Preparation Of High Temperature Superconducting YBa₂Cu₃O_7-δ Coatings And The Improvement Of The Weak Links

At present, because of the high manufacture cost of the high temperature superconductor YBa₂Cu₃O_7-δ (YBCO) tapes, it is very worthy of investigating possible YBCO coatings with high critical current density Jc at relatively low cost. In this paper, we prepared YBCO films by sol-gel method and by powder melting growth (PMG) technique. In order to enhance the superconducting properties, Ca was doped into YBCO films to improve the weak links between grain boundaries (GBs).The superconducting properties of the YBCO films, prepared by sol-gel method and by PMG technique, were studied. EDTA was chosen as complexant to prepare M(EDTA)n- gels, where M is the Y, Ba or Cu cation, or any combination of them. The sol in certain concentration was dipped onto the [100] MgO single-crystal substrate. The YBCO film with c-orientation formed after being sintered at 900oC. The relationship of resistance and temperature (R-T curve) showed low superconducting transition temperature Tc. The YBCO coatings with c-orientation were prepared on [100] MgO and [100] YSZ (yttrium stabilized zirconia) single-crystal substrates by PMG technique. Possibly because of the reaction of the YBCO liquid phase with the MgO substrates, and then the corresponding diffusion of Mg to the YBCO crystal at high temperature, especially along the grain boundary regions, we found the zero-resistance transition temperature dropped to Tc0=78K and the superconducting transition width increased (ΔTc=2.5K) for the coatings on [100] MgO substrates. The coatings exhibited the characteristic of weak links. In the cases of [100] YSZ substrate, the Tc0 of the YBCO coatings reached 92K.In order to examine the low Tc, we investigated the forming mechanism of YBCO films prepared by sol-gel method. Analysis of Fourier transform infrared (FTIR) spectra were carried out for M(EDTA)n- dried gels and the powders calcined at 250oC, 300oC and 500oC, respectively. It is showed that the M(EDTA)n- complexes did not decompose at the same temperature, which led to the composition segregation. The BaCO3 formed at low temperature was the main obstacle for the formation and the growth of YBCO.In order to increase the Jc of the YBCO coatings by improving the superconducting coupling between YBCO grains, we investigated the structure and superconducting properties of Y1-xCaxBa2Cu3O7-δ(x=0.05, 0.1, and 0.2, respectively) coatings prepared by PMG technique on [100] YSZ single-crystal substrate. It is showed that (1) The lattice parameters of the YCBCO powders, prepared by sol-gel method, supported the substitution of Ca for Y in the YBCO; (2) Ca-doping reduced the Y2BaCuO5 (Y211) phase in the YCBCO coatings on [100] YSZ single-crystal substrate; (3) By substituting Ca(â…¡) for the Y(â…¢), the YCBCO is of hole over-doping and the Tc0 decreased with the Ca content. The highest Jc(77K) value of YCBCO (x=0.05) coatings implied that Ca-doping improved the weak links between GBs of the polycrystalline YBCO film.