Optimization of buffer layers on rolled-ni substrates for high current YBCO and Tl,Bi-1223 coated conductors using ex-situ precursor approaches

High current YBa₂Cu₃O_7.y (YBCO) and T.7sB'o.₂₂Sr,.,(Ba.4Ca₂Cu₃O₉.y (Tl,Bi-1223) coated conductors were fabricated on Rolling-Assisted Biaxially Textured Substrates (RABiTS) with a layer sequence of CeO2/YSZ/CeO2/Ni. The top and bottom CeO2 (Cerium oxide) layers were deposited epitaxially on textured-Ni (100) substrates using reactive evaporation of Ce métal. The thickness of the CeO2 films was 200-400 A. The YSZ (Yttria Stabilized Zirconia) layers were also deposited epitaxially on CeO2-buffered Ni substrates either by rf magnetron sputtering or e-beam evaporation. The thickness of the YSZ films was typically 30009000 A. The e-beam CeO2 films were dense, crack-free and columnar. The microstructure of sputtered YSZ was dense and the e-beam deposited YSZ was porous. To understand the stability of these buffer layers, the as-grown CeO2-buffered YSZ (both sputtered and e-beam)/CeO2/Ni substrates were annealed in a controlled oxygen furnace at various temperatures. RBS studies indicate that the YSZ sputtered films were quite stable up to 780 °C and 200-mTorr oxygen. For e-beam YSZ films, there was an indication of diffusion of oxygen through these buffers into the Ni substrate. The Tl,Bi-1223 films were grown on all e-beam buffers using pulsed laser ablation of precursor films followed by post-annealing. The YBCO films were grown on e-beam/sputtered buffers using e-beam co-evaporated YBaF2-Cu precursors followed by post-annealing.