Low-pressure conversion studies for YBCO precursors derived by PVD and MOD methods

The rate at which thin film YBCO precursor converts to high J_c superconductor depends on many factors including the method by which the precursor is prepared, the precursor film thickness, and the conditions during precursor deposition and conversion. Using in-situ X-ray diffraction and reduced pressures, we have examined and compared conversion rates for precursors prepared by physical vapor deposition (PVD) and metal-organic deposition (MOD). For conversion conditions found to be optimal for obtaining high J_c and high precursor conversion rate (G_p) for PVD precursors, the G_p is nearly identical (∼0.2 nm/sec) for the both PVD and MOD precursors. The development of crystalline BaF₂ is, however, distinctly different for the two precursors. This may suggest that conversions of these PVD and MOD precursors follow different chemical pathways. For both precursors, higher G_p is realized using higher water pressure and ramp rate. Although G_p is higher, YBCO is randomly oriented and J_c is zero for the PVD precursor. The MOD precursor seems to be generally more tolerant of higher reaction rates than the PVD precursor. The reaction rate versus P_H2O determined for both precursor types shows a square root of P_H2O dependence of G_p. This behavior is consistent with a reaction rate limited by gas-phase HF removal.