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

High critical current density (J(c)) YBa2Cu3O7-x films were routinely prepared by metalorganic deposition using trifluoroacetates (TFA-MOD). Although the growth scheme of the YBa2Cu3O7-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 YBa2Cu3O7-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) YBa2Cu3O7-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 YBa2Cu3O7-x films on CeO2 buffered YSZ for which the J(c)'s were 10.5-11.4 MA /cm(2) (77 K, 0 T) by four-probe transport measurement.