Purified coating solution and growth scheme of the YBa2Cu3O7-x superconductors in metal organic deposition using trifluoroacetates

Metal organic deposition using trifluoroacetate (TFA-MOD) is one of the most promising approaches to YBa2Cu3O7-x superconductors. In this process, CuO nanocrystallites in calcined film never cause random orientation, which is a major problem in the other solution processes. A "quasi-liquid network" created during the firing process suppresses the influence of the nanocrystallites on the epitaxial structure. Impurities in the coating solution cause Y, Ba, and Cu compounds; a long calcining process leads to CuO grain growth in the YBa2Cu3O7-x, film. These compounds deteriorate superconducting properties in two ways. One is a direct physical obstacle and the other is an indirect chemical influence. In the latter mode, non-stoichiometric area, which is induced by the compound, breaks up the quasi-liquid and the quasi-liquid changes into the other phase. Therefore, the actual YBa2Cu3O7-x, film is thinner and the superconducting properties degrade. To reduce the influence of CuO nanocrystallites, excess humidity in the firing gas is effective because increased quasi-liquid consumes the nanocrystallites. However, the improvement is imperfect. A combination of highly purified coating solution and optimal calcining process is required to obtain high performance YBa2Cu3O7-x superconductors. With such a combination, YBa2Cu3O7-x, films having critical current density of 5-7 MA/cm(2) (77 K, 0 T) are routinely obtained.