[WBP1] Conductor Processing & Evaluation

[WBP1-26] Microstructural Observation for Understanding Delamination Mechanisms in YBa₂Cu₃O_y Coated Conductor

*Yusuke Kawagishi¹, Ryo Teranishi¹, Taiki Wada¹, Akiyoshi Matsumoto² (1. Kyushu Univ. (Japan), 2. National Institute for Materials Science (Japan))

Keywords:

REBCO CCs,Delamination,Microstructural Observation

[Purpose] YBa₂Cu₃O_y (YBCO) coated conductors (CCs) are multilayer superconductors consisting of a metallic substrate, buffer layers, a capping layer, a superconducting YBCO layer, and stabilization layers. YBCO CCs exhibit excellent critical current density in high magnetic fields and are being explored for use in superconducting magnets. One of the challenges for such applications is suppression of delamination. Because YBCO CCs have diverse architectures, delamination behavior must be characterized for each architecture. For the capping layer in REBCO coated conductors, LaMnO₃ (LMO) has recently begun to be used in place of the conventional CeO₂. While delamination in CeO₂-capped conductors has been investigated extensively, studies on LMO-capped conductors are very limited yet. This study aims to elucidate the delamination mechanisms of LMO-capped YBCO CCs via microstructural analysis under pin-pull loading. [Method] Commercial 4-mm-wide YBCO conductors ( LLC) were cut into 10 mm long specimens. Delamination was induced with a (Ref. 1), applying the load perpendicular to the tape’s longitudinal axis of the tape. The delaminated surfaces were observed by scanning electron microscopy (SEM), and elemental mapping images were obtained by energy-dispersive X-ray spectroscopy (EDS). [Results] Fig.1 shows SEM image of delaminated surface. A central step contrast separates three regions: a wavy morphology on the right part, a uniformly pitted morphology on the left part, and a smooth area near the center-right part. Fig. 2 is an EDS elemental map of delaminated surface. Correlation with Figs. 1 and 2, it was identified that the wavy region, the uniformly pitted region and the smooth region are YBCO, Hastelloy substrate and LMO, respectively. [Consideration] We focused on interfaces of Al₂O₃/Hastelloy and YBCO/LMO and compared existence of residue of each upper layer. It was confirmed little residual Al₂O₃ and much residual YBCO as seen in Fig. 1. It is suggested interfacial strength of YBCO/LMO interface is higher than Al₂O₃/Hastelloy interface. In addition, Fig.1 revealed defects and microcracks in YBCO. In bulk ceramics, it is well known that stress concentration at crack tips is a major cause of fracture (Ref. 2). From this reference, it is considered that intralayer delamination in YBCO occurred as stress was concentrated at these defects and the cracks propagated. [Conclusion]