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

Abstract

High current YBa₂Cu₃O_7-y (YBCO)
and Tl_0.78Bi_0.22Sr_1.6Ba_0.4Ca
₂Cu₃O_9-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 metal. The thickness
of the CeO₂ films was 200-400 Å. 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
3000-9000 Å. 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 Y-BaF
₂-Cu precursors followed by post-annealing