The growth of YBCO films with high critical current at reduced pressures using the BaF₂ ex situ process

The growth of 0.9-1.0 μm thick YBa₂Cu₃O _7-δ (YBCO) films on biaxially textured Ni-3 at.% W (NiW) substrates using the BaF₂ ex situ process was investigated at reduced pressures. By varying the water vapour pressure (P_H2O), Y-BaF ₂-Cu-O (YBFCO) precursor films deposited by e-beam co-evaporation were converted at a reduced total pressure (P_total) of 50-55 Torr and conversion temperature (T_S) of 740°C for a wet conversion time (t_W) of 60 min. Critical current density (J_C) values greater than 1 MA cm^-2 for the thick YBCO films were obtained under the condition of varying P_H2O from low pressure to 10 Torr. The transition temperatures (T_C) of the samples were over 90 K with ΔT_C = 1.8-2.5 K. Pre-heat treatment of the precursor films on CeO₂/YSZ/Y₂O₃/Ni/NiW substrates under an O ₂ atmosphere condition before the conversion resulted in smooth surfaces without large secondary phase particles embedded in the films.