A Double-Sided Oxide Buffer Architecture to Facilitate REBCO Growth for High Performance

In this article, we present an approach based on a double-sided oxide buffer architecture to reduce cost and improve the critical current of REBa₂Cu₃O_7+δ (RE = rare earth) superconductor tapes. We detail the successful reel-to-reel manufacturing process of a 20-m-long double-sided oxide buffer tape, which facilitates the growth of double-sided REBCO tapes. Our oxide buffer stack consists of a 45-μm-thick in-house electropolished Hastelloy substrate optimized for an average surface roughness (Ra) < 1 nm on both sides, followed by amorphous alumina diffusion barrier on both the sides using midfrequency reactive magnetron sputtering. We optimized the alumina thickness to prevent the delamination of thicker REBCO films. We deposited an 8-nm-thick yttria oxide seed layer and a 10-nm-thick biaxially textured magnesium oxide (MgO) template layer, using ion-beam-assisted deposition (IBAD), on both sides of the tape. This was followed by sequential deposition, on both sides of the tape, of a 60-nm-thick homo-epitaxial MgO film using midfrequency reactive magnetron sputtering and a 60-nm-thick cap layer of lanthanum manganate (LaMnO₃ = LMO) using radio frequency reactive magnetron sputtering. We employed a sacrificial tape in IBAD and other sputtering techniques to obtain scratch-free buffer films on both sides. We characterized the buffer quality using 2-D XRD omega and phi scans of the LMO film, and the full-width at half-maximum of out-of-plane texture and in-plane texture on both sides is in the range of 3.5 ± 0.2° and 7 ± 0.5°, respectively. In order to characterize the uniformity of the buffer quality over long tapes, 2-D XRD scans on both sides of the buffer were employed for every 2 m, and the statistical variation along the tape length was determined.