Studies of YBa sub 2 Cu sub 3 O sub 7 sub - sub x formation in coevaporated Y-BaF sub 2 -Cu-O thin films on sapphire substrates

Degradation mechanisms for YBCO films deposited on sapphire substrates are studied. The films are synthesized by post-deposition annealings in a wet O sub 2 atm of coevaporated layers of Y-BaF sub 2 -Cu-O. XRD reveals impurity phases in the annealed samples, the most significant being BaAl sub 2 O sub 4 , Y sub 2 BaCuO sub 5 , CuO, and Y sub 2 O sub 3 . While zero-resistivity temps. of 1 mu m thick films typically are between 80 and 88 K, magnetic characterization indicates weak coupling between the superconducting grains, resulting in low transport critical currents at 77 K. The film properties of YBCO /sapphire are compared with those of YBCO processed in parallel on SrTiO sub 3 or LaAlO sub 3 . 29 refs.