Strongly enhanced current densities in superconducting coated conductors of YBa₂Cu₃O_7−x + BaZrO₃

Abstract

There are numerous potential applications for superconducting tapes, based on YBa 2 Cu 3 O 7-x (YBCO) films coated onto metallic substrates [1]. A long established goal of more than 15 years has been to understand the magnetic flux pinning mechanisms which allow films to maintain high current densities out to high magnetic fields [2]. In fact, films carry 1-2 orders of magnitude higher current densities than any other form of the material [3]. For this reason, the idea of further improving pinning has received little attention. Now that commercialisation of conductors is much closer, for both better performance and lower fabrication costs, an important goal is to achieve enhanced pinning in a practical way. In this work, we demonstrate a simple and industrially scaleable route which yields a 1.5 to 5-fold improvement in the in-field current densities of already-high-quality conductors.The sources of enhanced pinning in vapour-grown YBCO films are the natural point, line and volume imperfections, probably the most significant of these being the dislocations perpendicular to the substrate plane of film [4]. In terms of dimensionality, dislocations are nearly ideal for pinning magnetic flux lines. However, the density of dislocations is dominated by the growth island size and their spacing is estimated to be rather large (~100 nm-~500 nm) [3,5]. To increase dislocation density, an obvious way would be to decrease the island size which the dislocations bound [6], e.g. by reducing growth temperature, but this is non-trivial because the crystalline quality of the film would be compromised.Heavy ion irradiation has been shown to reduce vortex mobility [7,8] but it is impractical for treatment of coated conductors. Other work involving growth of films on mis-cut single crystal substrates has demonstrated that introduction of columnar growth defects improves J c (77K) by up to 50%, but only at a particular field orientation and magnitude [9]. Other ideas for improving pinning are introduction of defects by multi-layering or addition of particles on the substrate surface [5,10]. Using these methods some improvements appear possible in thin films on single crystal substrates.In this work, prompted by our earlier report that suggested the possibility of enhanced pinning in the presence of epitaxial second phases [11], we study BaZrO 3 additions to YBCO. The main reasons for the choice of BaZrO 3 are a) while it can grow heteroepitaxially with YBCO it has a large lattice mismatch (~ 9%) so strain between the 2 phases could introduce defects for enhanced pinning, b) it is a high melting temperature phase and so growth kinetics should be slow, leading to small particles, and c) Zr does not substitute in the YBCO structure [12]. Indeed, single crystals of YBCO are often grown in BaZrO 3 -coated crucibles [13]. BaZrO 3 has also been previously investigated as a pinning centre in bulk, melt processed YBCO [14,15]. However, it was found that the BaZrO 3 agglomerated at the growth fronts of the grains, and, because of heteroepitaxi...

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“…After the seminal works demonstrated that second-phase additions 4,12 to RE-Ba-Cu-O (rare earth (RE)) (REBCO) thin films grown using pulsed laser deposition (PLD) significantly improved the current carrying performance in an applied magnetic field, 3,[13][14][15] it was shown that the morphology of the inclusions depends on the synthesis 1 Materials Physics and Applications Division, Los Alamos National Laboratory, Los Alamos, NM, USA; 2 Graduate School of Science and Technology, Seikei University, Tokyo, Japan; 3 Nanostructures Research Laboratory, Japan Fine Ceramics Center, Nagoya, Japan; 4 National Institute of Advanced Industrial Science and Technology, Ibaraki, Japan; method (that is, in situ vs ex situ). 3,16 Moreover, recent work has shown that large quantities (⩾10%) of second-phase additions are required to obtain the desired level of performance in superconductors.…”

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“…This approach resulted in pancake-like 211 particles with an average size of 15 nm distributed in the YBCO film, which produced some enhancement in pinning along c and much higher improvements for H//a,b. Correlated pinning along the c-axis was also observed when seemingly randomly distributed 10-nm-wide BaZrO 3 (BZO) particles were inserted in the YBCO film by ablating from a PLD target synthesized using Ba excess and added Zr [41]. In the last study the observed correlated pinning along the c-axis was related to misfit dislocations between the nanoparticles and the superconducting matrix, which aligned along the c direction, with an areal density of 400 µm -2 compared to 80 µm -2 for undoped YBCO films.…”