Carbon expelling scheme and required conditions for obtaining high-Jc YBa2Cu3O7-x film by metalorganic deposition using trifluoroacetates

High critical current density (J_c) YBa₂Cu₃O_7-x films were routinely prepared by metalorganic deposition using trifluoroacetates (TFA-MOD). Although the growth scheme of the YBa₂Cu₃O_7-x films has been studied, we still have two or more unsolved problems during heat-treatments. The first problem is chemical reaction, including the carbon-expelling scheme during the calcining process. Considering Gibbs free energy changes in the related chemical reactions and electronegativity of atoms, we explain the expelling scheme. During the calcining process metal-oxygen bonds remain after the decomposition. The fluorine partially attacks and replaces oxygen linked to Y and Ba. Consequently, these metal compounds become an amorphous matrix of metal oxyfluorides. Only CuO grows to be nanocrystalline. These results are consistent with the present precursor films derived from highly purified coating solution by the Solvent-Into-Gel method. The second problem is clarification of the required conditions to obtain high J_c YBCO film by TFA-MOD. Transmission electron microscopy revealed the existence of isolated Ba and Y oxides besides CuO grains within the YBa₂Cu₃O_7-x layer, which are derived from the normally purified coating solution. Optimal calcining processing, which certainly suppresses only the CuO grains, never improves the J_c of the films containing Ba and Y oxides. Thus the required condition for high J_c YBa₂Cu₃O_7-x films is a combination of a highly purified coating solution and an optimized calcining process. With that combination, we prepared 210 - 230 nm thick YBa₂Cu₃O_7-x films on CeO₂ buffered YSZ for which the J_c´s were 10.5 - 11.4 MA/cm² (77 K, 0 T) by four-probe transport measurement.