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,) for PVD precursors, the G(p) is nearly identical (similar to 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 Gp is realized using higher water pressure and ramp rate. Although Gp 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 Gp. This behavior is consistent with a reaction rate limited by gas-phase HF removal.