Ultralow-fluorine sol–gel deposition of thick YBCO multilayer films

Three kinds of low-fluorine YB₂Cu₃O_7−x (YBCO) starting sols with F/Ba = 6, 4 and 2 were applied to fabricate multilayer YBCO films on LaAlO₃ substrates by multi-coating technique. Influence of the fluorine content in the sols and the thickness of single-coated layer on the properties of multilayer YBCO films was discussed in detail. The critical current density (J _c) of multilayer YBCO films derived from F/Ba = 6 and 4 sols dropped sharply with thickness. However, using sol with F/Ba = 2, the J _c of multilayer films decreased slowly with thickness. Moreover, the thinner the single-coated film, the less the amount of a-axis-oriented grains inside the multilayer films, resulting in the improvement in J _c of the final thick films. Through multi-coating technique, YBCO films as thick as 1.1–1.2 μm were prepared. The J _c of the optimized thick YBCO films reached a high value of 4.23MA/cm² at 77 K, 0T.

Compared with the YBCO films derived from other low-fluorine sols, the YBCO films prepared using the ultralow-fluorine sol with mole ratio of F/Ba = 2 show the higher superconducting current transportation ability. Using this solution, the “thickness effect” of multilayer thick YBCO films is degraded obviously. Moreover, lowering the thickness of every layer film through controlling the withdrawal speed can also suppress the formation of a-axis grains, thus enhancing the critical current density of final thick YBCO films.