Superconducting Y-Ba-Cu-0 films were successfully prepared by a melt growth method without crucible on (100) MgO and sapphire substrates using a Ba,Cu,O,, flux. It was found that without a Y,BaCuO, (211) buffer layer, film formation is not feasible due to a wetting problem. The 21 1 layer reacts with the melt and is partly converted to theYBa,Cu,O, (123) film. The 123 film thickness is f to 3 that of the former 211 layer, whose quality in turn dominates the surface structure of the 123 film. The degree of preferred (OW) orientation is greater than 99% for the films on the 211 buffered (100) MgO, and 80% for those on the (100) sapphire. 2 to 3 pm superconducting films with T,~o,,,t > 80 K and T,~,,,, > 60 K were reproducibly obtainable on (100) MgO.
The effects were studied of surface buffer layers, a sputtered gold or platinum film and/or a screen-printed Y,BaCuO, (211) layer, on the microstructure, preferred orientation and superconducting transition of YBa,Cu,O,-, (123) films, grown by a melt method without crucible on polycrystalline alumina substrates. The degree of (ON) preferred orientation (DPO) reached 89% as Pt buffer alone was used, but this was without a superconducting transition. A combined 211 layer on a sputtered metal layer resulted in films with T,-,,,, of above 55 K, smoother film surfaces and DPO of 44%. The growth of preferred orientation is attributed to an inhomogeneous (ON) nucleation effect during solidification. Reduced surface cracks and better connection among crystallites of the films grown on a combined buffer layer are responsible for the measurable superconducting transition.
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