Water vapour pressure influence on the kinetics of the superconducting YBCO thin films epitaxic growth by the TFA–MOD method

We have found two different regimes in the kinetics of the YBCO formation depending on the water partial pressure at a constant temperature and total flow rate of the carrier gas. The first regime at low partial water pressure shows continual kinetics curves until the end of YBCO growth and the reaction is controlled chemically. The second regime at high partial water pressure shows irreproducible steps in the kinetics curves during the thin films YBCO growth. In this work we suggest that there is formation of a boundary layer of water (Nernst layer) when the partial water pressure is higher than 20 hPa at 795 °C for a total gas flow rate lower than 2.4 × 10⁻² m s⁻¹. These irreproducible steps dues probably to a water boundary layer formation can be eliminated by increasing the stirring rate of the carrier gas. The reaction order of YBa₂Cu₃O_7−x formation respect to the water pressure is n = 0.5 when the water boundary layer is not formed, but the apparent reaction order respect to the partial water pressure is zero or negative when the gas flow rate of the carrier gas is not big enough for the elimination of this water layer. This work also evidences that there is an intermediate step in the kinetics curves before the formation of YBCO. This step, which starts at low temperature during the heating ramp (400 °C) is attributed to the partial elimination of F from the BaF₂ precursor to form oxyfluoride compounds. So, at low total flow gas rates and low partial water pressures, the reaction is controlled by diffusion mechanism due to the formation of a HF boundary layer (Nernst layer), because the apparent order of YBCO formation is one respect to the stirring rate. Nevertheless, at high flow gas rates and low partial water pressures, the YBCO formation is controlled chemically, then the apparent order respect to the stirring rate is zero and the HF Nernst layer is eliminated. The apparent E_a for the oxyfluoride formation at low temperatures is only 18 Kj/mol indicating that this intermediate reaction is controlled by diffusion mechanism even at high stirring rate and relatively low partial water pressure. The apparent E_a for the YBCO formation at partial water pressures higher than 20 hPa for a total flow rate of 2.4 × 10⁻² m s⁻¹ is only 32 Kj/mol, indicating that the reaction control is mainly diffusive or mixed (diffusive and chemist).