Solution-processed lanthanum zirconium oxide as a barrier layer for high I_c-coated conductors

Abstract

High-quality lanthanum zirconium oxide (La₂Zr₂O₇ or LZO) films have been deposited and processed on Ni–W substrates using a sol-gel processing approach. It has been demonstrated that crack-free coatings with thicknesses up to 100 nm can be processed in a single step, while thicker coatings (200–225 nm) were processed using a multiple coating and annealing process. Using simulated metalorganic deposition (MOD)-YBa₂Cu₃O_7−δ (YBCO) processing conditions, the barrier properties of the sol-gel LZO coating with a thickness of 120 nm were found to be comparable to that of the standard 3-layer buffer stack deposited using physical vapor deposition. Secondary ion mass spectroscopy depth profile analysis of LZO films annealed in oxygen-18 shows that LZO effectively stops the diffusion of Ni within the first 80–100 nm. Using MOD processes, a CeO₂ cap layer and superconducting YBCO layer were deposited on sol-gel LZO/Ni–W. For the first time, using such an all-solution conductor architecture, a critical current (I_c) of 140 A/cm with a corresponding critical current density (J_c) of 1.75 MA/cm² has been demonstrated. Using a very thin Y₂O₃ seed layer (∼10 nm) deposited by electron beam evaporation; improved texture quality in the LZO layers has been demonstrated. The performance of the LZO deposited on these samples was evaluated using a sputtered CeO₂ cap layer and MOD YBCO layer. Critical currents of up to 255 A/cm (3.2 MA/cm²) with 0.8-μm-thick YBCO films have been demonstrated, comparable to the performance of films grown using physical vapor deposited yttria stabilized zirconia as a barrier layer. Similar experiments using an MOD-CeO₂ cap layer and MOD-YBCO layer yielded critical currents of 200 A/cm (2.5 MA/cm²) with 0.8-μm-thick YBCO films.