies on REBCO coated conductors. Chapter 7 presents the pinning mechanisms found in PLD thin films grown with designed pinning structures. The summary and future work is presented in chapter 8.
angular dependence, coated conductor, critical current density, high field magnet, superconductivity, thickness dependence
June 18, 2012.
A Dissertation submitted to the Department of Mechanical Engineering in partial fulfillment of the requirements for the degree of Doctor of Philosophy.
Includes bibliographical references.
David Larbalestier, Professor Directing Dissertation; Jan Jaroszynski, Professor Co-Directing Dissertation; James Brooks, University Representative; Eric Hellstrom, Committee Member; Steven Van Sciver, Committee Member.
Florida State University
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