Reel-to-reel ex situ conversion of high critical current density electron-beam co-evaporated BaF₂ precursor on RABiTS

A reel-to-reel furnace equipped with seven transverse flow modules has been developed to continuously process ex situ BaF₂ precursor coated conductors. Experimental results have revealed that as the precursor thickness increases, the processing parameters need to be adjusted in order to obtain high quality YBCO films with J_c's greater than 1 MA cm^-2 at 77 K and self-field. For thin precursors, a relatively aggressive conversion environment of high P(H₂O) can be used to obtain epitaxial YBCO with an extremely small amount of undesirable random or a-axis grains. Thin 0.3 μm YBCO was converted on a 2.3 m long by 1 cm wide Ni RABiTS substrate, yielding an end-to-end I_c of 18.9 A (J_c = 0.63 MA cm^-2). 16 cm sectional measurements showed that the tape performance is uniform, with a mean I_c of 19.1 A (0.64 MA cm^-2) and a 5.4% SD. In thick precursors, however, a less aggressive environment in the form of low P(H₂O) is better suited for epitaxial YBCO nucleation during the incubation period. By using a low initial P(H₂O) with progressively higher values during later stages of conversion, an end-to-end I_c of 92 A (1.02 MA cm^-2) was obtained in 1.1 m long by 1 cm wide 0.9 μm YBCO on better-textured Ni-W RABiTS. This conductor possesses uniform I_c according to 1 cm sectional measurements with a mean I_c of 94.4 A (1.05 MA cm^-2) and only 4.3% SD. Further adjustment in the P(H₂O) schedule resulted in a 45 cm long sample with end-to-end I_c of 111.8 A (1.24 MA cm^-2), a mean I_c of 114.8 A (1.28 MA cm^-2) and 4.8% SD. The highest I_c of 138 A (1.53 MA cm^-2) was obtained in a short 0.9 μm YBCO sample on a pre-treated Ni-W RABiTS substrate.